Interstate Power & Light (IPLDP) 10-K 23 Feb 18 2017 FY Annual report Financial data

Document And Entity Information

Document And Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2017	Jan. 31, 2018	Jun. 30, 2017
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Entity Registrant Name	ALLIANT ENERGY CORP
Entity Central Index Key	352,541
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		231,356,336
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 9.3
IPL [Member]
Entity Registrant Name	INTERSTATE POWER & LIGHT CO
Entity Central Index Key	52,485
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		13,370,788
WPL [Member]
Entity Registrant Name	WISCONSIN POWER & LIGHT CO
Entity Central Index Key	107,832
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		13,236,601

Consolidated Statements Of Inco

Consolidated Statements Of Income - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating revenues:
Electric utility	$ 2,894.7	$ 2,875.5	$ 2,770.5
Gas utility	400.9	355.4	381.2
Other utility	47.5	48.6	57.9
Non-utility	39.1	40.5	44
Total operating revenues	3,382.2	3,320	3,253.6
Operating expenses:
Electric production fuel and purchased power	818.1	854	837.7
Electric transmission service	480.9	527.9	485.3
Cost of gas sold	211.4	194.3	219.1
Asset valuation charges for Franklin County wind farm	0	86.4	0
Other operation and maintenance	651	606.5	629.5
Depreciation and amortization	461.8	411.6	401.3
Taxes other than income taxes	105.6	102.3	103.7
Total operating expenses	2,728.8	2,783	2,676.6
Operating income (loss)	653.4	537	577
Interest expense and other:
Interest expense	215.6	196.2	187.1
Equity (income) loss from unconsolidated investments, net	(44.8)	(39.6)	(33.8)
Allowance for funds used during construction	(49.7)	(62.5)	(36.9)
Interest income and other	(0.5)	(0.5)	(0.7)
Total interest expense and other	120.6	93.6	115.7
Income from continuing operations before income taxes	532.8	443.4	461.3
Income tax expense (benefit)	66.7	59.4	70.4
Income from continuing operations, net of tax	466.1	384	390.9
Income (loss) from discontinued operations, net of tax	1.4	(2.3)	(2.5)
Net income	467.5	381.7	388.4
Preferred dividend requirements of Interstate Power and Light Company	10.2	10.2	10.2
Net income attributable to common shareowners	$ 457.3	$ 371.5	$ 378.2
Weighted average number of common shares outstanding (basic and diluted) (in shares)	229.7	227.1	225.4
Earnings per weighted average common share attributable to Alliant Energy common shareowners (basic and diluted):
Income from continuing operations, net of tax (in dollars per share)	$ 1.99	$ 1.65	$ 1.69
Loss from discontinued operations, net of tax (in dollars per share)	0	(0.01)	(0.01)
Net income (in dollars per share)	$ 1.99	$ 1.64	$ 1.68
Amounts attributable to common shareowners:
Income from continuing operations, net of tax	$ 455.9	$ 373.8	$ 380.7
Income (loss) from discontinued operations, net of tax	1.4	(2.3)	(2.5)
Net income	457.3	371.5	378.2
IPL [Member]
Operating revenues:
Electric utility	1,598.9	1,569.7	1,503.8
Gas utility	226	204	217.3
Other utility	45.4	46.7	53.4
Total operating revenues	1,870.3	1,820.4	1,774.5
Operating expenses:
Electric production fuel and purchased power	443.6	430.5	428.4
Electric transmission service	310.4	359.7	328.2
Cost of gas sold	115.6	111	123.3
Other operation and maintenance	403.8	383.7	389.9
Depreciation and amortization	245	210.8	207.2
Taxes other than income taxes	55	53.9	55.6
Total operating expenses	1,573.4	1,549.6	1,532.6
Operating income (loss)	296.9	270.8	241.9
Interest expense and other:
Interest expense	112.4	103.2	96.8
Allowance for funds used during construction	(31.4)	(52)	(28.2)
Interest income and other	(0.2)	(0.3)	(0.2)
Total interest expense and other	80.8	50.9	68.4
Income from continuing operations before income taxes	216.1	219.9	173.5
Income tax expense (benefit)	(10.9)	(5.9)	(22.7)
Net income	227	225.8	196.2
Preferred dividend requirements of Interstate Power and Light Company	10.2	10.2	10.2
Net income attributable to common shareowners	216.8	215.6	186
Amounts attributable to common shareowners:
Net income	216.8	215.6	186
WPL [Member]
Operating revenues:
Electric utility	1,295.8	1,305.8	1,266.7
Gas utility	174.9	151.4	163.9
Other utility	2.1	1.9	4.5
Total operating revenues	1,472.8	1,459.1	1,435.1
Operating expenses:
Electric production fuel and purchased power	374.5	423.5	409.3
Electric transmission service	170.5	168.2	157.1
Cost of gas sold	95.8	83.3	95.8
Other operation and maintenance	249	219.8	235.4
Depreciation and amortization	212.9	192.5	184.3
Taxes other than income taxes	46.9	44.8	44.5
Total operating expenses	1,149.6	1,132.1	1,126.4
Operating income (loss)	323.2	327	308.7
Interest expense and other:
Interest expense	93.8	91.4	92.4
Equity (income) loss from unconsolidated investments, net	(0.7)	(39.8)	(35.1)
Allowance for funds used during construction	(18.3)	(10.5)	(8.7)
Interest income and other	(0.1)	(0.2)	(0.4)
Total interest expense and other	74.7	40.9	48.2
Income from continuing operations before income taxes	248.5	286.1	260.5
Income tax expense (benefit)	61.9	93.3	82.9
Net income	186.6	192.8	177.6
Net income attributable to noncontrolling interest	0	2.4	1.3
Net income attributable to common shareowners	186.6	190.4	176.3
Amounts attributable to common shareowners:
Net income	$ 186.6	$ 190.4	$ 176.3

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents		$ 27.9	$ 8.2
Accounts receivable, less allowance for doubtful accounts		482.8	493.3
Production fuel, at weighted average cost		72.3	98.1
Gas stored underground, at weighted average cost		44.5	37.6
Materials and supplies, at weighted average cost		105.6	86.6
Regulatory assets		84.3	57.8
Other		87.7	95.5
Total current assets		905.1	877.1
Property, plant and equipment, net		11,234.5	10,279.2
Investments:
ATC Investment		274.2	317.6
Other		121.9	20
Total investments		396.1	337.6
Other assets:
Regulatory assets		1,582.4	1,857.3
Deferred charges and other		69.7	22.6
Total other assets		1,652.1	1,879.9
Total assets		14,187.8	13,373.8
Current liabilities:
Current maturities of long-term debt		855.7	4.6
Commercial paper		320.2	244.1
Other short-term borrowings		95	0
Accounts payable		477.3	445.3
Regulatory liabilities		140	186.2
Other		260.8	281.8
Total current liabilities		2,149	1,162
Long-term debt, net (excluding current portion)	[1]	4,010.6	4,315.6
Other liabilities:
Deferred tax liabilities		1,478.4	2,570.2
Regulatory liabilities		1,357.2	494.8
Pension and other benefit obligations		504	489.9
Other		306.4	279.3
Total other liabilities		3,646	3,834.2
Commitments and contingencies (Note 16)
Common equity:
Common stock		2.3	2.3
Additional paid-in capital		1,845.5	1,693.1
Retained earnings		2,346	2,177
Accumulated other comprehensive loss		(0.5)	(0.4)
Shares in deferred compensation trust - 463,365 and 441,695 shares at a weighted average cost of $23.91 and $22.71 per share		(11.1)	(10)
Total common equity		4,182.2	3,862
Cumulative preferred stock of Interstate Power and Light Company		200	200
Total equity		4,382.2	4,062
Total liabilities and equity		14,187.8	13,373.8
IPL [Member]
Current assets:
Cash and cash equivalents		3.6	3.3
Accounts receivable, less allowance for doubtful accounts		264.9	240.7
Production fuel, at weighted average cost		52.4	70.3
Gas stored underground, at weighted average cost		20.3	16.3
Materials and supplies, at weighted average cost		60.6	46.5
Regulatory assets		41.9	17.7
Other		32.3	27.7
Total current assets		476	422.5
Property, plant and equipment, net		5,926.2	5,435.6
Other assets:
Regulatory assets		1,189.7	1,441.1
Deferred charges and other		14.1	5.5
Total other assets		1,203.8	1,446.6
Total assets		7,606	7,304.7
Current liabilities:
Current maturities of long-term debt		350	0
Commercial paper		0	0
Accounts payable		220.3	186.3
Accounts payable to associated companies		50.1	43.3
Regulatory liabilities		69.7	149.6
Accrued taxes		47.1	53.8
Other		90.5	88.8
Total current liabilities		827.7	521.8
Long-term debt, net (excluding current portion)	[1]	2,056	2,153.5
Other liabilities:
Deferred tax liabilities		910.7	1,511.8
Regulatory liabilities		685.7	281.2
Pension and other benefit obligations		173.8	173.2
Other		242.4	214.2
Total other liabilities		2,012.6	2,180.4
Commitments and contingencies (Note 16)
Common equity:
Common stock		33.4	33.4
Additional paid-in capital		1,797.8	1,597.8
Retained earnings		678.5	617.8
Total common equity		2,509.7	2,249
Cumulative preferred stock of Interstate Power and Light Company		200	200
Total equity		2,709.7	2,449
Total liabilities and equity		7,606	7,304.7
WPL [Member]
Current assets:
Cash and cash equivalents		23.1	4.2
Accounts receivable, less allowance for doubtful accounts		212.2	226.3
Production fuel, at weighted average cost		19.9	27.8
Gas stored underground, at weighted average cost		24.2	21.3
Materials and supplies, at weighted average cost		42.1	36.3
Regulatory assets		42.4	40.1
Prepaid gross receipts tax		41.3	39.8
Other		13.4	20.7
Total current assets		418.6	416.5
Property, plant and equipment, net		4,917.9	4,426.7
Other assets:
Regulatory assets		392.7	416.2
Deferred charges and other		27.3	30.9
Total other assets		420	447.1
Total assets		5,756.5	5,290.3
Current liabilities:
Current maturities of long-term debt		0	0
Commercial paper		25	52.3
Accounts payable		201.7	192.9
Accounts payable to associated companies		22.2	34.6
Regulatory liabilities		70.3	36.6
Accrued interest		25.6	23.6
Other		51.4	54.7
Total current liabilities		396.2	394.7
Long-term debt, net (excluding current portion)	[1]	1,833.4	1,535.2
Other liabilities:
Deferred tax liabilities		522.4	971.6
Regulatory liabilities		671.5	213.6
Capital lease obligations - Sheboygan Falls Energy Facility		70.2	77.2
Pension and other benefit obligations		213.7	207.8
Other		167.6	159.4
Total other liabilities		1,645.4	1,629.6
Commitments and contingencies (Note 16)
Common equity:
Common stock		66.2	66.2
Additional paid-in capital		1,109	1,019
Retained earnings		706.3	645.6
Total common equity		1,881.5	1,730.8
Total equity		1,881.5	1,730.8
Total liabilities and equity		$ 5,756.5	$ 5,290.3
[1]	There were no significant sinking fund requirements related to the outstanding long-term debt.

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	480,000,000	480,000,000
Common stock, shares outstanding (in shares)	231,348,646	227,673,654
Shares in deferred compensation trust (in shares)	463,365	441,695
Shares in deferred compensation trust, weighted average cost per share (in dollars per share)	$ 23.91	$ 22.71
IPL [Member]
Common stock, par value (in dollars per share)	$ 2.50	$ 2.50
Common stock, shares authorized (in shares)	24,000,000	24,000,000
Common stock, shares outstanding (in shares)	13,370,788	13,370,788
WPL [Member]
Common stock, par value (in dollars per share)	$ 5	$ 5
Common stock, shares authorized (in shares)	18,000,000	18,000,000
Common stock, shares outstanding (in shares)	13,236,601	13,236,601

Consolidated Statements Of Cash

Consolidated Statements Of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income	$ 467.5	$ 381.7	$ 388.4
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	461.8	411.6	401.3
Other amortizations	21.7	(4.8)	12.4
Deferred tax expense and tax credits	139.6	84.6	114.2
Equity (income) loss from unconsolidated investments, net	(44.8)	(39.6)	(33.8)
Distributions from equity method investments	38.1	28.3	30.6
Equity component of allowance for funds used during construction	(33.6)	(42.3)	(24.4)
Asset valuation charges for Franklin County wind farm	0	86.4	0
Other	6.7	0.8	15.7
Other changes in assets and liabilities:
Accounts receivable	29.6	(121.4)	36.8
Sales of accounts receivable	(9)	16	(17)
Regulatory assets	(130.8)	(3.6)	(104.5)
Regulatory liabilities	(83.8)	(63)	(67.8)
Deferred income taxes	81.7	102.4	94.6
Other	38.7	22.5	24.7
Net cash flows from operating activities	983.4	859.6	871.2
Cash flows from (used for) investing activities:
Utility construction and acquisition expenditures	(1,281.8)	(1,131.2)	(960.3)
Other construction and acquisition expenditures	(185.1)	(65.6)	(74)
Proceeds from Minnesota electric and natural gas distribution asset sales	0	0	139.9
Other	(29.4)	10.3	(24.8)
Net cash flows from (used for) investing activities	(1,496.3)	(1,186.5)	(919.2)
Cash flows from (used for) financing activities:
Common stock dividends	(288.3)	(266.5)	(247.3)
Proceeds from issuance of common stock, net	149.6	26.6	151.2
Proceeds from issuance of long-term debt	550	800	250.7
Payments to retire long-term debt	(4.6)	(313.4)	(183)
Net change in commercial paper and other short-term borrowings	171.1	84.3	18.5
Other	(45.2)	(1.7)	6.8
Net cash flows from (used for) financing activities	532.6	329.3	(3.1)
Net increase (decrease) in cash and cash equivalents	19.7	2.4	(51.1)
Cash and cash equivalents at beginning of period	8.2	5.8	56.9
Cash and cash equivalents at end of period	27.9	8.2	5.8
Supplemental cash flows information:
Interest, net of capitalized interest	(212.6)	(192.4)	(184.8)
Income taxes, net	(11.3)	(9.8)	0
Significant non-cash investing and financing activities:
Accrued capital expenditures	196.5	154.4	148.3
IPL [Member]
Cash flows from operating activities:
Net income	227	225.8	196.2
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	245	210.8	207.2
Deferred tax expense and tax credits	55.8	35.6	28.9
Equity component of allowance for funds used during construction	(21.1)	(35.2)	(18.6)
Other	1.5	2.9	19.5
Other changes in assets and liabilities:
Accounts receivable	(7.9)	(59.7)	20.4
Sales of accounts receivable	(9)	16	(17)
Regulatory assets	(126.2)	(54.7)	(76.3)
Accounts payable	24	8	(42.7)
Regulatory liabilities	(71.2)	(67.3)	(75.5)
Deferred income taxes	103.7	97.7	82.1
Other	18.4	(18)	60.8
Net cash flows from operating activities	440	361.9	385
Cash flows from (used for) investing activities:
Utility construction and acquisition expenditures	(676)	(689.7)	(619.3)
Proceeds from Minnesota electric and natural gas distribution asset sales	0	0	139.9
Other	(30.4)	(3.9)	(32.5)
Net cash flows from (used for) investing activities	(706.4)	(693.6)	(511.9)
Cash flows from (used for) financing activities:
Common stock dividends	(156.1)	(151.9)	(140)
Capital contributions from parent	200	190	165
Proceeds from issuance of long-term debt	250	300	250
Payments to retire long-term debt	0	0	(150)
Other	(27.2)	(7.6)	1.1
Net cash flows from (used for) financing activities	266.7	330.5	126.1
Net increase (decrease) in cash and cash equivalents	0.3	(1.2)	(0.8)
Cash and cash equivalents at beginning of period	3.3	4.5	5.3
Cash and cash equivalents at end of period	3.6	3.3	4.5
Supplemental cash flows information:
Interest, net of capitalized interest	(111.8)	(99.7)	(93.9)
Income taxes, net	8.6	(11.1)	19.3
Significant non-cash investing and financing activities:
Accrued capital expenditures	76.4	53.8	77
WPL [Member]
Cash flows from operating activities:
Net income	186.6	192.8	177.6
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	212.9	192.5	184.3
Other amortizations	17.7	(8.7)	5.3
Deferred tax expense and tax credits	53.9	114.5	77.6
Equity (income) loss from unconsolidated investments, net	(0.7)	(39.8)	(35.1)
Other	(13.3)	(17.8)	(10.2)
Other changes in assets and liabilities:
Accounts receivable	17.7	(47.6)	3.7
Regulatory assets	(4.7)	51.1	(28.2)
Other	(5.1)	44.6	39.7
Net cash flows from operating activities	465.7	521.4	449.8
Cash flows from (used for) investing activities:
Utility construction and acquisition expenditures	(637.4)	(453)	(344.3)
Other	(28.3)	(25.9)	(13.9)
Net cash flows from (used for) investing activities	(665.7)	(478.9)	(358.2)
Cash flows from (used for) financing activities:
Common stock dividends	(125.9)	(135)	(126.9)
Capital contributions from parent	90	60	0
Proceeds from issuance of long-term debt	300	0	0
Net change in commercial paper and other short-term borrowings	(27.3)	32.4	19.9
Other	(17.9)	3.9	(30.9)
Net cash flows from (used for) financing activities	218.9	(38.7)	(137.9)
Net increase (decrease) in cash and cash equivalents	18.9	3.8	(46.3)
Cash and cash equivalents at beginning of period	4.2	0.4	46.7
Cash and cash equivalents at end of period	23.1	4.2	0.4
Supplemental cash flows information:
Interest, net of capitalized interest	(91.7)	(91.5)	(93.1)
Income taxes, net	(8.4)	27.8	(7.4)
Significant non-cash investing and financing activities:
Accrued capital expenditures	114.5	93.1	55.2
Transfer of investment in ATC and tax liability to ATI	$ 0	$ (163.6)	$ 0

Consolidated Statements of Comm

Consolidated Statements of Common Equity - USD ($) $ in Millions	Total	IPL [Member]	WPL [Member]	Common Stock [Member]	Common Stock [Member]IPL [Member]	Common Stock [Member]WPL [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]IPL [Member]	Additional Paid-in Capital [Member]WPL [Member]	Retained Earnings [Member]	Retained Earnings [Member]IPL [Member]	Retained Earnings [Member]WPL [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Shares in Deferred Compensation Trust [Member]	Noncontrolling Interest [Member]WPL [Member]
Beginning balance at Dec. 31, 2014	$ 3,438.7	$ 1,784.3		$ 2.2	$ 33.4	$ 66.2	$ 1,508	$ 1,242.8	$ 959	$ 1,938	$ 508.1	$ 681.7	$ (0.6)	$ (8.9)	$ 8.5
Beginning balance at Dec. 31, 2014			$ 1,715.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to common shareowners	378.2	186	176.3							378.2	186	176.3			1.3
Net income	388.4	196.2	177.6
Common stock dividends	(247.3)	(140)	(126.9)							(247.3)	(140)	(126.9)
Common stock issued, net	151.2			0.1			151.1
Capital contributions from parent		165	0					165
Contributions from noncontrolling interest			3.4												3.4
Distributions to noncontrolling interest			(1.9)												(1.9)
Transfer of investment in ATC to ATI			0
Other	3.1						2.7							0.4
Other comprehensive income (loss), net of tax	0.2	0	0										0.2
Ending balance at Dec. 31, 2015	3,724.1	1,995.3		2.3	33.4	66.2	1,661.8	1,407.8	959	2,068.9	554.1	731.1	(0.4)	(8.5)	11.3
Ending balance at Dec. 31, 2015			1,767.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to common shareowners	371.5	215.6	190.4							371.5	215.6	190.4			2.4
Net income	381.7	225.8	192.8
Common stock dividends	(266.5)	(151.9)	(135)							(266.5)	(151.9)	(135)
Common stock issued, net	26.6						26.6
Capital contributions from parent		190	60					190	60
Contributions from noncontrolling interest			11.5												11.5
Distributions to noncontrolling interest			(2.5)												(2.5)
Transfer of investment in ATC to ATI			(163.6)									(140.9)			(22.7)
Other	6.3						4.7			3.1				(1.5)
Other comprehensive income (loss), net of tax	0	0	0
Ending balance at Dec. 31, 2016	3,862	2,249	1,730.8	2.3	33.4	66.2	1,693.1	1,597.8	1,019	2,177	617.8	645.6	(0.4)	(10)	0
Ending balance at Dec. 31, 2016	4,062	2,449	1,730.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to common shareowners	457.3	216.8	186.6							457.3	216.8	186.6
Net income	467.5	227	186.6
Common stock dividends	(288.3)	(156.1)	(125.9)							(288.3)	(156.1)	(125.9)
Common stock issued, net	149.6						149.6
Capital contributions from parent		200	90					200	90
Transfer of investment in ATC to ATI			0
Other	1.7						2.8							(1.1)
Other comprehensive income (loss), net of tax	(0.1)	0	0										(0.1)
Ending balance at Dec. 31, 2017	4,182.2	2,509.7	1,881.5	$ 2.3	$ 33.4	$ 66.2	$ 1,845.5	$ 1,797.8	$ 1,109	$ 2,346	$ 678.5	$ 706.3	$ (0.5)	$ (11.1)	$ 0
Ending balance at Dec. 31, 2017	$ 4,382.2	$ 2,709.7	$ 1,881.5

Consolidated Statements of Com7

Consolidated Statements of Common Equity (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Stockholders' Equity [Abstract]
Common stock dividends (in dollars per share)	$ 1.26	$ 1.175	$ 1.10

Summary Of Significant Accounti

Summary Of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Summary Of Significant Accounting Policies	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (a) General - Description of Business - Alliant Energy’s financial statements include the accounts of Alliant Energy and its consolidated subsidiaries. Alliant Energy is a Midwest U.S. energy holding company, whose primary wholly-owned subsidiaries are IPL, WPL, AEF and Corporate Services. IPL’s financial statements include the accounts of IPL and its consolidated subsidiary, IPL SPE LLC, which is used for IPL’s sales of accounts receivable program. IPL is a direct subsidiary of Alliant Energy and is a public utility engaged principally in the generation and distribution of electricity and the distribution and transportation of natural gas to retail customers in select markets in Iowa. IPL also sells electricity to wholesale customers in Minnesota, Illinois and Iowa, and is engaged in the generation and distribution of steam for two customers in Cedar Rapids, Iowa. WPL’s financial statements include the accounts of WPL and its consolidated subsidiary, WPL Transco, which held Alliant Energy’s investment in ATC until December 31, 2016. Refer to Note 6(a) for discussion of WPL’s transfer of its investment in ATC to ATI on December 31, 2016. WPL is a direct subsidiary of Alliant Energy and is a public utility engaged principally in the generation and distribution of electricity and the distribution and transportation of natural gas to retail customers in select markets in Wisconsin. WPL also sells electricity to wholesale customers in Wisconsin. AEF is comprised of Transportation, ATI, a non-utility wind investment, the Sheboygan Falls Energy Facility and other non-utility investments. Transportation includes a short-line railway that provides freight service between Cedar Rapids, Iowa and Iowa City, Iowa; barge terminal and hauling services on the Mississippi River; and other transfer and storage services. ATI, a wholly-owned subsidiary of AEF, holds all of Alliant Energy’s ATC Investment. The non-utility wind investment includes a 50% cash equity ownership interest in a 225 MW non-utility wind farm located in Oklahoma. The Sheboygan Falls Energy Facility is a 347 MW, simple-cycle, natural gas-fired EGU near Sheboygan Falls, Wisconsin, which is leased to WPL for an initial period of 20 years ending in 2025. Corporate Services is the subsidiary formed to provide administrative services to Alliant Energy and its subsidiaries. Basis of Presentation - The financial statements reflect investments in controlled subsidiaries on a consolidated basis and Alliant Energy’s, IPL’s and WPL’s proportionate shares of jointly-owned utility EGUs. Unconsolidated investments, which Alliant Energy and WPL do not control, but have the ability to exercise significant influence over operating and financial policies, are accounted for under the equity method of accounting. Under the equity method of accounting, Alliant Energy and WPL initially record the investment at cost, and adjust the carrying amount of the investment to recognize their respective share of the earnings or losses of the investee. Dividends received from an investee reduce the carrying amount of the equity investment. Investments that do not meet the criteria for consolidation or the equity method of accounting are accounted for under the cost method. Refer to Notes 1(m) and 6(a) for further discussion of VIEs and equity method investments, respectively. All intercompany balances and transactions, other than certain transactions affecting the rate-making process at IPL and WPL, have been eliminated from the financial statements. Such transactions not eliminated include costs that are recoverable from customers through rate-making processes. The financial statements are prepared in conformity with GAAP, which give recognition to the rate-making and accounting practices of FERC and state commissions having regulatory jurisdiction. Certain prior period amounts in the Financial Statements and Notes have been reclassified to conform to the current period presentation for comparative purposes, including reclassifications resulting from modifications to segment reporting as discussed in Note 17 . Discontinued operations reported in Alliant Energy’s income statements is related to various warranty claims associated with the sale of RMT, Inc. in 2013, which have resulted in operating expenses and income subsequent to the sale. Alliant Energy presents cash flows from continuing operations together with cash flows from discontinued operations in its cash flows statements. Use of Estimates - The preparation of the financial statements requires management to make estimates and assumptions that affect: (a) the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the financial statements; and (b) the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. (b) Regulatory Assets and Regulatory Liabilities - Alliant Energy, IPL and WPL are subject to regulation by FERC and various state regulatory commissions. As a result, Alliant Energy, IPL and WPL are subject to GAAP provisions for regulated operations, which provide that rate-regulated public utilities record certain costs and credits allowed in the rate-making process in different periods than for non-utility entities. Regulatory assets generally represent incurred costs that have been deferred as such costs are probable of recovery in future customer rates. Regulatory liabilities generally represent obligations to make refunds to customers or amounts collected in rates for which the related costs have not yet been incurred. Amounts recorded as regulatory assets or regulatory liabilities are generally recognized in the income statements at the time they are reflected in rates. Refer to Note 2 for additional discussion of regulatory assets and regulatory liabilities. (c) Income Taxes - The liability method of accounting is followed for deferred taxes, which requires the establishment of deferred tax assets and liabilities, as appropriate, for temporary differences between the tax basis of assets and liabilities and the amounts reported in the financial statements. Deferred taxes are recorded using currently enacted tax rates, including impacts from Tax Reform and estimates of state apportionment. Changes in deferred tax assets and liabilities associated with certain property-related differences at IPL are accounted for differently than other subsidiaries of Alliant Energy due to rate-making practices in Iowa. Rate-making practices in Iowa do not include the impact of certain deferred tax expenses (benefits) in the determination of retail rates. Based on these rate-making practices, deferred tax expense (benefit) related to these property-related differences at IPL is not recorded in the income statement but instead recorded to regulatory assets or regulatory liabilities until these temporary differences reverse. Refer to Note 2 for further discussion of these tax-related regulatory assets and regulatory liabilities associated with property-related differences at IPL. In Wisconsin, the PSCW allows rate recovery of deferred tax expense on all temporary differences. Investment tax credits are deferred and amortized to income over the average lives of the related property. Tax Reform repealed corporate federal AMT and allows unutilized AMT credits to be refunded over the next four tax years beginning with the U.S. federal tax return for calendar year 2018. Other tax credits reduce income tax expense in the year claimed. Alliant Energy files a consolidated federal income tax return and a combined return in Wisconsin, which include Alliant Energy and its subsidiaries. Alliant Energy subsidiaries with a presence in Iowa file as part of a consolidated return in Iowa. Alliant Energy allocates consolidated income tax expense to its subsidiaries that are members of the group that file a consolidated or combined income tax return. IPL and WPL use the modified separate return approach for calculating their income tax provisions and related deferred tax assets and liabilities. IPL and WPL are assumed to file separate tax returns with the federal and state taxing authorities, except that net operating losses (and other current or deferred tax attributes) are characterized as realized (or realizable) by IPL and WPL when those tax attributes are realized (or realizable) by the consolidated tax return group of Alliant Energy (even if IPL and WPL would not otherwise have realized the attributes on a stand-alone basis). The difference in the income taxes recorded for IPL and WPL under the modified separate return method compared to the income taxes recorded on a separate return basis was not material in 2017 , 2016 and 2015 . Refer to Note 11 for further discussion of Tax Reform, which was enacted in December 2017. (d) Cash and Cash Equivalents - Cash and cash equivalents include short-term liquid investments that have original maturities of less than 90 days. (e) Property, Plant and Equipment - Utility Plant - General - Utility plant is recorded at the original cost of acquisition or construction, which includes material, labor, contractor services, AFUDC and allocable overheads, such as supervision, engineering, benefits, certain taxes and transportation. Repairs, replacements and renewals of items of property determined to be less than a unit of property or that do not increase the property’s life or functionality are charged to maintenance expense. Generally, ordinary retirements of utility plant and salvage value are netted and charged to accumulated depreciation upon removal from utility plant accounts and no gain or loss is recognized consistent with rate-making principles. However, if regulators have approved recovery of the remaining net book value of property, plant and equipment that is retired early, or such approval by regulators is probable, the remaining net book value is reclassified from property, plant and equipment to regulatory assets upon retirement. Property, plant and equipment that is probable of being retired early is classified as plant anticipated to be retired early. Depreciation - IPL and WPL use a combination of remaining life and straight-line depreciation methods as approved by their respective regulatory commissions. The composite or group method of depreciation is used, in which a single depreciation rate is applied to the gross investment in a particular class of property. This method pools similar assets and then depreciates each group as a whole. Periodic depreciation studies are performed to determine the appropriate group lives, net salvage, estimated cost of removal and group depreciation rates. These depreciation studies are subject to review and approval by IPL’s and WPL’s respective regulatory commissions. Depreciation expense is included within the recoverable cost of service component of rates charged to customers. The average rates of depreciation for electric, gas and other properties, consistent with current rate-making practices, were as follows: IPL WPL 2017 2016 2015 2017 2016 2015 Electric - generation 3.5% 3.5% 3.6% 3.5% 3.1% 3.2% Electric - distribution 2.4% 2.4% 2.4% 2.6% 2.6% 2.7% Electric - other 4.5% 4.2% 4.0% 6.9% 4.7% 4.5% Gas 3.4% 3.3% 3.2% 2.5% 2.5% 2.5% Other 4.0% 3.9% 3.9% 6.0% 5.9% 6.0% In September 2016, the PSCW issued an order approving the implementation of updated depreciation rates for WPL effective January 1, 2017 as a result of an updated depreciation study. In February 2018, the IUB issued an order approving the implementation of updated depreciation rates for IPL, which are currently expected to be effective in the first half of 2018, as a result of an updated depreciation study. IPL estimates the new average rates of depreciation for its electric generation and electric distribution properties will be approximately 3.8% and 2.8% , respectively, during 2018. AFUDC - AFUDC represents costs to finance construction additions, including a return on equity component and cost of debt component as required by regulatory accounting. AFUDC for IPL’s construction projects is calculated in accordance with FERC guidelines. AFUDC for WPL’s retail and wholesale jurisdiction construction projects is calculated in accordance with PSCW and FERC guidelines, respectively. The AFUDC rates, computed in accordance with the prescribed regulatory formula, were as follows: 2017 2016 2015 IPL (Marshalltown CWIP) (a) 7.8% 7.9% 7.9% IPL (Wind generation CWIP) (b) 7.6% N/A N/A IPL (other CWIP) 7.6% 7.7% 7.7% WPL (retail jurisdiction) 7.6% 8.2% 8.2% WPL (wholesale jurisdiction) 6.0% 6.7% 7.9% (a) In 2013, the IUB issued an order establishing rate-making principles that require a 10.3% return on common equity for the calculation of AFUDC related to the construction of Marshalltown. (b) In 2016, the IUB issued an order establishing rate-making principles that require a return on common equity for the calculation of AFUDC related to the construction of up to 500 MW of new wind generation equal to the greater of 10.0% or whatever percentage the IUB finds reasonable during IPL’s most recent retail electric rate proceeding. In accordance with their most recent rate orders, IPL applies its AFUDC rates to 100% of applicable CWIP balances and WPL generally applies its AFUDC rates to 50% of applicable CWIP balances. WPL may apply its AFUDC rates to 100% of the retail portion of the CWIP balances for construction projects requiring a CA or CPCN that were approved by the PSCW after its most recent rate order, including West Riverside. Non-utility Property - General - Non-utility property is recorded at the original cost of acquisition or construction, which includes material, labor and contractor services. Repairs, replacements and renewals of items of property determined to be less than a unit of property or that do not increase the property’s life or functionality are charged to maintenance expense. Upon retirement or sale of non-utility property, the original cost and related accumulated depreciation are removed from the accounts and any gain or loss is included in the income statements. Costs related to software developed or obtained for internal use are capitalized and amortized on a straight-line basis over the estimated useful life of the related software. If software is retired prior to being fully amortized, the difference is recorded as a loss in the income statements. (f) Operating Revenues - Utility - Revenues from Alliant Energy’s utility business are primarily from electricity and natural gas sales and are recognized on an accrual basis as services are rendered or commodities are delivered to customers. Energy sales to individual customers are based on the reading of customers’ meters, which occurs on a systematic basis throughout each reporting period. Amounts of energy delivered to customers since the date of the last meter reading are estimated at the end of each reporting period and the corresponding estimated unbilled revenue is recorded in such reporting period. The unbilled revenue estimate is based on daily system demand volumes, estimated customer usage by class, temperature impacts, line losses and the most recent customer rates. IPL and WPL accrue revenues from their wholesale customers to the extent that the actual net revenue requirements calculated in accordance with FERC-approved formula rates for the reporting period are higher or lower than the amounts billed to wholesale customers during such period. Regulatory assets or regulatory liabilities are recorded as the offset for these accrued revenues under formulaic rate-making programs. IPL’s estimated recovery amount is recorded in the current period of service and is reflected in customer bills within two years under the provisions of approved formula rates. WPL’s estimated recovery amount is recorded in the current period of service and subject to final adjustments after a customer audit period in the subsequent year. Final settled recovery amounts are reflected in WPL’s customer bills within two years under the provisions of approved formula rates. IPL and WPL participate in bid/offer-based wholesale energy and ancillary services markets operated by MISO. MISO requires that all load serving entities and generation owners, including IPL and WPL, submit hourly bids and offers for energy and ancillary services. The MISO transactions are grouped together, resulting in a net supply to or net purchase from MISO for each hour of each day. The net supply to MISO is recorded in “Electric utility operating revenues” and the net purchase from MISO is recorded in “Electric production fuel and purchased power” in the income statements. Non-utility - Revenues from Alliant Energy’s non-utility businesses are primarily from its Transportation business and are recognized on an accrual basis as services are rendered or goods are delivered to customers. Taxes Collected from Customers - Sales or various other taxes collected by certain of Alliant Energy’s subsidiaries on behalf of other agencies are recorded on a net basis and are not included in operating revenues. Revenue Recognition - Refer to Note 1(n) for discussion of a new accounting standard related to revenue recognition, which Alliant Energy, IPL and WPL adopted on January 1, 2018. (g) Utility Cost Recovery Mechanisms - Electric Production Fuel and Purchased Power (Fuel-related Costs) - Fuel-related costs are incurred to generate and purchase electricity to meet the demand of IPL’s and WPL’s electric customers. These fuel-related costs include the cost of fossil fuels (primarily natural gas and coal) used to produce electricity at their EGUs and electricity purchased from MISO wholesale energy markets and under PPAs. These fuel-related costs are recorded in “Electric production fuel and purchased power” in the income statements. IPL Retail - The cost recovery mechanisms for IPL’s retail electric customers provide for monthly adjustments to their electric rates for changes in fuel-related costs. Changes in the under-/over-collection of these costs are recognized in “Electric production fuel and purchased power” in Alliant Energy’s and IPL’s income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and IPL’s balance sheets until they are reflected in future billings to customers. WPL Retail - The cost recovery mechanism for WPL’s retail electric customers is based on forecasts of certain fuel-related costs expected to be incurred during forward-looking test year periods and fuel monitoring ranges determined by the PSCW during each retail electric rate proceeding or in a separate fuel cost plan approval proceeding. If WPL’s actual fuel-related costs fall outside these fuel monitoring ranges, WPL is authorized to defer the incremental under-/over-collection of fuel-related costs that are outside the approved ranges. Deferral of under-collections are reduced to the extent actual return on common equity earned by WPL during the fuel cost plan year exceeds the most recently authorized return on common equity. Deferred amounts for fuel-related costs outside the approved fuel monitoring ranges are recognized in “Electric production fuel and purchased power” in Alliant Energy’s and WPL’s income statements. The cumulative effects of these deferred amounts are recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and WPL’s balance sheets until they are reflected in future billings to customers. IPL and WPL Wholesale - The cost recovery mechanisms for IPL’s and WPL’s wholesale electric customers provide for subsequent adjustments to their electric rates for changes in fuel-related costs. Changes in the under-/over-collection of these costs are recognized in “Electric production fuel and purchased power” in the income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on the balance sheets until they are reflected in future billings to customers. Purchased Electric Capacity - PPAs help meet the electricity demand of IPL’s and WPL’s customers. Certain of these PPAs include minimum payments for IPL’s and WPL’s rights to electric generating capacity, which are charged each period to “Electric production fuel and purchased power” in the income statements. Purchased electric capacity expenses are recovered from IPL’s and WPL’s retail electric customers through changes in base rates determined during periodic rate proceedings. Purchased electric capacity expenses are recovered from IPL’s and WPL’s wholesale electric customers through annual changes in base rates determined by a formula rate structure. Electric Transmission Service - Costs incurred for the transmission of electricity to meet the demands of IPL’s and WPL’s customers are charged to “Electric transmission service” in the income statements. IPL Retail - Electric transmission service expense is recovered from IPL’s retail electric customers through a transmission cost rider. This cost recovery mechanism provides for annual adjustments to electric rates charged to retail electric customers for changes in electric transmission service expense. Changes in the under-/over-collection of these costs are recognized in “Electric transmission service” in Alliant Energy’s and IPL’s income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and IPL’s balance sheets until they are reflected in future billings to customers. WPL Retail - Electric transmission service expense is recovered from WPL’s retail electric customers through changes in base rates determined during periodic rate proceedings. Pursuant to escrow accounting treatment approved by the PSCW, the difference between actual electric transmission service expense incurred and the amount of electric transmission service costs collected from customers as electric revenues is recognized in “Electric transmission service” in Alliant Energy’s and WPL’s income statements. An offsetting amount is recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and WPL’s balance sheets until reflected in future billings to customers. The PSCW’s December 2016 order for WPL’s retail electric rate review (2017/2018 Test Period) extends this escrow accounting treatment through 2018. IPL and WPL Wholesale - IPL and WPL arrange transmission service for the majority of their respective wholesale electric customers. Electric transmission service expense is allocated to and recovered from these customers based on a load ratio share computation. Cost of Gas Sold - Costs are incurred for the purchase, transportation and storage of natural gas to serve IPL’s and WPL’s gas customers and the costs associated with the natural gas delivered to customers are charged to “Cost of gas sold” in the income statements. The tariffs for IPL’s and WPL’s retail gas customers provide for subsequent adjustments to their rates each month for changes in the cost of gas sold. Changes in the under-/over-collection of these costs are also recognized in “Cost of gas sold” in the income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on the balance sheets until they are reflected in future billings to customers. Energy Efficiency Costs - Costs are incurred to fund energy efficiency programs and initiatives that help customers reduce their energy usage. The costs incurred for these programs and initiatives are charged to “Other operation and maintenance” in the income statements. Energy efficiency costs incurred by IPL are recovered from its retail electric and gas customers through an additional tariff called an energy efficiency cost recovery factor, which is revised annually and includes a reconciliation to eliminate any under-/over-collection of energy efficiency costs from prior periods. Energy efficiency costs incurred by WPL are recovered from retail electric and gas customers through changes in base rates determined during periodic rate proceedings. Reconciliations of any under-/over-collection of energy efficiency costs from prior periods are also addressed in WPL’s periodic rate proceedings. Changes in the under-/over-collection of energy efficiency costs for IPL and WPL are recognized in “Other operation and maintenance” in the income statements. The cumulative effects of the under-/over-collection of these costs for IPL and WPL are recorded in regulatory assets or regulatory liabilities on the balance sheets until they are reflected in future billings to customers. Refer to Note 2 for additional information regarding these utility cost recovery mechanisms. (h) Financial Instruments - Financial instruments are periodically used for risk management purposes to mitigate exposures to fluctuations in certain commodity prices and transmission congestion costs. The fair value of those financial instruments that are determined to be derivatives are recorded as assets or liabilities on the balance sheets. Certain commodity purchase and sales contracts qualify for and have been designated under the normal purchase and sale exception and are accounted for on the accrual basis of accounting. Alliant Energy, IPL and WPL have elected to not net the fair value amounts of derivatives subject to a master netting arrangement by counterparty. Alliant Energy, IPL and WPL do not offset fair value amounts recognized for the right to reclaim cash collateral (receivable) or the obligation to return cash collateral (payable) against fair value amounts recognized for derivative instruments that are executed with the same counterparty under the same master netting arrangement. Refer to Note 2 for discussion of the recognition of regulatory assets and regulatory liabilities related to the unrealized losses and gains on IPL’s and WPL’s derivative instruments. Refer to Notes 14 , 15 and 16(f) for further discussion of derivatives and related credit risk. (i) Asset Impairments - Property, Plant and Equipment of Regulated Operations - Property, plant and equipment of regulated operations are reviewed for possible impairment whenever events or changes in circumstances indicate all or a portion of the carrying value of the assets may be disallowed for rate-making purposes. If IPL or WPL are disallowed recovery of any portion of the carrying value of their regulated property, plant and equipment that is under construction, has been recently completed or is probable of abandonment, or conclude it is probable recovery will be disallowed, an impairment charge is recognized equal to the amount of the carrying value that was disallowed or is probable of being disallowed. If IPL or WPL are only allowed a partial return on the carrying value of their regulated property, plant and equipment that is under construction, has been recently completed or is probable of abandonment, or conclude it is probable a full return will not be allowed, an impairment charge is recognized equal to the difference between the carrying value and the present value of the future revenues expected from their regulated property, plant and equipment. Property, Plant and Equipment of Non-utility Operations - Property, plant and equipment of non-utility operations are reviewed for possible impairment whenever events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Impairment is indicated if the carrying value of an asset exceeds its undiscounted future cash flows. If an impairment is indicated, a charge is recognized equal to the amount the carrying value exceeds the asset’s fair value. Refer to Note 3 for discussion of Alliant Energy’s impairment analysis of the Franklin County wind farm assets and resulting asset valuation charges recorded by Alliant Energy in 2016. Unconsolidated Equity Investments - If events or circumstances indicate the carrying value of investments accounted for under the equity method of accounting exceeds fair value and the decline in value is other than temporary, potential impairment is assessed. If an impairment is indicated, a charge is recognized equal to the amount the carrying value exceeds the investment’s fair value. Refer to Note 6(a) for additional discussion of investments accounted for under the equity method of accounting. (j) Asset Retirement Obligations - The fair value of a legal obligation associated with the retirement of an asset is recorded as a liability when an asset is placed in service, when a legal obligation is subsequently identified or when sufficient information becomes available to determine a reasonable estimate of the fair value of future retirement costs. When an ARO is recorded as a liability, an equivalent amount is added to the asset cost. The fair value of AROs at inception is determined using discounted cash flows analyses. The liability is accreted to its present value and the capitalized cost is depreciated over the useful life of the related asset. Accretion and depreciation expenses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory assets on the balance sheets. Upon regulatory approval to recover IPL’s AROs expenditures, its regulatory assets are amortized to depreciation and amortization expenses in Alliant Energy’s and IPL’s income statements over the same time period the ARO expenditures are recovered from IPL’s customers. WPL’s regulatory assets related to AROs are being recovered as a component of depreciation rates pursuant to PSCW and FERC orders. Accretion and depreciation expenses related to AROs for Alliant Energy’s non-utility operations are recorded to depreciation and amortization expenses in Alliant Energy’s income statements. Upon settlement of the ARO liability, an entity settles the obligation for its recorded amount or incurs a gain or loss. Any gains or losses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory liabilities or regulatory assets on the balance sheets. Refer to Note 13 for additional discussion of AROs. (k) Debt Issuance and Retirement Costs - Debt issuance costs and debt premiums or discounts are presented on the balance sheet as a direct deduction from the carrying amount of the related debt liability, and are deferred and amortized over the expected life of each debt issue, considering maturity dates and, if applicable, redemption rights held by others. Alliant Energy’s non-utility businesses and Corporate Services expense in the period of retirement any unamortized debt issuance costs and debt premiums or discounts on debt retired early. (l) Allowance for Doubtful Accounts - Allowances for doubtful accounts are recorded for estimated losses resulting from the inability of customers to make required payments. Allowances for doubtful accounts are estimated based on historical write-offs, customer arrears and other economic factors within IPL’s and WPL’s service territories. Refer to Note 5(a) for details of allowance for doubtful accounts. (m) Variable Interest Entities - An entity is considered a VIE if its equity investors do not have sufficient equity at risk for the entity to finance its activities without additional subordinated financial support from other parties, or its equity investors lack any of the following characteristics: (1) power, through voting rights or similar rights, to direct the activities of the entity that most significantly impact the entity’s economic performance; (2) the obligation to absorb expected losses of the entity; or (3) the right to receive expected benefits of the entity. The primary beneficiary of a VIE is required to consolidate the VIE. The financial statements do not reflect any consolidated VIEs. (n) New Accounting Standards - Revenue Recognition - In May 2014, the Financial Accounting Standards Board issued an accounting standard providing principles for recognizing revenue for the transfer of promised goods or services to customers with the consideration to which the entity expects to be entitled in exchange for those goods or services. This standard also requires disclosures about the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers. Alliant Energy, IPL and WPL adopted this standard on January 1, 2018 and used the modified retrospective method of adoption and there will be no cumulative effect adjustments made to the opening retain
IPL [Member]
Summary Of Significant Accounting Policies	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (a) General - Description of Business - Alliant Energy’s financial statements include the accounts of Alliant Energy and its consolidated subsidiaries. Alliant Energy is a Midwest U.S. energy holding company, whose primary wholly-owned subsidiaries are IPL, WPL, AEF and Corporate Services. IPL’s financial statements include the accounts of IPL and its consolidated subsidiary, IPL SPE LLC, which is used for IPL’s sales of accounts receivable program. IPL is a direct subsidiary of Alliant Energy and is a public utility engaged principally in the generation and distribution of electricity and the distribution and transportation of natural gas to retail customers in select markets in Iowa. IPL also sells electricity to wholesale customers in Minnesota, Illinois and Iowa, and is engaged in the generation and distribution of steam for two customers in Cedar Rapids, Iowa. WPL’s financial statements include the accounts of WPL and its consolidated subsidiary, WPL Transco, which held Alliant Energy’s investment in ATC until December 31, 2016. Refer to Note 6(a) for discussion of WPL’s transfer of its investment in ATC to ATI on December 31, 2016. WPL is a direct subsidiary of Alliant Energy and is a public utility engaged principally in the generation and distribution of electricity and the distribution and transportation of natural gas to retail customers in select markets in Wisconsin. WPL also sells electricity to wholesale customers in Wisconsin. AEF is comprised of Transportation, ATI, a non-utility wind investment, the Sheboygan Falls Energy Facility and other non-utility investments. Transportation includes a short-line railway that provides freight service between Cedar Rapids, Iowa and Iowa City, Iowa; barge terminal and hauling services on the Mississippi River; and other transfer and storage services. ATI, a wholly-owned subsidiary of AEF, holds all of Alliant Energy’s ATC Investment. The non-utility wind investment includes a 50% cash equity ownership interest in a 225 MW non-utility wind farm located in Oklahoma. The Sheboygan Falls Energy Facility is a 347 MW, simple-cycle, natural gas-fired EGU near Sheboygan Falls, Wisconsin, which is leased to WPL for an initial period of 20 years ending in 2025. Corporate Services is the subsidiary formed to provide administrative services to Alliant Energy and its subsidiaries. Basis of Presentation - The financial statements reflect investments in controlled subsidiaries on a consolidated basis and Alliant Energy’s, IPL’s and WPL’s proportionate shares of jointly-owned utility EGUs. Unconsolidated investments, which Alliant Energy and WPL do not control, but have the ability to exercise significant influence over operating and financial policies, are accounted for under the equity method of accounting. Under the equity method of accounting, Alliant Energy and WPL initially record the investment at cost, and adjust the carrying amount of the investment to recognize their respective share of the earnings or losses of the investee. Dividends received from an investee reduce the carrying amount of the equity investment. Investments that do not meet the criteria for consolidation or the equity method of accounting are accounted for under the cost method. Refer to Notes 1(m) and 6(a) for further discussion of VIEs and equity method investments, respectively. All intercompany balances and transactions, other than certain transactions affecting the rate-making process at IPL and WPL, have been eliminated from the financial statements. Such transactions not eliminated include costs that are recoverable from customers through rate-making processes. The financial statements are prepared in conformity with GAAP, which give recognition to the rate-making and accounting practices of FERC and state commissions having regulatory jurisdiction. Certain prior period amounts in the Financial Statements and Notes have been reclassified to conform to the current period presentation for comparative purposes, including reclassifications resulting from modifications to segment reporting as discussed in Note 17 . Discontinued operations reported in Alliant Energy’s income statements is related to various warranty claims associated with the sale of RMT, Inc. in 2013, which have resulted in operating expenses and income subsequent to the sale. Alliant Energy presents cash flows from continuing operations together with cash flows from discontinued operations in its cash flows statements. Use of Estimates - The preparation of the financial statements requires management to make estimates and assumptions that affect: (a) the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the financial statements; and (b) the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. (b) Regulatory Assets and Regulatory Liabilities - Alliant Energy, IPL and WPL are subject to regulation by FERC and various state regulatory commissions. As a result, Alliant Energy, IPL and WPL are subject to GAAP provisions for regulated operations, which provide that rate-regulated public utilities record certain costs and credits allowed in the rate-making process in different periods than for non-utility entities. Regulatory assets generally represent incurred costs that have been deferred as such costs are probable of recovery in future customer rates. Regulatory liabilities generally represent obligations to make refunds to customers or amounts collected in rates for which the related costs have not yet been incurred. Amounts recorded as regulatory assets or regulatory liabilities are generally recognized in the income statements at the time they are reflected in rates. Refer to Note 2 for additional discussion of regulatory assets and regulatory liabilities. (c) Income Taxes - The liability method of accounting is followed for deferred taxes, which requires the establishment of deferred tax assets and liabilities, as appropriate, for temporary differences between the tax basis of assets and liabilities and the amounts reported in the financial statements. Deferred taxes are recorded using currently enacted tax rates, including impacts from Tax Reform and estimates of state apportionment. Changes in deferred tax assets and liabilities associated with certain property-related differences at IPL are accounted for differently than other subsidiaries of Alliant Energy due to rate-making practices in Iowa. Rate-making practices in Iowa do not include the impact of certain deferred tax expenses (benefits) in the determination of retail rates. Based on these rate-making practices, deferred tax expense (benefit) related to these property-related differences at IPL is not recorded in the income statement but instead recorded to regulatory assets or regulatory liabilities until these temporary differences reverse. Refer to Note 2 for further discussion of these tax-related regulatory assets and regulatory liabilities associated with property-related differences at IPL. In Wisconsin, the PSCW allows rate recovery of deferred tax expense on all temporary differences. Investment tax credits are deferred and amortized to income over the average lives of the related property. Tax Reform repealed corporate federal AMT and allows unutilized AMT credits to be refunded over the next four tax years beginning with the U.S. federal tax return for calendar year 2018. Other tax credits reduce income tax expense in the year claimed. Alliant Energy files a consolidated federal income tax return and a combined return in Wisconsin, which include Alliant Energy and its subsidiaries. Alliant Energy subsidiaries with a presence in Iowa file as part of a consolidated return in Iowa. Alliant Energy allocates consolidated income tax expense to its subsidiaries that are members of the group that file a consolidated or combined income tax return. IPL and WPL use the modified separate return approach for calculating their income tax provisions and related deferred tax assets and liabilities. IPL and WPL are assumed to file separate tax returns with the federal and state taxing authorities, except that net operating losses (and other current or deferred tax attributes) are characterized as realized (or realizable) by IPL and WPL when those tax attributes are realized (or realizable) by the consolidated tax return group of Alliant Energy (even if IPL and WPL would not otherwise have realized the attributes on a stand-alone basis). The difference in the income taxes recorded for IPL and WPL under the modified separate return method compared to the income taxes recorded on a separate return basis was not material in 2017 , 2016 and 2015 . Refer to Note 11 for further discussion of Tax Reform, which was enacted in December 2017. (d) Cash and Cash Equivalents - Cash and cash equivalents include short-term liquid investments that have original maturities of less than 90 days. (e) Property, Plant and Equipment - Utility Plant - General - Utility plant is recorded at the original cost of acquisition or construction, which includes material, labor, contractor services, AFUDC and allocable overheads, such as supervision, engineering, benefits, certain taxes and transportation. Repairs, replacements and renewals of items of property determined to be less than a unit of property or that do not increase the property’s life or functionality are charged to maintenance expense. Generally, ordinary retirements of utility plant and salvage value are netted and charged to accumulated depreciation upon removal from utility plant accounts and no gain or loss is recognized consistent with rate-making principles. However, if regulators have approved recovery of the remaining net book value of property, plant and equipment that is retired early, or such approval by regulators is probable, the remaining net book value is reclassified from property, plant and equipment to regulatory assets upon retirement. Property, plant and equipment that is probable of being retired early is classified as plant anticipated to be retired early. Depreciation - IPL and WPL use a combination of remaining life and straight-line depreciation methods as approved by their respective regulatory commissions. The composite or group method of depreciation is used, in which a single depreciation rate is applied to the gross investment in a particular class of property. This method pools similar assets and then depreciates each group as a whole. Periodic depreciation studies are performed to determine the appropriate group lives, net salvage, estimated cost of removal and group depreciation rates. These depreciation studies are subject to review and approval by IPL’s and WPL’s respective regulatory commissions. Depreciation expense is included within the recoverable cost of service component of rates charged to customers. The average rates of depreciation for electric, gas and other properties, consistent with current rate-making practices, were as follows: IPL WPL 2017 2016 2015 2017 2016 2015 Electric - generation 3.5% 3.5% 3.6% 3.5% 3.1% 3.2% Electric - distribution 2.4% 2.4% 2.4% 2.6% 2.6% 2.7% Electric - other 4.5% 4.2% 4.0% 6.9% 4.7% 4.5% Gas 3.4% 3.3% 3.2% 2.5% 2.5% 2.5% Other 4.0% 3.9% 3.9% 6.0% 5.9% 6.0% In September 2016, the PSCW issued an order approving the implementation of updated depreciation rates for WPL effective January 1, 2017 as a result of an updated depreciation study. In February 2018, the IUB issued an order approving the implementation of updated depreciation rates for IPL, which are currently expected to be effective in the first half of 2018, as a result of an updated depreciation study. IPL estimates the new average rates of depreciation for its electric generation and electric distribution properties will be approximately 3.8% and 2.8% , respectively, during 2018. AFUDC - AFUDC represents costs to finance construction additions, including a return on equity component and cost of debt component as required by regulatory accounting. AFUDC for IPL’s construction projects is calculated in accordance with FERC guidelines. AFUDC for WPL’s retail and wholesale jurisdiction construction projects is calculated in accordance with PSCW and FERC guidelines, respectively. The AFUDC rates, computed in accordance with the prescribed regulatory formula, were as follows: 2017 2016 2015 IPL (Marshalltown CWIP) (a) 7.8% 7.9% 7.9% IPL (Wind generation CWIP) (b) 7.6% N/A N/A IPL (other CWIP) 7.6% 7.7% 7.7% WPL (retail jurisdiction) 7.6% 8.2% 8.2% WPL (wholesale jurisdiction) 6.0% 6.7% 7.9% (a) In 2013, the IUB issued an order establishing rate-making principles that require a 10.3% return on common equity for the calculation of AFUDC related to the construction of Marshalltown. (b) In 2016, the IUB issued an order establishing rate-making principles that require a return on common equity for the calculation of AFUDC related to the construction of up to 500 MW of new wind generation equal to the greater of 10.0% or whatever percentage the IUB finds reasonable during IPL’s most recent retail electric rate proceeding. In accordance with their most recent rate orders, IPL applies its AFUDC rates to 100% of applicable CWIP balances and WPL generally applies its AFUDC rates to 50% of applicable CWIP balances. WPL may apply its AFUDC rates to 100% of the retail portion of the CWIP balances for construction projects requiring a CA or CPCN that were approved by the PSCW after its most recent rate order, including West Riverside. Non-utility Property - General - Non-utility property is recorded at the original cost of acquisition or construction, which includes material, labor and contractor services. Repairs, replacements and renewals of items of property determined to be less than a unit of property or that do not increase the property’s life or functionality are charged to maintenance expense. Upon retirement or sale of non-utility property, the original cost and related accumulated depreciation are removed from the accounts and any gain or loss is included in the income statements. Costs related to software developed or obtained for internal use are capitalized and amortized on a straight-line basis over the estimated useful life of the related software. If software is retired prior to being fully amortized, the difference is recorded as a loss in the income statements. (f) Operating Revenues - Utility - Revenues from Alliant Energy’s utility business are primarily from electricity and natural gas sales and are recognized on an accrual basis as services are rendered or commodities are delivered to customers. Energy sales to individual customers are based on the reading of customers’ meters, which occurs on a systematic basis throughout each reporting period. Amounts of energy delivered to customers since the date of the last meter reading are estimated at the end of each reporting period and the corresponding estimated unbilled revenue is recorded in such reporting period. The unbilled revenue estimate is based on daily system demand volumes, estimated customer usage by class, temperature impacts, line losses and the most recent customer rates. IPL and WPL accrue revenues from their wholesale customers to the extent that the actual net revenue requirements calculated in accordance with FERC-approved formula rates for the reporting period are higher or lower than the amounts billed to wholesale customers during such period. Regulatory assets or regulatory liabilities are recorded as the offset for these accrued revenues under formulaic rate-making programs. IPL’s estimated recovery amount is recorded in the current period of service and is reflected in customer bills within two years under the provisions of approved formula rates. WPL’s estimated recovery amount is recorded in the current period of service and subject to final adjustments after a customer audit period in the subsequent year. Final settled recovery amounts are reflected in WPL’s customer bills within two years under the provisions of approved formula rates. IPL and WPL participate in bid/offer-based wholesale energy and ancillary services markets operated by MISO. MISO requires that all load serving entities and generation owners, including IPL and WPL, submit hourly bids and offers for energy and ancillary services. The MISO transactions are grouped together, resulting in a net supply to or net purchase from MISO for each hour of each day. The net supply to MISO is recorded in “Electric utility operating revenues” and the net purchase from MISO is recorded in “Electric production fuel and purchased power” in the income statements. Non-utility - Revenues from Alliant Energy’s non-utility businesses are primarily from its Transportation business and are recognized on an accrual basis as services are rendered or goods are delivered to customers. Taxes Collected from Customers - Sales or various other taxes collected by certain of Alliant Energy’s subsidiaries on behalf of other agencies are recorded on a net basis and are not included in operating revenues. Revenue Recognition - Refer to Note 1(n) for discussion of a new accounting standard related to revenue recognition, which Alliant Energy, IPL and WPL adopted on January 1, 2018. (g) Utility Cost Recovery Mechanisms - Electric Production Fuel and Purchased Power (Fuel-related Costs) - Fuel-related costs are incurred to generate and purchase electricity to meet the demand of IPL’s and WPL’s electric customers. These fuel-related costs include the cost of fossil fuels (primarily natural gas and coal) used to produce electricity at their EGUs and electricity purchased from MISO wholesale energy markets and under PPAs. These fuel-related costs are recorded in “Electric production fuel and purchased power” in the income statements. IPL Retail - The cost recovery mechanisms for IPL’s retail electric customers provide for monthly adjustments to their electric rates for changes in fuel-related costs. Changes in the under-/over-collection of these costs are recognized in “Electric production fuel and purchased power” in Alliant Energy’s and IPL’s income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and IPL’s balance sheets until they are reflected in future billings to customers. WPL Retail - The cost recovery mechanism for WPL’s retail electric customers is based on forecasts of certain fuel-related costs expected to be incurred during forward-looking test year periods and fuel monitoring ranges determined by the PSCW during each retail electric rate proceeding or in a separate fuel cost plan approval proceeding. If WPL’s actual fuel-related costs fall outside these fuel monitoring ranges, WPL is authorized to defer the incremental under-/over-collection of fuel-related costs that are outside the approved ranges. Deferral of under-collections are reduced to the extent actual return on common equity earned by WPL during the fuel cost plan year exceeds the most recently authorized return on common equity. Deferred amounts for fuel-related costs outside the approved fuel monitoring ranges are recognized in “Electric production fuel and purchased power” in Alliant Energy’s and WPL’s income statements. The cumulative effects of these deferred amounts are recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and WPL’s balance sheets until they are reflected in future billings to customers. IPL and WPL Wholesale - The cost recovery mechanisms for IPL’s and WPL’s wholesale electric customers provide for subsequent adjustments to their electric rates for changes in fuel-related costs. Changes in the under-/over-collection of these costs are recognized in “Electric production fuel and purchased power” in the income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on the balance sheets until they are reflected in future billings to customers. Purchased Electric Capacity - PPAs help meet the electricity demand of IPL’s and WPL’s customers. Certain of these PPAs include minimum payments for IPL’s and WPL’s rights to electric generating capacity, which are charged each period to “Electric production fuel and purchased power” in the income statements. Purchased electric capacity expenses are recovered from IPL’s and WPL’s retail electric customers through changes in base rates determined during periodic rate proceedings. Purchased electric capacity expenses are recovered from IPL’s and WPL’s wholesale electric customers through annual changes in base rates determined by a formula rate structure. Electric Transmission Service - Costs incurred for the transmission of electricity to meet the demands of IPL’s and WPL’s customers are charged to “Electric transmission service” in the income statements. IPL Retail - Electric transmission service expense is recovered from IPL’s retail electric customers through a transmission cost rider. This cost recovery mechanism provides for annual adjustments to electric rates charged to retail electric customers for changes in electric transmission service expense. Changes in the under-/over-collection of these costs are recognized in “Electric transmission service” in Alliant Energy’s and IPL’s income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and IPL’s balance sheets until they are reflected in future billings to customers. WPL Retail - Electric transmission service expense is recovered from WPL’s retail electric customers through changes in base rates determined during periodic rate proceedings. Pursuant to escrow accounting treatment approved by the PSCW, the difference between actual electric transmission service expense incurred and the amount of electric transmission service costs collected from customers as electric revenues is recognized in “Electric transmission service” in Alliant Energy’s and WPL’s income statements. An offsetting amount is recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and WPL’s balance sheets until reflected in future billings to customers. The PSCW’s December 2016 order for WPL’s retail electric rate review (2017/2018 Test Period) extends this escrow accounting treatment through 2018. IPL and WPL Wholesale - IPL and WPL arrange transmission service for the majority of their respective wholesale electric customers. Electric transmission service expense is allocated to and recovered from these customers based on a load ratio share computation. Cost of Gas Sold - Costs are incurred for the purchase, transportation and storage of natural gas to serve IPL’s and WPL’s gas customers and the costs associated with the natural gas delivered to customers are charged to “Cost of gas sold” in the income statements. The tariffs for IPL’s and WPL’s retail gas customers provide for subsequent adjustments to their rates each month for changes in the cost of gas sold. Changes in the under-/over-collection of these costs are also recognized in “Cost of gas sold” in the income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on the balance sheets until they are reflected in future billings to customers. Energy Efficiency Costs - Costs are incurred to fund energy efficiency programs and initiatives that help customers reduce their energy usage. The costs incurred for these programs and initiatives are charged to “Other operation and maintenance” in the income statements. Energy efficiency costs incurred by IPL are recovered from its retail electric and gas customers through an additional tariff called an energy efficiency cost recovery factor, which is revised annually and includes a reconciliation to eliminate any under-/over-collection of energy efficiency costs from prior periods. Energy efficiency costs incurred by WPL are recovered from retail electric and gas customers through changes in base rates determined during periodic rate proceedings. Reconciliations of any under-/over-collection of energy efficiency costs from prior periods are also addressed in WPL’s periodic rate proceedings. Changes in the under-/over-collection of energy efficiency costs for IPL and WPL are recognized in “Other operation and maintenance” in the income statements. The cumulative effects of the under-/over-collection of these costs for IPL and WPL are recorded in regulatory assets or regulatory liabilities on the balance sheets until they are reflected in future billings to customers. Refer to Note 2 for additional information regarding these utility cost recovery mechanisms. (h) Financial Instruments - Financial instruments are periodically used for risk management purposes to mitigate exposures to fluctuations in certain commodity prices and transmission congestion costs. The fair value of those financial instruments that are determined to be derivatives are recorded as assets or liabilities on the balance sheets. Certain commodity purchase and sales contracts qualify for and have been designated under the normal purchase and sale exception and are accounted for on the accrual basis of accounting. Alliant Energy, IPL and WPL have elected to not net the fair value amounts of derivatives subject to a master netting arrangement by counterparty. Alliant Energy, IPL and WPL do not offset fair value amounts recognized for the right to reclaim cash collateral (receivable) or the obligation to return cash collateral (payable) against fair value amounts recognized for derivative instruments that are executed with the same counterparty under the same master netting arrangement. Refer to Note 2 for discussion of the recognition of regulatory assets and regulatory liabilities related to the unrealized losses and gains on IPL’s and WPL’s derivative instruments. Refer to Notes 14 , 15 and 16(f) for further discussion of derivatives and related credit risk. (i) Asset Impairments - Property, Plant and Equipment of Regulated Operations - Property, plant and equipment of regulated operations are reviewed for possible impairment whenever events or changes in circumstances indicate all or a portion of the carrying value of the assets may be disallowed for rate-making purposes. If IPL or WPL are disallowed recovery of any portion of the carrying value of their regulated property, plant and equipment that is under construction, has been recently completed or is probable of abandonment, or conclude it is probable recovery will be disallowed, an impairment charge is recognized equal to the amount of the carrying value that was disallowed or is probable of being disallowed. If IPL or WPL are only allowed a partial return on the carrying value of their regulated property, plant and equipment that is under construction, has been recently completed or is probable of abandonment, or conclude it is probable a full return will not be allowed, an impairment charge is recognized equal to the difference between the carrying value and the present value of the future revenues expected from their regulated property, plant and equipment. Property, Plant and Equipment of Non-utility Operations - Property, plant and equipment of non-utility operations are reviewed for possible impairment whenever events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Impairment is indicated if the carrying value of an asset exceeds its undiscounted future cash flows. If an impairment is indicated, a charge is recognized equal to the amount the carrying value exceeds the asset’s fair value. Refer to Note 3 for discussion of Alliant Energy’s impairment analysis of the Franklin County wind farm assets and resulting asset valuation charges recorded by Alliant Energy in 2016. Unconsolidated Equity Investments - If events or circumstances indicate the carrying value of investments accounted for under the equity method of accounting exceeds fair value and the decline in value is other than temporary, potential impairment is assessed. If an impairment is indicated, a charge is recognized equal to the amount the carrying value exceeds the investment’s fair value. Refer to Note 6(a) for additional discussion of investments accounted for under the equity method of accounting. (j) Asset Retirement Obligations - The fair value of a legal obligation associated with the retirement of an asset is recorded as a liability when an asset is placed in service, when a legal obligation is subsequently identified or when sufficient information becomes available to determine a reasonable estimate of the fair value of future retirement costs. When an ARO is recorded as a liability, an equivalent amount is added to the asset cost. The fair value of AROs at inception is determined using discounted cash flows analyses. The liability is accreted to its present value and the capitalized cost is depreciated over the useful life of the related asset. Accretion and depreciation expenses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory assets on the balance sheets. Upon regulatory approval to recover IPL’s AROs expenditures, its regulatory assets are amortized to depreciation and amortization expenses in Alliant Energy’s and IPL’s income statements over the same time period the ARO expenditures are recovered from IPL’s customers. WPL’s regulatory assets related to AROs are being recovered as a component of depreciation rates pursuant to PSCW and FERC orders. Accretion and depreciation expenses related to AROs for Alliant Energy’s non-utility operations are recorded to depreciation and amortization expenses in Alliant Energy’s income statements. Upon settlement of the ARO liability, an entity settles the obligation for its recorded amount or incurs a gain or loss. Any gains or losses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory liabilities or regulatory assets on the balance sheets. Refer to Note 13 for additional discussion of AROs. (k) Debt Issuance and Retirement Costs - Debt issuance costs and debt premiums or discounts are presented on the balance sheet as a direct deduction from the carrying amount of the related debt liability, and are deferred and amortized over the expected life of each debt issue, considering maturity dates and, if applicable, redemption rights held by others. Alliant Energy’s non-utility businesses and Corporate Services expense in the period of retirement any unamortized debt issuance costs and debt premiums or discounts on debt retired early. (l) Allowance for Doubtful Accounts - Allowances for doubtful accounts are recorded for estimated losses resulting from the inability of customers to make required payments. Allowances for doubtful accounts are estimated based on historical write-offs, customer arrears and other economic factors within IPL’s and WPL’s service territories. Refer to Note 5(a) for details of allowance for doubtful accounts. (m) Variable Interest Entities - An entity is considered a VIE if its equity investors do not have sufficient equity at risk for the entity to finance its activities without additional subordinated financial support from other parties, or its equity investors lack any of the following characteristics: (1) power, through voting rights or similar rights, to direct the activities of the entity that most significantly impact the entity’s economic performance; (2) the obligation to absorb expected losses of the entity; or (3) the right to receive expected benefits of the entity. The primary beneficiary of a VIE is required to consolidate the VIE. The financial statements do not reflect any consolidated VIEs. (n) New Accounting Standards - Revenue Recognition - In May 2014, the Financial Accounting Standards Board issued an accounting standard providing principles for recognizing revenue for the transfer of promised goods or services to customers with the consideration to which the entity expects to be entitled in exchange for those goods or services. This standard also requires disclosures about the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers. Alliant Energy, IPL and WPL adopted this standard on January 1, 2018 and used the modified retrospective method of adoption and there will be no cumulative effect adjustments made to the opening retain
WPL [Member]
Summary Of Significant Accounting Policies	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (a) General - Description of Business - Alliant Energy’s financial statements include the accounts of Alliant Energy and its consolidated subsidiaries. Alliant Energy is a Midwest U.S. energy holding company, whose primary wholly-owned subsidiaries are IPL, WPL, AEF and Corporate Services. IPL’s financial statements include the accounts of IPL and its consolidated subsidiary, IPL SPE LLC, which is used for IPL’s sales of accounts receivable program. IPL is a direct subsidiary of Alliant Energy and is a public utility engaged principally in the generation and distribution of electricity and the distribution and transportation of natural gas to retail customers in select markets in Iowa. IPL also sells electricity to wholesale customers in Minnesota, Illinois and Iowa, and is engaged in the generation and distribution of steam for two customers in Cedar Rapids, Iowa. WPL’s financial statements include the accounts of WPL and its consolidated subsidiary, WPL Transco, which held Alliant Energy’s investment in ATC until December 31, 2016. Refer to Note 6(a) for discussion of WPL’s transfer of its investment in ATC to ATI on December 31, 2016. WPL is a direct subsidiary of Alliant Energy and is a public utility engaged principally in the generation and distribution of electricity and the distribution and transportation of natural gas to retail customers in select markets in Wisconsin. WPL also sells electricity to wholesale customers in Wisconsin. AEF is comprised of Transportation, ATI, a non-utility wind investment, the Sheboygan Falls Energy Facility and other non-utility investments. Transportation includes a short-line railway that provides freight service between Cedar Rapids, Iowa and Iowa City, Iowa; barge terminal and hauling services on the Mississippi River; and other transfer and storage services. ATI, a wholly-owned subsidiary of AEF, holds all of Alliant Energy’s ATC Investment. The non-utility wind investment includes a 50% cash equity ownership interest in a 225 MW non-utility wind farm located in Oklahoma. The Sheboygan Falls Energy Facility is a 347 MW, simple-cycle, natural gas-fired EGU near Sheboygan Falls, Wisconsin, which is leased to WPL for an initial period of 20 years ending in 2025. Corporate Services is the subsidiary formed to provide administrative services to Alliant Energy and its subsidiaries. Basis of Presentation - The financial statements reflect investments in controlled subsidiaries on a consolidated basis and Alliant Energy’s, IPL’s and WPL’s proportionate shares of jointly-owned utility EGUs. Unconsolidated investments, which Alliant Energy and WPL do not control, but have the ability to exercise significant influence over operating and financial policies, are accounted for under the equity method of accounting. Under the equity method of accounting, Alliant Energy and WPL initially record the investment at cost, and adjust the carrying amount of the investment to recognize their respective share of the earnings or losses of the investee. Dividends received from an investee reduce the carrying amount of the equity investment. Investments that do not meet the criteria for consolidation or the equity method of accounting are accounted for under the cost method. Refer to Notes 1(m) and 6(a) for further discussion of VIEs and equity method investments, respectively. All intercompany balances and transactions, other than certain transactions affecting the rate-making process at IPL and WPL, have been eliminated from the financial statements. Such transactions not eliminated include costs that are recoverable from customers through rate-making processes. The financial statements are prepared in conformity with GAAP, which give recognition to the rate-making and accounting practices of FERC and state commissions having regulatory jurisdiction. Certain prior period amounts in the Financial Statements and Notes have been reclassified to conform to the current period presentation for comparative purposes, including reclassifications resulting from modifications to segment reporting as discussed in Note 17 . Discontinued operations reported in Alliant Energy’s income statements is related to various warranty claims associated with the sale of RMT, Inc. in 2013, which have resulted in operating expenses and income subsequent to the sale. Alliant Energy presents cash flows from continuing operations together with cash flows from discontinued operations in its cash flows statements. Use of Estimates - The preparation of the financial statements requires management to make estimates and assumptions that affect: (a) the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the financial statements; and (b) the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. (b) Regulatory Assets and Regulatory Liabilities - Alliant Energy, IPL and WPL are subject to regulation by FERC and various state regulatory commissions. As a result, Alliant Energy, IPL and WPL are subject to GAAP provisions for regulated operations, which provide that rate-regulated public utilities record certain costs and credits allowed in the rate-making process in different periods than for non-utility entities. Regulatory assets generally represent incurred costs that have been deferred as such costs are probable of recovery in future customer rates. Regulatory liabilities generally represent obligations to make refunds to customers or amounts collected in rates for which the related costs have not yet been incurred. Amounts recorded as regulatory assets or regulatory liabilities are generally recognized in the income statements at the time they are reflected in rates. Refer to Note 2 for additional discussion of regulatory assets and regulatory liabilities. (c) Income Taxes - The liability method of accounting is followed for deferred taxes, which requires the establishment of deferred tax assets and liabilities, as appropriate, for temporary differences between the tax basis of assets and liabilities and the amounts reported in the financial statements. Deferred taxes are recorded using currently enacted tax rates, including impacts from Tax Reform and estimates of state apportionment. Changes in deferred tax assets and liabilities associated with certain property-related differences at IPL are accounted for differently than other subsidiaries of Alliant Energy due to rate-making practices in Iowa. Rate-making practices in Iowa do not include the impact of certain deferred tax expenses (benefits) in the determination of retail rates. Based on these rate-making practices, deferred tax expense (benefit) related to these property-related differences at IPL is not recorded in the income statement but instead recorded to regulatory assets or regulatory liabilities until these temporary differences reverse. Refer to Note 2 for further discussion of these tax-related regulatory assets and regulatory liabilities associated with property-related differences at IPL. In Wisconsin, the PSCW allows rate recovery of deferred tax expense on all temporary differences. Investment tax credits are deferred and amortized to income over the average lives of the related property. Tax Reform repealed corporate federal AMT and allows unutilized AMT credits to be refunded over the next four tax years beginning with the U.S. federal tax return for calendar year 2018. Other tax credits reduce income tax expense in the year claimed. Alliant Energy files a consolidated federal income tax return and a combined return in Wisconsin, which include Alliant Energy and its subsidiaries. Alliant Energy subsidiaries with a presence in Iowa file as part of a consolidated return in Iowa. Alliant Energy allocates consolidated income tax expense to its subsidiaries that are members of the group that file a consolidated or combined income tax return. IPL and WPL use the modified separate return approach for calculating their income tax provisions and related deferred tax assets and liabilities. IPL and WPL are assumed to file separate tax returns with the federal and state taxing authorities, except that net operating losses (and other current or deferred tax attributes) are characterized as realized (or realizable) by IPL and WPL when those tax attributes are realized (or realizable) by the consolidated tax return group of Alliant Energy (even if IPL and WPL would not otherwise have realized the attributes on a stand-alone basis). The difference in the income taxes recorded for IPL and WPL under the modified separate return method compared to the income taxes recorded on a separate return basis was not material in 2017 , 2016 and 2015 . Refer to Note 11 for further discussion of Tax Reform, which was enacted in December 2017. (d) Cash and Cash Equivalents - Cash and cash equivalents include short-term liquid investments that have original maturities of less than 90 days. (e) Property, Plant and Equipment - Utility Plant - General - Utility plant is recorded at the original cost of acquisition or construction, which includes material, labor, contractor services, AFUDC and allocable overheads, such as supervision, engineering, benefits, certain taxes and transportation. Repairs, replacements and renewals of items of property determined to be less than a unit of property or that do not increase the property’s life or functionality are charged to maintenance expense. Generally, ordinary retirements of utility plant and salvage value are netted and charged to accumulated depreciation upon removal from utility plant accounts and no gain or loss is recognized consistent with rate-making principles. However, if regulators have approved recovery of the remaining net book value of property, plant and equipment that is retired early, or such approval by regulators is probable, the remaining net book value is reclassified from property, plant and equipment to regulatory assets upon retirement. Property, plant and equipment that is probable of being retired early is classified as plant anticipated to be retired early. Depreciation - IPL and WPL use a combination of remaining life and straight-line depreciation methods as approved by their respective regulatory commissions. The composite or group method of depreciation is used, in which a single depreciation rate is applied to the gross investment in a particular class of property. This method pools similar assets and then depreciates each group as a whole. Periodic depreciation studies are performed to determine the appropriate group lives, net salvage, estimated cost of removal and group depreciation rates. These depreciation studies are subject to review and approval by IPL’s and WPL’s respective regulatory commissions. Depreciation expense is included within the recoverable cost of service component of rates charged to customers. The average rates of depreciation for electric, gas and other properties, consistent with current rate-making practices, were as follows: IPL WPL 2017 2016 2015 2017 2016 2015 Electric - generation 3.5% 3.5% 3.6% 3.5% 3.1% 3.2% Electric - distribution 2.4% 2.4% 2.4% 2.6% 2.6% 2.7% Electric - other 4.5% 4.2% 4.0% 6.9% 4.7% 4.5% Gas 3.4% 3.3% 3.2% 2.5% 2.5% 2.5% Other 4.0% 3.9% 3.9% 6.0% 5.9% 6.0% In September 2016, the PSCW issued an order approving the implementation of updated depreciation rates for WPL effective January 1, 2017 as a result of an updated depreciation study. In February 2018, the IUB issued an order approving the implementation of updated depreciation rates for IPL, which are currently expected to be effective in the first half of 2018, as a result of an updated depreciation study. IPL estimates the new average rates of depreciation for its electric generation and electric distribution properties will be approximately 3.8% and 2.8% , respectively, during 2018. AFUDC - AFUDC represents costs to finance construction additions, including a return on equity component and cost of debt component as required by regulatory accounting. AFUDC for IPL’s construction projects is calculated in accordance with FERC guidelines. AFUDC for WPL’s retail and wholesale jurisdiction construction projects is calculated in accordance with PSCW and FERC guidelines, respectively. The AFUDC rates, computed in accordance with the prescribed regulatory formula, were as follows: 2017 2016 2015 IPL (Marshalltown CWIP) (a) 7.8% 7.9% 7.9% IPL (Wind generation CWIP) (b) 7.6% N/A N/A IPL (other CWIP) 7.6% 7.7% 7.7% WPL (retail jurisdiction) 7.6% 8.2% 8.2% WPL (wholesale jurisdiction) 6.0% 6.7% 7.9% (a) In 2013, the IUB issued an order establishing rate-making principles that require a 10.3% return on common equity for the calculation of AFUDC related to the construction of Marshalltown. (b) In 2016, the IUB issued an order establishing rate-making principles that require a return on common equity for the calculation of AFUDC related to the construction of up to 500 MW of new wind generation equal to the greater of 10.0% or whatever percentage the IUB finds reasonable during IPL’s most recent retail electric rate proceeding. In accordance with their most recent rate orders, IPL applies its AFUDC rates to 100% of applicable CWIP balances and WPL generally applies its AFUDC rates to 50% of applicable CWIP balances. WPL may apply its AFUDC rates to 100% of the retail portion of the CWIP balances for construction projects requiring a CA or CPCN that were approved by the PSCW after its most recent rate order, including West Riverside. Non-utility Property - General - Non-utility property is recorded at the original cost of acquisition or construction, which includes material, labor and contractor services. Repairs, replacements and renewals of items of property determined to be less than a unit of property or that do not increase the property’s life or functionality are charged to maintenance expense. Upon retirement or sale of non-utility property, the original cost and related accumulated depreciation are removed from the accounts and any gain or loss is included in the income statements. Costs related to software developed or obtained for internal use are capitalized and amortized on a straight-line basis over the estimated useful life of the related software. If software is retired prior to being fully amortized, the difference is recorded as a loss in the income statements. (f) Operating Revenues - Utility - Revenues from Alliant Energy’s utility business are primarily from electricity and natural gas sales and are recognized on an accrual basis as services are rendered or commodities are delivered to customers. Energy sales to individual customers are based on the reading of customers’ meters, which occurs on a systematic basis throughout each reporting period. Amounts of energy delivered to customers since the date of the last meter reading are estimated at the end of each reporting period and the corresponding estimated unbilled revenue is recorded in such reporting period. The unbilled revenue estimate is based on daily system demand volumes, estimated customer usage by class, temperature impacts, line losses and the most recent customer rates. IPL and WPL accrue revenues from their wholesale customers to the extent that the actual net revenue requirements calculated in accordance with FERC-approved formula rates for the reporting period are higher or lower than the amounts billed to wholesale customers during such period. Regulatory assets or regulatory liabilities are recorded as the offset for these accrued revenues under formulaic rate-making programs. IPL’s estimated recovery amount is recorded in the current period of service and is reflected in customer bills within two years under the provisions of approved formula rates. WPL’s estimated recovery amount is recorded in the current period of service and subject to final adjustments after a customer audit period in the subsequent year. Final settled recovery amounts are reflected in WPL’s customer bills within two years under the provisions of approved formula rates. IPL and WPL participate in bid/offer-based wholesale energy and ancillary services markets operated by MISO. MISO requires that all load serving entities and generation owners, including IPL and WPL, submit hourly bids and offers for energy and ancillary services. The MISO transactions are grouped together, resulting in a net supply to or net purchase from MISO for each hour of each day. The net supply to MISO is recorded in “Electric utility operating revenues” and the net purchase from MISO is recorded in “Electric production fuel and purchased power” in the income statements. Non-utility - Revenues from Alliant Energy’s non-utility businesses are primarily from its Transportation business and are recognized on an accrual basis as services are rendered or goods are delivered to customers. Taxes Collected from Customers - Sales or various other taxes collected by certain of Alliant Energy’s subsidiaries on behalf of other agencies are recorded on a net basis and are not included in operating revenues. Revenue Recognition - Refer to Note 1(n) for discussion of a new accounting standard related to revenue recognition, which Alliant Energy, IPL and WPL adopted on January 1, 2018. (g) Utility Cost Recovery Mechanisms - Electric Production Fuel and Purchased Power (Fuel-related Costs) - Fuel-related costs are incurred to generate and purchase electricity to meet the demand of IPL’s and WPL’s electric customers. These fuel-related costs include the cost of fossil fuels (primarily natural gas and coal) used to produce electricity at their EGUs and electricity purchased from MISO wholesale energy markets and under PPAs. These fuel-related costs are recorded in “Electric production fuel and purchased power” in the income statements. IPL Retail - The cost recovery mechanisms for IPL’s retail electric customers provide for monthly adjustments to their electric rates for changes in fuel-related costs. Changes in the under-/over-collection of these costs are recognized in “Electric production fuel and purchased power” in Alliant Energy’s and IPL’s income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and IPL’s balance sheets until they are reflected in future billings to customers. WPL Retail - The cost recovery mechanism for WPL’s retail electric customers is based on forecasts of certain fuel-related costs expected to be incurred during forward-looking test year periods and fuel monitoring ranges determined by the PSCW during each retail electric rate proceeding or in a separate fuel cost plan approval proceeding. If WPL’s actual fuel-related costs fall outside these fuel monitoring ranges, WPL is authorized to defer the incremental under-/over-collection of fuel-related costs that are outside the approved ranges. Deferral of under-collections are reduced to the extent actual return on common equity earned by WPL during the fuel cost plan year exceeds the most recently authorized return on common equity. Deferred amounts for fuel-related costs outside the approved fuel monitoring ranges are recognized in “Electric production fuel and purchased power” in Alliant Energy’s and WPL’s income statements. The cumulative effects of these deferred amounts are recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and WPL’s balance sheets until they are reflected in future billings to customers. IPL and WPL Wholesale - The cost recovery mechanisms for IPL’s and WPL’s wholesale electric customers provide for subsequent adjustments to their electric rates for changes in fuel-related costs. Changes in the under-/over-collection of these costs are recognized in “Electric production fuel and purchased power” in the income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on the balance sheets until they are reflected in future billings to customers. Purchased Electric Capacity - PPAs help meet the electricity demand of IPL’s and WPL’s customers. Certain of these PPAs include minimum payments for IPL’s and WPL’s rights to electric generating capacity, which are charged each period to “Electric production fuel and purchased power” in the income statements. Purchased electric capacity expenses are recovered from IPL’s and WPL’s retail electric customers through changes in base rates determined during periodic rate proceedings. Purchased electric capacity expenses are recovered from IPL’s and WPL’s wholesale electric customers through annual changes in base rates determined by a formula rate structure. Electric Transmission Service - Costs incurred for the transmission of electricity to meet the demands of IPL’s and WPL’s customers are charged to “Electric transmission service” in the income statements. IPL Retail - Electric transmission service expense is recovered from IPL’s retail electric customers through a transmission cost rider. This cost recovery mechanism provides for annual adjustments to electric rates charged to retail electric customers for changes in electric transmission service expense. Changes in the under-/over-collection of these costs are recognized in “Electric transmission service” in Alliant Energy’s and IPL’s income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and IPL’s balance sheets until they are reflected in future billings to customers. WPL Retail - Electric transmission service expense is recovered from WPL’s retail electric customers through changes in base rates determined during periodic rate proceedings. Pursuant to escrow accounting treatment approved by the PSCW, the difference between actual electric transmission service expense incurred and the amount of electric transmission service costs collected from customers as electric revenues is recognized in “Electric transmission service” in Alliant Energy’s and WPL’s income statements. An offsetting amount is recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and WPL’s balance sheets until reflected in future billings to customers. The PSCW’s December 2016 order for WPL’s retail electric rate review (2017/2018 Test Period) extends this escrow accounting treatment through 2018. IPL and WPL Wholesale - IPL and WPL arrange transmission service for the majority of their respective wholesale electric customers. Electric transmission service expense is allocated to and recovered from these customers based on a load ratio share computation. Cost of Gas Sold - Costs are incurred for the purchase, transportation and storage of natural gas to serve IPL’s and WPL’s gas customers and the costs associated with the natural gas delivered to customers are charged to “Cost of gas sold” in the income statements. The tariffs for IPL’s and WPL’s retail gas customers provide for subsequent adjustments to their rates each month for changes in the cost of gas sold. Changes in the under-/over-collection of these costs are also recognized in “Cost of gas sold” in the income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on the balance sheets until they are reflected in future billings to customers. Energy Efficiency Costs - Costs are incurred to fund energy efficiency programs and initiatives that help customers reduce their energy usage. The costs incurred for these programs and initiatives are charged to “Other operation and maintenance” in the income statements. Energy efficiency costs incurred by IPL are recovered from its retail electric and gas customers through an additional tariff called an energy efficiency cost recovery factor, which is revised annually and includes a reconciliation to eliminate any under-/over-collection of energy efficiency costs from prior periods. Energy efficiency costs incurred by WPL are recovered from retail electric and gas customers through changes in base rates determined during periodic rate proceedings. Reconciliations of any under-/over-collection of energy efficiency costs from prior periods are also addressed in WPL’s periodic rate proceedings. Changes in the under-/over-collection of energy efficiency costs for IPL and WPL are recognized in “Other operation and maintenance” in the income statements. The cumulative effects of the under-/over-collection of these costs for IPL and WPL are recorded in regulatory assets or regulatory liabilities on the balance sheets until they are reflected in future billings to customers. Refer to Note 2 for additional information regarding these utility cost recovery mechanisms. (h) Financial Instruments - Financial instruments are periodically used for risk management purposes to mitigate exposures to fluctuations in certain commodity prices and transmission congestion costs. The fair value of those financial instruments that are determined to be derivatives are recorded as assets or liabilities on the balance sheets. Certain commodity purchase and sales contracts qualify for and have been designated under the normal purchase and sale exception and are accounted for on the accrual basis of accounting. Alliant Energy, IPL and WPL have elected to not net the fair value amounts of derivatives subject to a master netting arrangement by counterparty. Alliant Energy, IPL and WPL do not offset fair value amounts recognized for the right to reclaim cash collateral (receivable) or the obligation to return cash collateral (payable) against fair value amounts recognized for derivative instruments that are executed with the same counterparty under the same master netting arrangement. Refer to Note 2 for discussion of the recognition of regulatory assets and regulatory liabilities related to the unrealized losses and gains on IPL’s and WPL’s derivative instruments. Refer to Notes 14 , 15 and 16(f) for further discussion of derivatives and related credit risk. (i) Asset Impairments - Property, Plant and Equipment of Regulated Operations - Property, plant and equipment of regulated operations are reviewed for possible impairment whenever events or changes in circumstances indicate all or a portion of the carrying value of the assets may be disallowed for rate-making purposes. If IPL or WPL are disallowed recovery of any portion of the carrying value of their regulated property, plant and equipment that is under construction, has been recently completed or is probable of abandonment, or conclude it is probable recovery will be disallowed, an impairment charge is recognized equal to the amount of the carrying value that was disallowed or is probable of being disallowed. If IPL or WPL are only allowed a partial return on the carrying value of their regulated property, plant and equipment that is under construction, has been recently completed or is probable of abandonment, or conclude it is probable a full return will not be allowed, an impairment charge is recognized equal to the difference between the carrying value and the present value of the future revenues expected from their regulated property, plant and equipment. Property, Plant and Equipment of Non-utility Operations - Property, plant and equipment of non-utility operations are reviewed for possible impairment whenever events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Impairment is indicated if the carrying value of an asset exceeds its undiscounted future cash flows. If an impairment is indicated, a charge is recognized equal to the amount the carrying value exceeds the asset’s fair value. Refer to Note 3 for discussion of Alliant Energy’s impairment analysis of the Franklin County wind farm assets and resulting asset valuation charges recorded by Alliant Energy in 2016. Unconsolidated Equity Investments - If events or circumstances indicate the carrying value of investments accounted for under the equity method of accounting exceeds fair value and the decline in value is other than temporary, potential impairment is assessed. If an impairment is indicated, a charge is recognized equal to the amount the carrying value exceeds the investment’s fair value. Refer to Note 6(a) for additional discussion of investments accounted for under the equity method of accounting. (j) Asset Retirement Obligations - The fair value of a legal obligation associated with the retirement of an asset is recorded as a liability when an asset is placed in service, when a legal obligation is subsequently identified or when sufficient information becomes available to determine a reasonable estimate of the fair value of future retirement costs. When an ARO is recorded as a liability, an equivalent amount is added to the asset cost. The fair value of AROs at inception is determined using discounted cash flows analyses. The liability is accreted to its present value and the capitalized cost is depreciated over the useful life of the related asset. Accretion and depreciation expenses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory assets on the balance sheets. Upon regulatory approval to recover IPL’s AROs expenditures, its regulatory assets are amortized to depreciation and amortization expenses in Alliant Energy’s and IPL’s income statements over the same time period the ARO expenditures are recovered from IPL’s customers. WPL’s regulatory assets related to AROs are being recovered as a component of depreciation rates pursuant to PSCW and FERC orders. Accretion and depreciation expenses related to AROs for Alliant Energy’s non-utility operations are recorded to depreciation and amortization expenses in Alliant Energy’s income statements. Upon settlement of the ARO liability, an entity settles the obligation for its recorded amount or incurs a gain or loss. Any gains or losses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory liabilities or regulatory assets on the balance sheets. Refer to Note 13 for additional discussion of AROs. (k) Debt Issuance and Retirement Costs - Debt issuance costs and debt premiums or discounts are presented on the balance sheet as a direct deduction from the carrying amount of the related debt liability, and are deferred and amortized over the expected life of each debt issue, considering maturity dates and, if applicable, redemption rights held by others. Alliant Energy’s non-utility businesses and Corporate Services expense in the period of retirement any unamortized debt issuance costs and debt premiums or discounts on debt retired early. (l) Allowance for Doubtful Accounts - Allowances for doubtful accounts are recorded for estimated losses resulting from the inability of customers to make required payments. Allowances for doubtful accounts are estimated based on historical write-offs, customer arrears and other economic factors within IPL’s and WPL’s service territories. Refer to Note 5(a) for details of allowance for doubtful accounts. (m) Variable Interest Entities - An entity is considered a VIE if its equity investors do not have sufficient equity at risk for the entity to finance its activities without additional subordinated financial support from other parties, or its equity investors lack any of the following characteristics: (1) power, through voting rights or similar rights, to direct the activities of the entity that most significantly impact the entity’s economic performance; (2) the obligation to absorb expected losses of the entity; or (3) the right to receive expected benefits of the entity. The primary beneficiary of a VIE is required to consolidate the VIE. The financial statements do not reflect any consolidated VIEs. (n) New Accounting Standards - Revenue Recognition - In May 2014, the Financial Accounting Standards Board issued an accounting standard providing principles for recognizing revenue for the transfer of promised goods or services to customers with the consideration to which the entity expects to be entitled in exchange for those goods or services. This standard also requires disclosures about the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers. Alliant Energy, IPL and WPL adopted this standard on January 1, 2018 and used the modified retrospective method of adoption and there will be no cumulative effect adjustments made to the opening retain

Regulatory Matters

Regulatory Matters	12 Months Ended
	Dec. 31, 2017
Regulatory Matters [Line Items]
Regulatory Matters	REGULATORY MATTERS Regulatory Assets - At December 31, regulatory assets were comprised of the following items (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Tax-related $778.2 $1,055.6 $750.5 $1,022.4 $27.7 $33.2 Pension and OPEB costs 548.0 578.7 274.4 294.0 273.6 284.7 AROs 109.3 105.9 72.5 64.3 36.8 41.6 EGUs retired early 63.8 41.4 31.6 — 32.2 41.4 Derivatives 45.3 30.7 21.8 10.0 23.5 20.7 Emission allowances 25.5 26.2 25.5 26.2 — — Other 96.6 76.6 55.3 41.9 41.3 34.7 $1,666.7 $1,915.1 $1,231.6 $1,458.8 $435.1 $456.3 A portion of the regulatory assets in the above table are not earning a return. These regulatory assets, but not the respective carrying costs of these regulatory assets, are expected to be recovered from customers in future rates. At December 31, 2017 , IPL and WPL had $56 million and $5 million , respectively, of regulatory assets representing past expenditures that were not earning a return. IPL’s regulatory assets that were not earning a return consisted primarily of emission allowances, debt redemption costs, and costs for clean air compliance and wind generation expansion projects. WPL’s regulatory assets that were not earning a return consisted primarily of environmental-related costs, and amounts related to the wholesale portion of under-collected fuel-related costs, which is discussed in Note 1(g) . The other regulatory assets reported in the above table either earn a return or the cash has not yet been expended, in which case the assets are offset by liabilities that also do not incur a carrying cost. Tax-related - IPL and WPL record regulatory assets for certain temporary differences (primarily related to utility property, plant and equipment at IPL) that result in a decrease in current rates charged to customers and an increase in future rates charged to customers based on the timing of income tax expense that is used to determine such rates. These temporary differences for IPL include the impacts of qualifying deductions for repairs expenditures, allocation of mixed service costs, and Iowa accelerated tax depreciation, which all contribute to lower current income tax expense during the first part of an asset’s useful life and higher current tax expense during the latter part of an asset’s useful life. These regulatory assets will be recovered from customers in the future when these temporary differences reverse resulting in additional current income tax expense used to determine customers’ rates. Tax-related regulatory assets are classified as non-current. During 2017 , Alliant Energy’s, IPL’s and WPL’s tax-related regulatory assets decreased primarily due to the impacts of Tax Reform, which is discussed in Note 11 . As a result of the reduced tax rate from Tax Reform, the amount of taxes needed to be collected from customers in the future has decreased, which resulted in a corresponding decrease in the associated regulatory assets. Pension and other postretirement benefits costs - The IUB and the PSCW have authorized IPL and WPL to record the retail portion of their respective previously unrecognized net actuarial gains and losses, and prior service costs and credits, as regulatory assets in lieu of AOCL on the balance sheets, as these amounts are expected to be recovered in future rates. IPL and WPL also recognize the wholesale portion of their previously unrecognized net actuarial gains and losses, and prior service costs and credits, as regulatory assets on the balance sheets because these amounts are expected to be recovered in rates in future periods under the formula rate structure. These regulatory assets will be increased or decreased as the net actuarial gains or losses, and prior service costs or credits, are subsequently amortized and recognized as a component of net periodic benefit costs. Regulatory assets are also increased or decreased as a result of the annual defined benefit plan measurement process. Pension and OPEB costs are included within the recoverable cost of service component of rates charged to IPL’s and WPL’s customers. The recoverable costs included in customers’ rates are based upon pension and OPEB costs determined in accordance with GAAP and are calculated using different methods for IPL’s and WPL’s respective regulatory jurisdictions. In February 2018, the IUB authorized IPL to recover from its Iowa retail electric customers an allocated portion of annual costs equal to a five -year inflation-adjusted average of actual costs incurred for 2013 through 2016 and an estimate of costs for its forward-looking post-Test Year (2017). The PSCW has authorized WPL to recover from its retail electric and gas customers an estimated allocated portion of annual costs equal to the costs expected to be incurred during each test period. IPL and WPL are authorized to recover from their wholesale customers an allocated portion of actual pension costs incurred each year through FERC-approved formula rates. Refer to Note 12(a) for additional details regarding pension and OPEB costs. AROs - Alliant Energy, IPL and WPL believe it is probable that any differences between expenses accrued for legal AROs related to their utility operations and expenses recovered currently in rates will be recoverable in future rates, and are deferring the differences as regulatory assets. IPL’s settlement reached in September 2017 and approved by the IUB in February 2018 did not include the recovery of certain ARO costs previously recorded as regulatory assets, and as a result, Alliant Energy and IPL recorded a write-down of regulatory assets in 2017 as discussed in “IPL’s Retail Electric Rate Review (2016 Test Year)” below. Refer to Note 13 for additional details of AROs. Electric generating units retired early - In June 2017, IPL retired Sutherland Units 1 and 3 and reclassified the remaining net book value of these EGUs from property, plant and equipment to a regulatory asset on Alliant Energy’s and IPL’s balance sheets. IPL was earning a return on the remaining net book value of these EGUs, as well as recovering the remaining net book value of these EGUs from both its retail and wholesale customers. IPL’s settlement reached in September 2017 and approved by the IUB in February 2018 authorized IPL to recover the remaining net book value of these EGUs from IPL’s retail customers over a 10 -year period; however, IPL is not allowed to earn a return on the remaining net book value of these EGUs from its retail customers. As a result, Alliant Energy and IPL recorded a write-down of regulatory assets in 2017 as discussed in “IPL’s Retail Electric Rate Review (2016 Test Year)” below. In September 2017, FERC approved continued recovery of the remaining net book value of these EGUs from IPL’s wholesale customers over a 10 -year period. In 2015, WPL retired Nelson Dewey Units 1 and 2 and Edgewater Unit 3. WPL received approval from the PSCW and FERC to reclassify the remaining net book value of these EGUs from property, plant and equipment to a regulatory asset on Alliant Energy’s and WPL’s balance sheets. The remaining net book value is included in WPL’s rate base and WPL is earning a return on the outstanding balance. WPL is currently recovering the remaining net book value of these EGUs from both its retail and wholesale customers over a 10 -year period beginning January 1, 2013 pursuant to PSCW and FERC orders. Derivatives - In accordance with IPL’s and WPL’s fuel and natural gas recovery mechanisms, prudently incurred costs from derivative instruments are recoverable from customers in the future after any losses are realized, and gains from derivative instruments are refundable to customers in the future after any gains are realized. Based on these recovery mechanisms, the changes in the fair value of derivative liabilities/assets resulted in comparable changes to regulatory assets/liabilities on the balance sheets. Refer to Note 15 for additional details of derivative assets and derivative liabilities. Emission allowances - IPL entered into forward contracts in 2007 to purchase SO2 emission allowances with vintage years of 2014 through 2017 from various counterparties to meet expected future emission reduction standards. Alliant Energy and IPL have recorded a regulatory asset for amounts paid under the forward contracts. In February 2018, the IUB authorized IPL to recover the unamortized forward contract costs for SO2 emission allowances through the energy adjustment clause over a 10 -year period. Other - Alliant Energy, IPL and WPL assess whether IPL’s and WPL’s regulatory assets are probable of future recovery by considering factors such as applicable regulations, recent orders by the applicable regulatory agencies, historical treatment of similar costs by the applicable regulatory agencies and regulatory environment changes. Based on these assessments, Alliant Energy, IPL and WPL believe the regulatory assets recognized as of December 31, 2017 in the above table are probable of future recovery. However, no assurance can be made that IPL and WPL will recover all of these regulatory assets in future rates. If future recovery of a regulatory asset ceases to be probable, the regulatory asset will be charged to expense in the period the likelihood of future recovery is less than probable. Regulatory Liabilities - At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Tax-related $899.4 $7.7 $399.5 $1.9 $499.9 $5.8 Cost of removal obligations 410.0 411.6 274.5 269.4 135.5 142.2 Electric transmission cost recovery 90.4 72.0 26.4 35.7 64.0 36.3 IPL’s tax benefit riders 25.0 83.5 25.0 83.5 — — Commodity cost recovery 21.0 30.8 14.6 17.8 6.4 13.0 Energy efficiency cost recovery 19.9 20.5 — — 19.9 20.5 Other 31.5 54.9 15.4 22.5 16.1 32.4 $1,497.2 $681.0 $755.4 $430.8 $741.8 $250.2 Regulatory liabilities related to cost of removal obligations, to the extent expensed through depreciation rates, reduce rate base. Excluding the regulatory liabilities arising from Tax Reform, a significant portion of the remaining regulatory liabilities are not used to reduce rate base in the revenue requirement calculations utilized in IPL’s and WPL’s respective rate proceedings. Tax-related - During 2017 , Alliant Energy’s, IPL’s and WPL’s tax-related regulatory liabilities increased primarily due to the impacts of Tax Reform, which is discussed in Note 11 . Tax-related regulatory liabilities are classified as non-current. Cost of removal obligations - Alliant Energy, IPL and WPL collect in rates future removal costs for many assets that do not have associated legal AROs. Alliant Energy, IPL and WPL record a regulatory liability for the estimated amounts they have collected in rates for these future removal costs and reduce the regulatory liability for amounts spent on removal activities. Electric transmission cost recovery - A group of MISO cooperative and municipal utilities previously filed two complaints with FERC requesting a reduction to the base return on equity used by MISO transmission owners, including ITC and ATC, to determine electric transmission costs billed to utilities, including IPL and WPL. In 2016, FERC issued an order on the first complaint and established a base return on equity of 10.32% , excluding any incentive adders granted by FERC. The base return on equity of 10.32% was effective September 28, 2016, and was for the refund period from November 12, 2013 through February 11, 2015 (first complaint period). In 2017, Alliant Energy, IPL and WPL received the refunds for the first complaint period of $50 million , $39 million and $11 million , respectively, after final true-ups. IPL and WPL each initially recorded the retail portion of the refunds to a regulatory liability. Pursuant to IUB approval, IPL’s retail portion of the refund from ITC was refunded to its retail customers in 2017. WPL’s retail portion of the refund from ATC will remain in a regulatory liability until such refunds are approved to be returned to retail customers in a future rate proceeding. IPL’s and WPL’s wholesale customers received their share of the refunds through normal monthly billing practices in 2017. The second complaint covers the period from February 12, 2015 through May 11, 2016. A decision from FERC on the second complaint is currently expected in 2018. Refer to Note 1(g) for additional details of IPL’s and WPL’s electric transmission service cost recovery mechanisms. IPL’s tax benefit riders - The IUB has approved electric and gas tax benefit riders proposed by IPL, which utilize regulatory liabilities to credit bills of IPL’s Iowa retail electric and gas customers to help offset the impact of rate increases on such customers. Alliant Energy and IPL recognize an offsetting reduction to income tax expense for the after-tax amounts credited to such customers, resulting in no impact to their net income from the electric and gas tax benefit riders. The tax benefit riders regulatory liabilities are related to tax benefits from tax accounting method changes for repairs expenditures and cost of removal expenditures, and a rate-making accounting change for capitalized interest. In 2017 , Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities increased (decreased) by ($59) million as follows (in millions): Electric tax benefit rider credits ($65 ) Gas tax benefit rider credits (6 ) Rate-making accounting change for capitalized interest 17 Tax Reform adjustment (Refer to Note 11 ) (5 ) ($59 ) In 2017, Alliant Energy and IPL implemented a rate-making accounting change for capitalized interest. IPL currently anticipates crediting its related tax benefits from this rate-making accounting change to its Iowa retail electric and gas customers in the future, and as a result, Alliant Energy and IPL recorded an increase of $17 million to IPL’s tax benefit riders regulatory liabilities in 2017. The remaining electric tax benefit rider regulatory liabilities are currently expected to be credited to IPL’s retail electric customers’ bills over a 12-month period after final rates are effective, which is currently expected in the first half of 2018. The remaining gas tax benefit rider regulatory liabilities are currently expected to be credited to IPL’s gas customers’ bills by December 2018, subject to final review by the IUB. Electric tax benefit rider - Details for IPL’s electric tax benefit rider are as follows (in millions): 2017 2016 2015 Credit to IPL’s Iowa retail electric customers’ bills with reduction to electric revenues (based on customers’ KWh usage) $65 $64 $72 Income tax benefit resulting from decreased taxable income caused by credits 27 27 30 Income tax benefit representing tax benefits realized from electric tax benefit rider 38 37 42 The IUB authorized IPL to reduce the electric tax benefit rider billing credits on customers’ bills for 2013 through 2016 to recognize the revenue requirement impact of the changes in tax accounting methods related to tangible property and mixed service costs. The revenue requirement adjustment resulted in increases to electric revenues in Alliant Energy’s and IPL’s income statements and was recognized through the energy adjustment clause as a reduction of the credits on IPL’s Iowa retail electric customers’ bills from the electric tax benefit rider as follows (in millions): 2016 2015 Revenue requirement adjustment $14 $14 Gas tax benefit rider - Details for IPL’s gas tax benefit rider are as follows (in millions): 2017 2016 2015 Credit to IPL’s Iowa retail gas customers’ bills with reduction to gas revenues (based on a fixed amount per day) $6 $12 $12 Income tax benefit resulting from decreased taxable income caused by credits 3 5 5 Income tax benefit representing tax benefits realized from gas tax benefit rider 3 7 7 Commodity cost recovery - Refer to Note 1(g) for additional details of IPL’s and WPL’s commodity cost recovery mechanisms. Energy efficiency cost recovery - WPL and IPL collect revenues from their customers to offset certain expenditures they each incur for energy efficiency programs, including state mandated programs and Shared Savings programs. Differences between forecasted costs used to set rates and actual costs for these programs are deferred as a regulatory asset or regulatory liability. Utility Rate Reviews - IPL’s Retail Electric Rate Review (2016 Test Year) - In April 2017, IPL filed a request with the IUB to increase annual electric base rates for its Iowa retail electric customers. The request was based on a 2016 historical Test Year as adjusted for certain known and measurable changes occurring up to 12 months after the commencement of the proceeding. The key drivers for the filing included recovery of capital projects, primarily power grid modernization and investments that advance cleaner energy, including Marshalltown. An interim retail electric base rate increase of $102 million , or approximately 7% , on an annual basis, was implemented effective April 13, 2017. Tax benefit rider credits and MISO transmission owner return on equity refunds were used to reduce the effect of the interim rate increase on customer bills in 2017. In 2017, Alliant Energy and IPL recorded increases in electric base rates of $77 million in conjunction with the interim retail electric base rate increase. In September 2017, IPL reached a partial, non-unanimous settlement agreement with the Iowa Office of Consumer Advocate, the Iowa Business Energy Coalition and the Large Energy Group for an annual retail electric base rate increase of $130 million , or approximately 9% . In February 2018, the IUB issued an order approving the settlement. Final rates are currently expected to be effective in the first half of 2018 once all motions for reconsideration have been addressed and final tariffs have been approved by the IUB. The final rate increase includes continuation of the electric transmission cost rider; increased depreciation expense resulting from an updated depreciation study; recovery over a four -year period of ARO expenditures since the last retail electric rate filing in 2010; recovery over a 10 -year period of the remaining net book value of Sutherland Units 1 and 3, unamortized forward contract costs for SO2 emission allowances through the energy adjustment clause and cancelled project costs approved in a prior EPB; and no double leverage applied to the weighted-average cost of capital. As a result of the partial settlement, in 2017, IPL recorded a write-down of regulatory assets of $9 million , including $4 million to “Other operation and maintenance” expenses primarily related to IPL being no longer probable of earning a return on the remaining net book value of Sutherland Units 1 and 3 from its retail customers when final rates are implemented, and $5 million to “Depreciation and amortization” expenses for certain AROs deemed no longer probable of recovery in future rates. WPL’s Retail Electric and Gas Rate Review (2017/2018 Test Period) - In December 2016, WPL received an order from the PSCW authorizing WPL to implement increases in annual retail electric and gas rates of $9 million and $9 million , respectively, effective January 1, 2017 followed by a freeze of such rates through the end of 2018. The $9 million net annual retail electric rate increase reflects a $60 million increase in base rates, partially offset by a $51 million reduction in fuel-related costs, using an estimate for 2017 fuel-related costs. The key drivers for the electric and gas base rate increases are recovery of the costs for environmental controls projects at Edgewater and Columbia, and investments in electric and gas distribution systems, including expansion of natural gas pipeline infrastructure. The filing also included utilization of amounts that WPL previously over-recovered from its customers for energy efficiency cost recovery and electric transmission cost recovery, as well as amounts deferred under the earnings sharing mechanism for the 2013/2014 Test Period to reduce the requested base rate increases. The order included provisions that require WPL to defer a portion of its earnings if its annual regulatory return on common equity exceeds certain levels in 2017 or 2018. In 2017, Alliant Energy and WPL recorded increases in electric and gas base rates of $63 million and $9 million , respectively, in conjunction with the base rate increases authorized in the PSCW’s December 2016 order. WPL’s Retail Electric and Gas Rate Review (2015/2016 Test Period) - In July 2014, WPL received an order from the PSCW authorizing WPL to maintain retail electric base rates at their then current levels through the end of 2016. The retail electric base rate review included a return of and return on costs for environmental controls projects, generation performance and reliability improvements, other ongoing capital expenditures and an increase in electric transmission service expense. The additional revenue requirement for these cost increases was offset by the impact of changes in the amortization of regulatory liabilities associated with energy efficiency recoveries and increased sales volumes. The order also authorized WPL to implement a $5 million decrease in annual base rates for its retail gas customers effective January 1, 2015 followed by a freeze of such gas base rates through the end of 2016. The order included provisions that required WPL to defer a portion of its earnings if its annual regulatory return on common equity exceeded certain levels in 2015 or 2016. As of December 31, 2017 , Alliant Energy and WPL deferred $5 million of WPL’s 2016 earnings for these provisions, which is included in “Other” in Alliant Energy’s and WPL’s regulatory liabilities tables above. IPL’s 2014 Retail Electric Rate Settlement Agreement - The IUB approved a settlement agreement in 2014 related to rates charged to IPL’s Iowa retail electric customers. The settlement agreement extended IPL’s Iowa retail electric base rates authorized in its 2009 Test Year rate review through 2016 and provided targeted retail electric customer billing credits of $105 million in aggregate. IPL recorded such billing credits as follows (in millions): 2016 2015 2014 Billing credits to reduce retail electric customers’ bills $9 $24 $72 WPL’s Retail Fuel-related Rate Filing (2016 Test Year) - Pursuant to a 2015 PSCW order, WPL’s 2016 fuel-related costs were subject to deferral if they were outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs. Retail fuel-related costs incurred by WPL in 2016 were lower than fuel-related costs used to determine rates for such period resulting in an over-collection of fuel-related costs. In August 2017, the PSCW authorized WPL to utilize $6 million of the over-collections as an offset to projected 2017 fuel-related cost under-collections. As of December 31, 2017 , $3 million of remaining fuel-related costs for 2016 outside of the approved bandwidth are included in “Commodity cost recovery” in Alliant Energy’s and WPL’s regulatory liabilities table above, and these costs are expected to offset any rate changes for WPL’s 2018 fuel-related costs. WPL’s Retail Fuel-related Rate Filing (2017 Test Year) - In March 2017, WPL filed an application with the PSCW for a mid-year fuel-related cost adjustment for 2017. Fuel-related costs for 2017 were expected to exceed the approved 2017 fuel-related cost plan by more than the 2% annual bandwidth. In August 2017, the PSCW authorized WPL to utilize $6 million of the 2016 fuel-related cost over-collections to offset a portion of the projected fuel-related cost under-collections for 2017. Retail fuel-related costs incurred by WPL in 2017 were higher than the projected fuel-related costs used to determine rates for such period resulting in an under-collection of fuel-related costs. As of December 31, 2017 , after applying the 2016 over-recovery amounts, the remaining fuel-related costs for 2017 outside of the approved bandwidth were $5 million and are included in “Other” in Alliant Energy’s and WPL’s regulatory assets table above. WPL’s Retail Fuel-related Rate Filing (2018 Test Year) - In December 2017, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $6 million , or approximately 1% , effective January 1, 2018. The increase primarily reflects a change in expected fuel-related costs in 2018, which was offset by $3 million of over-collections from WPL’s 2016 fuel-related costs as discussed above.
IPL [Member]
Regulatory Matters [Line Items]
Regulatory Matters	REGULATORY MATTERS Regulatory Assets - At December 31, regulatory assets were comprised of the following items (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Tax-related $778.2 $1,055.6 $750.5 $1,022.4 $27.7 $33.2 Pension and OPEB costs 548.0 578.7 274.4 294.0 273.6 284.7 AROs 109.3 105.9 72.5 64.3 36.8 41.6 EGUs retired early 63.8 41.4 31.6 — 32.2 41.4 Derivatives 45.3 30.7 21.8 10.0 23.5 20.7 Emission allowances 25.5 26.2 25.5 26.2 — — Other 96.6 76.6 55.3 41.9 41.3 34.7 $1,666.7 $1,915.1 $1,231.6 $1,458.8 $435.1 $456.3 A portion of the regulatory assets in the above table are not earning a return. These regulatory assets, but not the respective carrying costs of these regulatory assets, are expected to be recovered from customers in future rates. At December 31, 2017 , IPL and WPL had $56 million and $5 million , respectively, of regulatory assets representing past expenditures that were not earning a return. IPL’s regulatory assets that were not earning a return consisted primarily of emission allowances, debt redemption costs, and costs for clean air compliance and wind generation expansion projects. WPL’s regulatory assets that were not earning a return consisted primarily of environmental-related costs, and amounts related to the wholesale portion of under-collected fuel-related costs, which is discussed in Note 1(g) . The other regulatory assets reported in the above table either earn a return or the cash has not yet been expended, in which case the assets are offset by liabilities that also do not incur a carrying cost. Tax-related - IPL and WPL record regulatory assets for certain temporary differences (primarily related to utility property, plant and equipment at IPL) that result in a decrease in current rates charged to customers and an increase in future rates charged to customers based on the timing of income tax expense that is used to determine such rates. These temporary differences for IPL include the impacts of qualifying deductions for repairs expenditures, allocation of mixed service costs, and Iowa accelerated tax depreciation, which all contribute to lower current income tax expense during the first part of an asset’s useful life and higher current tax expense during the latter part of an asset’s useful life. These regulatory assets will be recovered from customers in the future when these temporary differences reverse resulting in additional current income tax expense used to determine customers’ rates. Tax-related regulatory assets are classified as non-current. During 2017 , Alliant Energy’s, IPL’s and WPL’s tax-related regulatory assets decreased primarily due to the impacts of Tax Reform, which is discussed in Note 11 . As a result of the reduced tax rate from Tax Reform, the amount of taxes needed to be collected from customers in the future has decreased, which resulted in a corresponding decrease in the associated regulatory assets. Pension and other postretirement benefits costs - The IUB and the PSCW have authorized IPL and WPL to record the retail portion of their respective previously unrecognized net actuarial gains and losses, and prior service costs and credits, as regulatory assets in lieu of AOCL on the balance sheets, as these amounts are expected to be recovered in future rates. IPL and WPL also recognize the wholesale portion of their previously unrecognized net actuarial gains and losses, and prior service costs and credits, as regulatory assets on the balance sheets because these amounts are expected to be recovered in rates in future periods under the formula rate structure. These regulatory assets will be increased or decreased as the net actuarial gains or losses, and prior service costs or credits, are subsequently amortized and recognized as a component of net periodic benefit costs. Regulatory assets are also increased or decreased as a result of the annual defined benefit plan measurement process. Pension and OPEB costs are included within the recoverable cost of service component of rates charged to IPL’s and WPL’s customers. The recoverable costs included in customers’ rates are based upon pension and OPEB costs determined in accordance with GAAP and are calculated using different methods for IPL’s and WPL’s respective regulatory jurisdictions. In February 2018, the IUB authorized IPL to recover from its Iowa retail electric customers an allocated portion of annual costs equal to a five -year inflation-adjusted average of actual costs incurred for 2013 through 2016 and an estimate of costs for its forward-looking post-Test Year (2017). The PSCW has authorized WPL to recover from its retail electric and gas customers an estimated allocated portion of annual costs equal to the costs expected to be incurred during each test period. IPL and WPL are authorized to recover from their wholesale customers an allocated portion of actual pension costs incurred each year through FERC-approved formula rates. Refer to Note 12(a) for additional details regarding pension and OPEB costs. AROs - Alliant Energy, IPL and WPL believe it is probable that any differences between expenses accrued for legal AROs related to their utility operations and expenses recovered currently in rates will be recoverable in future rates, and are deferring the differences as regulatory assets. IPL’s settlement reached in September 2017 and approved by the IUB in February 2018 did not include the recovery of certain ARO costs previously recorded as regulatory assets, and as a result, Alliant Energy and IPL recorded a write-down of regulatory assets in 2017 as discussed in “IPL’s Retail Electric Rate Review (2016 Test Year)” below. Refer to Note 13 for additional details of AROs. Electric generating units retired early - In June 2017, IPL retired Sutherland Units 1 and 3 and reclassified the remaining net book value of these EGUs from property, plant and equipment to a regulatory asset on Alliant Energy’s and IPL’s balance sheets. IPL was earning a return on the remaining net book value of these EGUs, as well as recovering the remaining net book value of these EGUs from both its retail and wholesale customers. IPL’s settlement reached in September 2017 and approved by the IUB in February 2018 authorized IPL to recover the remaining net book value of these EGUs from IPL’s retail customers over a 10 -year period; however, IPL is not allowed to earn a return on the remaining net book value of these EGUs from its retail customers. As a result, Alliant Energy and IPL recorded a write-down of regulatory assets in 2017 as discussed in “IPL’s Retail Electric Rate Review (2016 Test Year)” below. In September 2017, FERC approved continued recovery of the remaining net book value of these EGUs from IPL’s wholesale customers over a 10 -year period. In 2015, WPL retired Nelson Dewey Units 1 and 2 and Edgewater Unit 3. WPL received approval from the PSCW and FERC to reclassify the remaining net book value of these EGUs from property, plant and equipment to a regulatory asset on Alliant Energy’s and WPL’s balance sheets. The remaining net book value is included in WPL’s rate base and WPL is earning a return on the outstanding balance. WPL is currently recovering the remaining net book value of these EGUs from both its retail and wholesale customers over a 10 -year period beginning January 1, 2013 pursuant to PSCW and FERC orders. Derivatives - In accordance with IPL’s and WPL’s fuel and natural gas recovery mechanisms, prudently incurred costs from derivative instruments are recoverable from customers in the future after any losses are realized, and gains from derivative instruments are refundable to customers in the future after any gains are realized. Based on these recovery mechanisms, the changes in the fair value of derivative liabilities/assets resulted in comparable changes to regulatory assets/liabilities on the balance sheets. Refer to Note 15 for additional details of derivative assets and derivative liabilities. Emission allowances - IPL entered into forward contracts in 2007 to purchase SO2 emission allowances with vintage years of 2014 through 2017 from various counterparties to meet expected future emission reduction standards. Alliant Energy and IPL have recorded a regulatory asset for amounts paid under the forward contracts. In February 2018, the IUB authorized IPL to recover the unamortized forward contract costs for SO2 emission allowances through the energy adjustment clause over a 10 -year period. Other - Alliant Energy, IPL and WPL assess whether IPL’s and WPL’s regulatory assets are probable of future recovery by considering factors such as applicable regulations, recent orders by the applicable regulatory agencies, historical treatment of similar costs by the applicable regulatory agencies and regulatory environment changes. Based on these assessments, Alliant Energy, IPL and WPL believe the regulatory assets recognized as of December 31, 2017 in the above table are probable of future recovery. However, no assurance can be made that IPL and WPL will recover all of these regulatory assets in future rates. If future recovery of a regulatory asset ceases to be probable, the regulatory asset will be charged to expense in the period the likelihood of future recovery is less than probable. Regulatory Liabilities - At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Tax-related $899.4 $7.7 $399.5 $1.9 $499.9 $5.8 Cost of removal obligations 410.0 411.6 274.5 269.4 135.5 142.2 Electric transmission cost recovery 90.4 72.0 26.4 35.7 64.0 36.3 IPL’s tax benefit riders 25.0 83.5 25.0 83.5 — — Commodity cost recovery 21.0 30.8 14.6 17.8 6.4 13.0 Energy efficiency cost recovery 19.9 20.5 — — 19.9 20.5 Other 31.5 54.9 15.4 22.5 16.1 32.4 $1,497.2 $681.0 $755.4 $430.8 $741.8 $250.2 Regulatory liabilities related to cost of removal obligations, to the extent expensed through depreciation rates, reduce rate base. Excluding the regulatory liabilities arising from Tax Reform, a significant portion of the remaining regulatory liabilities are not used to reduce rate base in the revenue requirement calculations utilized in IPL’s and WPL’s respective rate proceedings. Tax-related - During 2017 , Alliant Energy’s, IPL’s and WPL’s tax-related regulatory liabilities increased primarily due to the impacts of Tax Reform, which is discussed in Note 11 . Tax-related regulatory liabilities are classified as non-current. Cost of removal obligations - Alliant Energy, IPL and WPL collect in rates future removal costs for many assets that do not have associated legal AROs. Alliant Energy, IPL and WPL record a regulatory liability for the estimated amounts they have collected in rates for these future removal costs and reduce the regulatory liability for amounts spent on removal activities. Electric transmission cost recovery - A group of MISO cooperative and municipal utilities previously filed two complaints with FERC requesting a reduction to the base return on equity used by MISO transmission owners, including ITC and ATC, to determine electric transmission costs billed to utilities, including IPL and WPL. In 2016, FERC issued an order on the first complaint and established a base return on equity of 10.32% , excluding any incentive adders granted by FERC. The base return on equity of 10.32% was effective September 28, 2016, and was for the refund period from November 12, 2013 through February 11, 2015 (first complaint period). In 2017, Alliant Energy, IPL and WPL received the refunds for the first complaint period of $50 million , $39 million and $11 million , respectively, after final true-ups. IPL and WPL each initially recorded the retail portion of the refunds to a regulatory liability. Pursuant to IUB approval, IPL’s retail portion of the refund from ITC was refunded to its retail customers in 2017. WPL’s retail portion of the refund from ATC will remain in a regulatory liability until such refunds are approved to be returned to retail customers in a future rate proceeding. IPL’s and WPL’s wholesale customers received their share of the refunds through normal monthly billing practices in 2017. The second complaint covers the period from February 12, 2015 through May 11, 2016. A decision from FERC on the second complaint is currently expected in 2018. Refer to Note 1(g) for additional details of IPL’s and WPL’s electric transmission service cost recovery mechanisms. IPL’s tax benefit riders - The IUB has approved electric and gas tax benefit riders proposed by IPL, which utilize regulatory liabilities to credit bills of IPL’s Iowa retail electric and gas customers to help offset the impact of rate increases on such customers. Alliant Energy and IPL recognize an offsetting reduction to income tax expense for the after-tax amounts credited to such customers, resulting in no impact to their net income from the electric and gas tax benefit riders. The tax benefit riders regulatory liabilities are related to tax benefits from tax accounting method changes for repairs expenditures and cost of removal expenditures, and a rate-making accounting change for capitalized interest. In 2017 , Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities increased (decreased) by ($59) million as follows (in millions): Electric tax benefit rider credits ($65 ) Gas tax benefit rider credits (6 ) Rate-making accounting change for capitalized interest 17 Tax Reform adjustment (Refer to Note 11 ) (5 ) ($59 ) In 2017, Alliant Energy and IPL implemented a rate-making accounting change for capitalized interest. IPL currently anticipates crediting its related tax benefits from this rate-making accounting change to its Iowa retail electric and gas customers in the future, and as a result, Alliant Energy and IPL recorded an increase of $17 million to IPL’s tax benefit riders regulatory liabilities in 2017. The remaining electric tax benefit rider regulatory liabilities are currently expected to be credited to IPL’s retail electric customers’ bills over a 12-month period after final rates are effective, which is currently expected in the first half of 2018. The remaining gas tax benefit rider regulatory liabilities are currently expected to be credited to IPL’s gas customers’ bills by December 2018, subject to final review by the IUB. Electric tax benefit rider - Details for IPL’s electric tax benefit rider are as follows (in millions): 2017 2016 2015 Credit to IPL’s Iowa retail electric customers’ bills with reduction to electric revenues (based on customers’ KWh usage) $65 $64 $72 Income tax benefit resulting from decreased taxable income caused by credits 27 27 30 Income tax benefit representing tax benefits realized from electric tax benefit rider 38 37 42 The IUB authorized IPL to reduce the electric tax benefit rider billing credits on customers’ bills for 2013 through 2016 to recognize the revenue requirement impact of the changes in tax accounting methods related to tangible property and mixed service costs. The revenue requirement adjustment resulted in increases to electric revenues in Alliant Energy’s and IPL’s income statements and was recognized through the energy adjustment clause as a reduction of the credits on IPL’s Iowa retail electric customers’ bills from the electric tax benefit rider as follows (in millions): 2016 2015 Revenue requirement adjustment $14 $14 Gas tax benefit rider - Details for IPL’s gas tax benefit rider are as follows (in millions): 2017 2016 2015 Credit to IPL’s Iowa retail gas customers’ bills with reduction to gas revenues (based on a fixed amount per day) $6 $12 $12 Income tax benefit resulting from decreased taxable income caused by credits 3 5 5 Income tax benefit representing tax benefits realized from gas tax benefit rider 3 7 7 Commodity cost recovery - Refer to Note 1(g) for additional details of IPL’s and WPL’s commodity cost recovery mechanisms. Energy efficiency cost recovery - WPL and IPL collect revenues from their customers to offset certain expenditures they each incur for energy efficiency programs, including state mandated programs and Shared Savings programs. Differences between forecasted costs used to set rates and actual costs for these programs are deferred as a regulatory asset or regulatory liability. Utility Rate Reviews - IPL’s Retail Electric Rate Review (2016 Test Year) - In April 2017, IPL filed a request with the IUB to increase annual electric base rates for its Iowa retail electric customers. The request was based on a 2016 historical Test Year as adjusted for certain known and measurable changes occurring up to 12 months after the commencement of the proceeding. The key drivers for the filing included recovery of capital projects, primarily power grid modernization and investments that advance cleaner energy, including Marshalltown. An interim retail electric base rate increase of $102 million , or approximately 7% , on an annual basis, was implemented effective April 13, 2017. Tax benefit rider credits and MISO transmission owner return on equity refunds were used to reduce the effect of the interim rate increase on customer bills in 2017. In 2017, Alliant Energy and IPL recorded increases in electric base rates of $77 million in conjunction with the interim retail electric base rate increase. In September 2017, IPL reached a partial, non-unanimous settlement agreement with the Iowa Office of Consumer Advocate, the Iowa Business Energy Coalition and the Large Energy Group for an annual retail electric base rate increase of $130 million , or approximately 9% . In February 2018, the IUB issued an order approving the settlement. Final rates are currently expected to be effective in the first half of 2018 once all motions for reconsideration have been addressed and final tariffs have been approved by the IUB. The final rate increase includes continuation of the electric transmission cost rider; increased depreciation expense resulting from an updated depreciation study; recovery over a four -year period of ARO expenditures since the last retail electric rate filing in 2010; recovery over a 10 -year period of the remaining net book value of Sutherland Units 1 and 3, unamortized forward contract costs for SO2 emission allowances through the energy adjustment clause and cancelled project costs approved in a prior EPB; and no double leverage applied to the weighted-average cost of capital. As a result of the partial settlement, in 2017, IPL recorded a write-down of regulatory assets of $9 million , including $4 million to “Other operation and maintenance” expenses primarily related to IPL being no longer probable of earning a return on the remaining net book value of Sutherland Units 1 and 3 from its retail customers when final rates are implemented, and $5 million to “Depreciation and amortization” expenses for certain AROs deemed no longer probable of recovery in future rates. WPL’s Retail Electric and Gas Rate Review (2017/2018 Test Period) - In December 2016, WPL received an order from the PSCW authorizing WPL to implement increases in annual retail electric and gas rates of $9 million and $9 million , respectively, effective January 1, 2017 followed by a freeze of such rates through the end of 2018. The $9 million net annual retail electric rate increase reflects a $60 million increase in base rates, partially offset by a $51 million reduction in fuel-related costs, using an estimate for 2017 fuel-related costs. The key drivers for the electric and gas base rate increases are recovery of the costs for environmental controls projects at Edgewater and Columbia, and investments in electric and gas distribution systems, including expansion of natural gas pipeline infrastructure. The filing also included utilization of amounts that WPL previously over-recovered from its customers for energy efficiency cost recovery and electric transmission cost recovery, as well as amounts deferred under the earnings sharing mechanism for the 2013/2014 Test Period to reduce the requested base rate increases. The order included provisions that require WPL to defer a portion of its earnings if its annual regulatory return on common equity exceeds certain levels in 2017 or 2018. In 2017, Alliant Energy and WPL recorded increases in electric and gas base rates of $63 million and $9 million , respectively, in conjunction with the base rate increases authorized in the PSCW’s December 2016 order. WPL’s Retail Electric and Gas Rate Review (2015/2016 Test Period) - In July 2014, WPL received an order from the PSCW authorizing WPL to maintain retail electric base rates at their then current levels through the end of 2016. The retail electric base rate review included a return of and return on costs for environmental controls projects, generation performance and reliability improvements, other ongoing capital expenditures and an increase in electric transmission service expense. The additional revenue requirement for these cost increases was offset by the impact of changes in the amortization of regulatory liabilities associated with energy efficiency recoveries and increased sales volumes. The order also authorized WPL to implement a $5 million decrease in annual base rates for its retail gas customers effective January 1, 2015 followed by a freeze of such gas base rates through the end of 2016. The order included provisions that required WPL to defer a portion of its earnings if its annual regulatory return on common equity exceeded certain levels in 2015 or 2016. As of December 31, 2017 , Alliant Energy and WPL deferred $5 million of WPL’s 2016 earnings for these provisions, which is included in “Other” in Alliant Energy’s and WPL’s regulatory liabilities tables above. IPL’s 2014 Retail Electric Rate Settlement Agreement - The IUB approved a settlement agreement in 2014 related to rates charged to IPL’s Iowa retail electric customers. The settlement agreement extended IPL’s Iowa retail electric base rates authorized in its 2009 Test Year rate review through 2016 and provided targeted retail electric customer billing credits of $105 million in aggregate. IPL recorded such billing credits as follows (in millions): 2016 2015 2014 Billing credits to reduce retail electric customers’ bills $9 $24 $72 WPL’s Retail Fuel-related Rate Filing (2016 Test Year) - Pursuant to a 2015 PSCW order, WPL’s 2016 fuel-related costs were subject to deferral if they were outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs. Retail fuel-related costs incurred by WPL in 2016 were lower than fuel-related costs used to determine rates for such period resulting in an over-collection of fuel-related costs. In August 2017, the PSCW authorized WPL to utilize $6 million of the over-collections as an offset to projected 2017 fuel-related cost under-collections. As of December 31, 2017 , $3 million of remaining fuel-related costs for 2016 outside of the approved bandwidth are included in “Commodity cost recovery” in Alliant Energy’s and WPL’s regulatory liabilities table above, and these costs are expected to offset any rate changes for WPL’s 2018 fuel-related costs. WPL’s Retail Fuel-related Rate Filing (2017 Test Year) - In March 2017, WPL filed an application with the PSCW for a mid-year fuel-related cost adjustment for 2017. Fuel-related costs for 2017 were expected to exceed the approved 2017 fuel-related cost plan by more than the 2% annual bandwidth. In August 2017, the PSCW authorized WPL to utilize $6 million of the 2016 fuel-related cost over-collections to offset a portion of the projected fuel-related cost under-collections for 2017. Retail fuel-related costs incurred by WPL in 2017 were higher than the projected fuel-related costs used to determine rates for such period resulting in an under-collection of fuel-related costs. As of December 31, 2017 , after applying the 2016 over-recovery amounts, the remaining fuel-related costs for 2017 outside of the approved bandwidth were $5 million and are included in “Other” in Alliant Energy’s and WPL’s regulatory assets table above. WPL’s Retail Fuel-related Rate Filing (2018 Test Year) - In December 2017, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $6 million , or approximately 1% , effective January 1, 2018. The increase primarily reflects a change in expected fuel-related costs in 2018, which was offset by $3 million of over-collections from WPL’s 2016 fuel-related costs as discussed above.
WPL [Member]
Regulatory Matters [Line Items]
Regulatory Matters	REGULATORY MATTERS Regulatory Assets - At December 31, regulatory assets were comprised of the following items (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Tax-related $778.2 $1,055.6 $750.5 $1,022.4 $27.7 $33.2 Pension and OPEB costs 548.0 578.7 274.4 294.0 273.6 284.7 AROs 109.3 105.9 72.5 64.3 36.8 41.6 EGUs retired early 63.8 41.4 31.6 — 32.2 41.4 Derivatives 45.3 30.7 21.8 10.0 23.5 20.7 Emission allowances 25.5 26.2 25.5 26.2 — — Other 96.6 76.6 55.3 41.9 41.3 34.7 $1,666.7 $1,915.1 $1,231.6 $1,458.8 $435.1 $456.3 A portion of the regulatory assets in the above table are not earning a return. These regulatory assets, but not the respective carrying costs of these regulatory assets, are expected to be recovered from customers in future rates. At December 31, 2017 , IPL and WPL had $56 million and $5 million , respectively, of regulatory assets representing past expenditures that were not earning a return. IPL’s regulatory assets that were not earning a return consisted primarily of emission allowances, debt redemption costs, and costs for clean air compliance and wind generation expansion projects. WPL’s regulatory assets that were not earning a return consisted primarily of environmental-related costs, and amounts related to the wholesale portion of under-collected fuel-related costs, which is discussed in Note 1(g) . The other regulatory assets reported in the above table either earn a return or the cash has not yet been expended, in which case the assets are offset by liabilities that also do not incur a carrying cost. Tax-related - IPL and WPL record regulatory assets for certain temporary differences (primarily related to utility property, plant and equipment at IPL) that result in a decrease in current rates charged to customers and an increase in future rates charged to customers based on the timing of income tax expense that is used to determine such rates. These temporary differences for IPL include the impacts of qualifying deductions for repairs expenditures, allocation of mixed service costs, and Iowa accelerated tax depreciation, which all contribute to lower current income tax expense during the first part of an asset’s useful life and higher current tax expense during the latter part of an asset’s useful life. These regulatory assets will be recovered from customers in the future when these temporary differences reverse resulting in additional current income tax expense used to determine customers’ rates. Tax-related regulatory assets are classified as non-current. During 2017 , Alliant Energy’s, IPL’s and WPL’s tax-related regulatory assets decreased primarily due to the impacts of Tax Reform, which is discussed in Note 11 . As a result of the reduced tax rate from Tax Reform, the amount of taxes needed to be collected from customers in the future has decreased, which resulted in a corresponding decrease in the associated regulatory assets. Pension and other postretirement benefits costs - The IUB and the PSCW have authorized IPL and WPL to record the retail portion of their respective previously unrecognized net actuarial gains and losses, and prior service costs and credits, as regulatory assets in lieu of AOCL on the balance sheets, as these amounts are expected to be recovered in future rates. IPL and WPL also recognize the wholesale portion of their previously unrecognized net actuarial gains and losses, and prior service costs and credits, as regulatory assets on the balance sheets because these amounts are expected to be recovered in rates in future periods under the formula rate structure. These regulatory assets will be increased or decreased as the net actuarial gains or losses, and prior service costs or credits, are subsequently amortized and recognized as a component of net periodic benefit costs. Regulatory assets are also increased or decreased as a result of the annual defined benefit plan measurement process. Pension and OPEB costs are included within the recoverable cost of service component of rates charged to IPL’s and WPL’s customers. The recoverable costs included in customers’ rates are based upon pension and OPEB costs determined in accordance with GAAP and are calculated using different methods for IPL’s and WPL’s respective regulatory jurisdictions. In February 2018, the IUB authorized IPL to recover from its Iowa retail electric customers an allocated portion of annual costs equal to a five -year inflation-adjusted average of actual costs incurred for 2013 through 2016 and an estimate of costs for its forward-looking post-Test Year (2017). The PSCW has authorized WPL to recover from its retail electric and gas customers an estimated allocated portion of annual costs equal to the costs expected to be incurred during each test period. IPL and WPL are authorized to recover from their wholesale customers an allocated portion of actual pension costs incurred each year through FERC-approved formula rates. Refer to Note 12(a) for additional details regarding pension and OPEB costs. AROs - Alliant Energy, IPL and WPL believe it is probable that any differences between expenses accrued for legal AROs related to their utility operations and expenses recovered currently in rates will be recoverable in future rates, and are deferring the differences as regulatory assets. IPL’s settlement reached in September 2017 and approved by the IUB in February 2018 did not include the recovery of certain ARO costs previously recorded as regulatory assets, and as a result, Alliant Energy and IPL recorded a write-down of regulatory assets in 2017 as discussed in “IPL’s Retail Electric Rate Review (2016 Test Year)” below. Refer to Note 13 for additional details of AROs. Electric generating units retired early - In June 2017, IPL retired Sutherland Units 1 and 3 and reclassified the remaining net book value of these EGUs from property, plant and equipment to a regulatory asset on Alliant Energy’s and IPL’s balance sheets. IPL was earning a return on the remaining net book value of these EGUs, as well as recovering the remaining net book value of these EGUs from both its retail and wholesale customers. IPL’s settlement reached in September 2017 and approved by the IUB in February 2018 authorized IPL to recover the remaining net book value of these EGUs from IPL’s retail customers over a 10 -year period; however, IPL is not allowed to earn a return on the remaining net book value of these EGUs from its retail customers. As a result, Alliant Energy and IPL recorded a write-down of regulatory assets in 2017 as discussed in “IPL’s Retail Electric Rate Review (2016 Test Year)” below. In September 2017, FERC approved continued recovery of the remaining net book value of these EGUs from IPL’s wholesale customers over a 10 -year period. In 2015, WPL retired Nelson Dewey Units 1 and 2 and Edgewater Unit 3. WPL received approval from the PSCW and FERC to reclassify the remaining net book value of these EGUs from property, plant and equipment to a regulatory asset on Alliant Energy’s and WPL’s balance sheets. The remaining net book value is included in WPL’s rate base and WPL is earning a return on the outstanding balance. WPL is currently recovering the remaining net book value of these EGUs from both its retail and wholesale customers over a 10 -year period beginning January 1, 2013 pursuant to PSCW and FERC orders. Derivatives - In accordance with IPL’s and WPL’s fuel and natural gas recovery mechanisms, prudently incurred costs from derivative instruments are recoverable from customers in the future after any losses are realized, and gains from derivative instruments are refundable to customers in the future after any gains are realized. Based on these recovery mechanisms, the changes in the fair value of derivative liabilities/assets resulted in comparable changes to regulatory assets/liabilities on the balance sheets. Refer to Note 15 for additional details of derivative assets and derivative liabilities. Emission allowances - IPL entered into forward contracts in 2007 to purchase SO2 emission allowances with vintage years of 2014 through 2017 from various counterparties to meet expected future emission reduction standards. Alliant Energy and IPL have recorded a regulatory asset for amounts paid under the forward contracts. In February 2018, the IUB authorized IPL to recover the unamortized forward contract costs for SO2 emission allowances through the energy adjustment clause over a 10 -year period. Other - Alliant Energy, IPL and WPL assess whether IPL’s and WPL’s regulatory assets are probable of future recovery by considering factors such as applicable regulations, recent orders by the applicable regulatory agencies, historical treatment of similar costs by the applicable regulatory agencies and regulatory environment changes. Based on these assessments, Alliant Energy, IPL and WPL believe the regulatory assets recognized as of December 31, 2017 in the above table are probable of future recovery. However, no assurance can be made that IPL and WPL will recover all of these regulatory assets in future rates. If future recovery of a regulatory asset ceases to be probable, the regulatory asset will be charged to expense in the period the likelihood of future recovery is less than probable. Regulatory Liabilities - At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Tax-related $899.4 $7.7 $399.5 $1.9 $499.9 $5.8 Cost of removal obligations 410.0 411.6 274.5 269.4 135.5 142.2 Electric transmission cost recovery 90.4 72.0 26.4 35.7 64.0 36.3 IPL’s tax benefit riders 25.0 83.5 25.0 83.5 — — Commodity cost recovery 21.0 30.8 14.6 17.8 6.4 13.0 Energy efficiency cost recovery 19.9 20.5 — — 19.9 20.5 Other 31.5 54.9 15.4 22.5 16.1 32.4 $1,497.2 $681.0 $755.4 $430.8 $741.8 $250.2 Regulatory liabilities related to cost of removal obligations, to the extent expensed through depreciation rates, reduce rate base. Excluding the regulatory liabilities arising from Tax Reform, a significant portion of the remaining regulatory liabilities are not used to reduce rate base in the revenue requirement calculations utilized in IPL’s and WPL’s respective rate proceedings. Tax-related - During 2017 , Alliant Energy’s, IPL’s and WPL’s tax-related regulatory liabilities increased primarily due to the impacts of Tax Reform, which is discussed in Note 11 . Tax-related regulatory liabilities are classified as non-current. Cost of removal obligations - Alliant Energy, IPL and WPL collect in rates future removal costs for many assets that do not have associated legal AROs. Alliant Energy, IPL and WPL record a regulatory liability for the estimated amounts they have collected in rates for these future removal costs and reduce the regulatory liability for amounts spent on removal activities. Electric transmission cost recovery - A group of MISO cooperative and municipal utilities previously filed two complaints with FERC requesting a reduction to the base return on equity used by MISO transmission owners, including ITC and ATC, to determine electric transmission costs billed to utilities, including IPL and WPL. In 2016, FERC issued an order on the first complaint and established a base return on equity of 10.32% , excluding any incentive adders granted by FERC. The base return on equity of 10.32% was effective September 28, 2016, and was for the refund period from November 12, 2013 through February 11, 2015 (first complaint period). In 2017, Alliant Energy, IPL and WPL received the refunds for the first complaint period of $50 million , $39 million and $11 million , respectively, after final true-ups. IPL and WPL each initially recorded the retail portion of the refunds to a regulatory liability. Pursuant to IUB approval, IPL’s retail portion of the refund from ITC was refunded to its retail customers in 2017. WPL’s retail portion of the refund from ATC will remain in a regulatory liability until such refunds are approved to be returned to retail customers in a future rate proceeding. IPL’s and WPL’s wholesale customers received their share of the refunds through normal monthly billing practices in 2017. The second complaint covers the period from February 12, 2015 through May 11, 2016. A decision from FERC on the second complaint is currently expected in 2018. Refer to Note 1(g) for additional details of IPL’s and WPL’s electric transmission service cost recovery mechanisms. IPL’s tax benefit riders - The IUB has approved electric and gas tax benefit riders proposed by IPL, which utilize regulatory liabilities to credit bills of IPL’s Iowa retail electric and gas customers to help offset the impact of rate increases on such customers. Alliant Energy and IPL recognize an offsetting reduction to income tax expense for the after-tax amounts credited to such customers, resulting in no impact to their net income from the electric and gas tax benefit riders. The tax benefit riders regulatory liabilities are related to tax benefits from tax accounting method changes for repairs expenditures and cost of removal expenditures, and a rate-making accounting change for capitalized interest. In 2017 , Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities increased (decreased) by ($59) million as follows (in millions): Electric tax benefit rider credits ($65 ) Gas tax benefit rider credits (6 ) Rate-making accounting change for capitalized interest 17 Tax Reform adjustment (Refer to Note 11 ) (5 ) ($59 ) In 2017, Alliant Energy and IPL implemented a rate-making accounting change for capitalized interest. IPL currently anticipates crediting its related tax benefits from this rate-making accounting change to its Iowa retail electric and gas customers in the future, and as a result, Alliant Energy and IPL recorded an increase of $17 million to IPL’s tax benefit riders regulatory liabilities in 2017. The remaining electric tax benefit rider regulatory liabilities are currently expected to be credited to IPL’s retail electric customers’ bills over a 12-month period after final rates are effective, which is currently expected in the first half of 2018. The remaining gas tax benefit rider regulatory liabilities are currently expected to be credited to IPL’s gas customers’ bills by December 2018, subject to final review by the IUB. Electric tax benefit rider - Details for IPL’s electric tax benefit rider are as follows (in millions): 2017 2016 2015 Credit to IPL’s Iowa retail electric customers’ bills with reduction to electric revenues (based on customers’ KWh usage) $65 $64 $72 Income tax benefit resulting from decreased taxable income caused by credits 27 27 30 Income tax benefit representing tax benefits realized from electric tax benefit rider 38 37 42 The IUB authorized IPL to reduce the electric tax benefit rider billing credits on customers’ bills for 2013 through 2016 to recognize the revenue requirement impact of the changes in tax accounting methods related to tangible property and mixed service costs. The revenue requirement adjustment resulted in increases to electric revenues in Alliant Energy’s and IPL’s income statements and was recognized through the energy adjustment clause as a reduction of the credits on IPL’s Iowa retail electric customers’ bills from the electric tax benefit rider as follows (in millions): 2016 2015 Revenue requirement adjustment $14 $14 Gas tax benefit rider - Details for IPL’s gas tax benefit rider are as follows (in millions): 2017 2016 2015 Credit to IPL’s Iowa retail gas customers’ bills with reduction to gas revenues (based on a fixed amount per day) $6 $12 $12 Income tax benefit resulting from decreased taxable income caused by credits 3 5 5 Income tax benefit representing tax benefits realized from gas tax benefit rider 3 7 7 Commodity cost recovery - Refer to Note 1(g) for additional details of IPL’s and WPL’s commodity cost recovery mechanisms. Energy efficiency cost recovery - WPL and IPL collect revenues from their customers to offset certain expenditures they each incur for energy efficiency programs, including state mandated programs and Shared Savings programs. Differences between forecasted costs used to set rates and actual costs for these programs are deferred as a regulatory asset or regulatory liability. Utility Rate Reviews - IPL’s Retail Electric Rate Review (2016 Test Year) - In April 2017, IPL filed a request with the IUB to increase annual electric base rates for its Iowa retail electric customers. The request was based on a 2016 historical Test Year as adjusted for certain known and measurable changes occurring up to 12 months after the commencement of the proceeding. The key drivers for the filing included recovery of capital projects, primarily power grid modernization and investments that advance cleaner energy, including Marshalltown. An interim retail electric base rate increase of $102 million , or approximately 7% , on an annual basis, was implemented effective April 13, 2017. Tax benefit rider credits and MISO transmission owner return on equity refunds were used to reduce the effect of the interim rate increase on customer bills in 2017. In 2017, Alliant Energy and IPL recorded increases in electric base rates of $77 million in conjunction with the interim retail electric base rate increase. In September 2017, IPL reached a partial, non-unanimous settlement agreement with the Iowa Office of Consumer Advocate, the Iowa Business Energy Coalition and the Large Energy Group for an annual retail electric base rate increase of $130 million , or approximately 9% . In February 2018, the IUB issued an order approving the settlement. Final rates are currently expected to be effective in the first half of 2018 once all motions for reconsideration have been addressed and final tariffs have been approved by the IUB. The final rate increase includes continuation of the electric transmission cost rider; increased depreciation expense resulting from an updated depreciation study; recovery over a four -year period of ARO expenditures since the last retail electric rate filing in 2010; recovery over a 10 -year period of the remaining net book value of Sutherland Units 1 and 3, unamortized forward contract costs for SO2 emission allowances through the energy adjustment clause and cancelled project costs approved in a prior EPB; and no double leverage applied to the weighted-average cost of capital. As a result of the partial settlement, in 2017, IPL recorded a write-down of regulatory assets of $9 million , including $4 million to “Other operation and maintenance” expenses primarily related to IPL being no longer probable of earning a return on the remaining net book value of Sutherland Units 1 and 3 from its retail customers when final rates are implemented, and $5 million to “Depreciation and amortization” expenses for certain AROs deemed no longer probable of recovery in future rates. WPL’s Retail Electric and Gas Rate Review (2017/2018 Test Period) - In December 2016, WPL received an order from the PSCW authorizing WPL to implement increases in annual retail electric and gas rates of $9 million and $9 million , respectively, effective January 1, 2017 followed by a freeze of such rates through the end of 2018. The $9 million net annual retail electric rate increase reflects a $60 million increase in base rates, partially offset by a $51 million reduction in fuel-related costs, using an estimate for 2017 fuel-related costs. The key drivers for the electric and gas base rate increases are recovery of the costs for environmental controls projects at Edgewater and Columbia, and investments in electric and gas distribution systems, including expansion of natural gas pipeline infrastructure. The filing also included utilization of amounts that WPL previously over-recovered from its customers for energy efficiency cost recovery and electric transmission cost recovery, as well as amounts deferred under the earnings sharing mechanism for the 2013/2014 Test Period to reduce the requested base rate increases. The order included provisions that require WPL to defer a portion of its earnings if its annual regulatory return on common equity exceeds certain levels in 2017 or 2018. In 2017, Alliant Energy and WPL recorded increases in electric and gas base rates of $63 million and $9 million , respectively, in conjunction with the base rate increases authorized in the PSCW’s December 2016 order. WPL’s Retail Electric and Gas Rate Review (2015/2016 Test Period) - In July 2014, WPL received an order from the PSCW authorizing WPL to maintain retail electric base rates at their then current levels through the end of 2016. The retail electric base rate review included a return of and return on costs for environmental controls projects, generation performance and reliability improvements, other ongoing capital expenditures and an increase in electric transmission service expense. The additional revenue requirement for these cost increases was offset by the impact of changes in the amortization of regulatory liabilities associated with energy efficiency recoveries and increased sales volumes. The order also authorized WPL to implement a $5 million decrease in annual base rates for its retail gas customers effective January 1, 2015 followed by a freeze of such gas base rates through the end of 2016. The order included provisions that required WPL to defer a portion of its earnings if its annual regulatory return on common equity exceeded certain levels in 2015 or 2016. As of December 31, 2017 , Alliant Energy and WPL deferred $5 million of WPL’s 2016 earnings for these provisions, which is included in “Other” in Alliant Energy’s and WPL’s regulatory liabilities tables above. IPL’s 2014 Retail Electric Rate Settlement Agreement - The IUB approved a settlement agreement in 2014 related to rates charged to IPL’s Iowa retail electric customers. The settlement agreement extended IPL’s Iowa retail electric base rates authorized in its 2009 Test Year rate review through 2016 and provided targeted retail electric customer billing credits of $105 million in aggregate. IPL recorded such billing credits as follows (in millions): 2016 2015 2014 Billing credits to reduce retail electric customers’ bills $9 $24 $72 WPL’s Retail Fuel-related Rate Filing (2016 Test Year) - Pursuant to a 2015 PSCW order, WPL’s 2016 fuel-related costs were subject to deferral if they were outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs. Retail fuel-related costs incurred by WPL in 2016 were lower than fuel-related costs used to determine rates for such period resulting in an over-collection of fuel-related costs. In August 2017, the PSCW authorized WPL to utilize $6 million of the over-collections as an offset to projected 2017 fuel-related cost under-collections. As of December 31, 2017 , $3 million of remaining fuel-related costs for 2016 outside of the approved bandwidth are included in “Commodity cost recovery” in Alliant Energy’s and WPL’s regulatory liabilities table above, and these costs are expected to offset any rate changes for WPL’s 2018 fuel-related costs. WPL’s Retail Fuel-related Rate Filing (2017 Test Year) - In March 2017, WPL filed an application with the PSCW for a mid-year fuel-related cost adjustment for 2017. Fuel-related costs for 2017 were expected to exceed the approved 2017 fuel-related cost plan by more than the 2% annual bandwidth. In August 2017, the PSCW authorized WPL to utilize $6 million of the 2016 fuel-related cost over-collections to offset a portion of the projected fuel-related cost under-collections for 2017. Retail fuel-related costs incurred by WPL in 2017 were higher than the projected fuel-related costs used to determine rates for such period resulting in an under-collection of fuel-related costs. As of December 31, 2017 , after applying the 2016 over-recovery amounts, the remaining fuel-related costs for 2017 outside of the approved bandwidth were $5 million and are included in “Other” in Alliant Energy’s and WPL’s regulatory assets table above. WPL’s Retail Fuel-related Rate Filing (2018 Test Year) - In December 2017, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $6 million , or approximately 1% , effective January 1, 2018. The increase primarily reflects a change in expected fuel-related costs in 2018, which was offset by $3 million of over-collections from WPL’s 2016 fuel-related costs as discussed above.

Property, Plant and Equipment

Property, Plant and Equipment	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	PROPERTY, PLANT AND EQUIPMENT At December 31, details of property, plant and equipment on the balance sheets were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Utility: Electric plant: Generation in service $6,655.3 $5,866.9 $3,715.9 $2,916.8 $2,939.4 $2,950.1 Distribution in service 5,123.5 4,739.2 2,820.9 2,589.3 2,302.6 2,149.9 Other in service 425.1 329.1 282.3 223.5 142.8 105.6 Anticipated to be retired early (a)(b) 93.0 108.3 — 108.3 93.0 — Total electric plant 12,296.9 11,043.5 6,819.1 5,837.9 5,477.8 5,205.6 Gas plant in service 1,244.0 1,107.6 654.8 556.7 589.2 550.9 Other plant in service 571.9 549.3 333.4 313.0 238.5 236.3 Accumulated depreciation (4,283.1 ) (4,135.7 ) (2,311.0 ) (2,258.3 ) (1,972.1 ) (1,877.4 ) Net plant 9,829.7 8,564.7 5,496.3 4,449.3 4,333.4 4,115.4 Leased Sheboygan Falls Energy Facility, net (c) — — — — 46.2 52.4 Construction work in progress 962.2 1,226.8 424.4 968.1 537.8 258.7 Other, net 6.0 18.4 5.5 18.2 0.5 0.2 Total utility 10,797.9 9,809.9 5,926.2 5,435.6 4,917.9 4,426.7 Non-utility and other: Non-utility Generation, net (d) 90.9 135.0 — — — — Corporate Services and other, net (e) 345.7 334.3 — — — — Total non-utility and other 436.6 469.3 — — — — Total property, plant and equipment $11,234.5 $10,279.2 $5,926.2 $5,435.6 $4,917.9 $4,426.7 (a) In 2017, IPL retired Sutherland Unit 3 and reclassified the remaining net book value of this EGU from property, plant and equipment to a regulatory asset on Alliant Energy’s and IPL’s balance sheets. Refer to Note 2 for further discussion, including recovery of the remaining net book value of Sutherland Unit 3. (b) In 2017, WPL received approval from MISO to retire Edgewater Unit 4 and currently anticipates retiring this EGU by September 30, 2018. In 2016, the PSCW authorized WPL to recover the remaining net book value of Edgewater Unit 4 over a 10 -year period beginning the later of the retirement date of the EGU or January 1, 2019. (c) Less accumulated amortization of $77.6 million and $71.4 million for WPL as of December 31, 2017 and 2016 , respectively. The Sheboygan Falls Energy Facility is eliminated from WPL upon consolidation and is included in the “Non-utility Generation, net” line within Alliant Energy’s consolidated property, plant and equipment. (d) Less accumulated depreciation of $50.5 million and $46.5 million for Alliant Energy as of December 31, 2017 and 2016 , respectively. Refer to “Franklin County Wind Farm” below for discussion of the April 2017 transfer of the Franklin County wind farm from AEF to IPL pursuant to a February 2017 FERC order. (e) Less accumulated depreciation of $285.6 million and $272.0 million for Alliant Energy as of December 31, 2017 and 2016 , respectively. Utility - Natural Gas-Fired Generation Projects - IPL’s Marshalltown Generating Station - Construction of Marshalltown, a 706 MW natural gas-fired combined-cycle EGU, began in 2014 and was completed in 2017, which resulted in a transfer of the capitalized project costs from “Construction work in progress” to “Electric plant - Generation in service” in the above table for Alliant Energy and IPL in 2017. As of December 31, 2017 and 2016 , the capitalized project costs for the EGU consisted of capitalized expenditures of $645 million and CWIP of $612 million , and AFUDC of $81 million and $68 million , respectively. WPL’s West Riverside Energy Center - WPL is currently constructing West Riverside, an approximate 730 MW natural gas-fired combined-cycle EGU. Construction began in 2016 and is currently expected to be completed by early 2020. As of December 31, 2017 and 2016 , Alliant Energy and WPL recorded capitalized expenditures for CWIP of $339 million and $81 million , and AFUDC of $14 million and $2 million , respectively, for West Riverside in “Construction work in progress” in the above table for Alliant Energy and WPL. These capital expenditures do not yet reflect any potential impacts from the exercise of purchase options by certain WPL electric cooperatives for a partial ownership interest in West Riverside. Wind Generation - IPL’s Expansion of Wind Generation - IPL currently plans to add up to 1,000 MW of new wind projects to its existing generation portfolio. These wind projects are expected to be placed into service in 2019 and 2020. As of December 31, 2017 and 2016 , Alliant Energy and IPL recorded capitalized expenditures for CWIP of $264 million and $102 million , and AFUDC of $11 million and $1 million , respectively, for this expansion of wind generation in “Construction work in progress” in the above table for Alliant Energy and IPL. Franklin County Wind Farm - In 2016, based on an evaluation of the strategic options for the Franklin County wind farm, Alliant Energy concluded it was probable the Franklin County wind farm would be transferred to IPL. As a result, Alliant Energy performed an impairment analysis of such assets and recorded non-cash, pre-tax asset valuation charges of $86 million (after-tax charges of $51 million , or $0.23 per share) in 2016. Alliant Energy recorded such charges as a reduction to property, plant and equipment on its balance sheet and charges to “Asset valuation charges for Franklin County wind farm” in its income statement in 2016. In April 2017, the Franklin County wind farm was transferred from AEF to IPL as approved by a February 2017 FERC order. IPL’s purchase price, including certain transaction-related costs, was $32 million . As of the closing date, the estimated fair values of the assets purchased and liabilities assumed by IPL were as follows (in millions): Electric plant in service $40 Current assets 2 Total assets acquired 42 Other liabilities 10 Net assets acquired $32 WPL’s Proposed Acquisition of Forward Wind Energy Center - In October 2017, WPL entered into definitive agreements to acquire the assets of FWEC, which is a 129 MW wind farm located in Wisconsin. WPL currently expects to acquire 55 MW of FWEC for approximately $74 million . In November 2017, WPL filed for approval from the PSCW to acquire the assets of FWEC, and a decision from the PSCW is currently expected in the first half of 2018. In January 2018, FERC approved WPL’s request to acquire the assets of FWEC. Sales of IPL’s Minnesota Electric and Natural Gas Distribution Assets - In 2015, IPL completed the sale of its Minnesota natural gas distribution assets (primarily related to property, plant and equipment) and received proceeds of $11 million and a promissory note of $2 million . In 2015, IPL completed the sale of its Minnesota electric distribution assets (primarily related to property, plant and equipment) to Southern Minnesota Energy Cooperative, a combined group of various neighboring electric cooperatives, and received proceeds of $129 million . The proceeds from the natural gas distribution assets were used for general corporate purposes and the proceeds from the electric distribution assets were used to reduce cash amounts received from IPL’s sales of accounts receivable program. The premium received over the book value of the property, plant and equipment sold was more than offset by a reduction in tax-related regulatory assets associated with the distribution assets. As a result, Alliant Energy and IPL recorded pre-tax charges of $11 million and $3 million for the Minnesota electric and natural gas distribution asset transactions, respectively, in “Other operation and maintenance” in their income statements in 2015. The electric distribution asset sales agreement includes a wholesale power supply agreement between IPL and Southern Minnesota Energy Cooperative, which was approved by FERC in 2015 and became effective upon the sale of IPL’s Minnesota electric distribution assets. The wholesale power supply agreement contains a five -year termination notice, which may not be given until the fifth anniversary of the effective date of the agreement, resulting in a minimum term of 10 years . The agreement remains in effect indefinitely, unless notice to terminate is provided by either party. This wholesale power supply agreement includes standardized pricing mechanisms that are detailed in IPL’s current tariffs accepted by FERC through wholesale rate review proceedings. IPL’s current return on common equity authorized by FERC related to its wholesale electric rates is 10.97% . As a result of IPL’s requirement to supply electricity to Southern Minnesota Energy Cooperative under the wholesale power supply agreement, the sale of the electric distribution assets did not have a significant impact on IPL’s generation plans or operating results. AFUDC - AFUDC represents costs to finance construction additions, including a return on equity component and cost of debt component as required by regulatory accounting. The concurrent credit for the amount of AFUDC capitalized is recorded as “Allowance for funds used during construction” in the income statements. The amount of AFUDC generated by equity and debt components was as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Equity $33.6 $42.3 $24.4 $21.1 $35.2 $18.6 $12.5 $7.1 $5.8 Debt 16.1 20.2 12.5 10.3 16.8 9.6 5.8 3.4 2.9 $49.7 $62.5 $36.9 $31.4 $52.0 $28.2 $18.3 $10.5 $8.7 Non-utility and Other - The non-utility and other property, plant and equipment recorded on Alliant Energy’s balance sheets includes the following: Non-utility Generation - The Sheboygan Falls Energy Facility was placed into service in 2005 and is depreciated using the straight-line method over a 35 -year period. As of December 31, 2017 , Alliant Energy recorded $91 million on its balance sheet related to the Sheboygan Falls Energy Facility. Corporate Services and Other - Property, plant and equipment related to Corporate Services includes a customer billing and information system for IPL and WPL and other computer software, and the corporate headquarters building located in Madison, Wisconsin. The customer billing and information system is amortized using the straight-line method over a 12 -year period. The majority of the remaining software is amortized over a 5 -year period. Property, plant and equipment related to Transportation includes a short-line railway in Iowa and a barge terminal on the Mississippi River. The Corporate Services and Other property, plant and equipment is depreciated using the straight-line method over periods ranging from 5 to 30 years.
IPL [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	PROPERTY, PLANT AND EQUIPMENT At December 31, details of property, plant and equipment on the balance sheets were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Utility: Electric plant: Generation in service $6,655.3 $5,866.9 $3,715.9 $2,916.8 $2,939.4 $2,950.1 Distribution in service 5,123.5 4,739.2 2,820.9 2,589.3 2,302.6 2,149.9 Other in service 425.1 329.1 282.3 223.5 142.8 105.6 Anticipated to be retired early (a)(b) 93.0 108.3 — 108.3 93.0 — Total electric plant 12,296.9 11,043.5 6,819.1 5,837.9 5,477.8 5,205.6 Gas plant in service 1,244.0 1,107.6 654.8 556.7 589.2 550.9 Other plant in service 571.9 549.3 333.4 313.0 238.5 236.3 Accumulated depreciation (4,283.1 ) (4,135.7 ) (2,311.0 ) (2,258.3 ) (1,972.1 ) (1,877.4 ) Net plant 9,829.7 8,564.7 5,496.3 4,449.3 4,333.4 4,115.4 Leased Sheboygan Falls Energy Facility, net (c) — — — — 46.2 52.4 Construction work in progress 962.2 1,226.8 424.4 968.1 537.8 258.7 Other, net 6.0 18.4 5.5 18.2 0.5 0.2 Total utility 10,797.9 9,809.9 5,926.2 5,435.6 4,917.9 4,426.7 Non-utility and other: Non-utility Generation, net (d) 90.9 135.0 — — — — Corporate Services and other, net (e) 345.7 334.3 — — — — Total non-utility and other 436.6 469.3 — — — — Total property, plant and equipment $11,234.5 $10,279.2 $5,926.2 $5,435.6 $4,917.9 $4,426.7 (a) In 2017, IPL retired Sutherland Unit 3 and reclassified the remaining net book value of this EGU from property, plant and equipment to a regulatory asset on Alliant Energy’s and IPL’s balance sheets. Refer to Note 2 for further discussion, including recovery of the remaining net book value of Sutherland Unit 3. (b) In 2017, WPL received approval from MISO to retire Edgewater Unit 4 and currently anticipates retiring this EGU by September 30, 2018. In 2016, the PSCW authorized WPL to recover the remaining net book value of Edgewater Unit 4 over a 10 -year period beginning the later of the retirement date of the EGU or January 1, 2019. (c) Less accumulated amortization of $77.6 million and $71.4 million for WPL as of December 31, 2017 and 2016 , respectively. The Sheboygan Falls Energy Facility is eliminated from WPL upon consolidation and is included in the “Non-utility Generation, net” line within Alliant Energy’s consolidated property, plant and equipment. (d) Less accumulated depreciation of $50.5 million and $46.5 million for Alliant Energy as of December 31, 2017 and 2016 , respectively. Refer to “Franklin County Wind Farm” below for discussion of the April 2017 transfer of the Franklin County wind farm from AEF to IPL pursuant to a February 2017 FERC order. (e) Less accumulated depreciation of $285.6 million and $272.0 million for Alliant Energy as of December 31, 2017 and 2016 , respectively. Utility - Natural Gas-Fired Generation Projects - IPL’s Marshalltown Generating Station - Construction of Marshalltown, a 706 MW natural gas-fired combined-cycle EGU, began in 2014 and was completed in 2017, which resulted in a transfer of the capitalized project costs from “Construction work in progress” to “Electric plant - Generation in service” in the above table for Alliant Energy and IPL in 2017. As of December 31, 2017 and 2016 , the capitalized project costs for the EGU consisted of capitalized expenditures of $645 million and CWIP of $612 million , and AFUDC of $81 million and $68 million , respectively. WPL’s West Riverside Energy Center - WPL is currently constructing West Riverside, an approximate 730 MW natural gas-fired combined-cycle EGU. Construction began in 2016 and is currently expected to be completed by early 2020. As of December 31, 2017 and 2016 , Alliant Energy and WPL recorded capitalized expenditures for CWIP of $339 million and $81 million , and AFUDC of $14 million and $2 million , respectively, for West Riverside in “Construction work in progress” in the above table for Alliant Energy and WPL. These capital expenditures do not yet reflect any potential impacts from the exercise of purchase options by certain WPL electric cooperatives for a partial ownership interest in West Riverside. Wind Generation - IPL’s Expansion of Wind Generation - IPL currently plans to add up to 1,000 MW of new wind projects to its existing generation portfolio. These wind projects are expected to be placed into service in 2019 and 2020. As of December 31, 2017 and 2016 , Alliant Energy and IPL recorded capitalized expenditures for CWIP of $264 million and $102 million , and AFUDC of $11 million and $1 million , respectively, for this expansion of wind generation in “Construction work in progress” in the above table for Alliant Energy and IPL. Franklin County Wind Farm - In 2016, based on an evaluation of the strategic options for the Franklin County wind farm, Alliant Energy concluded it was probable the Franklin County wind farm would be transferred to IPL. As a result, Alliant Energy performed an impairment analysis of such assets and recorded non-cash, pre-tax asset valuation charges of $86 million (after-tax charges of $51 million , or $0.23 per share) in 2016. Alliant Energy recorded such charges as a reduction to property, plant and equipment on its balance sheet and charges to “Asset valuation charges for Franklin County wind farm” in its income statement in 2016. In April 2017, the Franklin County wind farm was transferred from AEF to IPL as approved by a February 2017 FERC order. IPL’s purchase price, including certain transaction-related costs, was $32 million . As of the closing date, the estimated fair values of the assets purchased and liabilities assumed by IPL were as follows (in millions): Electric plant in service $40 Current assets 2 Total assets acquired 42 Other liabilities 10 Net assets acquired $32 WPL’s Proposed Acquisition of Forward Wind Energy Center - In October 2017, WPL entered into definitive agreements to acquire the assets of FWEC, which is a 129 MW wind farm located in Wisconsin. WPL currently expects to acquire 55 MW of FWEC for approximately $74 million . In November 2017, WPL filed for approval from the PSCW to acquire the assets of FWEC, and a decision from the PSCW is currently expected in the first half of 2018. In January 2018, FERC approved WPL’s request to acquire the assets of FWEC. Sales of IPL’s Minnesota Electric and Natural Gas Distribution Assets - In 2015, IPL completed the sale of its Minnesota natural gas distribution assets (primarily related to property, plant and equipment) and received proceeds of $11 million and a promissory note of $2 million . In 2015, IPL completed the sale of its Minnesota electric distribution assets (primarily related to property, plant and equipment) to Southern Minnesota Energy Cooperative, a combined group of various neighboring electric cooperatives, and received proceeds of $129 million . The proceeds from the natural gas distribution assets were used for general corporate purposes and the proceeds from the electric distribution assets were used to reduce cash amounts received from IPL’s sales of accounts receivable program. The premium received over the book value of the property, plant and equipment sold was more than offset by a reduction in tax-related regulatory assets associated with the distribution assets. As a result, Alliant Energy and IPL recorded pre-tax charges of $11 million and $3 million for the Minnesota electric and natural gas distribution asset transactions, respectively, in “Other operation and maintenance” in their income statements in 2015. The electric distribution asset sales agreement includes a wholesale power supply agreement between IPL and Southern Minnesota Energy Cooperative, which was approved by FERC in 2015 and became effective upon the sale of IPL’s Minnesota electric distribution assets. The wholesale power supply agreement contains a five -year termination notice, which may not be given until the fifth anniversary of the effective date of the agreement, resulting in a minimum term of 10 years . The agreement remains in effect indefinitely, unless notice to terminate is provided by either party. This wholesale power supply agreement includes standardized pricing mechanisms that are detailed in IPL’s current tariffs accepted by FERC through wholesale rate review proceedings. IPL’s current return on common equity authorized by FERC related to its wholesale electric rates is 10.97% . As a result of IPL’s requirement to supply electricity to Southern Minnesota Energy Cooperative under the wholesale power supply agreement, the sale of the electric distribution assets did not have a significant impact on IPL’s generation plans or operating results. AFUDC - AFUDC represents costs to finance construction additions, including a return on equity component and cost of debt component as required by regulatory accounting. The concurrent credit for the amount of AFUDC capitalized is recorded as “Allowance for funds used during construction” in the income statements. The amount of AFUDC generated by equity and debt components was as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Equity $33.6 $42.3 $24.4 $21.1 $35.2 $18.6 $12.5 $7.1 $5.8 Debt 16.1 20.2 12.5 10.3 16.8 9.6 5.8 3.4 2.9 $49.7 $62.5 $36.9 $31.4 $52.0 $28.2 $18.3 $10.5 $8.7
WPL [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	PROPERTY, PLANT AND EQUIPMENT At December 31, details of property, plant and equipment on the balance sheets were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Utility: Electric plant: Generation in service $6,655.3 $5,866.9 $3,715.9 $2,916.8 $2,939.4 $2,950.1 Distribution in service 5,123.5 4,739.2 2,820.9 2,589.3 2,302.6 2,149.9 Other in service 425.1 329.1 282.3 223.5 142.8 105.6 Anticipated to be retired early (a)(b) 93.0 108.3 — 108.3 93.0 — Total electric plant 12,296.9 11,043.5 6,819.1 5,837.9 5,477.8 5,205.6 Gas plant in service 1,244.0 1,107.6 654.8 556.7 589.2 550.9 Other plant in service 571.9 549.3 333.4 313.0 238.5 236.3 Accumulated depreciation (4,283.1 ) (4,135.7 ) (2,311.0 ) (2,258.3 ) (1,972.1 ) (1,877.4 ) Net plant 9,829.7 8,564.7 5,496.3 4,449.3 4,333.4 4,115.4 Leased Sheboygan Falls Energy Facility, net (c) — — — — 46.2 52.4 Construction work in progress 962.2 1,226.8 424.4 968.1 537.8 258.7 Other, net 6.0 18.4 5.5 18.2 0.5 0.2 Total utility 10,797.9 9,809.9 5,926.2 5,435.6 4,917.9 4,426.7 Non-utility and other: Non-utility Generation, net (d) 90.9 135.0 — — — — Corporate Services and other, net (e) 345.7 334.3 — — — — Total non-utility and other 436.6 469.3 — — — — Total property, plant and equipment $11,234.5 $10,279.2 $5,926.2 $5,435.6 $4,917.9 $4,426.7 (a) In 2017, IPL retired Sutherland Unit 3 and reclassified the remaining net book value of this EGU from property, plant and equipment to a regulatory asset on Alliant Energy’s and IPL’s balance sheets. Refer to Note 2 for further discussion, including recovery of the remaining net book value of Sutherland Unit 3. (b) In 2017, WPL received approval from MISO to retire Edgewater Unit 4 and currently anticipates retiring this EGU by September 30, 2018. In 2016, the PSCW authorized WPL to recover the remaining net book value of Edgewater Unit 4 over a 10 -year period beginning the later of the retirement date of the EGU or January 1, 2019. (c) Less accumulated amortization of $77.6 million and $71.4 million for WPL as of December 31, 2017 and 2016 , respectively. The Sheboygan Falls Energy Facility is eliminated from WPL upon consolidation and is included in the “Non-utility Generation, net” line within Alliant Energy’s consolidated property, plant and equipment. (d) Less accumulated depreciation of $50.5 million and $46.5 million for Alliant Energy as of December 31, 2017 and 2016 , respectively. Refer to “Franklin County Wind Farm” below for discussion of the April 2017 transfer of the Franklin County wind farm from AEF to IPL pursuant to a February 2017 FERC order. (e) Less accumulated depreciation of $285.6 million and $272.0 million for Alliant Energy as of December 31, 2017 and 2016 , respectively. Utility - Natural Gas-Fired Generation Projects - IPL’s Marshalltown Generating Station - Construction of Marshalltown, a 706 MW natural gas-fired combined-cycle EGU, began in 2014 and was completed in 2017, which resulted in a transfer of the capitalized project costs from “Construction work in progress” to “Electric plant - Generation in service” in the above table for Alliant Energy and IPL in 2017. As of December 31, 2017 and 2016 , the capitalized project costs for the EGU consisted of capitalized expenditures of $645 million and CWIP of $612 million , and AFUDC of $81 million and $68 million , respectively. WPL’s West Riverside Energy Center - WPL is currently constructing West Riverside, an approximate 730 MW natural gas-fired combined-cycle EGU. Construction began in 2016 and is currently expected to be completed by early 2020. As of December 31, 2017 and 2016 , Alliant Energy and WPL recorded capitalized expenditures for CWIP of $339 million and $81 million , and AFUDC of $14 million and $2 million , respectively, for West Riverside in “Construction work in progress” in the above table for Alliant Energy and WPL. These capital expenditures do not yet reflect any potential impacts from the exercise of purchase options by certain WPL electric cooperatives for a partial ownership interest in West Riverside. Wind Generation - IPL’s Expansion of Wind Generation - IPL currently plans to add up to 1,000 MW of new wind projects to its existing generation portfolio. These wind projects are expected to be placed into service in 2019 and 2020. As of December 31, 2017 and 2016 , Alliant Energy and IPL recorded capitalized expenditures for CWIP of $264 million and $102 million , and AFUDC of $11 million and $1 million , respectively, for this expansion of wind generation in “Construction work in progress” in the above table for Alliant Energy and IPL. Franklin County Wind Farm - In 2016, based on an evaluation of the strategic options for the Franklin County wind farm, Alliant Energy concluded it was probable the Franklin County wind farm would be transferred to IPL. As a result, Alliant Energy performed an impairment analysis of such assets and recorded non-cash, pre-tax asset valuation charges of $86 million (after-tax charges of $51 million , or $0.23 per share) in 2016. Alliant Energy recorded such charges as a reduction to property, plant and equipment on its balance sheet and charges to “Asset valuation charges for Franklin County wind farm” in its income statement in 2016. In April 2017, the Franklin County wind farm was transferred from AEF to IPL as approved by a February 2017 FERC order. IPL’s purchase price, including certain transaction-related costs, was $32 million . As of the closing date, the estimated fair values of the assets purchased and liabilities assumed by IPL were as follows (in millions): Electric plant in service $40 Current assets 2 Total assets acquired 42 Other liabilities 10 Net assets acquired $32 WPL’s Proposed Acquisition of Forward Wind Energy Center - In October 2017, WPL entered into definitive agreements to acquire the assets of FWEC, which is a 129 MW wind farm located in Wisconsin. WPL currently expects to acquire 55 MW of FWEC for approximately $74 million . In November 2017, WPL filed for approval from the PSCW to acquire the assets of FWEC, and a decision from the PSCW is currently expected in the first half of 2018. In January 2018, FERC approved WPL’s request to acquire the assets of FWEC. Sales of IPL’s Minnesota Electric and Natural Gas Distribution Assets - In 2015, IPL completed the sale of its Minnesota natural gas distribution assets (primarily related to property, plant and equipment) and received proceeds of $11 million and a promissory note of $2 million . In 2015, IPL completed the sale of its Minnesota electric distribution assets (primarily related to property, plant and equipment) to Southern Minnesota Energy Cooperative, a combined group of various neighboring electric cooperatives, and received proceeds of $129 million . The proceeds from the natural gas distribution assets were used for general corporate purposes and the proceeds from the electric distribution assets were used to reduce cash amounts received from IPL’s sales of accounts receivable program. The premium received over the book value of the property, plant and equipment sold was more than offset by a reduction in tax-related regulatory assets associated with the distribution assets. As a result, Alliant Energy and IPL recorded pre-tax charges of $11 million and $3 million for the Minnesota electric and natural gas distribution asset transactions, respectively, in “Other operation and maintenance” in their income statements in 2015. The electric distribution asset sales agreement includes a wholesale power supply agreement between IPL and Southern Minnesota Energy Cooperative, which was approved by FERC in 2015 and became effective upon the sale of IPL’s Minnesota electric distribution assets. The wholesale power supply agreement contains a five -year termination notice, which may not be given until the fifth anniversary of the effective date of the agreement, resulting in a minimum term of 10 years . The agreement remains in effect indefinitely, unless notice to terminate is provided by either party. This wholesale power supply agreement includes standardized pricing mechanisms that are detailed in IPL’s current tariffs accepted by FERC through wholesale rate review proceedings. IPL’s current return on common equity authorized by FERC related to its wholesale electric rates is 10.97% . As a result of IPL’s requirement to supply electricity to Southern Minnesota Energy Cooperative under the wholesale power supply agreement, the sale of the electric distribution assets did not have a significant impact on IPL’s generation plans or operating results. AFUDC - AFUDC represents costs to finance construction additions, including a return on equity component and cost of debt component as required by regulatory accounting. The concurrent credit for the amount of AFUDC capitalized is recorded as “Allowance for funds used during construction” in the income statements. The amount of AFUDC generated by equity and debt components was as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Equity $33.6 $42.3 $24.4 $21.1 $35.2 $18.6 $12.5 $7.1 $5.8 Debt 16.1 20.2 12.5 10.3 16.8 9.6 5.8 3.4 2.9 $49.7 $62.5 $36.9 $31.4 $52.0 $28.2 $18.3 $10.5 $8.7

Jointly-Owned Electric Utility

Jointly-Owned Electric Utility Plant	12 Months Ended
	Dec. 31, 2017
Jointly-Owned Electric Utility Plant	JOINTLY-OWNED ELECTRIC UTILITY PLANT Under joint ownership agreements with other utilities, IPL and WPL have undivided ownership interests in jointly-owned coal-fired EGUs. Each of the respective owners is responsible for the financing of its portion of the construction costs. KWh generation and operating expenses are primarily divided between the joint owners on the same basis as ownership. IPL’s and WPL’s shares of expenses from jointly-owned coal-fired EGUs are included in the corresponding operating expenses (e.g., electric production fuel, other operation and maintenance, etc.) in their income statements. Information relative to IPL’s and WPL’s ownership interest in these jointly-owned coal-fired EGUs at December 31, 2017 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0 % $501.8 $144.9 $37.3 George Neal Unit 4 25.7 % 187.2 85.2 0.8 George Neal Unit 3 28.0 % 150.6 53.0 2.5 Louisa Unit 1 4.0 % 37.8 22.4 1.4 877.4 305.5 42.0 WPL Columbia Units 1-2 50.1 % 714.7 216.8 46.9 Edgewater Unit 4 68.2 % 99.9 60.4 0.1 814.6 277.2 47.0 Alliant Energy $1,692.0 $582.7 $89.0 In 2016, WPL received an order from the PSCW approving amendments to the Columbia joint operating agreement, which allow the co-owners to forgo certain capital expenditures at Columbia (excluding capital expenditures related to the Columbia Unit 2 SCR currently being constructed), resulting in WPL incurring these additional capital expenditures in exchange for a proportional increase in its ownership share of Columbia. Based on the additional capital expenditures WPL currently expects to incur through June 1, 2020, WPL’s ownership interest in Columbia is expected to increase in the future. In June 2017, FERC approved these amendments to the Columbia joint operating agreement. In October 2017, WPL received an order from the PSCW authorizing various electric cooperatives, which currently have wholesale power supply agreements with WPL, to acquire a partial ownership interest in West Riverside while the EGU is being constructed.
IPL [Member]
Jointly-Owned Electric Utility Plant	JOINTLY-OWNED ELECTRIC UTILITY PLANT Under joint ownership agreements with other utilities, IPL and WPL have undivided ownership interests in jointly-owned coal-fired EGUs. Each of the respective owners is responsible for the financing of its portion of the construction costs. KWh generation and operating expenses are primarily divided between the joint owners on the same basis as ownership. IPL’s and WPL’s shares of expenses from jointly-owned coal-fired EGUs are included in the corresponding operating expenses (e.g., electric production fuel, other operation and maintenance, etc.) in their income statements. Information relative to IPL’s and WPL’s ownership interest in these jointly-owned coal-fired EGUs at December 31, 2017 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0 % $501.8 $144.9 $37.3 George Neal Unit 4 25.7 % 187.2 85.2 0.8 George Neal Unit 3 28.0 % 150.6 53.0 2.5 Louisa Unit 1 4.0 % 37.8 22.4 1.4 877.4 305.5 42.0 WPL Columbia Units 1-2 50.1 % 714.7 216.8 46.9 Edgewater Unit 4 68.2 % 99.9 60.4 0.1 814.6 277.2 47.0 Alliant Energy $1,692.0 $582.7 $89.0
WPL [Member]
Jointly-Owned Electric Utility Plant	JOINTLY-OWNED ELECTRIC UTILITY PLANT Under joint ownership agreements with other utilities, IPL and WPL have undivided ownership interests in jointly-owned coal-fired EGUs. Each of the respective owners is responsible for the financing of its portion of the construction costs. KWh generation and operating expenses are primarily divided between the joint owners on the same basis as ownership. IPL’s and WPL’s shares of expenses from jointly-owned coal-fired EGUs are included in the corresponding operating expenses (e.g., electric production fuel, other operation and maintenance, etc.) in their income statements. Information relative to IPL’s and WPL’s ownership interest in these jointly-owned coal-fired EGUs at December 31, 2017 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0 % $501.8 $144.9 $37.3 George Neal Unit 4 25.7 % 187.2 85.2 0.8 George Neal Unit 3 28.0 % 150.6 53.0 2.5 Louisa Unit 1 4.0 % 37.8 22.4 1.4 877.4 305.5 42.0 WPL Columbia Units 1-2 50.1 % 714.7 216.8 46.9 Edgewater Unit 4 68.2 % 99.9 60.4 0.1 814.6 277.2 47.0 Alliant Energy $1,692.0 $582.7 $89.0 In 2016, WPL received an order from the PSCW approving amendments to the Columbia joint operating agreement, which allow the co-owners to forgo certain capital expenditures at Columbia (excluding capital expenditures related to the Columbia Unit 2 SCR currently being constructed), resulting in WPL incurring these additional capital expenditures in exchange for a proportional increase in its ownership share of Columbia. Based on the additional capital expenditures WPL currently expects to incur through June 1, 2020, WPL’s ownership interest in Columbia is expected to increase in the future. In June 2017, FERC approved these amendments to the Columbia joint operating agreement. In October 2017, WPL received an order from the PSCW authorizing various electric cooperatives, which currently have wholesale power supply agreements with WPL, to acquire a partial ownership interest in West Riverside while the EGU is being constructed.

Receivables

Receivables	12 Months Ended
	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables	RECEIVABLES (a) Accounts Receivable - Details for accounts receivable included on the balance sheets as of December 31 were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Customer $103.3 $111.7 $— $— $97.7 $104.4 Unbilled utility revenues 85.1 90.2 — — 85.1 90.2 Deferred proceeds 222.1 211.1 222.1 211.1 — — Other 84.3 89.0 44.1 30.7 40.1 38.8 Allowance for doubtful accounts (12.0 ) (8.7 ) (1.3 ) (1.1 ) (10.7 ) (7.1 ) $482.8 $493.3 $264.9 $240.7 $212.2 $226.3 (b) Sales of Accounts Receivable - IPL maintains a Receivables Agreement whereby it may sell its customer accounts receivables, unbilled revenues and certain other accounts receivables to a third party through wholly-owned and consolidated special purpose entities. The purchase commitment from the third party to which IPL sells its receivables expires in March 2018. IPL currently expects to amend and extend the purchase commitment. IPL pays a monthly fee to the third party that varies based on interest rates, limits on cash proceeds and cash amounts received from the third party. Deferred proceeds represent IPL’s interest in the receivables sold to the third party. At IPL’s request, deferred proceeds are paid to IPL from collections of receivables, after paying any required expenses incurred by the third party and the collection agent. Corporate Services acts as collection agent for the third party and receives a fee for collection services. The Receivables Agreement can be terminated by the third party if arrears or write-offs exceed certain levels. IPL was in compliance with all related covenants as of December 31, 2017 . Under the Receivables Agreement, IPL has the right to receive cash proceeds, up to a certain limit, from the third party in exchange for the receivables sold. Effective February 2017, the limit on cash proceeds was changed to $125 million . Cash proceeds are used by IPL to meet short-term financing needs, and cannot exceed the current limit or amount of receivables available for sale, whichever is less. As of December 31, 2017 , IPL had $91.7 million of available capacity under its sales of accounts receivable program. The transfers of receivables meet the criteria for sale accounting established by the transfer of financial assets accounting rules. IPL believes that the allowance for doubtful accounts related to its sales of receivables is a reasonable approximation of credit risk of the customers that generated the receivables. In 2017 , 2016 and 2015 , IPL’s costs incurred related to the sales of accounts receivable program were not material. Refer to Note 14 for discussion of the fair value of deferred proceeds. IPL’s maximum and average outstanding aggregate cash proceeds (based on daily outstanding balances) related to the sales of accounts receivable program were as follows (in millions): Maximum Average 2017 2016 2015 2017 2016 2015 Outstanding aggregate cash proceeds $112.0 $172.0 $137.0 $62.2 $73.2 $46.7 As of December 31, the attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2017 2016 Customer accounts receivable $133.8 $157.6 Unbilled utility revenues 112.7 90.4 Other receivables 0.3 0.1 Receivables sold to third party 246.8 248.1 Less: cash proceeds (a) 12.0 21.0 Deferred proceeds 234.8 227.1 Less: allowance for doubtful accounts 12.7 16.0 Fair value of deferred proceeds $222.1 $211.1 Outstanding receivables past due $44.7 $68.0 (a) Changes in cash proceeds are presented in “Sales of accounts receivable” in operating activities in Alliant Energy’s and IPL’s cash flows statements. Additional attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2017 2016 2015 Collections reinvested in receivables $1,647.1 $1,818.1 $1,812.9 Write-offs, net of recoveries 17.7 4.8 8.8 In connection with the implementation of IPL’s new customer billing and information system in 2016, IPL postponed the write-off of customer bills for a portion of 2016, resulting in lower write-offs in 2016, higher write-offs in 2017 and higher outstanding receivables past due as of December 31, 2016.
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables	RECEIVABLES (a) Accounts Receivable - Details for accounts receivable included on the balance sheets as of December 31 were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Customer $103.3 $111.7 $— $— $97.7 $104.4 Unbilled utility revenues 85.1 90.2 — — 85.1 90.2 Deferred proceeds 222.1 211.1 222.1 211.1 — — Other 84.3 89.0 44.1 30.7 40.1 38.8 Allowance for doubtful accounts (12.0 ) (8.7 ) (1.3 ) (1.1 ) (10.7 ) (7.1 ) $482.8 $493.3 $264.9 $240.7 $212.2 $226.3 (b) Sales of Accounts Receivable - IPL maintains a Receivables Agreement whereby it may sell its customer accounts receivables, unbilled revenues and certain other accounts receivables to a third party through wholly-owned and consolidated special purpose entities. The purchase commitment from the third party to which IPL sells its receivables expires in March 2018. IPL currently expects to amend and extend the purchase commitment. IPL pays a monthly fee to the third party that varies based on interest rates, limits on cash proceeds and cash amounts received from the third party. Deferred proceeds represent IPL’s interest in the receivables sold to the third party. At IPL’s request, deferred proceeds are paid to IPL from collections of receivables, after paying any required expenses incurred by the third party and the collection agent. Corporate Services acts as collection agent for the third party and receives a fee for collection services. The Receivables Agreement can be terminated by the third party if arrears or write-offs exceed certain levels. IPL was in compliance with all related covenants as of December 31, 2017 . Under the Receivables Agreement, IPL has the right to receive cash proceeds, up to a certain limit, from the third party in exchange for the receivables sold. Effective February 2017, the limit on cash proceeds was changed to $125 million . Cash proceeds are used by IPL to meet short-term financing needs, and cannot exceed the current limit or amount of receivables available for sale, whichever is less. As of December 31, 2017 , IPL had $91.7 million of available capacity under its sales of accounts receivable program. The transfers of receivables meet the criteria for sale accounting established by the transfer of financial assets accounting rules. IPL believes that the allowance for doubtful accounts related to its sales of receivables is a reasonable approximation of credit risk of the customers that generated the receivables. In 2017 , 2016 and 2015 , IPL’s costs incurred related to the sales of accounts receivable program were not material. Refer to Note 14 for discussion of the fair value of deferred proceeds. IPL’s maximum and average outstanding aggregate cash proceeds (based on daily outstanding balances) related to the sales of accounts receivable program were as follows (in millions): Maximum Average 2017 2016 2015 2017 2016 2015 Outstanding aggregate cash proceeds $112.0 $172.0 $137.0 $62.2 $73.2 $46.7 As of December 31, the attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2017 2016 Customer accounts receivable $133.8 $157.6 Unbilled utility revenues 112.7 90.4 Other receivables 0.3 0.1 Receivables sold to third party 246.8 248.1 Less: cash proceeds (a) 12.0 21.0 Deferred proceeds 234.8 227.1 Less: allowance for doubtful accounts 12.7 16.0 Fair value of deferred proceeds $222.1 $211.1 Outstanding receivables past due $44.7 $68.0 (a) Changes in cash proceeds are presented in “Sales of accounts receivable” in operating activities in Alliant Energy’s and IPL’s cash flows statements. Additional attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2017 2016 2015 Collections reinvested in receivables $1,647.1 $1,818.1 $1,812.9 Write-offs, net of recoveries 17.7 4.8 8.8 In connection with the implementation of IPL’s new customer billing and information system in 2016, IPL postponed the write-off of customer bills for a portion of 2016, resulting in lower write-offs in 2016, higher write-offs in 2017 and higher outstanding receivables past due as of December 31, 2016.
WPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables	RECEIVABLES (a) Accounts Receivable - Details for accounts receivable included on the balance sheets as of December 31 were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Customer $103.3 $111.7 $— $— $97.7 $104.4 Unbilled utility revenues 85.1 90.2 — — 85.1 90.2 Deferred proceeds 222.1 211.1 222.1 211.1 — — Other 84.3 89.0 44.1 30.7 40.1 38.8 Allowance for doubtful accounts (12.0 ) (8.7 ) (1.3 ) (1.1 ) (10.7 ) (7.1 ) $482.8 $493.3 $264.9 $240.7 $212.2 $226.3

Investments

Investments	12 Months Ended
	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Investments	INVESTMENTS (a) Unconsolidated Equity Investments - Unconsolidated investments accounted for under the equity method of accounting are as follows (in millions): Ownership Interest at Carrying Value at December 31, Equity (Income) / Loss December 31, 2017 2017 2016 2017 2016 2015 Alliant Energy ATC Investment (a) 16%-20% $274.2 $317.6 ($42.4 ) ($39.1 ) ($34.2 ) Wind Investment in Oklahoma 50% 98.3 — (1.8 ) — — Other Various 8.9 8.4 (0.6 ) (0.5 ) 0.4 $381.4 $326.0 ($44.8 ) ($39.6 ) ($33.8 ) WPL ATC —% $— $— $— ($39.1 ) ($34.2 ) Wisconsin River Power Company 50% 8.3 7.7 (0.7 ) (0.7 ) (0.9 ) $8.3 $7.7 ($0.7 ) ($39.8 ) ($35.1 ) (a) As of December 31, 2017 , Alliant Energy’s ATC Investment is comprised of a 16% ownership interest in ATC and a 20% ownership interest in ATC Holdco LLC, which are described below. In 2017, Alliant Energy’s investment in ATC decreased due to the impacts of Tax Reform. Refer to Note 11 for further discussion. Alliant Energy currently has the ability to exercise significant influence over ATC’s financial and operating policies through its participation on ATC’s Board of Directors. Refer to Note 18 for information regarding related party transactions with ATC. Summary aggregate financial information from the financial statements of these investments is as follows (in millions): Alliant Energy WPL 2017 2016 2015 2017 2016 2015 Operating revenues $741 $658 $624 $8 $658 $624 Operating income 374 331 299 4 331 299 Net income 267 232 186 2 234 202 As of December 31: Current assets 104 82 7 6 Non-current assets 5,041 4,340 20 19 Current liabilities 770 498 2 3 Non-current liabilities 2,038 2,144 8 7 Minority interest 255 — — — Investment in ATC and WPL’s Noncontrolling Interest - Prior to 2014, WPL owned 100% of WPL Transco, which held Alliant Energy’s investment in ATC. In 2014, WPL Transco’s operating agreement was amended to allow ATI, a wholly-owned subsidiary of AEF, to become a member of WPL Transco in addition to WPL. In 2014, ATI began funding capital contributions that WPL Transco made to ATC. WPL Transco’s equity income from ATC and ATC dividends received by WPL Transco were allocated between WPL and ATI based on their respective ownership interests at the time the equity income was generated and at the time of the dividend payments. Prior to the transfer of the investment in ATC to ATI discussed below, WPL consolidated WPL Transco, and ATI’s ownership in WPL Transco was recorded as a noncontrolling interest in total equity on WPL’s balance sheets. On December 31, 2016, pursuant to a June 2016 PSCW order, WPL Transco was liquidated and WPL transferred its investment in ATC to ATI. As a result, WPL no longer records equity income from its prior investment in ATC. In conjunction with the transfer of the investment in ATC, a deferred intercompany tax gain recognized by WPL was assumed by ATI. The impact of WPL’s transfer of the ATC investment, including the assumption of such intercompany tax gain by ATI, was recorded as a net reduction in total equity of $163.6 million on WPL’s balance sheet. WPL’s income statement includes all of the equity earnings from ATC through December 31, 2016, the date of transfer. There were no impacts of this transfer to Alliant Energy’s consolidated financial statements. As of December 31, 2016, ATI owns Alliant Energy’s entire ATC Investment. Investment in ATC Holdco LLC - In 2011, Duke Energy Corporation and ATC announced the creation of DATC, a joint venture that is expected to acquire, build, own and operate new electric transmission infrastructure in North America. DATC continues to evaluate new projects and opportunities, and participates in a competitive bidding process on projects it considers to be viable. In October 2017, ATC transferred a portion of its interest in DATC to ATC Holdco LLC, and as a result, Alliant Energy contributed additional equity capital funding based on its ownership interest in ATC Holdco LLC. A portion of proceeds from the transfer was distributed to all ATC Holdco LLC’s owners based on their ATC ownership percentage. Non-utility Wind Investment in Oklahoma - In July 2017, a wholly-owned subsidiary of AEF acquired 50% of a cash equity ownership interest in a 225 MW non-utility wind farm located in Oklahoma, which started commercial operations in December 2016. This wind farm has both cash and tax equity ownership. The wind farm provides electricity to a third-party under a long-term PPA. In 2017, Alliant Energy’s “Other investments” assets increased $98 million from this acquisition. Alliant Energy will not maintain or operate the wind farm, and provided a parent guarantee of its subsidiary’s indemnification obligations under the operating agreement and PPA. Refer to Note 16(d) for discussion of the guarantee. Alliant Energy accounts for this non-utility investment under the equity method of accounting. In conjunction with the acquisition, in July 2017, AEF entered into a $95 million , 364 -day variable-rate term loan credit agreement (with Alliant Energy as guarantor). (b) Cash Surrender Value of Life Insurance Policies - Various life insurance policies cover certain current and former employees and directors. In 2016, certain of Alliant Energy’s and IPL’s company-owned life insurance policies were liquidated. The related proceeds of $31 million and $19 million were recorded in investing activities in Alliant Energy’s and IPL’s cash flows statements, respectively, in 2016. At December 31, the cash surrender value of these investments was as follows (in millions): Alliant Energy WPL 2017 2016 2017 2016 Cash surrender value $10.4 $10.6 $5.6 $5.8
IPL [Member]
Schedule of Equity Method Investments [Line Items]
Investments	INVESTMENTS (b) Cash Surrender Value of Life Insurance Policies - Various life insurance policies cover certain current and former employees and directors. In 2016, certain of Alliant Energy’s and IPL’s company-owned life insurance policies were liquidated. The related proceeds of $31 million and $19 million were recorded in investing activities in Alliant Energy’s and IPL’s cash flows statements, respectively, in 2016. At December 31, the cash surrender value of these investments was as follows (in millions): Alliant Energy WPL 2017 2016 2017 2016 Cash surrender value $10.4 $10.6 $5.6 $5.8
WPL [Member]
Schedule of Equity Method Investments [Line Items]
Investments	INVESTMENTS (a) Unconsolidated Equity Investments - Unconsolidated investments accounted for under the equity method of accounting are as follows (in millions): Ownership Interest at Carrying Value at December 31, Equity (Income) / Loss December 31, 2017 2017 2016 2017 2016 2015 Alliant Energy ATC Investment (a) 16%-20% $274.2 $317.6 ($42.4 ) ($39.1 ) ($34.2 ) Wind Investment in Oklahoma 50% 98.3 — (1.8 ) — — Other Various 8.9 8.4 (0.6 ) (0.5 ) 0.4 $381.4 $326.0 ($44.8 ) ($39.6 ) ($33.8 ) WPL ATC —% $— $— $— ($39.1 ) ($34.2 ) Wisconsin River Power Company 50% 8.3 7.7 (0.7 ) (0.7 ) (0.9 ) $8.3 $7.7 ($0.7 ) ($39.8 ) ($35.1 ) (a) As of December 31, 2017 , Alliant Energy’s ATC Investment is comprised of a 16% ownership interest in ATC and a 20% ownership interest in ATC Holdco LLC, which are described below. In 2017, Alliant Energy’s investment in ATC decreased due to the impacts of Tax Reform. Refer to Note 11 for further discussion. Alliant Energy currently has the ability to exercise significant influence over ATC’s financial and operating policies through its participation on ATC’s Board of Directors. Refer to Note 18 for information regarding related party transactions with ATC. Summary aggregate financial information from the financial statements of these investments is as follows (in millions): Alliant Energy WPL 2017 2016 2015 2017 2016 2015 Operating revenues $741 $658 $624 $8 $658 $624 Operating income 374 331 299 4 331 299 Net income 267 232 186 2 234 202 As of December 31: Current assets 104 82 7 6 Non-current assets 5,041 4,340 20 19 Current liabilities 770 498 2 3 Non-current liabilities 2,038 2,144 8 7 Minority interest 255 — — — Investment in ATC and WPL’s Noncontrolling Interest - Prior to 2014, WPL owned 100% of WPL Transco, which held Alliant Energy’s investment in ATC. In 2014, WPL Transco’s operating agreement was amended to allow ATI, a wholly-owned subsidiary of AEF, to become a member of WPL Transco in addition to WPL. In 2014, ATI began funding capital contributions that WPL Transco made to ATC. WPL Transco’s equity income from ATC and ATC dividends received by WPL Transco were allocated between WPL and ATI based on their respective ownership interests at the time the equity income was generated and at the time of the dividend payments. Prior to the transfer of the investment in ATC to ATI discussed below, WPL consolidated WPL Transco, and ATI’s ownership in WPL Transco was recorded as a noncontrolling interest in total equity on WPL’s balance sheets. On December 31, 2016, pursuant to a June 2016 PSCW order, WPL Transco was liquidated and WPL transferred its investment in ATC to ATI. As a result, WPL no longer records equity income from its prior investment in ATC. In conjunction with the transfer of the investment in ATC, a deferred intercompany tax gain recognized by WPL was assumed by ATI. The impact of WPL’s transfer of the ATC investment, including the assumption of such intercompany tax gain by ATI, was recorded as a net reduction in total equity of $163.6 million on WPL’s balance sheet. WPL’s income statement includes all of the equity earnings from ATC through December 31, 2016, the date of transfer. There were no impacts of this transfer to Alliant Energy’s consolidated financial statements. As of December 31, 2016, ATI owns Alliant Energy’s entire ATC Investment. Investment in ATC Holdco LLC - In 2011, Duke Energy Corporation and ATC announced the creation of DATC, a joint venture that is expected to acquire, build, own and operate new electric transmission infrastructure in North America. DATC continues to evaluate new projects and opportunities, and participates in a competitive bidding process on projects it considers to be viable. In October 2017, ATC transferred a portion of its interest in DATC to ATC Holdco LLC, and as a result, Alliant Energy contributed additional equity capital funding based on its ownership interest in ATC Holdco LLC. A portion of proceeds from the transfer was distributed to all ATC Holdco LLC’s owners based on their ATC ownership percentage. (b) Cash Surrender Value of Life Insurance Policies - Various life insurance policies cover certain current and former employees and directors. In 2016, certain of Alliant Energy’s and IPL’s company-owned life insurance policies were liquidated. The related proceeds of $31 million and $19 million were recorded in investing activities in Alliant Energy’s and IPL’s cash flows statements, respectively, in 2016. At December 31, the cash surrender value of these investments was as follows (in millions): Alliant Energy WPL 2017 2016 2017 2016 Cash surrender value $10.4 $10.6 $5.6 $5.8

Common Equity

Common Equity	12 Months Ended
	Dec. 31, 2017
Schedule of Common Equity [Line Items]
Common Equity	COMMON EQUITY Common Share Activity - A summary of Alliant Energy’s common stock activity was as follows: 2017 2016 2015 Shares outstanding, January 1 227,673,654 226,918,432 221,871,360 At-the-market offering programs 3,074,931 — 4,373,234 Shareowner Direct Plan issuances 640,723 732,814 606,010 Equity-based compensation plans ( Note 12(b) ) 5,185 22,408 112,756 Other (45,847 ) — (44,928 ) Shares outstanding, December 31 231,348,646 227,673,654 226,918,432 At December 31, 2017 , Alliant Energy had a total of 12.4 million shares available for issuance in the aggregate, pursuant to its Amended and Restated OIP, Shareowner Direct Plan and 401(k) Savings Plan. At-the-Market Offering Programs - In 2017 and 2015, Alliant Energy filed prospectus supplements under which it could sell up to $125 million and $150 million of its common stock, respectively, through at-the-market offering programs. In 2017, Alliant Energy issued 3,074,931 shares of common stock through this program and received cash proceeds of $124 million , net of $1 million in commissions and fees. In 2015, Alliant Energy issued 4,373,234 shares of common stock through this program and received cash proceeds of $133 million , net of $2 million in commissions and fees. The proceeds from the issuances of common stock were used for general corporate purposes. The 2015 at-the-market offering program expired in 2016, and Alliant Energy currently has no plans to issue any additional common stock through the 2017 at-the-market offering program. Shareowner Direct Plan - Alliant Energy satisfies its requirements under the Shareowner Direct Plan (dividend reinvestment and stock purchase plan) by acquiring Alliant Energy common stock through original issue, rather than on the open market. Shareowner Rights Agreement - Alliant Energy previously established an amended and restated Shareowner Rights Agreement. The rights under this agreement were exercisable if a person or group acquired, or announced an intention to acquire, 15% or more of Alliant Energy’s outstanding common stock. In January 2018, Alliant Energy’s Board of Directors authorized the redemption of the rights, which were redeemed to shareowners as of the close of business on January 31, 2018. Dividend Restrictions - Alliant Energy does not have any significant common stock dividend restrictions. IPL and WPL each have common stock dividend restrictions based on applicable regulatory limitations. IPL also has common stock dividend restrictions based on the terms of its outstanding preferred stock. As of December 31, 2017 , IPL and WPL were in compliance with all such dividend restrictions. IPL is restricted from paying common stock dividends to its parent company, Alliant Energy, if for any past or current dividend period, dividends on its preferred stock have not been paid, or declared and set apart for payment. IPL has paid all dividends on its preferred stock through 2017 . Under the Federal Power Act, IPL may not pay dividends to its parent company in excess of the current amount of its retained earnings. As of December 31, 2017 , IPL’s amount of retained earnings that were free of dividend restrictions was $679 million . If IPL’s actual 13 -month average common equity ratio (calculated on a financial basis consistent with IPL’s rate reviews) falls below 42% of total capitalization, IPL is required to notify the IUB. Pursuant to a December 2016 PSCW order, WPL has a regulatory limitation on distributions to its parent company. WPL is prohibited from paying annual common stock dividends to its parent company in excess of forecasted dividend levels of $140 million in 2018 if WPL’s actual 13 -month average common equity ratio (calculated on a financial basis consistent with WPL’s rate reviews) would fall below 51.00% for 2018. As of December 31, 2017 , WPL’s amount of retained earnings that were free of dividend restrictions was $140 million for 2018 . Restricted Net Assets of Subsidiaries - IPL and WPL do not have regulatory authority to lend or advance any amounts to their parent company. As of December 31, 2017 , the amount of IPL’s and WPL’s net assets that were not available to be transferred to their parent company, Alliant Energy, in the form of loans, advances or cash dividends without the consent of IPL’s and WPL’s regulatory authorities was $1.8 billion and $1.7 billion , respectively. Comprehensive Income - In 2017 , 2016 and 2015 , Alliant Energy’s other comprehensive income (loss) was ($0.1) million , $0 million and $0.2 million , respectively; therefore, its comprehensive income was substantially equal to its net income and its comprehensive income attributable to Alliant Energy common shareowners was substantially equal to its net income attributable to Alliant Energy common shareowners for such periods. In 2017 , 2016 and 2015 , IPL and WPL had no other comprehensive income; therefore their comprehensive income was equal to their net income and their comprehensive income available for common stock was equal to their earnings available for common stock for such periods.
IPL [Member]
Schedule of Common Equity [Line Items]
Common Equity	COMMON EQUITY Common Share Activity - A summary of Alliant Energy’s common stock activity was as follows: 2017 2016 2015 Shares outstanding, January 1 227,673,654 226,918,432 221,871,360 At-the-market offering programs 3,074,931 — 4,373,234 Shareowner Direct Plan issuances 640,723 732,814 606,010 Equity-based compensation plans ( Note 12(b) ) 5,185 22,408 112,756 Other (45,847 ) — (44,928 ) Shares outstanding, December 31 231,348,646 227,673,654 226,918,432 At December 31, 2017 , Alliant Energy had a total of 12.4 million shares available for issuance in the aggregate, pursuant to its Amended and Restated OIP, Shareowner Direct Plan and 401(k) Savings Plan. At-the-Market Offering Programs - In 2017 and 2015, Alliant Energy filed prospectus supplements under which it could sell up to $125 million and $150 million of its common stock, respectively, through at-the-market offering programs. In 2017, Alliant Energy issued 3,074,931 shares of common stock through this program and received cash proceeds of $124 million , net of $1 million in commissions and fees. In 2015, Alliant Energy issued 4,373,234 shares of common stock through this program and received cash proceeds of $133 million , net of $2 million in commissions and fees. The proceeds from the issuances of common stock were used for general corporate purposes. The 2015 at-the-market offering program expired in 2016, and Alliant Energy currently has no plans to issue any additional common stock through the 2017 at-the-market offering program. Shareowner Direct Plan - Alliant Energy satisfies its requirements under the Shareowner Direct Plan (dividend reinvestment and stock purchase plan) by acquiring Alliant Energy common stock through original issue, rather than on the open market. Shareowner Rights Agreement - Alliant Energy previously established an amended and restated Shareowner Rights Agreement. The rights under this agreement were exercisable if a person or group acquired, or announced an intention to acquire, 15% or more of Alliant Energy’s outstanding common stock. In January 2018, Alliant Energy’s Board of Directors authorized the redemption of the rights, which were redeemed to shareowners as of the close of business on January 31, 2018. Dividend Restrictions - Alliant Energy does not have any significant common stock dividend restrictions. IPL and WPL each have common stock dividend restrictions based on applicable regulatory limitations. IPL also has common stock dividend restrictions based on the terms of its outstanding preferred stock. As of December 31, 2017 , IPL and WPL were in compliance with all such dividend restrictions. IPL is restricted from paying common stock dividends to its parent company, Alliant Energy, if for any past or current dividend period, dividends on its preferred stock have not been paid, or declared and set apart for payment. IPL has paid all dividends on its preferred stock through 2017 . Under the Federal Power Act, IPL may not pay dividends to its parent company in excess of the current amount of its retained earnings. As of December 31, 2017 , IPL’s amount of retained earnings that were free of dividend restrictions was $679 million . If IPL’s actual 13 -month average common equity ratio (calculated on a financial basis consistent with IPL’s rate reviews) falls below 42% of total capitalization, IPL is required to notify the IUB. Pursuant to a December 2016 PSCW order, WPL has a regulatory limitation on distributions to its parent company. WPL is prohibited from paying annual common stock dividends to its parent company in excess of forecasted dividend levels of $140 million in 2018 if WPL’s actual 13 -month average common equity ratio (calculated on a financial basis consistent with WPL’s rate reviews) would fall below 51.00% for 2018. As of December 31, 2017 , WPL’s amount of retained earnings that were free of dividend restrictions was $140 million for 2018 . Restricted Net Assets of Subsidiaries - IPL and WPL do not have regulatory authority to lend or advance any amounts to their parent company. As of December 31, 2017 , the amount of IPL’s and WPL’s net assets that were not available to be transferred to their parent company, Alliant Energy, in the form of loans, advances or cash dividends without the consent of IPL’s and WPL’s regulatory authorities was $1.8 billion and $1.7 billion , respectively. Comprehensive Income - In 2017 , 2016 and 2015 , Alliant Energy’s other comprehensive income (loss) was ($0.1) million , $0 million and $0.2 million , respectively; therefore, its comprehensive income was substantially equal to its net income and its comprehensive income attributable to Alliant Energy common shareowners was substantially equal to its net income attributable to Alliant Energy common shareowners for such periods. In 2017 , 2016 and 2015 , IPL and WPL had no other comprehensive income; therefore their comprehensive income was equal to their net income and their comprehensive income available for common stock was equal to their earnings available for common stock for such periods.
WPL [Member]
Schedule of Common Equity [Line Items]
Common Equity	COMMON EQUITY Common Share Activity - A summary of Alliant Energy’s common stock activity was as follows: 2017 2016 2015 Shares outstanding, January 1 227,673,654 226,918,432 221,871,360 At-the-market offering programs 3,074,931 — 4,373,234 Shareowner Direct Plan issuances 640,723 732,814 606,010 Equity-based compensation plans ( Note 12(b) ) 5,185 22,408 112,756 Other (45,847 ) — (44,928 ) Shares outstanding, December 31 231,348,646 227,673,654 226,918,432 At December 31, 2017 , Alliant Energy had a total of 12.4 million shares available for issuance in the aggregate, pursuant to its Amended and Restated OIP, Shareowner Direct Plan and 401(k) Savings Plan. At-the-Market Offering Programs - In 2017 and 2015, Alliant Energy filed prospectus supplements under which it could sell up to $125 million and $150 million of its common stock, respectively, through at-the-market offering programs. In 2017, Alliant Energy issued 3,074,931 shares of common stock through this program and received cash proceeds of $124 million , net of $1 million in commissions and fees. In 2015, Alliant Energy issued 4,373,234 shares of common stock through this program and received cash proceeds of $133 million , net of $2 million in commissions and fees. The proceeds from the issuances of common stock were used for general corporate purposes. The 2015 at-the-market offering program expired in 2016, and Alliant Energy currently has no plans to issue any additional common stock through the 2017 at-the-market offering program. Shareowner Direct Plan - Alliant Energy satisfies its requirements under the Shareowner Direct Plan (dividend reinvestment and stock purchase plan) by acquiring Alliant Energy common stock through original issue, rather than on the open market. Shareowner Rights Agreement - Alliant Energy previously established an amended and restated Shareowner Rights Agreement. The rights under this agreement were exercisable if a person or group acquired, or announced an intention to acquire, 15% or more of Alliant Energy’s outstanding common stock. In January 2018, Alliant Energy’s Board of Directors authorized the redemption of the rights, which were redeemed to shareowners as of the close of business on January 31, 2018. Dividend Restrictions - Alliant Energy does not have any significant common stock dividend restrictions. IPL and WPL each have common stock dividend restrictions based on applicable regulatory limitations. IPL also has common stock dividend restrictions based on the terms of its outstanding preferred stock. As of December 31, 2017 , IPL and WPL were in compliance with all such dividend restrictions. IPL is restricted from paying common stock dividends to its parent company, Alliant Energy, if for any past or current dividend period, dividends on its preferred stock have not been paid, or declared and set apart for payment. IPL has paid all dividends on its preferred stock through 2017 . Under the Federal Power Act, IPL may not pay dividends to its parent company in excess of the current amount of its retained earnings. As of December 31, 2017 , IPL’s amount of retained earnings that were free of dividend restrictions was $679 million . If IPL’s actual 13 -month average common equity ratio (calculated on a financial basis consistent with IPL’s rate reviews) falls below 42% of total capitalization, IPL is required to notify the IUB. Pursuant to a December 2016 PSCW order, WPL has a regulatory limitation on distributions to its parent company. WPL is prohibited from paying annual common stock dividends to its parent company in excess of forecasted dividend levels of $140 million in 2018 if WPL’s actual 13 -month average common equity ratio (calculated on a financial basis consistent with WPL’s rate reviews) would fall below 51.00% for 2018. As of December 31, 2017 , WPL’s amount of retained earnings that were free of dividend restrictions was $140 million for 2018 . Restricted Net Assets of Subsidiaries - IPL and WPL do not have regulatory authority to lend or advance any amounts to their parent company. As of December 31, 2017 , the amount of IPL’s and WPL’s net assets that were not available to be transferred to their parent company, Alliant Energy, in the form of loans, advances or cash dividends without the consent of IPL’s and WPL’s regulatory authorities was $1.8 billion and $1.7 billion , respectively. Comprehensive Income - In 2017 , 2016 and 2015 , Alliant Energy’s other comprehensive income (loss) was ($0.1) million , $0 million and $0.2 million , respectively; therefore, its comprehensive income was substantially equal to its net income and its comprehensive income attributable to Alliant Energy common shareowners was substantially equal to its net income attributable to Alliant Energy common shareowners for such periods. In 2017 , 2016 and 2015 , IPL and WPL had no other comprehensive income; therefore their comprehensive income was equal to their net income and their comprehensive income available for common stock was equal to their earnings available for common stock for such periods.

Redeemable Preferred Stock

Redeemable Preferred Stock	12 Months Ended
	Dec. 31, 2017
Redeemable Preferred Stock	REDEEMABLE PREFERRED STOCK IPL is authorized to issue up to 16,000,000 shares of cumulative preferred stock in aggregate. Information related to the carrying value of IPL’s cumulative preferred stock at December 31 was as follows: Series Liquidation Preference/Stated Value Shares Authorized Shares Outstanding 2017 2016 (in millions) 5.1% $25 8,000,000 8,000,000 $200.0 $200.0 On or after March 15, 2018, IPL may, at its option, redeem the 5.1% cumulative preferred stock for cash at a redemption price of $25 per share plus accrued and unpaid dividends up to the redemption date. The current articles of incorporation of IPL contain a provision that grants the holders of its cumulative preferred stock voting rights to elect two members of IPL’s Board of Directors if preferred dividends equal to six or more quarterly dividend requirements (whether or not consecutive) are in arrears. Such voting rights would not provide the holders of IPL’s preferred stock control of the decision on redemption of IPL’s preferred stock and could not force IPL to exercise its call option. Therefore, IPL’s cumulative preferred stock is presented in total equity on Alliant Energy’s and IPL’s balance sheets in a manner consistent with noncontrolling interests. Refer to Note 14 for information on the fair value of cumulative preferred stock.
IPL [Member]
Redeemable Preferred Stock	REDEEMABLE PREFERRED STOCK IPL is authorized to issue up to 16,000,000 shares of cumulative preferred stock in aggregate. Information related to the carrying value of IPL’s cumulative preferred stock at December 31 was as follows: Series Liquidation Preference/Stated Value Shares Authorized Shares Outstanding 2017 2016 (in millions) 5.1% $25 8,000,000 8,000,000 $200.0 $200.0 On or after March 15, 2018, IPL may, at its option, redeem the 5.1% cumulative preferred stock for cash at a redemption price of $25 per share plus accrued and unpaid dividends up to the redemption date. The current articles of incorporation of IPL contain a provision that grants the holders of its cumulative preferred stock voting rights to elect two members of IPL’s Board of Directors if preferred dividends equal to six or more quarterly dividend requirements (whether or not consecutive) are in arrears. Such voting rights would not provide the holders of IPL’s preferred stock control of the decision on redemption of IPL’s preferred stock and could not force IPL to exercise its call option. Therefore, IPL’s cumulative preferred stock is presented in total equity on Alliant Energy’s and IPL’s balance sheets in a manner consistent with noncontrolling interests. Refer to Note 14 for information on the fair value of cumulative preferred stock.

Debt

Debt	12 Months Ended
	Dec. 31, 2017
Debt Instrument [Line Items]
Debt	DEBT (a) Short-term Debt - Alliant Energy and its subsidiaries maintain committed bank lines of credit to provide short-term borrowing flexibility and back-stop liquidity for commercial paper outstanding. In August 2017, Alliant Energy, IPL and WPL entered into a single new credit facility agreement, which expires in August 2022. At December 31, 2017 , the short-term borrowing capacity under the credit facility agreement totaled $1 billion ( $400 million for Alliant Energy at the parent company level, $250 million for IPL and $350 million for WPL). Subject to certain conditions, Alliant Energy (at the parent company level), IPL and WPL may each reallocate and change its sublimit up to $500 million , $400 million and $500 million , respectively, within the $1 billion total commitment. Information regarding commercial paper classified as short-term debt and back-stopped by the credit facility was as follows (dollars in millions): Alliant Energy IPL WPL December 31 2017 2016 2017 2016 2017 2016 Commercial paper outstanding $320.2 $244.1 $— $— $25.0 $52.3 Commercial paper weighted average interest rates 2.0% 0.9% N/A N/A 1.5% 0.7% Available credit facility capacity $679.8 $755.9 $250.0 $300.0 $325.0 $347.7 Alliant Energy IPL WPL For the year ended 2017 2016 2017 2016 2017 2016 Maximum amount outstanding (based on daily outstanding balances) $424.4 $251.8 $20.0 $3.1 $271.2 $118.3 Average amount outstanding (based on daily outstanding balances) $294.3 $179.0 $0.5 $— $118.2 $38.1 Weighted average interest rates 1.2% 0.6% 1.3% 0.7% 1.0% 0.4% In July 2017, AEF entered into a $95 million , 364 -day variable-rate ( 2.2% at December 31, 2017 ) term loan credit agreement (with Alliant Energy as guarantor) related to the acquisition of a cash equity ownership interest in a non-utility wind farm located in Oklahoma, which includes substantially the same financial covenants that are included in the credit facility agreement. Refer to Note 6(a) for further discussion of the non-utility wind farm investment. Financial Covenants - The single new credit facility agreement contains a financial covenant, which requires Alliant Energy, IPL and WPL to maintain certain debt-to-capital ratios in order to borrow under the credit facility. AEF’s term loan credit agreement contains a financial covenant, which requires Alliant Energy to maintain a certain debt-to-capital ratio in order to borrow under the term loan credit agreement. The required debt-to-capital ratios compared to the actual debt-to-capital ratios at December 31, 2017 were as follows: Alliant Energy IPL WPL Requirement, not to exceed 65% 65% 65% Actual 54% 47% 51% The debt component of the capital ratios includes long- and short-term debt (excluding non-recourse debt and hybrid securities to the extent the total carrying value of such hybrid securities does not exceed 15% of consolidated capital of the applicable borrower), capital lease obligations, certain letters of credit, guarantees of the foregoing and new synthetic leases. Unfunded vested benefits under qualified pension plans and sales of accounts receivable are not included in the debt-to-capital ratios. The equity component of the capital ratios excludes accumulated other comprehensive income (loss). (b) Long-Term Debt - Long-term debt, net as of December 31 was as follows (dollars in millions): 2017 2016 Alliant Energy IPL WPL Alliant Energy IPL WPL Senior Debentures (a): 5.875%, due 2018 $100.0 $100.0 $— $100.0 $100.0 $— 7.25%, due 2018 250.0 250.0 — 250.0 250.0 — 3.65%, due 2020 200.0 200.0 — 200.0 200.0 — 3.25%, due 2024 (b) 500.0 500.0 — 250.0 250.0 — 3.4%, due 2025 250.0 250.0 — 250.0 250.0 — 5.5%, due 2025 50.0 50.0 — 50.0 50.0 — 6.45%, due 2033 100.0 100.0 — 100.0 100.0 — 6.3%, due 2034 125.0 125.0 — 125.0 125.0 — 6.25%, due 2039 300.0 300.0 — 300.0 300.0 — 4.7%, due 2043 250.0 250.0 — 250.0 250.0 — 3.7%, due 2046 300.0 300.0 — 300.0 300.0 — 2,425.0 2,425.0 — 2,175.0 2,175.0 — Debentures (a): 5%, due 2019 250.0 — 250.0 250.0 — 250.0 4.6%, due 2020 150.0 — 150.0 150.0 — 150.0 2.25%, due 2022 250.0 — 250.0 250.0 — 250.0 3.05%, due 2027 (c) 300.0 — 300.0 — — — 6.25%, due 2034 100.0 — 100.0 100.0 — 100.0 6.375%, due 2037 300.0 — 300.0 300.0 — 300.0 7.6%, due 2038 250.0 — 250.0 250.0 — 250.0 4.1%, due 2044 250.0 — 250.0 250.0 — 250.0 1,850.0 — 1,850.0 1,550.0 — 1,550.0 Other: AEF term loan credit agreement through 2018, 2% at December 31, 2017 (with Alliant Energy as guarantor) (d) 500.0 — — 500.0 — — Corporate Services 3.45% senior notes, due 2022 (a) 75.0 — — 75.0 — — Sheboygan Power, LLC 5.06% senior secured notes, due 2018 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 49.6 — — 53.8 — — Other, 1% at December 31, 2017, due 2018 to 2025 2.9 — — 3.3 — — 627.5 — — 632.1 — — Subtotal 4,902.5 2,425.0 1,850.0 4,357.1 2,175.0 1,550.0 Current maturities (855.7 ) (350.0 ) — (4.6 ) — — Unamortized debt issuance costs (25.4 ) (14.3 ) (10.5 ) (23.4 ) (13.7 ) (9.1 ) Unamortized debt (discount) and premium, net (10.8 ) (4.7 ) (6.1 ) (13.5 ) (7.8 ) (5.7 ) Long-term debt, net (e) $4,010.6 $2,056.0 $1,833.4 $4,315.6 $2,153.5 $1,535.2 (a) Contains optional redemption provisions which, if elected by the issuer at its sole discretion, could require material redemption premium payments by the issuer. The redemption premium payments under these optional redemption provisions are variable and dependent on applicable U.S. Treasury rates at the time of redemption. (b) In 2017, IPL issued $250 million of 3.25% senior debentures due 2024. The proceeds from the issuance were used by IPL to reduce commercial paper classified as long-term debt, reduce cash amounts received from its sales of accounts receivable program and for general corporate purposes. (c) In 2017, WPL issued $300 million of 3.05% debentures due 2027. The proceeds from the issuance were used by WPL to reduce commercial paper and for general corporate purposes. (d) Refer to Note 9(a) for discussion of a financial covenant contained in AEF’s term loan credit agreement. (e) There were no significant sinking fund requirements related to the outstanding long-term debt. Five-Year Schedule of Long-term Debt Maturities - At December 31, 2017 , long-term debt maturities for 2018 through 2022 were as follows (in millions): 2018 2019 2020 2021 2022 IPL $350 $— $200 $— $— WPL — 250 150 — 250 Corporate Services — — — — 75 AEF 506 6 7 8 8 Alliant Energy $856 $256 $357 $8 $333 Fair Value of Long-term Debt - Refer to Note 14 for information on the fair value of long-term debt outstanding.
IPL [Member]
Debt Instrument [Line Items]
Debt	DEBT (a) Short-term Debt - Alliant Energy and its subsidiaries maintain committed bank lines of credit to provide short-term borrowing flexibility and back-stop liquidity for commercial paper outstanding. In August 2017, Alliant Energy, IPL and WPL entered into a single new credit facility agreement, which expires in August 2022. At December 31, 2017 , the short-term borrowing capacity under the credit facility agreement totaled $1 billion ( $400 million for Alliant Energy at the parent company level, $250 million for IPL and $350 million for WPL). Subject to certain conditions, Alliant Energy (at the parent company level), IPL and WPL may each reallocate and change its sublimit up to $500 million , $400 million and $500 million , respectively, within the $1 billion total commitment. Information regarding commercial paper classified as short-term debt and back-stopped by the credit facility was as follows (dollars in millions): Alliant Energy IPL WPL December 31 2017 2016 2017 2016 2017 2016 Commercial paper outstanding $320.2 $244.1 $— $— $25.0 $52.3 Commercial paper weighted average interest rates 2.0% 0.9% N/A N/A 1.5% 0.7% Available credit facility capacity $679.8 $755.9 $250.0 $300.0 $325.0 $347.7 Alliant Energy IPL WPL For the year ended 2017 2016 2017 2016 2017 2016 Maximum amount outstanding (based on daily outstanding balances) $424.4 $251.8 $20.0 $3.1 $271.2 $118.3 Average amount outstanding (based on daily outstanding balances) $294.3 $179.0 $0.5 $— $118.2 $38.1 Weighted average interest rates 1.2% 0.6% 1.3% 0.7% 1.0% 0.4% In July 2017, AEF entered into a $95 million , 364 -day variable-rate ( 2.2% at December 31, 2017 ) term loan credit agreement (with Alliant Energy as guarantor) related to the acquisition of a cash equity ownership interest in a non-utility wind farm located in Oklahoma, which includes substantially the same financial covenants that are included in the credit facility agreement. Refer to Note 6(a) for further discussion of the non-utility wind farm investment. Financial Covenants - The single new credit facility agreement contains a financial covenant, which requires Alliant Energy, IPL and WPL to maintain certain debt-to-capital ratios in order to borrow under the credit facility. AEF’s term loan credit agreement contains a financial covenant, which requires Alliant Energy to maintain a certain debt-to-capital ratio in order to borrow under the term loan credit agreement. The required debt-to-capital ratios compared to the actual debt-to-capital ratios at December 31, 2017 were as follows: Alliant Energy IPL WPL Requirement, not to exceed 65% 65% 65% Actual 54% 47% 51% The debt component of the capital ratios includes long- and short-term debt (excluding non-recourse debt and hybrid securities to the extent the total carrying value of such hybrid securities does not exceed 15% of consolidated capital of the applicable borrower), capital lease obligations, certain letters of credit, guarantees of the foregoing and new synthetic leases. Unfunded vested benefits under qualified pension plans and sales of accounts receivable are not included in the debt-to-capital ratios. The equity component of the capital ratios excludes accumulated other comprehensive income (loss). (b) Long-Term Debt - Long-term debt, net as of December 31 was as follows (dollars in millions): 2017 2016 Alliant Energy IPL WPL Alliant Energy IPL WPL Senior Debentures (a): 5.875%, due 2018 $100.0 $100.0 $— $100.0 $100.0 $— 7.25%, due 2018 250.0 250.0 — 250.0 250.0 — 3.65%, due 2020 200.0 200.0 — 200.0 200.0 — 3.25%, due 2024 (b) 500.0 500.0 — 250.0 250.0 — 3.4%, due 2025 250.0 250.0 — 250.0 250.0 — 5.5%, due 2025 50.0 50.0 — 50.0 50.0 — 6.45%, due 2033 100.0 100.0 — 100.0 100.0 — 6.3%, due 2034 125.0 125.0 — 125.0 125.0 — 6.25%, due 2039 300.0 300.0 — 300.0 300.0 — 4.7%, due 2043 250.0 250.0 — 250.0 250.0 — 3.7%, due 2046 300.0 300.0 — 300.0 300.0 — 2,425.0 2,425.0 — 2,175.0 2,175.0 — Debentures (a): 5%, due 2019 250.0 — 250.0 250.0 — 250.0 4.6%, due 2020 150.0 — 150.0 150.0 — 150.0 2.25%, due 2022 250.0 — 250.0 250.0 — 250.0 3.05%, due 2027 (c) 300.0 — 300.0 — — — 6.25%, due 2034 100.0 — 100.0 100.0 — 100.0 6.375%, due 2037 300.0 — 300.0 300.0 — 300.0 7.6%, due 2038 250.0 — 250.0 250.0 — 250.0 4.1%, due 2044 250.0 — 250.0 250.0 — 250.0 1,850.0 — 1,850.0 1,550.0 — 1,550.0 Other: AEF term loan credit agreement through 2018, 2% at December 31, 2017 (with Alliant Energy as guarantor) (d) 500.0 — — 500.0 — — Corporate Services 3.45% senior notes, due 2022 (a) 75.0 — — 75.0 — — Sheboygan Power, LLC 5.06% senior secured notes, due 2018 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 49.6 — — 53.8 — — Other, 1% at December 31, 2017, due 2018 to 2025 2.9 — — 3.3 — — 627.5 — — 632.1 — — Subtotal 4,902.5 2,425.0 1,850.0 4,357.1 2,175.0 1,550.0 Current maturities (855.7 ) (350.0 ) — (4.6 ) — — Unamortized debt issuance costs (25.4 ) (14.3 ) (10.5 ) (23.4 ) (13.7 ) (9.1 ) Unamortized debt (discount) and premium, net (10.8 ) (4.7 ) (6.1 ) (13.5 ) (7.8 ) (5.7 ) Long-term debt, net (e) $4,010.6 $2,056.0 $1,833.4 $4,315.6 $2,153.5 $1,535.2 (a) Contains optional redemption provisions which, if elected by the issuer at its sole discretion, could require material redemption premium payments by the issuer. The redemption premium payments under these optional redemption provisions are variable and dependent on applicable U.S. Treasury rates at the time of redemption. (b) In 2017, IPL issued $250 million of 3.25% senior debentures due 2024. The proceeds from the issuance were used by IPL to reduce commercial paper classified as long-term debt, reduce cash amounts received from its sales of accounts receivable program and for general corporate purposes. (c) In 2017, WPL issued $300 million of 3.05% debentures due 2027. The proceeds from the issuance were used by WPL to reduce commercial paper and for general corporate purposes. (d) Refer to Note 9(a) for discussion of a financial covenant contained in AEF’s term loan credit agreement. (e) There were no significant sinking fund requirements related to the outstanding long-term debt. Five-Year Schedule of Long-term Debt Maturities - At December 31, 2017 , long-term debt maturities for 2018 through 2022 were as follows (in millions): 2018 2019 2020 2021 2022 IPL $350 $— $200 $— $— WPL — 250 150 — 250 Corporate Services — — — — 75 AEF 506 6 7 8 8 Alliant Energy $856 $256 $357 $8 $333 Fair Value of Long-term Debt - Refer to Note 14 for information on the fair value of long-term debt outstanding.
WPL [Member]
Debt Instrument [Line Items]
Debt	DEBT (a) Short-term Debt - Alliant Energy and its subsidiaries maintain committed bank lines of credit to provide short-term borrowing flexibility and back-stop liquidity for commercial paper outstanding. In August 2017, Alliant Energy, IPL and WPL entered into a single new credit facility agreement, which expires in August 2022. At December 31, 2017 , the short-term borrowing capacity under the credit facility agreement totaled $1 billion ( $400 million for Alliant Energy at the parent company level, $250 million for IPL and $350 million for WPL). Subject to certain conditions, Alliant Energy (at the parent company level), IPL and WPL may each reallocate and change its sublimit up to $500 million , $400 million and $500 million , respectively, within the $1 billion total commitment. Information regarding commercial paper classified as short-term debt and back-stopped by the credit facility was as follows (dollars in millions): Alliant Energy IPL WPL December 31 2017 2016 2017 2016 2017 2016 Commercial paper outstanding $320.2 $244.1 $— $— $25.0 $52.3 Commercial paper weighted average interest rates 2.0% 0.9% N/A N/A 1.5% 0.7% Available credit facility capacity $679.8 $755.9 $250.0 $300.0 $325.0 $347.7 Alliant Energy IPL WPL For the year ended 2017 2016 2017 2016 2017 2016 Maximum amount outstanding (based on daily outstanding balances) $424.4 $251.8 $20.0 $3.1 $271.2 $118.3 Average amount outstanding (based on daily outstanding balances) $294.3 $179.0 $0.5 $— $118.2 $38.1 Weighted average interest rates 1.2% 0.6% 1.3% 0.7% 1.0% 0.4% In July 2017, AEF entered into a $95 million , 364 -day variable-rate ( 2.2% at December 31, 2017 ) term loan credit agreement (with Alliant Energy as guarantor) related to the acquisition of a cash equity ownership interest in a non-utility wind farm located in Oklahoma, which includes substantially the same financial covenants that are included in the credit facility agreement. Refer to Note 6(a) for further discussion of the non-utility wind farm investment. Financial Covenants - The single new credit facility agreement contains a financial covenant, which requires Alliant Energy, IPL and WPL to maintain certain debt-to-capital ratios in order to borrow under the credit facility. AEF’s term loan credit agreement contains a financial covenant, which requires Alliant Energy to maintain a certain debt-to-capital ratio in order to borrow under the term loan credit agreement. The required debt-to-capital ratios compared to the actual debt-to-capital ratios at December 31, 2017 were as follows: Alliant Energy IPL WPL Requirement, not to exceed 65% 65% 65% Actual 54% 47% 51% The debt component of the capital ratios includes long- and short-term debt (excluding non-recourse debt and hybrid securities to the extent the total carrying value of such hybrid securities does not exceed 15% of consolidated capital of the applicable borrower), capital lease obligations, certain letters of credit, guarantees of the foregoing and new synthetic leases. Unfunded vested benefits under qualified pension plans and sales of accounts receivable are not included in the debt-to-capital ratios. The equity component of the capital ratios excludes accumulated other comprehensive income (loss). (b) Long-Term Debt - Long-term debt, net as of December 31 was as follows (dollars in millions): 2017 2016 Alliant Energy IPL WPL Alliant Energy IPL WPL Senior Debentures (a): 5.875%, due 2018 $100.0 $100.0 $— $100.0 $100.0 $— 7.25%, due 2018 250.0 250.0 — 250.0 250.0 — 3.65%, due 2020 200.0 200.0 — 200.0 200.0 — 3.25%, due 2024 (b) 500.0 500.0 — 250.0 250.0 — 3.4%, due 2025 250.0 250.0 — 250.0 250.0 — 5.5%, due 2025 50.0 50.0 — 50.0 50.0 — 6.45%, due 2033 100.0 100.0 — 100.0 100.0 — 6.3%, due 2034 125.0 125.0 — 125.0 125.0 — 6.25%, due 2039 300.0 300.0 — 300.0 300.0 — 4.7%, due 2043 250.0 250.0 — 250.0 250.0 — 3.7%, due 2046 300.0 300.0 — 300.0 300.0 — 2,425.0 2,425.0 — 2,175.0 2,175.0 — Debentures (a): 5%, due 2019 250.0 — 250.0 250.0 — 250.0 4.6%, due 2020 150.0 — 150.0 150.0 — 150.0 2.25%, due 2022 250.0 — 250.0 250.0 — 250.0 3.05%, due 2027 (c) 300.0 — 300.0 — — — 6.25%, due 2034 100.0 — 100.0 100.0 — 100.0 6.375%, due 2037 300.0 — 300.0 300.0 — 300.0 7.6%, due 2038 250.0 — 250.0 250.0 — 250.0 4.1%, due 2044 250.0 — 250.0 250.0 — 250.0 1,850.0 — 1,850.0 1,550.0 — 1,550.0 Other: AEF term loan credit agreement through 2018, 2% at December 31, 2017 (with Alliant Energy as guarantor) (d) 500.0 — — 500.0 — — Corporate Services 3.45% senior notes, due 2022 (a) 75.0 — — 75.0 — — Sheboygan Power, LLC 5.06% senior secured notes, due 2018 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 49.6 — — 53.8 — — Other, 1% at December 31, 2017, due 2018 to 2025 2.9 — — 3.3 — — 627.5 — — 632.1 — — Subtotal 4,902.5 2,425.0 1,850.0 4,357.1 2,175.0 1,550.0 Current maturities (855.7 ) (350.0 ) — (4.6 ) — — Unamortized debt issuance costs (25.4 ) (14.3 ) (10.5 ) (23.4 ) (13.7 ) (9.1 ) Unamortized debt (discount) and premium, net (10.8 ) (4.7 ) (6.1 ) (13.5 ) (7.8 ) (5.7 ) Long-term debt, net (e) $4,010.6 $2,056.0 $1,833.4 $4,315.6 $2,153.5 $1,535.2 (a) Contains optional redemption provisions which, if elected by the issuer at its sole discretion, could require material redemption premium payments by the issuer. The redemption premium payments under these optional redemption provisions are variable and dependent on applicable U.S. Treasury rates at the time of redemption. (b) In 2017, IPL issued $250 million of 3.25% senior debentures due 2024. The proceeds from the issuance were used by IPL to reduce commercial paper classified as long-term debt, reduce cash amounts received from its sales of accounts receivable program and for general corporate purposes. (c) In 2017, WPL issued $300 million of 3.05% debentures due 2027. The proceeds from the issuance were used by WPL to reduce commercial paper and for general corporate purposes. (d) Refer to Note 9(a) for discussion of a financial covenant contained in AEF’s term loan credit agreement. (e) There were no significant sinking fund requirements related to the outstanding long-term debt. Five-Year Schedule of Long-term Debt Maturities - At December 31, 2017 , long-term debt maturities for 2018 through 2022 were as follows (in millions): 2018 2019 2020 2021 2022 IPL $350 $— $200 $— $— WPL — 250 150 — 250 Corporate Services — — — — 75 AEF 506 6 7 8 8 Alliant Energy $856 $256 $357 $8 $333 Fair Value of Long-term Debt - Refer to Note 14 for information on the fair value of long-term debt outstanding.

Leases

Leases	12 Months Ended
	Dec. 31, 2017
Leases	LEASES (a) Operating Leases - Various agreements have been entered into related to property, plant and equipment rights that are accounted for as operating leases. In 2017 , 2016 and 2015 , rental expenses associated with operating leases were not material. At December 31, 2017 , future minimum operating lease payments, excluding contingent rentals, were as follows (in millions): 2018 2019 2020 2021 2022 Thereafter Total Alliant Energy $6 $5 $2 $2 $1 $13 $29 IPL 3 2 1 1 1 13 21 WPL 3 3 1 — — — 7
IPL [Member]
Leases	LEASES (a) Operating Leases - Various agreements have been entered into related to property, plant and equipment rights that are accounted for as operating leases. In 2017 , 2016 and 2015 , rental expenses associated with operating leases were not material. At December 31, 2017 , future minimum operating lease payments, excluding contingent rentals, were as follows (in millions): 2018 2019 2020 2021 2022 Thereafter Total Alliant Energy $6 $5 $2 $2 $1 $13 $29 IPL 3 2 1 1 1 13 21 WPL 3 3 1 — — — 7
WPL [Member]
Leases	LEASES (a) Operating Leases - Various agreements have been entered into related to property, plant and equipment rights that are accounted for as operating leases. In 2017 , 2016 and 2015 , rental expenses associated with operating leases were not material. At December 31, 2017 , future minimum operating lease payments, excluding contingent rentals, were as follows (in millions): 2018 2019 2020 2021 2022 Thereafter Total Alliant Energy $6 $5 $2 $2 $1 $13 $29 IPL 3 2 1 1 1 13 21 WPL 3 3 1 — — — 7 (b) Capital Leases - WPL - In 2005, WPL entered into a 20 -year agreement with AEF’s Non-utility Generation business to lease the Sheboygan Falls Energy Facility, with an option for two lease renewal periods thereafter. The lease became effective in 2005 when the EGU began commercial operation. WPL is responsible for the operation of the EGU and has exclusive rights to its output, and the PSCW approved this affiliated lease agreement in 2005. The capital lease asset is amortized using the straight-line method over the 20 -year lease term. WPL’s retail and wholesale rates include recovery of the Sheboygan Falls Energy Facility lease payments. The Sheboygan Falls Energy Facility lease expenses were included in WPL’s income statements as follows (in millions): 2017 2016 2015 Interest expense $8.7 $9.3 $9.9 Depreciation and amortization 6.2 6.2 6.2 $14.9 $15.5 $16.1 At December 31, 2017 , WPL’s estimated future minimum capital lease payments for the Sheboygan Falls Energy Facility were as follows (in millions): 2018 2019 2020 2021 2022 Thereafter Total Less: amount representing interest Present value of minimum capital lease payments Sheboygan Falls Energy Facility $15 $15 $15 $15 $15 $35 $110 $33 $77

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax [Line Items]
Income Taxes	INCOME TAXES Tax Reform - In December 2017, Tax Reform was enacted. Substantially all of the provisions of Tax Reform are effective for taxable years beginning after December 31, 2017. The most significant provisions of Tax Reform that impact Alliant Energy, IPL and WPL were the reduction in the federal corporate tax rate from 35% to 21% , the repeal of the federal corporate alternative minimum tax (AMT), and the elimination of bonus depreciation deductions on utility property placed in service after September 27, 2017 unless a binding contract existed as of that date. The enactment of Tax Reform in December 2017 had a material impact on Alliant Energy’s, IPL’s and WPL’s 2017 financial statements as the tax effects of the changes in tax laws must be recognized in the period in which the law is enacted. Alliant Energy, IPL and WPL have completed their assessment of the applicable provisions of Tax Reform, and the impacts to their 2017 balance sheets and income statements are as follows (in millions): Alliant Energy IPL WPL Balance Sheet Other current assets $— $4.7 ($1.4 ) Investments: ATC Investment (75.0 ) — — Other assets: Regulatory assets (387.6 ) (375.0 ) (12.7 ) Deferred charges and other 41.0 — — Total Tax Reform impact on assets ($421.6 ) ($370.3 ) ($14.1 ) Other liabilities: Deferred tax liabilities ($1,331.9 ) ($757.2 ) ($523.8 ) Regulatory liabilities 885.9 390.7 495.2 Other 6.3 — — Total Tax Reform impact on liabilities ($439.7 ) ($366.5 ) ($28.6 ) Income Statement Income tax expense (benefit) ($18.1 ) $3.8 ($14.5 ) Deferred Tax Liabilities - Deferred tax assets and liabilities are measured at the enacted tax rate expected to be applied when temporary differences are to be realized or settled. Given the enactment of Tax Reform in December 2017, Alliant Energy’s, IPL’s and WPL’s deferred tax assets and liabilities at December 31, 2017 were re-measured based upon the new corporate tax rate. Alliant Energy’s utility operations in IPL and WPL recorded the net changes from re-measuring deferred tax assets and liabilities as a change in regulatory liabilities or regulatory assets as described in further detail below. Alliant Energy’s, IPL’s and WPL’s non-utility operations recorded the net change in deferred tax assets and liabilities to “Income tax expense (benefit)” in their respective income statement or as an increase to “Other liabilities” or decrease in “ATC Investment” on Alliant Energy’s balance sheet. Regulatory Assets and Regulatory Liabilities - Tax Reform requires regulated utilities to account for certain changes in deferred taxes related to accelerated depreciation and net operating losses using a normalization method of accounting. This method of accounting requires regulated utilities to record the normalized changes in deferred tax liabilities and assets to a regulatory liability or regulatory asset. For other changes to regulatory assets and regulatory liabilities resulting from Tax Reform, IPL and WPL are awaiting decisions from the IUB, PCSW and FERC to determine the appropriate regulatory treatment. IPL and WPL have recorded the offset to the changes in utility deferred tax liabilities and assets in a regulatory liability or regulatory asset to be reflected in future customer billings. Future changes may occur based on decisions received from the IUB, PSCW and FERC. In January 2018, the IUB issued an order initiating investigation of the impacts of Tax Reform. The order requires IPL to track all calculated differences since January 1, 2018 resulting from Tax Reform, such that any over-collections can be refunded to its customers at a future date, if appropriate. In January 2018, the PSCW issued an order directing WPL to defer and to accrue carrying costs on the revenue requirement impacts resulting from Tax Reform since its inception. ATC Investment - Tax Reform required a re-measurement of deferred tax assets and liabilities associated with Alliant Energy’s ATC Investment. These deferred tax assets and liabilities have been utilized to determine the amount of costs billed by ATC to its customers. Due to the regulated nature of ATC and FERC policy on income tax allowances, Alliant Energy expects the changes in deferred taxes that have been collected in ATC’s customers’ rates to be returned to such customers over the regulatory life of ATC’s assets. As a result, Alliant Energy has reduced the deferred tax liabilities associated with the ATC Investment and recorded the offset to “ATC Investment” on its balance sheet. Starting in 2018, the adjustment as a result of the change in deferred taxes will be amortized as a reduction in deferred tax expense over the remaining life of ATC’s assets. Deferred Charges and Other - Tax Reform repealed corporate federal AMT and allows unutilized AMT credits to be refunded over the next four tax years beginning with the U.S. federal tax return for calendar year 2018. As a result of these changes , Alliant Energy reclassified $41 million of unutilized AMT credits from a deferred tax asset into a long-term tax receivable in 2017 in anticipation of future refunds of such tax credits. The long-term tax receivable is included in “Deferred charges and other” on Alliant Energy’s balance sheet. Income Tax Expense (Benefit) - The changes in deferred taxes for Alliant Energy’s, IPL’s and WPL’s non-utility operations were primarily recorded in the income statements. In addition, Alliant Energy recognized valuation allowances for certain tax credit carryforwards as a result of Tax Reform that are discussed in more detail below. Income Tax Expense (Benefit) - The components of “Income tax expense (benefit)” in the income statements were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Current tax expense (benefit): Federal ($41.0 ) $1.8 $2.0 ($27.9 ) ($12.8 ) ($14.1 ) $5.5 ($22.3 ) $4.7 State 8.5 17.2 3.2 1.6 15.5 11.5 2.5 1.1 0.6 IPL’s tax benefit riders (40.4 ) (44.2 ) (49.0 ) (40.4 ) (44.2 ) (49.0 ) — — — Deferred tax expense (benefit): Federal 159.5 112.8 120.8 72.5 59.1 40.7 55.0 112.3 76.8 State 12.3 4.9 27.9 (2.2 ) (9.0 ) 3.3 16.6 20.8 20.2 Production tax credits (31.1 ) (31.8 ) (33.1 ) (14.1 ) (14.0 ) (14.5 ) (17.0 ) (17.8 ) (18.6 ) Investment tax credits (1.1 ) (1.3 ) (1.4 ) (0.4 ) (0.5 ) (0.6 ) (0.7 ) (0.8 ) (0.8 ) $66.7 $59.4 $70.4 ($10.9 ) ($5.9 ) ($22.7 ) $61.9 $93.3 $82.9 Income Tax Rates - The overall income tax rates shown in the following table were computed by dividing income tax expense (benefit) by income from continuing operations before income taxes. Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Statutory federal income tax rate 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % State income taxes, net of federal benefits 5.5 5.4 5.2 6.5 6.4 6.2 5.1 5.1 5.1 Effect of rate-making on property-related differences (8.5 ) (8.5 ) (6.8 ) (19.1 ) (16.2 ) (17.2 ) (1.7 ) (0.7 ) (0.5 ) IPL’s tax benefit riders (7.6 ) (10.0 ) (10.6 ) (18.7 ) (20.1 ) (28.3 ) — — — Production tax credits (6.1 ) (7.2 ) (7.2 ) (6.7 ) (6.3 ) (8.3 ) (7.1 ) (6.2 ) (7.1 ) Impact of Tax Reform (3.4 ) — — 1.7 — — (5.8 ) — — Other items, net (2.4 ) (1.3 ) (0.3 ) (3.7 ) (1.5 ) (0.5 ) (0.6 ) (0.6 ) (0.7 ) Overall income tax rate 12.5 % 13.4 % 15.3 % (5.0 %) (2.7 %) (13.1 %) 24.9 % 32.6 % 31.8 % Deferred Tax Assets and Liabilities - The deferred tax assets and liabilities included on the balance sheets at December 31 arise from the following temporary differences (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Deferred tax liabilities: Property $1,852.7 $2,919.0 $1,102.6 $1,677.0 $674.2 $1,124.5 ATC Investment 86.4 153.1 — — — — Other 75.9 95.1 63.4 71.4 36.5 58.8 Total deferred tax liabilities 2,015.0 3,167.2 1,166.0 1,748.4 710.7 1,183.3 Deferred tax assets: Federal credit carryforwards 260.7 268.4 113.1 95.9 131.0 112.9 Net operating losses carryforwards - federal 174.4 173.3 107.4 69.6 43.7 75.4 Regulatory liability - IPL’s tax benefit riders 7.4 34.7 7.4 34.7 — — Net operating losses carryforwards - state 41.3 32.9 0.9 0.6 0.2 0.1 Other 61.8 87.9 27.1 35.8 14.7 23.6 Subtotal deferred tax assets 545.6 597.2 255.9 236.6 189.6 212.0 Valuation allowance (9.0 ) (0.2 ) (0.6 ) — (1.3 ) (0.3 ) Total deferred tax assets 536.6 597.0 255.3 236.6 188.3 211.7 Total deferred tax liabilities, net $1,478.4 $2,570.2 $910.7 $1,511.8 $522.4 $971.6 The decreases in deferred tax liabilities were primarily due to the reduction in the federal corporate tax rate from 35% to 21% as a result of Tax Reform, as discussed above. Property - The decrease in property-related deferred tax liabilities from Tax Reform was partially offset by bonus depreciation deductions from property placed in service in 2017, including IPL’s Marshalltown Generating Station. Alliant Energy currently estimates its total bonus depreciation deductions to be claimed on its U.S. federal income tax return for calendar year 2017 will be approximately $670 million ( $500 million for IPL and $140 million for WPL). Carryforwards - The decrease in Alliant Energy’s federal credit carryforwards was due to the reclassification of AMT credits to a long-term tax receivable as a result of Tax Reform, partially offset by production tax credits generated in 2017. The increase in Alliant Energy’s and IPL’s federal net operating losses carryforwards was primarily due to bonus depreciation deductions for property placed in service in 2017, including IPL’s Marshalltown Generating Station, largely offset by the impacts of Tax Reform. At December 31, 2017 , carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2030-2037 $852 $537 $208 State net operating losses 2018-2037 700 13 2 Federal tax credits 2022-2037 267 119 131 Valuation Allowances - Due to the anticipated future reductions in revenues from utility customers due to Tax Reform, Alliant Energy expects a reduction in its future consolidated taxable income, which will extend the period to which prior unutilized operating losses will be utilized. Taxable income must be reduced by net operating losses carryforwards prior to utilizing federal tax credit carryforwards. Alliant Energy expects to utilize its net operating losses carryforwards by 2024 and therefore, currently does not expect to utilize 2002 and 2003 vintage federal credit carryforwards prior to their expiration in 2022 and 2023, respectively. This has resulted in valuation allowance charges recorded to “Income tax expense (benefit)” on the income statements as noted in the table above. Uncertain Tax Positions - At December 31, 2017 , 2016 and 2015 , there were no uncertain tax positions or penalties accrued related to uncertain tax positions, and interest accrued and tax positions favorably impacting future effective tax rates for continuing operations were not material. As of December 31, 2017 , no material changes to unrecognized tax benefits are expected during the next 12 months. Open tax years - Tax years that remain subject to the statute of limitations in the major jurisdictions for each of Alliant Energy, IPL and WPL are as follows: Consolidated federal income tax returns (a) 2014 - 2016 Consolidated Iowa income tax returns (b) 2014 - 2016 Wisconsin combined tax returns (c) 2013 - 2016 (a) These federal tax returns are effectively settled as a result of participation in the IRS Compliance Assurance Program, which allows Alliant Energy and the IRS to work together to resolve issues related to Alliant Energy’s current tax year before filing its federal income tax return. The statute of limitations for these federal tax returns expires three years from each filing date. (b) The statute of limitations for these Iowa tax returns expires three years from each filing date. (c) The statute of limitations for these Wisconsin combined tax returns expires four years from each filing date.
IPL [Member]
Income Tax [Line Items]
Income Taxes	INCOME TAXES Tax Reform - In December 2017, Tax Reform was enacted. Substantially all of the provisions of Tax Reform are effective for taxable years beginning after December 31, 2017. The most significant provisions of Tax Reform that impact Alliant Energy, IPL and WPL were the reduction in the federal corporate tax rate from 35% to 21% , the repeal of the federal corporate alternative minimum tax (AMT), and the elimination of bonus depreciation deductions on utility property placed in service after September 27, 2017 unless a binding contract existed as of that date. The enactment of Tax Reform in December 2017 had a material impact on Alliant Energy’s, IPL’s and WPL’s 2017 financial statements as the tax effects of the changes in tax laws must be recognized in the period in which the law is enacted. Alliant Energy, IPL and WPL have completed their assessment of the applicable provisions of Tax Reform, and the impacts to their 2017 balance sheets and income statements are as follows (in millions): Alliant Energy IPL WPL Balance Sheet Other current assets $— $4.7 ($1.4 ) Investments: ATC Investment (75.0 ) — — Other assets: Regulatory assets (387.6 ) (375.0 ) (12.7 ) Deferred charges and other 41.0 — — Total Tax Reform impact on assets ($421.6 ) ($370.3 ) ($14.1 ) Other liabilities: Deferred tax liabilities ($1,331.9 ) ($757.2 ) ($523.8 ) Regulatory liabilities 885.9 390.7 495.2 Other 6.3 — — Total Tax Reform impact on liabilities ($439.7 ) ($366.5 ) ($28.6 ) Income Statement Income tax expense (benefit) ($18.1 ) $3.8 ($14.5 ) Deferred Tax Liabilities - Deferred tax assets and liabilities are measured at the enacted tax rate expected to be applied when temporary differences are to be realized or settled. Given the enactment of Tax Reform in December 2017, Alliant Energy’s, IPL’s and WPL’s deferred tax assets and liabilities at December 31, 2017 were re-measured based upon the new corporate tax rate. Alliant Energy’s utility operations in IPL and WPL recorded the net changes from re-measuring deferred tax assets and liabilities as a change in regulatory liabilities or regulatory assets as described in further detail below. Alliant Energy’s, IPL’s and WPL’s non-utility operations recorded the net change in deferred tax assets and liabilities to “Income tax expense (benefit)” in their respective income statement or as an increase to “Other liabilities” or decrease in “ATC Investment” on Alliant Energy’s balance sheet. Regulatory Assets and Regulatory Liabilities - Tax Reform requires regulated utilities to account for certain changes in deferred taxes related to accelerated depreciation and net operating losses using a normalization method of accounting. This method of accounting requires regulated utilities to record the normalized changes in deferred tax liabilities and assets to a regulatory liability or regulatory asset. For other changes to regulatory assets and regulatory liabilities resulting from Tax Reform, IPL and WPL are awaiting decisions from the IUB, PCSW and FERC to determine the appropriate regulatory treatment. IPL and WPL have recorded the offset to the changes in utility deferred tax liabilities and assets in a regulatory liability or regulatory asset to be reflected in future customer billings. Future changes may occur based on decisions received from the IUB, PSCW and FERC. In January 2018, the IUB issued an order initiating investigation of the impacts of Tax Reform. The order requires IPL to track all calculated differences since January 1, 2018 resulting from Tax Reform, such that any over-collections can be refunded to its customers at a future date, if appropriate. In January 2018, the PSCW issued an order directing WPL to defer and to accrue carrying costs on the revenue requirement impacts resulting from Tax Reform since its inception. ATC Investment - Tax Reform required a re-measurement of deferred tax assets and liabilities associated with Alliant Energy’s ATC Investment. These deferred tax assets and liabilities have been utilized to determine the amount of costs billed by ATC to its customers. Due to the regulated nature of ATC and FERC policy on income tax allowances, Alliant Energy expects the changes in deferred taxes that have been collected in ATC’s customers’ rates to be returned to such customers over the regulatory life of ATC’s assets. As a result, Alliant Energy has reduced the deferred tax liabilities associated with the ATC Investment and recorded the offset to “ATC Investment” on its balance sheet. Starting in 2018, the adjustment as a result of the change in deferred taxes will be amortized as a reduction in deferred tax expense over the remaining life of ATC’s assets. Deferred Charges and Other - Tax Reform repealed corporate federal AMT and allows unutilized AMT credits to be refunded over the next four tax years beginning with the U.S. federal tax return for calendar year 2018. As a result of these changes , Alliant Energy reclassified $41 million of unutilized AMT credits from a deferred tax asset into a long-term tax receivable in 2017 in anticipation of future refunds of such tax credits. The long-term tax receivable is included in “Deferred charges and other” on Alliant Energy’s balance sheet. Income Tax Expense (Benefit) - The changes in deferred taxes for Alliant Energy’s, IPL’s and WPL’s non-utility operations were primarily recorded in the income statements. In addition, Alliant Energy recognized valuation allowances for certain tax credit carryforwards as a result of Tax Reform that are discussed in more detail below. Income Tax Expense (Benefit) - The components of “Income tax expense (benefit)” in the income statements were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Current tax expense (benefit): Federal ($41.0 ) $1.8 $2.0 ($27.9 ) ($12.8 ) ($14.1 ) $5.5 ($22.3 ) $4.7 State 8.5 17.2 3.2 1.6 15.5 11.5 2.5 1.1 0.6 IPL’s tax benefit riders (40.4 ) (44.2 ) (49.0 ) (40.4 ) (44.2 ) (49.0 ) — — — Deferred tax expense (benefit): Federal 159.5 112.8 120.8 72.5 59.1 40.7 55.0 112.3 76.8 State 12.3 4.9 27.9 (2.2 ) (9.0 ) 3.3 16.6 20.8 20.2 Production tax credits (31.1 ) (31.8 ) (33.1 ) (14.1 ) (14.0 ) (14.5 ) (17.0 ) (17.8 ) (18.6 ) Investment tax credits (1.1 ) (1.3 ) (1.4 ) (0.4 ) (0.5 ) (0.6 ) (0.7 ) (0.8 ) (0.8 ) $66.7 $59.4 $70.4 ($10.9 ) ($5.9 ) ($22.7 ) $61.9 $93.3 $82.9 Income Tax Rates - The overall income tax rates shown in the following table were computed by dividing income tax expense (benefit) by income from continuing operations before income taxes. Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Statutory federal income tax rate 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % State income taxes, net of federal benefits 5.5 5.4 5.2 6.5 6.4 6.2 5.1 5.1 5.1 Effect of rate-making on property-related differences (8.5 ) (8.5 ) (6.8 ) (19.1 ) (16.2 ) (17.2 ) (1.7 ) (0.7 ) (0.5 ) IPL’s tax benefit riders (7.6 ) (10.0 ) (10.6 ) (18.7 ) (20.1 ) (28.3 ) — — — Production tax credits (6.1 ) (7.2 ) (7.2 ) (6.7 ) (6.3 ) (8.3 ) (7.1 ) (6.2 ) (7.1 ) Impact of Tax Reform (3.4 ) — — 1.7 — — (5.8 ) — — Other items, net (2.4 ) (1.3 ) (0.3 ) (3.7 ) (1.5 ) (0.5 ) (0.6 ) (0.6 ) (0.7 ) Overall income tax rate 12.5 % 13.4 % 15.3 % (5.0 %) (2.7 %) (13.1 %) 24.9 % 32.6 % 31.8 % Deferred Tax Assets and Liabilities - The deferred tax assets and liabilities included on the balance sheets at December 31 arise from the following temporary differences (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Deferred tax liabilities: Property $1,852.7 $2,919.0 $1,102.6 $1,677.0 $674.2 $1,124.5 ATC Investment 86.4 153.1 — — — — Other 75.9 95.1 63.4 71.4 36.5 58.8 Total deferred tax liabilities 2,015.0 3,167.2 1,166.0 1,748.4 710.7 1,183.3 Deferred tax assets: Federal credit carryforwards 260.7 268.4 113.1 95.9 131.0 112.9 Net operating losses carryforwards - federal 174.4 173.3 107.4 69.6 43.7 75.4 Regulatory liability - IPL’s tax benefit riders 7.4 34.7 7.4 34.7 — — Net operating losses carryforwards - state 41.3 32.9 0.9 0.6 0.2 0.1 Other 61.8 87.9 27.1 35.8 14.7 23.6 Subtotal deferred tax assets 545.6 597.2 255.9 236.6 189.6 212.0 Valuation allowance (9.0 ) (0.2 ) (0.6 ) — (1.3 ) (0.3 ) Total deferred tax assets 536.6 597.0 255.3 236.6 188.3 211.7 Total deferred tax liabilities, net $1,478.4 $2,570.2 $910.7 $1,511.8 $522.4 $971.6 The decreases in deferred tax liabilities were primarily due to the reduction in the federal corporate tax rate from 35% to 21% as a result of Tax Reform, as discussed above. Property - The decrease in property-related deferred tax liabilities from Tax Reform was partially offset by bonus depreciation deductions from property placed in service in 2017, including IPL’s Marshalltown Generating Station. Alliant Energy currently estimates its total bonus depreciation deductions to be claimed on its U.S. federal income tax return for calendar year 2017 will be approximately $670 million ( $500 million for IPL and $140 million for WPL). Carryforwards - The decrease in Alliant Energy’s federal credit carryforwards was due to the reclassification of AMT credits to a long-term tax receivable as a result of Tax Reform, partially offset by production tax credits generated in 2017. The increase in Alliant Energy’s and IPL’s federal net operating losses carryforwards was primarily due to bonus depreciation deductions for property placed in service in 2017, including IPL’s Marshalltown Generating Station, largely offset by the impacts of Tax Reform. At December 31, 2017 , carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2030-2037 $852 $537 $208 State net operating losses 2018-2037 700 13 2 Federal tax credits 2022-2037 267 119 131 Valuation Allowances - Due to the anticipated future reductions in revenues from utility customers due to Tax Reform, Alliant Energy expects a reduction in its future consolidated taxable income, which will extend the period to which prior unutilized operating losses will be utilized. Taxable income must be reduced by net operating losses carryforwards prior to utilizing federal tax credit carryforwards. Alliant Energy expects to utilize its net operating losses carryforwards by 2024 and therefore, currently does not expect to utilize 2002 and 2003 vintage federal credit carryforwards prior to their expiration in 2022 and 2023, respectively. This has resulted in valuation allowance charges recorded to “Income tax expense (benefit)” on the income statements as noted in the table above. Uncertain Tax Positions - At December 31, 2017 , 2016 and 2015 , there were no uncertain tax positions or penalties accrued related to uncertain tax positions, and interest accrued and tax positions favorably impacting future effective tax rates for continuing operations were not material. As of December 31, 2017 , no material changes to unrecognized tax benefits are expected during the next 12 months. Open tax years - Tax years that remain subject to the statute of limitations in the major jurisdictions for each of Alliant Energy, IPL and WPL are as follows: Consolidated federal income tax returns (a) 2014 - 2016 Consolidated Iowa income tax returns (b) 2014 - 2016 Wisconsin combined tax returns (c) 2013 - 2016 (a) These federal tax returns are effectively settled as a result of participation in the IRS Compliance Assurance Program, which allows Alliant Energy and the IRS to work together to resolve issues related to Alliant Energy’s current tax year before filing its federal income tax return. The statute of limitations for these federal tax returns expires three years from each filing date. (b) The statute of limitations for these Iowa tax returns expires three years from each filing date. (c) The statute of limitations for these Wisconsin combined tax returns expires four years from each filing date.
WPL [Member]
Income Tax [Line Items]
Income Taxes	INCOME TAXES Tax Reform - In December 2017, Tax Reform was enacted. Substantially all of the provisions of Tax Reform are effective for taxable years beginning after December 31, 2017. The most significant provisions of Tax Reform that impact Alliant Energy, IPL and WPL were the reduction in the federal corporate tax rate from 35% to 21% , the repeal of the federal corporate alternative minimum tax (AMT), and the elimination of bonus depreciation deductions on utility property placed in service after September 27, 2017 unless a binding contract existed as of that date. The enactment of Tax Reform in December 2017 had a material impact on Alliant Energy’s, IPL’s and WPL’s 2017 financial statements as the tax effects of the changes in tax laws must be recognized in the period in which the law is enacted. Alliant Energy, IPL and WPL have completed their assessment of the applicable provisions of Tax Reform, and the impacts to their 2017 balance sheets and income statements are as follows (in millions): Alliant Energy IPL WPL Balance Sheet Other current assets $— $4.7 ($1.4 ) Investments: ATC Investment (75.0 ) — — Other assets: Regulatory assets (387.6 ) (375.0 ) (12.7 ) Deferred charges and other 41.0 — — Total Tax Reform impact on assets ($421.6 ) ($370.3 ) ($14.1 ) Other liabilities: Deferred tax liabilities ($1,331.9 ) ($757.2 ) ($523.8 ) Regulatory liabilities 885.9 390.7 495.2 Other 6.3 — — Total Tax Reform impact on liabilities ($439.7 ) ($366.5 ) ($28.6 ) Income Statement Income tax expense (benefit) ($18.1 ) $3.8 ($14.5 ) Deferred Tax Liabilities - Deferred tax assets and liabilities are measured at the enacted tax rate expected to be applied when temporary differences are to be realized or settled. Given the enactment of Tax Reform in December 2017, Alliant Energy’s, IPL’s and WPL’s deferred tax assets and liabilities at December 31, 2017 were re-measured based upon the new corporate tax rate. Alliant Energy’s utility operations in IPL and WPL recorded the net changes from re-measuring deferred tax assets and liabilities as a change in regulatory liabilities or regulatory assets as described in further detail below. Alliant Energy’s, IPL’s and WPL’s non-utility operations recorded the net change in deferred tax assets and liabilities to “Income tax expense (benefit)” in their respective income statement or as an increase to “Other liabilities” or decrease in “ATC Investment” on Alliant Energy’s balance sheet. Regulatory Assets and Regulatory Liabilities - Tax Reform requires regulated utilities to account for certain changes in deferred taxes related to accelerated depreciation and net operating losses using a normalization method of accounting. This method of accounting requires regulated utilities to record the normalized changes in deferred tax liabilities and assets to a regulatory liability or regulatory asset. For other changes to regulatory assets and regulatory liabilities resulting from Tax Reform, IPL and WPL are awaiting decisions from the IUB, PCSW and FERC to determine the appropriate regulatory treatment. IPL and WPL have recorded the offset to the changes in utility deferred tax liabilities and assets in a regulatory liability or regulatory asset to be reflected in future customer billings. Future changes may occur based on decisions received from the IUB, PSCW and FERC. In January 2018, the IUB issued an order initiating investigation of the impacts of Tax Reform. The order requires IPL to track all calculated differences since January 1, 2018 resulting from Tax Reform, such that any over-collections can be refunded to its customers at a future date, if appropriate. In January 2018, the PSCW issued an order directing WPL to defer and to accrue carrying costs on the revenue requirement impacts resulting from Tax Reform since its inception. ATC Investment - Tax Reform required a re-measurement of deferred tax assets and liabilities associated with Alliant Energy’s ATC Investment. These deferred tax assets and liabilities have been utilized to determine the amount of costs billed by ATC to its customers. Due to the regulated nature of ATC and FERC policy on income tax allowances, Alliant Energy expects the changes in deferred taxes that have been collected in ATC’s customers’ rates to be returned to such customers over the regulatory life of ATC’s assets. As a result, Alliant Energy has reduced the deferred tax liabilities associated with the ATC Investment and recorded the offset to “ATC Investment” on its balance sheet. Starting in 2018, the adjustment as a result of the change in deferred taxes will be amortized as a reduction in deferred tax expense over the remaining life of ATC’s assets. Deferred Charges and Other - Tax Reform repealed corporate federal AMT and allows unutilized AMT credits to be refunded over the next four tax years beginning with the U.S. federal tax return for calendar year 2018. As a result of these changes , Alliant Energy reclassified $41 million of unutilized AMT credits from a deferred tax asset into a long-term tax receivable in 2017 in anticipation of future refunds of such tax credits. The long-term tax receivable is included in “Deferred charges and other” on Alliant Energy’s balance sheet. Income Tax Expense (Benefit) - The changes in deferred taxes for Alliant Energy’s, IPL’s and WPL’s non-utility operations were primarily recorded in the income statements. In addition, Alliant Energy recognized valuation allowances for certain tax credit carryforwards as a result of Tax Reform that are discussed in more detail below. Income Tax Expense (Benefit) - The components of “Income tax expense (benefit)” in the income statements were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Current tax expense (benefit): Federal ($41.0 ) $1.8 $2.0 ($27.9 ) ($12.8 ) ($14.1 ) $5.5 ($22.3 ) $4.7 State 8.5 17.2 3.2 1.6 15.5 11.5 2.5 1.1 0.6 IPL’s tax benefit riders (40.4 ) (44.2 ) (49.0 ) (40.4 ) (44.2 ) (49.0 ) — — — Deferred tax expense (benefit): Federal 159.5 112.8 120.8 72.5 59.1 40.7 55.0 112.3 76.8 State 12.3 4.9 27.9 (2.2 ) (9.0 ) 3.3 16.6 20.8 20.2 Production tax credits (31.1 ) (31.8 ) (33.1 ) (14.1 ) (14.0 ) (14.5 ) (17.0 ) (17.8 ) (18.6 ) Investment tax credits (1.1 ) (1.3 ) (1.4 ) (0.4 ) (0.5 ) (0.6 ) (0.7 ) (0.8 ) (0.8 ) $66.7 $59.4 $70.4 ($10.9 ) ($5.9 ) ($22.7 ) $61.9 $93.3 $82.9 Income Tax Rates - The overall income tax rates shown in the following table were computed by dividing income tax expense (benefit) by income from continuing operations before income taxes. Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Statutory federal income tax rate 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % State income taxes, net of federal benefits 5.5 5.4 5.2 6.5 6.4 6.2 5.1 5.1 5.1 Effect of rate-making on property-related differences (8.5 ) (8.5 ) (6.8 ) (19.1 ) (16.2 ) (17.2 ) (1.7 ) (0.7 ) (0.5 ) IPL’s tax benefit riders (7.6 ) (10.0 ) (10.6 ) (18.7 ) (20.1 ) (28.3 ) — — — Production tax credits (6.1 ) (7.2 ) (7.2 ) (6.7 ) (6.3 ) (8.3 ) (7.1 ) (6.2 ) (7.1 ) Impact of Tax Reform (3.4 ) — — 1.7 — — (5.8 ) — — Other items, net (2.4 ) (1.3 ) (0.3 ) (3.7 ) (1.5 ) (0.5 ) (0.6 ) (0.6 ) (0.7 ) Overall income tax rate 12.5 % 13.4 % 15.3 % (5.0 %) (2.7 %) (13.1 %) 24.9 % 32.6 % 31.8 % Deferred Tax Assets and Liabilities - The deferred tax assets and liabilities included on the balance sheets at December 31 arise from the following temporary differences (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Deferred tax liabilities: Property $1,852.7 $2,919.0 $1,102.6 $1,677.0 $674.2 $1,124.5 ATC Investment 86.4 153.1 — — — — Other 75.9 95.1 63.4 71.4 36.5 58.8 Total deferred tax liabilities 2,015.0 3,167.2 1,166.0 1,748.4 710.7 1,183.3 Deferred tax assets: Federal credit carryforwards 260.7 268.4 113.1 95.9 131.0 112.9 Net operating losses carryforwards - federal 174.4 173.3 107.4 69.6 43.7 75.4 Regulatory liability - IPL’s tax benefit riders 7.4 34.7 7.4 34.7 — — Net operating losses carryforwards - state 41.3 32.9 0.9 0.6 0.2 0.1 Other 61.8 87.9 27.1 35.8 14.7 23.6 Subtotal deferred tax assets 545.6 597.2 255.9 236.6 189.6 212.0 Valuation allowance (9.0 ) (0.2 ) (0.6 ) — (1.3 ) (0.3 ) Total deferred tax assets 536.6 597.0 255.3 236.6 188.3 211.7 Total deferred tax liabilities, net $1,478.4 $2,570.2 $910.7 $1,511.8 $522.4 $971.6 The decreases in deferred tax liabilities were primarily due to the reduction in the federal corporate tax rate from 35% to 21% as a result of Tax Reform, as discussed above. Property - The decrease in property-related deferred tax liabilities from Tax Reform was partially offset by bonus depreciation deductions from property placed in service in 2017, including IPL’s Marshalltown Generating Station. Alliant Energy currently estimates its total bonus depreciation deductions to be claimed on its U.S. federal income tax return for calendar year 2017 will be approximately $670 million ( $500 million for IPL and $140 million for WPL). Carryforwards - The decrease in Alliant Energy’s federal credit carryforwards was due to the reclassification of AMT credits to a long-term tax receivable as a result of Tax Reform, partially offset by production tax credits generated in 2017. The increase in Alliant Energy’s and IPL’s federal net operating losses carryforwards was primarily due to bonus depreciation deductions for property placed in service in 2017, including IPL’s Marshalltown Generating Station, largely offset by the impacts of Tax Reform. At December 31, 2017 , carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2030-2037 $852 $537 $208 State net operating losses 2018-2037 700 13 2 Federal tax credits 2022-2037 267 119 131 Valuation Allowances - Due to the anticipated future reductions in revenues from utility customers due to Tax Reform, Alliant Energy expects a reduction in its future consolidated taxable income, which will extend the period to which prior unutilized operating losses will be utilized. Taxable income must be reduced by net operating losses carryforwards prior to utilizing federal tax credit carryforwards. Alliant Energy expects to utilize its net operating losses carryforwards by 2024 and therefore, currently does not expect to utilize 2002 and 2003 vintage federal credit carryforwards prior to their expiration in 2022 and 2023, respectively. This has resulted in valuation allowance charges recorded to “Income tax expense (benefit)” on the income statements as noted in the table above. Uncertain Tax Positions - At December 31, 2017 , 2016 and 2015 , there were no uncertain tax positions or penalties accrued related to uncertain tax positions, and interest accrued and tax positions favorably impacting future effective tax rates for continuing operations were not material. As of December 31, 2017 , no material changes to unrecognized tax benefits are expected during the next 12 months. Open tax years - Tax years that remain subject to the statute of limitations in the major jurisdictions for each of Alliant Energy, IPL and WPL are as follows: Consolidated federal income tax returns (a) 2014 - 2016 Consolidated Iowa income tax returns (b) 2014 - 2016 Wisconsin combined tax returns (c) 2013 - 2016 (a) These federal tax returns are effectively settled as a result of participation in the IRS Compliance Assurance Program, which allows Alliant Energy and the IRS to work together to resolve issues related to Alliant Energy’s current tax year before filing its federal income tax return. The statute of limitations for these federal tax returns expires three years from each filing date. (b) The statute of limitations for these Iowa tax returns expires three years from each filing date. (c) The statute of limitations for these Wisconsin combined tax returns expires four years from each filing date.

Benefit Plans

Benefit Plans	12 Months Ended
	Dec. 31, 2017
Benefit Plans	BENEFIT PLANS (a) Pension and Other Postretirement Benefits Plans - Retirement benefits are provided to substantially all employees through various qualified and non-qualified non-contributory defined benefit pension plans (currently closed to new hires), and/or through defined contribution plans (including 401(k) savings plans). Benefits of the non-contributory defined benefit pension plans are based on the plan participant’s years of service, age and compensation. Benefits of the defined contribution plans are based on the plan participant’s years of service, age, compensation and contributions. Certain defined benefit postretirement health care and life benefits are provided to eligible retirees. In general, the retiree health care plans consist of fixed benefit subsidy structures and the retiree life insurance plans are non-contributory. IPL and WPL account for their participation in Alliant Energy and Corporate Services sponsored plans as multiple-employer plans. In IPL’s and WPL’s tables below, the defined benefit pension plan amounts represent those respective amounts for their bargaining unit employees covered under the qualified plans that they sponsor, as well as amounts directly assigned to them related to their current and former non-bargaining employees who are participants in the Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans. In IPL’s and WPL’s tables below, the OPEB plan amounts represent respective amounts for their employees, as well as amounts directly assigned to them related to their current and former non-bargaining employees who are participants in the Corporate Services sponsored OPEB plan. Assumptions - The assumptions for defined benefit pension and OPEB plans at the measurement date of December 31 were as follows: Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2015 2017 2016 2015 Discount rate for benefit obligations 3.66% 4.19% 4.47% 3.53% 3.98% 4.30% Discount rate for net periodic cost 4.19% 4.47% 4.18% 3.98% 4.30% 3.97% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.80% 6.30% 6.20% Rate of compensation increase 3.65 % - 4.50% 3.65 % - 4.50% 3.65 % - 4.50% N/A N/A N/A Medical cost trend on covered charges: Initial trend rate (end of year) N/A N/A N/A 6.75% 7.00% 7.25% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans IPL 2017 2016 2015 2017 2016 2015 Discount rate for benefit obligations 3.68% 4.22% 4.50% 3.51% 3.95% 4.28% Discount rate for net periodic cost 4.22% 4.50% 4.20% 3.95% 4.28% 3.94% Expected rate of return on plan assets 7.60% 7.60% 7.60% 6.20% 6.60% 6.60% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Medical cost trend on covered charges: Initial trend rate (end of year) N/A N/A N/A 6.75% 7.00% 7.25% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans WPL 2017 2016 2015 2017 2016 2015 Discount rate for benefit obligations 3.69% 4.23% 4.51% 3.51% 3.96% 4.28% Discount rate for net periodic cost 4.23% 4.51% 4.20% 3.96% 4.28% 3.96% Expected rate of return on plan assets 7.60% 7.60% 7.60% 3.50% 4.70% 4.60% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Medical cost trend on covered charges: Initial trend rate (end of year) N/A N/A N/A 6.75% 7.00% 7.25% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Expected rate of return on plan assets - The expected rate of return on plan assets is based on projected asset class returns using target allocations. A forward-looking building blocks approach is used, and historical returns, survey information and capital market information are analyzed to support the expected rate of return on plan assets assumption. Refer to “Investment Strategy for Plan Assets” below for additional information related to investment strategy and mix of assets for the pension and OPEB plans. Life Expectancy - The life expectancy assumption is used in determining the benefit obligation and net periodic benefit cost for defined benefit pension and OPEB plans. This assumption was updated to utilize mortality tables that were released in 2014 by the Society of Actuaries and updated in 2015 and 2016. Net Periodic Benefit Costs (Credits) - The components of net periodic benefit costs (credits) for sponsored defined benefit pension and OPEB plans are included in the tables below (in millions). Net periodic benefit costs are primarily included in “Other operation and maintenance” in the income statements. Alliant Energy Defined Benefit Pension Plans OPEB Plans 2017 2016 2015 2017 2016 2015 Service cost $12.5 $12.6 $15.9 $5.0 $5.3 $5.5 Interest cost 51.0 53.0 53.6 8.6 9.4 9.1 Expected return on plan assets (a) (65.5 ) (65.5 ) (75.0 ) (6.1 ) (6.1 ) (8.4 ) Amortization of prior service credit (b) (0.4 ) (0.3 ) (0.2 ) (0.2 ) (4.1 ) (11.3 ) Amortization of actuarial loss (c) 37.6 37.4 35.4 3.8 4.7 4.8 Additional benefit costs — — 0.5 — — — Settlement losses (d) 0.9 — — — — — $36.1 $37.2 $30.2 $11.1 $9.2 ($0.3 ) IPL Defined Benefit Pension Plans OPEB Plans 2017 2016 2015 2017 2016 2015 Service cost $7.3 $7.5 $8.8 $2.1 $2.3 $2.4 Interest cost 23.5 24.5 25.0 3.5 3.8 3.8 Expected return on plan assets (a) (30.8 ) (30.9 ) (35.8 ) (4.3 ) (4.3 ) (5.7 ) Amortization of prior service credit (b) (0.2 ) (0.2 ) (0.1 ) — (2.6 ) (6.1 ) Amortization of actuarial loss (c) 16.1 16.5 15.3 2.0 2.6 2.3 $15.9 $17.4 $13.2 $3.3 $1.8 ($3.3 ) WPL Defined Benefit Pension Plans OPEB Plans 2017 2016 2015 2017 2016 2015 Service cost $4.9 $4.9 $5.8 $1.9 $2.0 $2.1 Interest cost 21.8 22.3 22.6 3.4 3.8 3.7 Expected return on plan assets (a) (28.5 ) (28.3 ) (32.4 ) (0.8 ) (0.8 ) (1.5 ) Amortization of prior service cost (credit) (b) 0.1 0.2 0.2 (0.2 ) (0.9 ) (3.5 ) Amortization of actuarial loss (c) 18.5 17.6 16.8 1.6 1.8 2.2 Additional benefit costs — — 0.5 — — — $16.8 $16.7 $13.5 $5.9 $5.9 $3.0 (a) The expected return on plan assets is based on the expected rate of return on plan assets and the fair value approach to the market-related value of plan assets. (b) Unrecognized prior service costs (credits) for the OPEB plans are amortized over the average future service period to full eligibility of the participants of each plan. (c) Unrecognized net actuarial gains or losses in excess of 10% of the greater of the plans’ benefit obligations or assets are amortized over the average future service lives of plan participants, except for the Alliant Energy Cash Balance Pension Plan where gains or losses outside the 10% threshold are amortized over the time period the participants are expected to receive benefits. (d) Settlement losses related to payments made to retired executives of Alliant Energy. The estimated amortization from “Regulatory assets” and “Regulatory liabilities” on the balance sheets and AOCL on Alliant Energy’s balance sheet into net periodic benefit cost in 2018 is as follows (in millions): Alliant Energy IPL WPL Defined Benefit Defined Benefit Defined Benefit Pension Plans OPEB Plans Pension Plans OPEB Plans Pension Plans OPEB Plans Actuarial loss $35.2 $3.4 $15.0 $1.2 $17.2 $2.0 Prior service credit (0.7 ) (0.2 ) (0.2 ) — (0.2 ) (0.2 ) $34.5 $3.2 $14.8 $1.2 $17.0 $1.8 Benefit Plan Assets and Obligations - A reconciliation of the funded status of qualified and non-qualified defined benefit pension and OPEB plans to the amounts recognized on the balance sheets at December 31 was as follows (in millions): Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2017 2016 Change in benefit obligation: Net benefit obligation at January 1 $1,244.3 $1,206.3 $220.1 $221.4 Service cost 12.5 12.6 5.0 5.3 Interest cost 51.0 53.0 8.6 9.4 Plan participants’ contributions — — 2.9 2.4 Actuarial (gain) loss 83.6 48.3 5.4 (0.3 ) Gross benefits paid (88.3 ) (75.9 ) (19.7 ) (18.1 ) Net benefit obligation at December 31 1,303.1 1,244.3 222.3 220.1 Change in plan assets: Fair value of plan assets at January 1 895.7 895.0 105.8 106.9 Actual return on plan assets 136.7 74.3 12.9 8.2 Employer contributions 6.6 2.3 9.2 6.4 Plan participants’ contributions — — 2.9 2.4 Gross benefits paid (88.3 ) (75.9 ) (19.7 ) (18.1 ) Fair value of plan assets at December 31 950.7 895.7 111.1 105.8 Under funded status at December 31 ($352.4 ) ($348.6 ) ($111.2 ) ($114.3 ) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2017 2016 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $8.8 $3.2 Current liabilities (2.2 ) (6.5 ) (9.1 ) (8.6 ) Pension and other benefit obligations (350.2 ) (342.1 ) (110.9 ) (108.9 ) Net amounts recognized at December 31 ($352.4 ) ($348.6 ) ($111.2 ) ($114.3 ) Amounts recognized in Regulatory Assets (refer to Note 2 for details) and AOCL (refer to Alliant Energy’s common equity statements for details) consist of: Net actuarial loss $509.1 $535.1 $47.4 $52.6 Prior service credit (6.5 ) (6.9 ) (1.3 ) (1.5 ) $502.6 $528.2 $46.1 $51.1 Defined Benefit Pension Plans OPEB Plans IPL 2017 2016 2017 2016 Change in benefit obligation: Net benefit obligation at January 1 $570.4 $556.1 $90.1 $91.3 Service cost 7.3 7.5 2.1 2.3 Interest cost 23.5 24.5 3.5 3.8 Plan participants’ contributions — — 1.0 0.9 Actuarial (gain) loss 34.9 19.1 (0.1 ) (0.7 ) Gross benefits paid (43.2 ) (36.8 ) (7.2 ) (7.5 ) Net benefit obligation at December 31 592.9 570.4 89.4 90.1 Change in plan assets: Fair value of plan assets at January 1 422.0 422.7 68.2 69.2 Actual return on plan assets 64.3 35.3 8.9 5.3 Employer contributions 0.6 0.8 2.0 0.3 Plan participants’ contributions — — 1.0 0.9 Gross benefits paid (43.2 ) (36.8 ) (7.2 ) (7.5 ) Fair value of plan assets at December 31 443.7 422.0 72.9 68.2 Under funded status at December 31 ($149.2 ) ($148.4 ) ($16.5 ) ($21.9 ) Defined Benefit Pension Plans OPEB Plans IPL 2017 2016 2017 2016 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $5.9 $0.4 Current liabilities (0.5 ) (0.7 ) (2.0 ) (1.9 ) Pension and other benefit obligations (148.7 ) (147.7 ) (20.4 ) (20.4 ) Net amounts recognized at December 31 ($149.2 ) ($148.4 ) ($16.5 ) ($21.9 ) Amounts recognized in Regulatory Assets consist of (refer to Note 2 for details): Net actuarial loss $218.9 $233.6 $18.7 $25.4 Prior service credit (2.1 ) (2.3 ) — — $216.8 $231.3 $18.7 $25.4 Defined Benefit Pension Plans OPEB Plans WPL 2017 2016 2017 2016 Change in benefit obligation: Net benefit obligation at January 1 $529.2 $505.9 $88.9 $89.7 Service cost 4.9 4.9 1.9 2.0 Interest cost 21.8 22.3 3.4 3.8 Plan participants’ contributions — — 1.4 1.2 Actuarial loss 38.3 25.7 4.1 0.5 Gross benefits paid (34.4 ) (29.6 ) (9.3 ) (8.3 ) Net benefit obligation at December 31 559.8 529.2 90.4 88.9 Change in plan assets: Fair value of plan assets at January 1 389.7 386.8 18.6 18.7 Actual return on plan assets 59.6 32.4 1.2 1.2 Employer contributions 0.1 0.1 6.8 5.8 Plan participants’ contributions — — 1.4 1.2 Gross benefits paid (34.4 ) (29.6 ) (9.3 ) (8.3 ) Fair value of plan assets at December 31 415.0 389.7 18.7 18.6 Under funded status at December 31 ($144.8 ) ($139.5 ) ($71.7 ) ($70.3 ) Defined Benefit Pension Plans OPEB Plans WPL 2017 2016 2017 2016 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $2.9 $2.7 Current liabilities (0.1 ) (0.1 ) (6.8 ) (6.4 ) Pension and other benefit obligations (144.7 ) (139.4 ) (67.8 ) (66.6 ) Net amounts recognized at December 31 ($144.8 ) ($139.5 ) ($71.7 ) ($70.3 ) Amounts recognized in Regulatory Assets consist of (refer to Note 2 for details): Net actuarial loss $224.7 $236.1 $23.6 $21.5 Prior service credit (1.5 ) (1.4 ) (1.3 ) (1.5 ) $223.2 $234.7 $22.3 $20.0 Included in the following tables are accumulated benefit obligations, aggregate amounts applicable to defined benefit pension and OPEB plans with accumulated benefit obligations in excess of plan assets, as well as defined benefit pension plans with projected benefit obligations in excess of plan assets as of the December 31 measurement date (in millions): Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2017 2016 Accumulated benefit obligations $1,269.0 $1,201.5 $222.3 $220.1 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,269.0 1,201.5 222.3 220.1 Fair value of plan assets 950.7 895.7 111.1 105.8 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,303.1 1,244.3 N/A N/A Fair value of plan assets 950.7 895.7 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2017 2016 2017 2016 Accumulated benefit obligations $573.1 $546.7 $89.4 $90.1 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 573.1 546.7 89.4 90.1 Fair value of plan assets 443.7 422.0 72.9 68.2 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 592.9 570.4 N/A N/A Fair value of plan assets 443.7 422.0 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2017 2016 2017 2016 Accumulated benefit obligations $548.1 $513.2 $90.4 $88.9 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 548.1 513.2 90.4 88.9 Fair value of plan assets 415.0 389.7 18.7 18.6 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 559.8 529.2 N/A N/A Fair value of plan assets 415.0 389.7 N/A N/A In addition to the amounts recognized in regulatory assets in the above tables for IPL and WPL, regulatory assets were recognized for amounts associated with Corporate Services employees participating in other Alliant Energy sponsored benefit plans that were allocated to IPL and WPL at December 31 as follows (in millions): IPL WPL 2017 2016 2017 2016 Regulatory assets $38.9 $37.3 $28.1 $30.0 Estimated Future Employer Contributions and Benefit Payments - Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2018 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $6.3 $4.4 $0.3 OPEB plans 9.0 2.0 6.8 (a) Alliant Energy sponsors several non-qualified defined benefit pension plans that cover certain current and former key employees of IPL and WPL. Alliant Energy allocates pension costs to IPL and WPL for these plans. In addition, IPL and WPL amounts reflect funding for their non-bargaining employees who are participants in the Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans. Expected benefit payments for the qualified and non-qualified defined benefit plans, which reflect expected future service, as appropriate, are as follows (in millions): Alliant Energy 2018 2019 2020 2021 2022 2023 - 2027 Defined benefit pension benefits $72.2 $73.9 $76.4 $77.1 $92.9 $398.2 OPEB 18.2 18.4 17.7 17.5 17.2 80.1 $90.4 $92.3 $94.1 $94.6 $110.1 $478.3 IPL 2018 2019 2020 2021 2022 2023 - 2027 Defined benefit pension benefits $33.9 $33.8 $36.5 $36.6 $37.6 $188.4 OPEB 7.1 7.1 7.2 7.1 7.0 32.6 $41.0 $40.9 $43.7 $43.7 $44.6 $221.0 WPL 2018 2019 2020 2021 2022 2023 - 2027 Defined benefit pension benefits $31.4 $32.0 $32.4 $32.4 $32.7 $167.8 OPEB 8.3 8.3 7.5 7.3 7.0 31.8 $39.7 $40.3 $39.9 $39.7 $39.7 $199.6 Investment Strategy for Plan Assets - Investment strategies for defined benefit pension and OPEB plan assets combine preservation of principal and prudent risk-taking to protect the integrity of plan assets, in order to meet the obligations to plan participants while minimizing benefit costs over the long term. Investment risk of plan assets is mitigated through diversification, including broad U.S. equity, international equity and fixed income exposure, and global asset and risk parity strategies. Global asset and risk parity strategies include investments in global equity, global debt, commodities and currencies. Defined Benefit Pension Plan Assets - The asset mix of defined benefit pension plans is governed by allocation targets. Historical performance results and future expectations suggest that equity securities will provide higher total investment returns than fixed income securities over a long-term investment horizon. Consistent with the goals of meeting obligations to plan participants and minimizing benefit costs over the long-term, the defined benefit pension plans have a long-term investment posture more heavily weighted toward equity holdings. The asset allocation is monitored regularly and appropriate steps are taken as needed to rebalance the assets within the prescribed ranges. An overlay management service is also used to help maintain target allocations and meet liquidity needs. The overlay manager is authorized to use derivative financial instruments to facilitate this service. For separately managed accounts, prohibited investments include, but may not be limited to, direct ownership of real estate, margin trading, oil and gas limited partnerships and securities of the managers’ firms or affiliate firms. At December 31, 2017 , the current target ranges and actual allocations for the defined benefit pension plan assets were as follows: Target Range Actual Allocation Allocation Cash and equivalents 0 % - 5% 3% Equity securities - U.S. 11 % - 41% 24% Equity securities - international 14 % - 34% 23% Global asset securities 5 % - 15% 10% Risk parity securities 5 % - 15% 10% Fixed income securities 20 % - 40% 30% Other Postretirement Benefits Plan Assets - OPEB plan assets are comprised of specific assets within certain defined benefit pension plans (401(h) assets) as well as assets held in VEBA trusts. The investment strategy of the Corporate Services 401(h) assets mirrors those of the defined benefit pension plans, which are discussed above. For VEBA trusts with assets greater than $5 million and the WPL 401(h) assets, the mix among asset classes is governed by allocation targets. The asset allocation is monitored regularly and appropriate steps are taken as needed to rebalance the assets within the prescribed ranges. Mutual funds are used to achieve the desired diversification. At December 31, 2017 , the current target ranges and actual allocations for VEBA trusts with assets greater than $5 million and the WPL 401(h) assets were as follows: Target Range Actual Allocation Allocation Cash and equivalents 0 % - 5% 1% Equity securities - U.S. 0 % - 50% 26% Equity securities - international 0 % - 34% 10% Fixed income securities 20 % - 100% 63% Fair Value Measurements - Fair value measurement accounting establishes three levels of fair value hierarchy that prioritize the inputs to valuation techniques used to measure fair value. Refer to Note 14 for discussion of levels within the fair value hierarchy. Level 1 items include investments in securities held in registered investment companies, treasury bills and directly held equity securities, which are valued at the closing price reported in the active market in which the securities are traded. Level 2 items include fixed income securities consisting of corporate and government bonds and agency obligations, which are valued at the closing price reported in the active market for similar assets in which the individual securities are traded or based on yields currently available on comparable securities of issuers with similar credit ratings. Certain investments that are measured at fair value using the net asset value practical expedient have not been classified in the fair value hierarchy. These fair value amounts are included in the tables below to reconcile the fair value hierarchy to the respective total plan assets. At December 31, the fair values of qualified and non-qualified defined benefit pension plan assets were as follows (in millions): 2017 2016 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $28.2 $4.5 $23.7 $— $30.4 $5.0 $25.4 $— Equity securities - U.S. 158.3 158.3 — — 183.6 183.6 — — Equity securities - international 137.5 137.5 — — 97.4 97.4 — — Global asset securities 49.4 49.4 — — 53.0 53.0 — — Fixed income securities 135.9 55.8 80.1 — 125.4 53.6 71.8 — Total assets in fair value hierarchy 509.3 $405.5 $103.8 $— 489.8 $392.6 $97.2 $— Assets measured at net asset value 441.1 405.9 Accrued investment income 1.0 1.1 Due to brokers, net (pending trades with brokers) (0.7 ) (1.1 ) Total pension plan assets $950.7 $895.7 2017 2016 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $13.2 $2.2 $11.0 $— $14.4 $2.4 $12.0 $— Equity securities - U.S. 73.9 73.9 — — 86.5 86.5 — — Equity securities - international 64.2 64.2 — — 45.9 45.9 — — Global asset securities 23.0 23.0 — — 24.9 24.9 — — Fixed income securities 63.4 26.0 37.4 — 59.1 25.3 33.8 — Total assets in fair value hierarchy 237.7 $189.3 $48.4 $— 230.8 $185.0 $45.8 $— Assets measured at net asset value 205.8 191.2 Accrued investment income 0.5 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.5 ) Total pension plan assets $443.7 $422.0 2017 2016 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $12.3 $2.0 $10.3 $— $13.3 $2.2 $11.1 $— Equity securities - U.S. 69.1 69.1 — — 79.9 79.9 — — Equity securities - international 60.0 60.0 — — 42.4 42.4 — — Global asset securities 21.6 21.6 — — 23.0 23.0 — — Fixed income securities 59.3 24.3 35.0 — 54.5 23.3 31.2 — Total assets in fair value hierarchy 222.3 $177.0 $45.3 $— 213.1 $170.8 $42.3 $— Assets measured at net asset value 192.5 176.6 Accrued investment income 0.5 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.5 ) Total pension plan assets $415.0 $389.7 At December 31, the fair values of OPEB plan assets were as follows (in millions): 2017 2016 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $1.2 $0.7 $0.5 $— $3.5 $2.0 $1.5 $— Equity securities - U.S. 27.9 27.9 — — 22.5 22.5 — — Equity securities - international 11.4 11.4 — — 13.5 13.5 — — Global asset securities 0.4 0.4 — — 16.5 16.5 — — Fixed income securities 66.6 66.0 0.6 — 46.8 46.2 0.6 — Total assets in fair value hierarchy 107.5 $106.4 $1.1 $— 102.8 $100.7 $2.1 $— Assets measured at net asset value 3.6 3.0 Total OPEB plan assets $111.1 $105.8 2017 2016 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.3 $0.3 $— $— $0.8 $0.8 $— $— Equity securities - U.S. 22.3 22.3 — — 17.0 17.0 — — Equity securities - international 7.5 7.5 — — 11.0 11.0 — — Global asset securities — — — — 7.0 7.0 — — Fixed income securities 42.8 42.8 — — 32.4 32.4 — — Total OPEB plan assets $72.9 $72.9 $— $— $68.2 $68.2 $— $— 2017 2016 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.6 $0.3 $0.3 $— $2.0 $0.7 $1.3 $— Global asset securities — — — — 5.5 5.5 — — Fixed income securities 18.1 18.1 — — 11.1 11.1 — — Total OPEB plan assets $18.7 $18.4 $0.3 $— $18.6 $17.3 $1.3 $— For the various defined benefit pension and OPEB plans, Alliant Energy common stock represented less than 1% of assets directly held in the plans at December 31, 2017 and 2016 . 401(k) Savings Plans - A significant number of employees participate in defined contribution retirement plans (401(k) savings plans). Alliant Energy common stock directly held by participants represented 11.5% and 12.6% of total assets in the 401(k) savings plans at December 31, 2017 and 2016 , respectively. Costs related to the 401(k) savings plans, which are partially based on the participants’ contributions and include allocated costs associated with Corporate Services employees for IPL and WPL, were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 401(k) costs $24.8 $23.6 $24.9 $12.8 $12.0 $12.7 $11.1 $10.7 $11.2 (b) Equity-based Compensation Plans - In 2015, Alliant Energy’s shareowners approved the Amended and Restated OIP, which permits the grant of shares of Alliant Energy common stock, restricted stock, restricted stock units, performance shares, performance units, and other stock-based or cash-based awards to key employees. At December 31, 2017 , performance shares, performance-contingent restricted stock and restricted stock units (performance- and time-vesting) were outstanding under the Amended and Restated OIP, and 7.1 million shares of Alliant Energy’s common stock remained available for grants under the Amended and Restated OIP. Alliant Energy satisfies share payouts related to equity awards under the Amended and Restated OIP through the issuance of new shares of its common stock. Alliant Energy also has the DLIP, which permits the grant of cash-based long-term awards, including performance units, restricted cash awards and restricted units, to certain key employees. At December 31, 2017 , performance units, performance-contingent cash awards and restricted units (performance- and time-vesting) were outstanding under the DLIP. There is no limit to the number of grants that can be made under the DLIP and Alliant Energy satisfies all payouts under the DLIP through cash payments. Nonvested awards generally do not have non-forfeitable rights to dividends or dividend equivalents when dividends are paid to common shareowners. A summary of compensation expense, including amounts allocated to IPL and WPL, and the related income tax benefits recognized for share-based compensation awards was as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Compensation expense $15.1 $18.0 $10.7 $8.3 $9.5 $5.7 $6.4 $7.9 $4.7 Income tax benefits 6.2 7.4 4.4 3.4 4.0 2.4 2.6 3.2 1.9 As of December 31, 2017 , Alliant Energy’s, IPL’s and WPL’s total unrecognized compensation cost related to share-based compensation awards was $5.6 million , $3.1 million and $2.2 million , respectively, which is expected to be recognized over a weighted average period of between one and two years. Share-based compensation expense is recognized on a straight-line basis over the requisite service periods and is primarily recorded in “Other operation and maintenance” in the income statements. Performance Shares and Performance Units - Payouts of performance shares under the Amended and Restated OIP and performance units under the DLIP to key employees are contingent upon achievement over three -year periods of specified performance criteria, which currently include metrics of total shareowner return relative to an investor-owned utility peer group. Performance shares can be paid out in shares of Alliant Energy’s common stock, cash or a combination of cash and stock. Performance units must be paid out in cash. Alliant Energy assumes it will make future payouts of its performance shares and performance units in cash; therefore, performance shares and performance units are accounted for as liability awards. A summary of the performance shares and performance units activity, with amounts representing the target number of awards, was as follows: Performance Shares Performance Units 2017 2016 2015 2017 2016 2015 Nonvested awards, January 1 257,599 288,430 288,848 93,320 116,412 127,330 Granted 65,350 68,585 90,806 21,558 23,918 35,674 Vested (99,438 ) (98,186 ) (91,224 ) (37,395 ) (42,760 ) (45,690 ) Forfeited — (1,230 ) — (5,746 ) (4,250 ) (902 ) Nonvested awards, December 31 223,511 257,599 288,430 71,737 93,320 116,412 Granted Awards - Each performance share’s value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the performance period. For performance units granted in 2017 and 2016, the value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the performance period. For performance units granted in 2015, the value is based on the closing market price of one share of Alliant Energy’s common stock on the grant date of the award. The actual payout for performance shares and performance units is dependent upon actual performance and may range from zero to 200% of the target number of awards. Compensation expense for performance shares and performance units is recorded ratably over the performance period based on the fair value of the awards at each reporting period. Vested Awards - Certain performance shares and performance units vested, resulting in payouts (a combination of cash and common stock for the performance shares and cash only for the performance units) as follows: Performance Shares Performance Units 2017 2016 2015 2017 2016 2015 2014 Grant 2013 Grant 2012 Grant 2014 Grant 2013 Grant 2012 Grant Performance awards vested 99,438 98,186 91,224 37,395 42,760 45,690 Percentage of target number of performance awards 147.5% 165.0% 167.5% 147.5% 165.0% 167.5% Aggregate payout value (in millions) $5.6 $5.1 $5.1 $1.5 $1.7 $1.6 Payout - cash (in millions) $5.1 $2.9 $3.2 $1.5 $1.7 $1.6 Payout - common stock shares issued 5,185 22,408 21,950 N/A N/A N/A Fair Value of Awards - Information related to fair values of nonvested performance shares and performance units at December 31, 2017 , by year of grant, were as follows: Performance Shares Performance Units 2017 Grant 2016 Grant 2015 Grant 2017 Grant 2016 Grant 2015 Grant Nonvested awards at target 65,350 67,355 90,806 18,600 21,227 31,910 Alliant Energy common stock closing price on December 29, 2017 $42.61 $42.61 $42.61 $42.61 $42.61 N/A Alliant Energy common stock closing price on grant date N/A N/A N/A N/A N/A $32.55 Estimated payout percentage based on performance criteria 105 % 150 % 138 % 105 % 150 % 138 % Fair values of each nonvested award $44.74 $63.92 $58.80 $44.74 $63.92 $44.92 Performance Restricted Stock Units and Performance Restricted Units - Alliant Energy granted new types of share-based compensation awards to key employees beginning in 2016 referred to as performance restricted stock units under the Amended and Restated OIP, and performance restricted units and key employee performance restricted units under the DLIP. Payouts of these units are based on the achievement of certain performance targets (currently specified growth of consolidated income from continuing operations) during a three -year performance period. The actual number of units that will be paid out upon vesting is dependent upon actual performance and may range from zero to 200% of the target number of units. If performance targets are not met during the performance period, these units are forfeited. As of December 31, 2017 , the amount of nonvested performance restricted units and key employee performance restricted units was not material. Performance Restricted Stock Units - Performance restricted stock units generally must be paid out in shares and are accounted for as equity awards. Each performance restricted stock unit’s value is based on the closing market price of one share of Alliant Energy’s common stock on the grant date of the award. Compensation expense is recorded ratably over the performance period based on a probability assessment of payouts for the awards at each reporting period. A summary of the performance restricted stock units activity, with amounts representing the target number of units, was as follows: 2017 2016 Units Weighted Average Grant Date Fair Value Units Weighted Average Grant Date Fair Value Nonvested units, January 1 67,355 $33.96 — $— Granted 65,350 39.12 68,585 33.96 Forfeited — — (1,230 ) 33.90 Nonvested units, December 31 132,705 36.50 67,355 33.96 Restricted Stock Units and Restricted Units - Alliant Energy granted new types of share-based compensation awards to key employees beginning in 2016 referred to as restricted stock units under the Amended and Restated OIP and restricted units under the DLIP. Payouts of these units are based on the expiration of a three -year time-vesting period. Each restricted stock unit’s value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the time-vesting period. Compensation expense is recorded ratably over the performance period based on the fair value of the awards at each reporting period. Restricted stock units can be paid out in shares of Alliant Energy common stock, cash or a combination of cash and stock. Alliant Energy assumes it will make future payouts of its restricted stock units in cash; therefore, restricted stock units are accounted for as liability awards. As of December 31, 2017 , the amount of nonvested restricted units was not material. A summary of the restricted stock units activity was as follows: 2017 2016 Nonvested units, January 1 57,736 — Granted 56,013 58,790 Forfeited — (1,054 ) Nonvested units, December 31 113,749 57,736 Performance-Contingent Restricted
IPL [Member]
Benefit Plans	BENEFIT PLANS (a) Pension and Other Postretirement Benefits Plans - Retirement benefits are provided to substantially all employees through various qualified and non-qualified non-contributory defined benefit pension plans (currently closed to new hires), and/or through defined contribution plans (including 401(k) savings plans). Benefits of the non-contributory defined benefit pension plans are based on the plan participant’s years of service, age and compensation. Benefits of the defined contribution plans are based on the plan participant’s years of service, age, compensation and contributions. Certain defined benefit postretirement health care and life benefits are provided to eligible retirees. In general, the retiree health care plans consist of fixed benefit subsidy structures and the retiree life insurance plans are non-contributory. IPL and WPL account for their participation in Alliant Energy and Corporate Services sponsored plans as multiple-employer plans. In IPL’s and WPL’s tables below, the defined benefit pension plan amounts represent those respective amounts for their bargaining unit employees covered under the qualified plans that they sponsor, as well as amounts directly assigned to them related to their current and former non-bargaining employees who are participants in the Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans. In IPL’s and WPL’s tables below, the OPEB plan amounts represent respective amounts for their employees, as well as amounts directly assigned to them related to their current and former non-bargaining employees who are participants in the Corporate Services sponsored OPEB plan. Assumptions - The assumptions for defined benefit pension and OPEB plans at the measurement date of December 31 were as follows: Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2015 2017 2016 2015 Discount rate for benefit obligations 3.66% 4.19% 4.47% 3.53% 3.98% 4.30% Discount rate for net periodic cost 4.19% 4.47% 4.18% 3.98% 4.30% 3.97% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.80% 6.30% 6.20% Rate of compensation increase 3.65 % - 4.50% 3.65 % - 4.50% 3.65 % - 4.50% N/A N/A N/A Medical cost trend on covered charges: Initial trend rate (end of year) N/A N/A N/A 6.75% 7.00% 7.25% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans IPL 2017 2016 2015 2017 2016 2015 Discount rate for benefit obligations 3.68% 4.22% 4.50% 3.51% 3.95% 4.28% Discount rate for net periodic cost 4.22% 4.50% 4.20% 3.95% 4.28% 3.94% Expected rate of return on plan assets 7.60% 7.60% 7.60% 6.20% 6.60% 6.60% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Medical cost trend on covered charges: Initial trend rate (end of year) N/A N/A N/A 6.75% 7.00% 7.25% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans WPL 2017 2016 2015 2017 2016 2015 Discount rate for benefit obligations 3.69% 4.23% 4.51% 3.51% 3.96% 4.28% Discount rate for net periodic cost 4.23% 4.51% 4.20% 3.96% 4.28% 3.96% Expected rate of return on plan assets 7.60% 7.60% 7.60% 3.50% 4.70% 4.60% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Medical cost trend on covered charges: Initial trend rate (end of year) N/A N/A N/A 6.75% 7.00% 7.25% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Expected rate of return on plan assets - The expected rate of return on plan assets is based on projected asset class returns using target allocations. A forward-looking building blocks approach is used, and historical returns, survey information and capital market information are analyzed to support the expected rate of return on plan assets assumption. Refer to “Investment Strategy for Plan Assets” below for additional information related to investment strategy and mix of assets for the pension and OPEB plans. Life Expectancy - The life expectancy assumption is used in determining the benefit obligation and net periodic benefit cost for defined benefit pension and OPEB plans. This assumption was updated to utilize mortality tables that were released in 2014 by the Society of Actuaries and updated in 2015 and 2016. Net Periodic Benefit Costs (Credits) - The components of net periodic benefit costs (credits) for sponsored defined benefit pension and OPEB plans are included in the tables below (in millions). Net periodic benefit costs are primarily included in “Other operation and maintenance” in the income statements. Alliant Energy Defined Benefit Pension Plans OPEB Plans 2017 2016 2015 2017 2016 2015 Service cost $12.5 $12.6 $15.9 $5.0 $5.3 $5.5 Interest cost 51.0 53.0 53.6 8.6 9.4 9.1 Expected return on plan assets (a) (65.5 ) (65.5 ) (75.0 ) (6.1 ) (6.1 ) (8.4 ) Amortization of prior service credit (b) (0.4 ) (0.3 ) (0.2 ) (0.2 ) (4.1 ) (11.3 ) Amortization of actuarial loss (c) 37.6 37.4 35.4 3.8 4.7 4.8 Additional benefit costs — — 0.5 — — — Settlement losses (d) 0.9 — — — — — $36.1 $37.2 $30.2 $11.1 $9.2 ($0.3 ) IPL Defined Benefit Pension Plans OPEB Plans 2017 2016 2015 2017 2016 2015 Service cost $7.3 $7.5 $8.8 $2.1 $2.3 $2.4 Interest cost 23.5 24.5 25.0 3.5 3.8 3.8 Expected return on plan assets (a) (30.8 ) (30.9 ) (35.8 ) (4.3 ) (4.3 ) (5.7 ) Amortization of prior service credit (b) (0.2 ) (0.2 ) (0.1 ) — (2.6 ) (6.1 ) Amortization of actuarial loss (c) 16.1 16.5 15.3 2.0 2.6 2.3 $15.9 $17.4 $13.2 $3.3 $1.8 ($3.3 ) WPL Defined Benefit Pension Plans OPEB Plans 2017 2016 2015 2017 2016 2015 Service cost $4.9 $4.9 $5.8 $1.9 $2.0 $2.1 Interest cost 21.8 22.3 22.6 3.4 3.8 3.7 Expected return on plan assets (a) (28.5 ) (28.3 ) (32.4 ) (0.8 ) (0.8 ) (1.5 ) Amortization of prior service cost (credit) (b) 0.1 0.2 0.2 (0.2 ) (0.9 ) (3.5 ) Amortization of actuarial loss (c) 18.5 17.6 16.8 1.6 1.8 2.2 Additional benefit costs — — 0.5 — — — $16.8 $16.7 $13.5 $5.9 $5.9 $3.0 (a) The expected return on plan assets is based on the expected rate of return on plan assets and the fair value approach to the market-related value of plan assets. (b) Unrecognized prior service costs (credits) for the OPEB plans are amortized over the average future service period to full eligibility of the participants of each plan. (c) Unrecognized net actuarial gains or losses in excess of 10% of the greater of the plans’ benefit obligations or assets are amortized over the average future service lives of plan participants, except for the Alliant Energy Cash Balance Pension Plan where gains or losses outside the 10% threshold are amortized over the time period the participants are expected to receive benefits. (d) Settlement losses related to payments made to retired executives of Alliant Energy. The estimated amortization from “Regulatory assets” and “Regulatory liabilities” on the balance sheets and AOCL on Alliant Energy’s balance sheet into net periodic benefit cost in 2018 is as follows (in millions): Alliant Energy IPL WPL Defined Benefit Defined Benefit Defined Benefit Pension Plans OPEB Plans Pension Plans OPEB Plans Pension Plans OPEB Plans Actuarial loss $35.2 $3.4 $15.0 $1.2 $17.2 $2.0 Prior service credit (0.7 ) (0.2 ) (0.2 ) — (0.2 ) (0.2 ) $34.5 $3.2 $14.8 $1.2 $17.0 $1.8 Benefit Plan Assets and Obligations - A reconciliation of the funded status of qualified and non-qualified defined benefit pension and OPEB plans to the amounts recognized on the balance sheets at December 31 was as follows (in millions): Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2017 2016 Change in benefit obligation: Net benefit obligation at January 1 $1,244.3 $1,206.3 $220.1 $221.4 Service cost 12.5 12.6 5.0 5.3 Interest cost 51.0 53.0 8.6 9.4 Plan participants’ contributions — — 2.9 2.4 Actuarial (gain) loss 83.6 48.3 5.4 (0.3 ) Gross benefits paid (88.3 ) (75.9 ) (19.7 ) (18.1 ) Net benefit obligation at December 31 1,303.1 1,244.3 222.3 220.1 Change in plan assets: Fair value of plan assets at January 1 895.7 895.0 105.8 106.9 Actual return on plan assets 136.7 74.3 12.9 8.2 Employer contributions 6.6 2.3 9.2 6.4 Plan participants’ contributions — — 2.9 2.4 Gross benefits paid (88.3 ) (75.9 ) (19.7 ) (18.1 ) Fair value of plan assets at December 31 950.7 895.7 111.1 105.8 Under funded status at December 31 ($352.4 ) ($348.6 ) ($111.2 ) ($114.3 ) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2017 2016 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $8.8 $3.2 Current liabilities (2.2 ) (6.5 ) (9.1 ) (8.6 ) Pension and other benefit obligations (350.2 ) (342.1 ) (110.9 ) (108.9 ) Net amounts recognized at December 31 ($352.4 ) ($348.6 ) ($111.2 ) ($114.3 ) Amounts recognized in Regulatory Assets (refer to Note 2 for details) and AOCL (refer to Alliant Energy’s common equity statements for details) consist of: Net actuarial loss $509.1 $535.1 $47.4 $52.6 Prior service credit (6.5 ) (6.9 ) (1.3 ) (1.5 ) $502.6 $528.2 $46.1 $51.1 Defined Benefit Pension Plans OPEB Plans IPL 2017 2016 2017 2016 Change in benefit obligation: Net benefit obligation at January 1 $570.4 $556.1 $90.1 $91.3 Service cost 7.3 7.5 2.1 2.3 Interest cost 23.5 24.5 3.5 3.8 Plan participants’ contributions — — 1.0 0.9 Actuarial (gain) loss 34.9 19.1 (0.1 ) (0.7 ) Gross benefits paid (43.2 ) (36.8 ) (7.2 ) (7.5 ) Net benefit obligation at December 31 592.9 570.4 89.4 90.1 Change in plan assets: Fair value of plan assets at January 1 422.0 422.7 68.2 69.2 Actual return on plan assets 64.3 35.3 8.9 5.3 Employer contributions 0.6 0.8 2.0 0.3 Plan participants’ contributions — — 1.0 0.9 Gross benefits paid (43.2 ) (36.8 ) (7.2 ) (7.5 ) Fair value of plan assets at December 31 443.7 422.0 72.9 68.2 Under funded status at December 31 ($149.2 ) ($148.4 ) ($16.5 ) ($21.9 ) Defined Benefit Pension Plans OPEB Plans IPL 2017 2016 2017 2016 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $5.9 $0.4 Current liabilities (0.5 ) (0.7 ) (2.0 ) (1.9 ) Pension and other benefit obligations (148.7 ) (147.7 ) (20.4 ) (20.4 ) Net amounts recognized at December 31 ($149.2 ) ($148.4 ) ($16.5 ) ($21.9 ) Amounts recognized in Regulatory Assets consist of (refer to Note 2 for details): Net actuarial loss $218.9 $233.6 $18.7 $25.4 Prior service credit (2.1 ) (2.3 ) — — $216.8 $231.3 $18.7 $25.4 Defined Benefit Pension Plans OPEB Plans WPL 2017 2016 2017 2016 Change in benefit obligation: Net benefit obligation at January 1 $529.2 $505.9 $88.9 $89.7 Service cost 4.9 4.9 1.9 2.0 Interest cost 21.8 22.3 3.4 3.8 Plan participants’ contributions — — 1.4 1.2 Actuarial loss 38.3 25.7 4.1 0.5 Gross benefits paid (34.4 ) (29.6 ) (9.3 ) (8.3 ) Net benefit obligation at December 31 559.8 529.2 90.4 88.9 Change in plan assets: Fair value of plan assets at January 1 389.7 386.8 18.6 18.7 Actual return on plan assets 59.6 32.4 1.2 1.2 Employer contributions 0.1 0.1 6.8 5.8 Plan participants’ contributions — — 1.4 1.2 Gross benefits paid (34.4 ) (29.6 ) (9.3 ) (8.3 ) Fair value of plan assets at December 31 415.0 389.7 18.7 18.6 Under funded status at December 31 ($144.8 ) ($139.5 ) ($71.7 ) ($70.3 ) Defined Benefit Pension Plans OPEB Plans WPL 2017 2016 2017 2016 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $2.9 $2.7 Current liabilities (0.1 ) (0.1 ) (6.8 ) (6.4 ) Pension and other benefit obligations (144.7 ) (139.4 ) (67.8 ) (66.6 ) Net amounts recognized at December 31 ($144.8 ) ($139.5 ) ($71.7 ) ($70.3 ) Amounts recognized in Regulatory Assets consist of (refer to Note 2 for details): Net actuarial loss $224.7 $236.1 $23.6 $21.5 Prior service credit (1.5 ) (1.4 ) (1.3 ) (1.5 ) $223.2 $234.7 $22.3 $20.0 Included in the following tables are accumulated benefit obligations, aggregate amounts applicable to defined benefit pension and OPEB plans with accumulated benefit obligations in excess of plan assets, as well as defined benefit pension plans with projected benefit obligations in excess of plan assets as of the December 31 measurement date (in millions): Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2017 2016 Accumulated benefit obligations $1,269.0 $1,201.5 $222.3 $220.1 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,269.0 1,201.5 222.3 220.1 Fair value of plan assets 950.7 895.7 111.1 105.8 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,303.1 1,244.3 N/A N/A Fair value of plan assets 950.7 895.7 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2017 2016 2017 2016 Accumulated benefit obligations $573.1 $546.7 $89.4 $90.1 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 573.1 546.7 89.4 90.1 Fair value of plan assets 443.7 422.0 72.9 68.2 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 592.9 570.4 N/A N/A Fair value of plan assets 443.7 422.0 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2017 2016 2017 2016 Accumulated benefit obligations $548.1 $513.2 $90.4 $88.9 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 548.1 513.2 90.4 88.9 Fair value of plan assets 415.0 389.7 18.7 18.6 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 559.8 529.2 N/A N/A Fair value of plan assets 415.0 389.7 N/A N/A In addition to the amounts recognized in regulatory assets in the above tables for IPL and WPL, regulatory assets were recognized for amounts associated with Corporate Services employees participating in other Alliant Energy sponsored benefit plans that were allocated to IPL and WPL at December 31 as follows (in millions): IPL WPL 2017 2016 2017 2016 Regulatory assets $38.9 $37.3 $28.1 $30.0 Estimated Future Employer Contributions and Benefit Payments - Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2018 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $6.3 $4.4 $0.3 OPEB plans 9.0 2.0 6.8 (a) Alliant Energy sponsors several non-qualified defined benefit pension plans that cover certain current and former key employees of IPL and WPL. Alliant Energy allocates pension costs to IPL and WPL for these plans. In addition, IPL and WPL amounts reflect funding for their non-bargaining employees who are participants in the Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans. Expected benefit payments for the qualified and non-qualified defined benefit plans, which reflect expected future service, as appropriate, are as follows (in millions): Alliant Energy 2018 2019 2020 2021 2022 2023 - 2027 Defined benefit pension benefits $72.2 $73.9 $76.4 $77.1 $92.9 $398.2 OPEB 18.2 18.4 17.7 17.5 17.2 80.1 $90.4 $92.3 $94.1 $94.6 $110.1 $478.3 IPL 2018 2019 2020 2021 2022 2023 - 2027 Defined benefit pension benefits $33.9 $33.8 $36.5 $36.6 $37.6 $188.4 OPEB 7.1 7.1 7.2 7.1 7.0 32.6 $41.0 $40.9 $43.7 $43.7 $44.6 $221.0 WPL 2018 2019 2020 2021 2022 2023 - 2027 Defined benefit pension benefits $31.4 $32.0 $32.4 $32.4 $32.7 $167.8 OPEB 8.3 8.3 7.5 7.3 7.0 31.8 $39.7 $40.3 $39.9 $39.7 $39.7 $199.6 Investment Strategy for Plan Assets - Investment strategies for defined benefit pension and OPEB plan assets combine preservation of principal and prudent risk-taking to protect the integrity of plan assets, in order to meet the obligations to plan participants while minimizing benefit costs over the long term. Investment risk of plan assets is mitigated through diversification, including broad U.S. equity, international equity and fixed income exposure, and global asset and risk parity strategies. Global asset and risk parity strategies include investments in global equity, global debt, commodities and currencies. Defined Benefit Pension Plan Assets - The asset mix of defined benefit pension plans is governed by allocation targets. Historical performance results and future expectations suggest that equity securities will provide higher total investment returns than fixed income securities over a long-term investment horizon. Consistent with the goals of meeting obligations to plan participants and minimizing benefit costs over the long-term, the defined benefit pension plans have a long-term investment posture more heavily weighted toward equity holdings. The asset allocation is monitored regularly and appropriate steps are taken as needed to rebalance the assets within the prescribed ranges. An overlay management service is also used to help maintain target allocations and meet liquidity needs. The overlay manager is authorized to use derivative financial instruments to facilitate this service. For separately managed accounts, prohibited investments include, but may not be limited to, direct ownership of real estate, margin trading, oil and gas limited partnerships and securities of the managers’ firms or affiliate firms. At December 31, 2017 , the current target ranges and actual allocations for the defined benefit pension plan assets were as follows: Target Range Actual Allocation Allocation Cash and equivalents 0 % - 5% 3% Equity securities - U.S. 11 % - 41% 24% Equity securities - international 14 % - 34% 23% Global asset securities 5 % - 15% 10% Risk parity securities 5 % - 15% 10% Fixed income securities 20 % - 40% 30% Other Postretirement Benefits Plan Assets - OPEB plan assets are comprised of specific assets within certain defined benefit pension plans (401(h) assets) as well as assets held in VEBA trusts. The investment strategy of the Corporate Services 401(h) assets mirrors those of the defined benefit pension plans, which are discussed above. For VEBA trusts with assets greater than $5 million and the WPL 401(h) assets, the mix among asset classes is governed by allocation targets. The asset allocation is monitored regularly and appropriate steps are taken as needed to rebalance the assets within the prescribed ranges. Mutual funds are used to achieve the desired diversification. At December 31, 2017 , the current target ranges and actual allocations for VEBA trusts with assets greater than $5 million and the WPL 401(h) assets were as follows: Target Range Actual Allocation Allocation Cash and equivalents 0 % - 5% 1% Equity securities - U.S. 0 % - 50% 26% Equity securities - international 0 % - 34% 10% Fixed income securities 20 % - 100% 63% Fair Value Measurements - Fair value measurement accounting establishes three levels of fair value hierarchy that prioritize the inputs to valuation techniques used to measure fair value. Refer to Note 14 for discussion of levels within the fair value hierarchy. Level 1 items include investments in securities held in registered investment companies, treasury bills and directly held equity securities, which are valued at the closing price reported in the active market in which the securities are traded. Level 2 items include fixed income securities consisting of corporate and government bonds and agency obligations, which are valued at the closing price reported in the active market for similar assets in which the individual securities are traded or based on yields currently available on comparable securities of issuers with similar credit ratings. Certain investments that are measured at fair value using the net asset value practical expedient have not been classified in the fair value hierarchy. These fair value amounts are included in the tables below to reconcile the fair value hierarchy to the respective total plan assets. At December 31, the fair values of qualified and non-qualified defined benefit pension plan assets were as follows (in millions): 2017 2016 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $28.2 $4.5 $23.7 $— $30.4 $5.0 $25.4 $— Equity securities - U.S. 158.3 158.3 — — 183.6 183.6 — — Equity securities - international 137.5 137.5 — — 97.4 97.4 — — Global asset securities 49.4 49.4 — — 53.0 53.0 — — Fixed income securities 135.9 55.8 80.1 — 125.4 53.6 71.8 — Total assets in fair value hierarchy 509.3 $405.5 $103.8 $— 489.8 $392.6 $97.2 $— Assets measured at net asset value 441.1 405.9 Accrued investment income 1.0 1.1 Due to brokers, net (pending trades with brokers) (0.7 ) (1.1 ) Total pension plan assets $950.7 $895.7 2017 2016 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $13.2 $2.2 $11.0 $— $14.4 $2.4 $12.0 $— Equity securities - U.S. 73.9 73.9 — — 86.5 86.5 — — Equity securities - international 64.2 64.2 — — 45.9 45.9 — — Global asset securities 23.0 23.0 — — 24.9 24.9 — — Fixed income securities 63.4 26.0 37.4 — 59.1 25.3 33.8 — Total assets in fair value hierarchy 237.7 $189.3 $48.4 $— 230.8 $185.0 $45.8 $— Assets measured at net asset value 205.8 191.2 Accrued investment income 0.5 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.5 ) Total pension plan assets $443.7 $422.0 2017 2016 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $12.3 $2.0 $10.3 $— $13.3 $2.2 $11.1 $— Equity securities - U.S. 69.1 69.1 — — 79.9 79.9 — — Equity securities - international 60.0 60.0 — — 42.4 42.4 — — Global asset securities 21.6 21.6 — — 23.0 23.0 — — Fixed income securities 59.3 24.3 35.0 — 54.5 23.3 31.2 — Total assets in fair value hierarchy 222.3 $177.0 $45.3 $— 213.1 $170.8 $42.3 $— Assets measured at net asset value 192.5 176.6 Accrued investment income 0.5 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.5 ) Total pension plan assets $415.0 $389.7 At December 31, the fair values of OPEB plan assets were as follows (in millions): 2017 2016 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $1.2 $0.7 $0.5 $— $3.5 $2.0 $1.5 $— Equity securities - U.S. 27.9 27.9 — — 22.5 22.5 — — Equity securities - international 11.4 11.4 — — 13.5 13.5 — — Global asset securities 0.4 0.4 — — 16.5 16.5 — — Fixed income securities 66.6 66.0 0.6 — 46.8 46.2 0.6 — Total assets in fair value hierarchy 107.5 $106.4 $1.1 $— 102.8 $100.7 $2.1 $— Assets measured at net asset value 3.6 3.0 Total OPEB plan assets $111.1 $105.8 2017 2016 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.3 $0.3 $— $— $0.8 $0.8 $— $— Equity securities - U.S. 22.3 22.3 — — 17.0 17.0 — — Equity securities - international 7.5 7.5 — — 11.0 11.0 — — Global asset securities — — — — 7.0 7.0 — — Fixed income securities 42.8 42.8 — — 32.4 32.4 — — Total OPEB plan assets $72.9 $72.9 $— $— $68.2 $68.2 $— $— 2017 2016 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.6 $0.3 $0.3 $— $2.0 $0.7 $1.3 $— Global asset securities — — — — 5.5 5.5 — — Fixed income securities 18.1 18.1 — — 11.1 11.1 — — Total OPEB plan assets $18.7 $18.4 $0.3 $— $18.6 $17.3 $1.3 $— For the various defined benefit pension and OPEB plans, Alliant Energy common stock represented less than 1% of assets directly held in the plans at December 31, 2017 and 2016 . 401(k) Savings Plans - A significant number of employees participate in defined contribution retirement plans (401(k) savings plans). Alliant Energy common stock directly held by participants represented 11.5% and 12.6% of total assets in the 401(k) savings plans at December 31, 2017 and 2016 , respectively. Costs related to the 401(k) savings plans, which are partially based on the participants’ contributions and include allocated costs associated with Corporate Services employees for IPL and WPL, were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 401(k) costs $24.8 $23.6 $24.9 $12.8 $12.0 $12.7 $11.1 $10.7 $11.2 (b) Equity-based Compensation Plans - In 2015, Alliant Energy’s shareowners approved the Amended and Restated OIP, which permits the grant of shares of Alliant Energy common stock, restricted stock, restricted stock units, performance shares, performance units, and other stock-based or cash-based awards to key employees. At December 31, 2017 , performance shares, performance-contingent restricted stock and restricted stock units (performance- and time-vesting) were outstanding under the Amended and Restated OIP, and 7.1 million shares of Alliant Energy’s common stock remained available for grants under the Amended and Restated OIP. Alliant Energy satisfies share payouts related to equity awards under the Amended and Restated OIP through the issuance of new shares of its common stock. Alliant Energy also has the DLIP, which permits the grant of cash-based long-term awards, including performance units, restricted cash awards and restricted units, to certain key employees. At December 31, 2017 , performance units, performance-contingent cash awards and restricted units (performance- and time-vesting) were outstanding under the DLIP. There is no limit to the number of grants that can be made under the DLIP and Alliant Energy satisfies all payouts under the DLIP through cash payments. Nonvested awards generally do not have non-forfeitable rights to dividends or dividend equivalents when dividends are paid to common shareowners. A summary of compensation expense, including amounts allocated to IPL and WPL, and the related income tax benefits recognized for share-based compensation awards was as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Compensation expense $15.1 $18.0 $10.7 $8.3 $9.5 $5.7 $6.4 $7.9 $4.7 Income tax benefits 6.2 7.4 4.4 3.4 4.0 2.4 2.6 3.2 1.9 As of December 31, 2017 , Alliant Energy’s, IPL’s and WPL’s total unrecognized compensation cost related to share-based compensation awards was $5.6 million , $3.1 million and $2.2 million , respectively, which is expected to be recognized over a weighted average period of between one and two years. Share-based compensation expense is recognized on a straight-line basis over the requisite service periods and is primarily recorded in “Other operation and maintenance” in the income statements. Performance Shares and Performance Units - Payouts of performance shares under the Amended and Restated OIP and performance units under the DLIP to key employees are contingent upon achievement over three -year periods of specified performance criteria, which currently include metrics of total shareowner return relative to an investor-owned utility peer group. Performance shares can be paid out in shares of Alliant Energy’s common stock, cash or a combination of cash and stock. Performance units must be paid out in cash. Alliant Energy assumes it will make future payouts of its performance shares and performance units in cash; therefore, performance shares and performance units are accounted for as liability awards. A summary of the performance shares and performance units activity, with amounts representing the target number of awards, was as follows: Performance Shares Performance Units 2017 2016 2015 2017 2016 2015 Nonvested awards, January 1 257,599 288,430 288,848 93,320 116,412 127,330 Granted 65,350 68,585 90,806 21,558 23,918 35,674 Vested (99,438 ) (98,186 ) (91,224 ) (37,395 ) (42,760 ) (45,690 ) Forfeited — (1,230 ) — (5,746 ) (4,250 ) (902 ) Nonvested awards, December 31 223,511 257,599 288,430 71,737 93,320 116,412 Granted Awards - Each performance share’s value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the performance period. For performance units granted in 2017 and 2016, the value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the performance period. For performance units granted in 2015, the value is based on the closing market price of one share of Alliant Energy’s common stock on the grant date of the award. The actual payout for performance shares and performance units is dependent upon actual performance and may range from zero to 200% of the target number of awards. Compensation expense for performance shares and performance units is recorded ratably over the performance period based on the fair value of the awards at each reporting period. Vested Awards - Certain performance shares and performance units vested, resulting in payouts (a combination of cash and common stock for the performance shares and cash only for the performance units) as follows: Performance Shares Performance Units 2017 2016 2015 2017 2016 2015 2014 Grant 2013 Grant 2012 Grant 2014 Grant 2013 Grant 2012 Grant Performance awards vested 99,438 98,186 91,224 37,395 42,760 45,690 Percentage of target number of performance awards 147.5% 165.0% 167.5% 147.5% 165.0% 167.5% Aggregate payout value (in millions) $5.6 $5.1 $5.1 $1.5 $1.7 $1.6 Payout - cash (in millions) $5.1 $2.9 $3.2 $1.5 $1.7 $1.6 Payout - common stock shares issued 5,185 22,408 21,950 N/A N/A N/A Fair Value of Awards - Information related to fair values of nonvested performance shares and performance units at December 31, 2017 , by year of grant, were as follows: Performance Shares Performance Units 2017 Grant 2016 Grant 2015 Grant 2017 Grant 2016 Grant 2015 Grant Nonvested awards at target 65,350 67,355 90,806 18,600 21,227 31,910 Alliant Energy common stock closing price on December 29, 2017 $42.61 $42.61 $42.61 $42.61 $42.61 N/A Alliant Energy common stock closing price on grant date N/A N/A N/A N/A N/A $32.55 Estimated payout percentage based on performance criteria 105 % 150 % 138 % 105 % 150 % 138 % Fair values of each nonvested award $44.74 $63.92 $58.80 $44.74 $63.92 $44.92 Performance Restricted Stock Units and Performance Restricted Units - Alliant Energy granted new types of share-based compensation awards to key employees beginning in 2016 referred to as performance restricted stock units under the Amended and Restated OIP, and performance restricted units and key employee performance restricted units under the DLIP. Payouts of these units are based on the achievement of certain performance targets (currently specified growth of consolidated income from continuing operations) during a three -year performance period. The actual number of units that will be paid out upon vesting is dependent upon actual performance and may range from zero to 200% of the target number of units. If performance targets are not met during the performance period, these units are forfeited. As of December 31, 2017 , the amount of nonvested performance restricted units and key employee performance restricted units was not material. Performance Restricted Stock Units - Performance restricted stock units generally must be paid out in shares and are accounted for as equity awards. Each performance restricted stock unit’s value is based on the closing market price of one share of Alliant Energy’s common stock on the grant date of the award. Compensation expense is recorded ratably over the performance period based on a probability assessment of payouts for the awards at each reporting period. A summary of the performance restricted stock units activity, with amounts representing the target number of units, was as follows: 2017 2016 Units Weighted Average Grant Date Fair Value Units Weighted Average Grant Date Fair Value Nonvested units, January 1 67,355 $33.96 — $— Granted 65,350 39.12 68,585 33.96 Forfeited — — (1,230 ) 33.90 Nonvested units, December 31 132,705 36.50 67,355 33.96 Restricted Stock Units and Restricted Units - Alliant Energy granted new types of share-based compensation awards to key employees beginning in 2016 referred to as restricted stock units under the Amended and Restated OIP and restricted units under the DLIP. Payouts of these units are based on the expiration of a three -year time-vesting period. Each restricted stock unit’s value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the time-vesting period. Compensation expense is recorded ratably over the performance period based on the fair value of the awards at each reporting period. Restricted stock units can be paid out in shares of Alliant Energy common stock, cash or a combination of cash and stock. Alliant Energy assumes it will make future payouts of its restricted stock units in cash; therefore, restricted stock units are accounted for as liability awards. As of December 31, 2017 , the amount of nonvested restricted units was not material. A summary of the restricted stock units activity was as follows: 2017 2016 Nonvested units, January 1 57,736 — Granted 56,013 58,790 Forfeited — (1,054 ) Nonvested units, December 31 113,749 57,736 Performance-Contingent Restricted
WPL [Member]
Benefit Plans	BENEFIT PLANS (a) Pension and Other Postretirement Benefits Plans - Retirement benefits are provided to substantially all employees through various qualified and non-qualified non-contributory defined benefit pension plans (currently closed to new hires), and/or through defined contribution plans (including 401(k) savings plans). Benefits of the non-contributory defined benefit pension plans are based on the plan participant’s years of service, age and compensation. Benefits of the defined contribution plans are based on the plan participant’s years of service, age, compensation and contributions. Certain defined benefit postretirement health care and life benefits are provided to eligible retirees. In general, the retiree health care plans consist of fixed benefit subsidy structures and the retiree life insurance plans are non-contributory. IPL and WPL account for their participation in Alliant Energy and Corporate Services sponsored plans as multiple-employer plans. In IPL’s and WPL’s tables below, the defined benefit pension plan amounts represent those respective amounts for their bargaining unit employees covered under the qualified plans that they sponsor, as well as amounts directly assigned to them related to their current and former non-bargaining employees who are participants in the Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans. In IPL’s and WPL’s tables below, the OPEB plan amounts represent respective amounts for their employees, as well as amounts directly assigned to them related to their current and former non-bargaining employees who are participants in the Corporate Services sponsored OPEB plan. Assumptions - The assumptions for defined benefit pension and OPEB plans at the measurement date of December 31 were as follows: Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2015 2017 2016 2015 Discount rate for benefit obligations 3.66% 4.19% 4.47% 3.53% 3.98% 4.30% Discount rate for net periodic cost 4.19% 4.47% 4.18% 3.98% 4.30% 3.97% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.80% 6.30% 6.20% Rate of compensation increase 3.65 % - 4.50% 3.65 % - 4.50% 3.65 % - 4.50% N/A N/A N/A Medical cost trend on covered charges: Initial trend rate (end of year) N/A N/A N/A 6.75% 7.00% 7.25% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans IPL 2017 2016 2015 2017 2016 2015 Discount rate for benefit obligations 3.68% 4.22% 4.50% 3.51% 3.95% 4.28% Discount rate for net periodic cost 4.22% 4.50% 4.20% 3.95% 4.28% 3.94% Expected rate of return on plan assets 7.60% 7.60% 7.60% 6.20% 6.60% 6.60% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Medical cost trend on covered charges: Initial trend rate (end of year) N/A N/A N/A 6.75% 7.00% 7.25% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans WPL 2017 2016 2015 2017 2016 2015 Discount rate for benefit obligations 3.69% 4.23% 4.51% 3.51% 3.96% 4.28% Discount rate for net periodic cost 4.23% 4.51% 4.20% 3.96% 4.28% 3.96% Expected rate of return on plan assets 7.60% 7.60% 7.60% 3.50% 4.70% 4.60% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Medical cost trend on covered charges: Initial trend rate (end of year) N/A N/A N/A 6.75% 7.00% 7.25% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Expected rate of return on plan assets - The expected rate of return on plan assets is based on projected asset class returns using target allocations. A forward-looking building blocks approach is used, and historical returns, survey information and capital market information are analyzed to support the expected rate of return on plan assets assumption. Refer to “Investment Strategy for Plan Assets” below for additional information related to investment strategy and mix of assets for the pension and OPEB plans. Life Expectancy - The life expectancy assumption is used in determining the benefit obligation and net periodic benefit cost for defined benefit pension and OPEB plans. This assumption was updated to utilize mortality tables that were released in 2014 by the Society of Actuaries and updated in 2015 and 2016. Net Periodic Benefit Costs (Credits) - The components of net periodic benefit costs (credits) for sponsored defined benefit pension and OPEB plans are included in the tables below (in millions). Net periodic benefit costs are primarily included in “Other operation and maintenance” in the income statements. Alliant Energy Defined Benefit Pension Plans OPEB Plans 2017 2016 2015 2017 2016 2015 Service cost $12.5 $12.6 $15.9 $5.0 $5.3 $5.5 Interest cost 51.0 53.0 53.6 8.6 9.4 9.1 Expected return on plan assets (a) (65.5 ) (65.5 ) (75.0 ) (6.1 ) (6.1 ) (8.4 ) Amortization of prior service credit (b) (0.4 ) (0.3 ) (0.2 ) (0.2 ) (4.1 ) (11.3 ) Amortization of actuarial loss (c) 37.6 37.4 35.4 3.8 4.7 4.8 Additional benefit costs — — 0.5 — — — Settlement losses (d) 0.9 — — — — — $36.1 $37.2 $30.2 $11.1 $9.2 ($0.3 ) IPL Defined Benefit Pension Plans OPEB Plans 2017 2016 2015 2017 2016 2015 Service cost $7.3 $7.5 $8.8 $2.1 $2.3 $2.4 Interest cost 23.5 24.5 25.0 3.5 3.8 3.8 Expected return on plan assets (a) (30.8 ) (30.9 ) (35.8 ) (4.3 ) (4.3 ) (5.7 ) Amortization of prior service credit (b) (0.2 ) (0.2 ) (0.1 ) — (2.6 ) (6.1 ) Amortization of actuarial loss (c) 16.1 16.5 15.3 2.0 2.6 2.3 $15.9 $17.4 $13.2 $3.3 $1.8 ($3.3 ) WPL Defined Benefit Pension Plans OPEB Plans 2017 2016 2015 2017 2016 2015 Service cost $4.9 $4.9 $5.8 $1.9 $2.0 $2.1 Interest cost 21.8 22.3 22.6 3.4 3.8 3.7 Expected return on plan assets (a) (28.5 ) (28.3 ) (32.4 ) (0.8 ) (0.8 ) (1.5 ) Amortization of prior service cost (credit) (b) 0.1 0.2 0.2 (0.2 ) (0.9 ) (3.5 ) Amortization of actuarial loss (c) 18.5 17.6 16.8 1.6 1.8 2.2 Additional benefit costs — — 0.5 — — — $16.8 $16.7 $13.5 $5.9 $5.9 $3.0 (a) The expected return on plan assets is based on the expected rate of return on plan assets and the fair value approach to the market-related value of plan assets. (b) Unrecognized prior service costs (credits) for the OPEB plans are amortized over the average future service period to full eligibility of the participants of each plan. (c) Unrecognized net actuarial gains or losses in excess of 10% of the greater of the plans’ benefit obligations or assets are amortized over the average future service lives of plan participants, except for the Alliant Energy Cash Balance Pension Plan where gains or losses outside the 10% threshold are amortized over the time period the participants are expected to receive benefits. (d) Settlement losses related to payments made to retired executives of Alliant Energy. The estimated amortization from “Regulatory assets” and “Regulatory liabilities” on the balance sheets and AOCL on Alliant Energy’s balance sheet into net periodic benefit cost in 2018 is as follows (in millions): Alliant Energy IPL WPL Defined Benefit Defined Benefit Defined Benefit Pension Plans OPEB Plans Pension Plans OPEB Plans Pension Plans OPEB Plans Actuarial loss $35.2 $3.4 $15.0 $1.2 $17.2 $2.0 Prior service credit (0.7 ) (0.2 ) (0.2 ) — (0.2 ) (0.2 ) $34.5 $3.2 $14.8 $1.2 $17.0 $1.8 Benefit Plan Assets and Obligations - A reconciliation of the funded status of qualified and non-qualified defined benefit pension and OPEB plans to the amounts recognized on the balance sheets at December 31 was as follows (in millions): Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2017 2016 Change in benefit obligation: Net benefit obligation at January 1 $1,244.3 $1,206.3 $220.1 $221.4 Service cost 12.5 12.6 5.0 5.3 Interest cost 51.0 53.0 8.6 9.4 Plan participants’ contributions — — 2.9 2.4 Actuarial (gain) loss 83.6 48.3 5.4 (0.3 ) Gross benefits paid (88.3 ) (75.9 ) (19.7 ) (18.1 ) Net benefit obligation at December 31 1,303.1 1,244.3 222.3 220.1 Change in plan assets: Fair value of plan assets at January 1 895.7 895.0 105.8 106.9 Actual return on plan assets 136.7 74.3 12.9 8.2 Employer contributions 6.6 2.3 9.2 6.4 Plan participants’ contributions — — 2.9 2.4 Gross benefits paid (88.3 ) (75.9 ) (19.7 ) (18.1 ) Fair value of plan assets at December 31 950.7 895.7 111.1 105.8 Under funded status at December 31 ($352.4 ) ($348.6 ) ($111.2 ) ($114.3 ) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2017 2016 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $8.8 $3.2 Current liabilities (2.2 ) (6.5 ) (9.1 ) (8.6 ) Pension and other benefit obligations (350.2 ) (342.1 ) (110.9 ) (108.9 ) Net amounts recognized at December 31 ($352.4 ) ($348.6 ) ($111.2 ) ($114.3 ) Amounts recognized in Regulatory Assets (refer to Note 2 for details) and AOCL (refer to Alliant Energy’s common equity statements for details) consist of: Net actuarial loss $509.1 $535.1 $47.4 $52.6 Prior service credit (6.5 ) (6.9 ) (1.3 ) (1.5 ) $502.6 $528.2 $46.1 $51.1 Defined Benefit Pension Plans OPEB Plans IPL 2017 2016 2017 2016 Change in benefit obligation: Net benefit obligation at January 1 $570.4 $556.1 $90.1 $91.3 Service cost 7.3 7.5 2.1 2.3 Interest cost 23.5 24.5 3.5 3.8 Plan participants’ contributions — — 1.0 0.9 Actuarial (gain) loss 34.9 19.1 (0.1 ) (0.7 ) Gross benefits paid (43.2 ) (36.8 ) (7.2 ) (7.5 ) Net benefit obligation at December 31 592.9 570.4 89.4 90.1 Change in plan assets: Fair value of plan assets at January 1 422.0 422.7 68.2 69.2 Actual return on plan assets 64.3 35.3 8.9 5.3 Employer contributions 0.6 0.8 2.0 0.3 Plan participants’ contributions — — 1.0 0.9 Gross benefits paid (43.2 ) (36.8 ) (7.2 ) (7.5 ) Fair value of plan assets at December 31 443.7 422.0 72.9 68.2 Under funded status at December 31 ($149.2 ) ($148.4 ) ($16.5 ) ($21.9 ) Defined Benefit Pension Plans OPEB Plans IPL 2017 2016 2017 2016 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $5.9 $0.4 Current liabilities (0.5 ) (0.7 ) (2.0 ) (1.9 ) Pension and other benefit obligations (148.7 ) (147.7 ) (20.4 ) (20.4 ) Net amounts recognized at December 31 ($149.2 ) ($148.4 ) ($16.5 ) ($21.9 ) Amounts recognized in Regulatory Assets consist of (refer to Note 2 for details): Net actuarial loss $218.9 $233.6 $18.7 $25.4 Prior service credit (2.1 ) (2.3 ) — — $216.8 $231.3 $18.7 $25.4 Defined Benefit Pension Plans OPEB Plans WPL 2017 2016 2017 2016 Change in benefit obligation: Net benefit obligation at January 1 $529.2 $505.9 $88.9 $89.7 Service cost 4.9 4.9 1.9 2.0 Interest cost 21.8 22.3 3.4 3.8 Plan participants’ contributions — — 1.4 1.2 Actuarial loss 38.3 25.7 4.1 0.5 Gross benefits paid (34.4 ) (29.6 ) (9.3 ) (8.3 ) Net benefit obligation at December 31 559.8 529.2 90.4 88.9 Change in plan assets: Fair value of plan assets at January 1 389.7 386.8 18.6 18.7 Actual return on plan assets 59.6 32.4 1.2 1.2 Employer contributions 0.1 0.1 6.8 5.8 Plan participants’ contributions — — 1.4 1.2 Gross benefits paid (34.4 ) (29.6 ) (9.3 ) (8.3 ) Fair value of plan assets at December 31 415.0 389.7 18.7 18.6 Under funded status at December 31 ($144.8 ) ($139.5 ) ($71.7 ) ($70.3 ) Defined Benefit Pension Plans OPEB Plans WPL 2017 2016 2017 2016 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $2.9 $2.7 Current liabilities (0.1 ) (0.1 ) (6.8 ) (6.4 ) Pension and other benefit obligations (144.7 ) (139.4 ) (67.8 ) (66.6 ) Net amounts recognized at December 31 ($144.8 ) ($139.5 ) ($71.7 ) ($70.3 ) Amounts recognized in Regulatory Assets consist of (refer to Note 2 for details): Net actuarial loss $224.7 $236.1 $23.6 $21.5 Prior service credit (1.5 ) (1.4 ) (1.3 ) (1.5 ) $223.2 $234.7 $22.3 $20.0 Included in the following tables are accumulated benefit obligations, aggregate amounts applicable to defined benefit pension and OPEB plans with accumulated benefit obligations in excess of plan assets, as well as defined benefit pension plans with projected benefit obligations in excess of plan assets as of the December 31 measurement date (in millions): Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2017 2016 Accumulated benefit obligations $1,269.0 $1,201.5 $222.3 $220.1 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,269.0 1,201.5 222.3 220.1 Fair value of plan assets 950.7 895.7 111.1 105.8 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,303.1 1,244.3 N/A N/A Fair value of plan assets 950.7 895.7 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2017 2016 2017 2016 Accumulated benefit obligations $573.1 $546.7 $89.4 $90.1 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 573.1 546.7 89.4 90.1 Fair value of plan assets 443.7 422.0 72.9 68.2 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 592.9 570.4 N/A N/A Fair value of plan assets 443.7 422.0 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2017 2016 2017 2016 Accumulated benefit obligations $548.1 $513.2 $90.4 $88.9 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 548.1 513.2 90.4 88.9 Fair value of plan assets 415.0 389.7 18.7 18.6 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 559.8 529.2 N/A N/A Fair value of plan assets 415.0 389.7 N/A N/A In addition to the amounts recognized in regulatory assets in the above tables for IPL and WPL, regulatory assets were recognized for amounts associated with Corporate Services employees participating in other Alliant Energy sponsored benefit plans that were allocated to IPL and WPL at December 31 as follows (in millions): IPL WPL 2017 2016 2017 2016 Regulatory assets $38.9 $37.3 $28.1 $30.0 Estimated Future Employer Contributions and Benefit Payments - Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2018 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $6.3 $4.4 $0.3 OPEB plans 9.0 2.0 6.8 (a) Alliant Energy sponsors several non-qualified defined benefit pension plans that cover certain current and former key employees of IPL and WPL. Alliant Energy allocates pension costs to IPL and WPL for these plans. In addition, IPL and WPL amounts reflect funding for their non-bargaining employees who are participants in the Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans. Expected benefit payments for the qualified and non-qualified defined benefit plans, which reflect expected future service, as appropriate, are as follows (in millions): Alliant Energy 2018 2019 2020 2021 2022 2023 - 2027 Defined benefit pension benefits $72.2 $73.9 $76.4 $77.1 $92.9 $398.2 OPEB 18.2 18.4 17.7 17.5 17.2 80.1 $90.4 $92.3 $94.1 $94.6 $110.1 $478.3 IPL 2018 2019 2020 2021 2022 2023 - 2027 Defined benefit pension benefits $33.9 $33.8 $36.5 $36.6 $37.6 $188.4 OPEB 7.1 7.1 7.2 7.1 7.0 32.6 $41.0 $40.9 $43.7 $43.7 $44.6 $221.0 WPL 2018 2019 2020 2021 2022 2023 - 2027 Defined benefit pension benefits $31.4 $32.0 $32.4 $32.4 $32.7 $167.8 OPEB 8.3 8.3 7.5 7.3 7.0 31.8 $39.7 $40.3 $39.9 $39.7 $39.7 $199.6 Investment Strategy for Plan Assets - Investment strategies for defined benefit pension and OPEB plan assets combine preservation of principal and prudent risk-taking to protect the integrity of plan assets, in order to meet the obligations to plan participants while minimizing benefit costs over the long term. Investment risk of plan assets is mitigated through diversification, including broad U.S. equity, international equity and fixed income exposure, and global asset and risk parity strategies. Global asset and risk parity strategies include investments in global equity, global debt, commodities and currencies. Defined Benefit Pension Plan Assets - The asset mix of defined benefit pension plans is governed by allocation targets. Historical performance results and future expectations suggest that equity securities will provide higher total investment returns than fixed income securities over a long-term investment horizon. Consistent with the goals of meeting obligations to plan participants and minimizing benefit costs over the long-term, the defined benefit pension plans have a long-term investment posture more heavily weighted toward equity holdings. The asset allocation is monitored regularly and appropriate steps are taken as needed to rebalance the assets within the prescribed ranges. An overlay management service is also used to help maintain target allocations and meet liquidity needs. The overlay manager is authorized to use derivative financial instruments to facilitate this service. For separately managed accounts, prohibited investments include, but may not be limited to, direct ownership of real estate, margin trading, oil and gas limited partnerships and securities of the managers’ firms or affiliate firms. At December 31, 2017 , the current target ranges and actual allocations for the defined benefit pension plan assets were as follows: Target Range Actual Allocation Allocation Cash and equivalents 0 % - 5% 3% Equity securities - U.S. 11 % - 41% 24% Equity securities - international 14 % - 34% 23% Global asset securities 5 % - 15% 10% Risk parity securities 5 % - 15% 10% Fixed income securities 20 % - 40% 30% Other Postretirement Benefits Plan Assets - OPEB plan assets are comprised of specific assets within certain defined benefit pension plans (401(h) assets) as well as assets held in VEBA trusts. The investment strategy of the Corporate Services 401(h) assets mirrors those of the defined benefit pension plans, which are discussed above. For VEBA trusts with assets greater than $5 million and the WPL 401(h) assets, the mix among asset classes is governed by allocation targets. The asset allocation is monitored regularly and appropriate steps are taken as needed to rebalance the assets within the prescribed ranges. Mutual funds are used to achieve the desired diversification. At December 31, 2017 , the current target ranges and actual allocations for VEBA trusts with assets greater than $5 million and the WPL 401(h) assets were as follows: Target Range Actual Allocation Allocation Cash and equivalents 0 % - 5% 1% Equity securities - U.S. 0 % - 50% 26% Equity securities - international 0 % - 34% 10% Fixed income securities 20 % - 100% 63% Fair Value Measurements - Fair value measurement accounting establishes three levels of fair value hierarchy that prioritize the inputs to valuation techniques used to measure fair value. Refer to Note 14 for discussion of levels within the fair value hierarchy. Level 1 items include investments in securities held in registered investment companies, treasury bills and directly held equity securities, which are valued at the closing price reported in the active market in which the securities are traded. Level 2 items include fixed income securities consisting of corporate and government bonds and agency obligations, which are valued at the closing price reported in the active market for similar assets in which the individual securities are traded or based on yields currently available on comparable securities of issuers with similar credit ratings. Certain investments that are measured at fair value using the net asset value practical expedient have not been classified in the fair value hierarchy. These fair value amounts are included in the tables below to reconcile the fair value hierarchy to the respective total plan assets. At December 31, the fair values of qualified and non-qualified defined benefit pension plan assets were as follows (in millions): 2017 2016 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $28.2 $4.5 $23.7 $— $30.4 $5.0 $25.4 $— Equity securities - U.S. 158.3 158.3 — — 183.6 183.6 — — Equity securities - international 137.5 137.5 — — 97.4 97.4 — — Global asset securities 49.4 49.4 — — 53.0 53.0 — — Fixed income securities 135.9 55.8 80.1 — 125.4 53.6 71.8 — Total assets in fair value hierarchy 509.3 $405.5 $103.8 $— 489.8 $392.6 $97.2 $— Assets measured at net asset value 441.1 405.9 Accrued investment income 1.0 1.1 Due to brokers, net (pending trades with brokers) (0.7 ) (1.1 ) Total pension plan assets $950.7 $895.7 2017 2016 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $13.2 $2.2 $11.0 $— $14.4 $2.4 $12.0 $— Equity securities - U.S. 73.9 73.9 — — 86.5 86.5 — — Equity securities - international 64.2 64.2 — — 45.9 45.9 — — Global asset securities 23.0 23.0 — — 24.9 24.9 — — Fixed income securities 63.4 26.0 37.4 — 59.1 25.3 33.8 — Total assets in fair value hierarchy 237.7 $189.3 $48.4 $— 230.8 $185.0 $45.8 $— Assets measured at net asset value 205.8 191.2 Accrued investment income 0.5 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.5 ) Total pension plan assets $443.7 $422.0 2017 2016 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $12.3 $2.0 $10.3 $— $13.3 $2.2 $11.1 $— Equity securities - U.S. 69.1 69.1 — — 79.9 79.9 — — Equity securities - international 60.0 60.0 — — 42.4 42.4 — — Global asset securities 21.6 21.6 — — 23.0 23.0 — — Fixed income securities 59.3 24.3 35.0 — 54.5 23.3 31.2 — Total assets in fair value hierarchy 222.3 $177.0 $45.3 $— 213.1 $170.8 $42.3 $— Assets measured at net asset value 192.5 176.6 Accrued investment income 0.5 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.5 ) Total pension plan assets $415.0 $389.7 At December 31, the fair values of OPEB plan assets were as follows (in millions): 2017 2016 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $1.2 $0.7 $0.5 $— $3.5 $2.0 $1.5 $— Equity securities - U.S. 27.9 27.9 — — 22.5 22.5 — — Equity securities - international 11.4 11.4 — — 13.5 13.5 — — Global asset securities 0.4 0.4 — — 16.5 16.5 — — Fixed income securities 66.6 66.0 0.6 — 46.8 46.2 0.6 — Total assets in fair value hierarchy 107.5 $106.4 $1.1 $— 102.8 $100.7 $2.1 $— Assets measured at net asset value 3.6 3.0 Total OPEB plan assets $111.1 $105.8 2017 2016 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.3 $0.3 $— $— $0.8 $0.8 $— $— Equity securities - U.S. 22.3 22.3 — — 17.0 17.0 — — Equity securities - international 7.5 7.5 — — 11.0 11.0 — — Global asset securities — — — — 7.0 7.0 — — Fixed income securities 42.8 42.8 — — 32.4 32.4 — — Total OPEB plan assets $72.9 $72.9 $— $— $68.2 $68.2 $— $— 2017 2016 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.6 $0.3 $0.3 $— $2.0 $0.7 $1.3 $— Global asset securities — — — — 5.5 5.5 — — Fixed income securities 18.1 18.1 — — 11.1 11.1 — — Total OPEB plan assets $18.7 $18.4 $0.3 $— $18.6 $17.3 $1.3 $— For the various defined benefit pension and OPEB plans, Alliant Energy common stock represented less than 1% of assets directly held in the plans at December 31, 2017 and 2016 . 401(k) Savings Plans - A significant number of employees participate in defined contribution retirement plans (401(k) savings plans). Alliant Energy common stock directly held by participants represented 11.5% and 12.6% of total assets in the 401(k) savings plans at December 31, 2017 and 2016 , respectively. Costs related to the 401(k) savings plans, which are partially based on the participants’ contributions and include allocated costs associated with Corporate Services employees for IPL and WPL, were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 401(k) costs $24.8 $23.6 $24.9 $12.8 $12.0 $12.7 $11.1 $10.7 $11.2 (b) Equity-based Compensation Plans - In 2015, Alliant Energy’s shareowners approved the Amended and Restated OIP, which permits the grant of shares of Alliant Energy common stock, restricted stock, restricted stock units, performance shares, performance units, and other stock-based or cash-based awards to key employees. At December 31, 2017 , performance shares, performance-contingent restricted stock and restricted stock units (performance- and time-vesting) were outstanding under the Amended and Restated OIP, and 7.1 million shares of Alliant Energy’s common stock remained available for grants under the Amended and Restated OIP. Alliant Energy satisfies share payouts related to equity awards under the Amended and Restated OIP through the issuance of new shares of its common stock. Alliant Energy also has the DLIP, which permits the grant of cash-based long-term awards, including performance units, restricted cash awards and restricted units, to certain key employees. At December 31, 2017 , performance units, performance-contingent cash awards and restricted units (performance- and time-vesting) were outstanding under the DLIP. There is no limit to the number of grants that can be made under the DLIP and Alliant Energy satisfies all payouts under the DLIP through cash payments. Nonvested awards generally do not have non-forfeitable rights to dividends or dividend equivalents when dividends are paid to common shareowners. A summary of compensation expense, including amounts allocated to IPL and WPL, and the related income tax benefits recognized for share-based compensation awards was as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Compensation expense $15.1 $18.0 $10.7 $8.3 $9.5 $5.7 $6.4 $7.9 $4.7 Income tax benefits 6.2 7.4 4.4 3.4 4.0 2.4 2.6 3.2 1.9 As of December 31, 2017 , Alliant Energy’s, IPL’s and WPL’s total unrecognized compensation cost related to share-based compensation awards was $5.6 million , $3.1 million and $2.2 million , respectively, which is expected to be recognized over a weighted average period of between one and two years. Share-based compensation expense is recognized on a straight-line basis over the requisite service periods and is primarily recorded in “Other operation and maintenance” in the income statements. Performance Shares and Performance Units - Payouts of performance shares under the Amended and Restated OIP and performance units under the DLIP to key employees are contingent upon achievement over three -year periods of specified performance criteria, which currently include metrics of total shareowner return relative to an investor-owned utility peer group. Performance shares can be paid out in shares of Alliant Energy’s common stock, cash or a combination of cash and stock. Performance units must be paid out in cash. Alliant Energy assumes it will make future payouts of its performance shares and performance units in cash; therefore, performance shares and performance units are accounted for as liability awards. A summary of the performance shares and performance units activity, with amounts representing the target number of awards, was as follows: Performance Shares Performance Units 2017 2016 2015 2017 2016 2015 Nonvested awards, January 1 257,599 288,430 288,848 93,320 116,412 127,330 Granted 65,350 68,585 90,806 21,558 23,918 35,674 Vested (99,438 ) (98,186 ) (91,224 ) (37,395 ) (42,760 ) (45,690 ) Forfeited — (1,230 ) — (5,746 ) (4,250 ) (902 ) Nonvested awards, December 31 223,511 257,599 288,430 71,737 93,320 116,412 Granted Awards - Each performance share’s value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the performance period. For performance units granted in 2017 and 2016, the value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the performance period. For performance units granted in 2015, the value is based on the closing market price of one share of Alliant Energy’s common stock on the grant date of the award. The actual payout for performance shares and performance units is dependent upon actual performance and may range from zero to 200% of the target number of awards. Compensation expense for performance shares and performance units is recorded ratably over the performance period based on the fair value of the awards at each reporting period. Vested Awards - Certain performance shares and performance units vested, resulting in payouts (a combination of cash and common stock for the performance shares and cash only for the performance units) as follows: Performance Shares Performance Units 2017 2016 2015 2017 2016 2015 2014 Grant 2013 Grant 2012 Grant 2014 Grant 2013 Grant 2012 Grant Performance awards vested 99,438 98,186 91,224 37,395 42,760 45,690 Percentage of target number of performance awards 147.5% 165.0% 167.5% 147.5% 165.0% 167.5% Aggregate payout value (in millions) $5.6 $5.1 $5.1 $1.5 $1.7 $1.6 Payout - cash (in millions) $5.1 $2.9 $3.2 $1.5 $1.7 $1.6 Payout - common stock shares issued 5,185 22,408 21,950 N/A N/A N/A Fair Value of Awards - Information related to fair values of nonvested performance shares and performance units at December 31, 2017 , by year of grant, were as follows: Performance Shares Performance Units 2017 Grant 2016 Grant 2015 Grant 2017 Grant 2016 Grant 2015 Grant Nonvested awards at target 65,350 67,355 90,806 18,600 21,227 31,910 Alliant Energy common stock closing price on December 29, 2017 $42.61 $42.61 $42.61 $42.61 $42.61 N/A Alliant Energy common stock closing price on grant date N/A N/A N/A N/A N/A $32.55 Estimated payout percentage based on performance criteria 105 % 150 % 138 % 105 % 150 % 138 % Fair values of each nonvested award $44.74 $63.92 $58.80 $44.74 $63.92 $44.92 Performance Restricted Stock Units and Performance Restricted Units - Alliant Energy granted new types of share-based compensation awards to key employees beginning in 2016 referred to as performance restricted stock units under the Amended and Restated OIP, and performance restricted units and key employee performance restricted units under the DLIP. Payouts of these units are based on the achievement of certain performance targets (currently specified growth of consolidated income from continuing operations) during a three -year performance period. The actual number of units that will be paid out upon vesting is dependent upon actual performance and may range from zero to 200% of the target number of units. If performance targets are not met during the performance period, these units are forfeited. As of December 31, 2017 , the amount of nonvested performance restricted units and key employee performance restricted units was not material. Performance Restricted Stock Units - Performance restricted stock units generally must be paid out in shares and are accounted for as equity awards. Each performance restricted stock unit’s value is based on the closing market price of one share of Alliant Energy’s common stock on the grant date of the award. Compensation expense is recorded ratably over the performance period based on a probability assessment of payouts for the awards at each reporting period. A summary of the performance restricted stock units activity, with amounts representing the target number of units, was as follows: 2017 2016 Units Weighted Average Grant Date Fair Value Units Weighted Average Grant Date Fair Value Nonvested units, January 1 67,355 $33.96 — $— Granted 65,350 39.12 68,585 33.96 Forfeited — — (1,230 ) 33.90 Nonvested units, December 31 132,705 36.50 67,355 33.96 Restricted Stock Units and Restricted Units - Alliant Energy granted new types of share-based compensation awards to key employees beginning in 2016 referred to as restricted stock units under the Amended and Restated OIP and restricted units under the DLIP. Payouts of these units are based on the expiration of a three -year time-vesting period. Each restricted stock unit’s value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the time-vesting period. Compensation expense is recorded ratably over the performance period based on the fair value of the awards at each reporting period. Restricted stock units can be paid out in shares of Alliant Energy common stock, cash or a combination of cash and stock. Alliant Energy assumes it will make future payouts of its restricted stock units in cash; therefore, restricted stock units are accounted for as liability awards. As of December 31, 2017 , the amount of nonvested restricted units was not material. A summary of the restricted stock units activity was as follows: 2017 2016 Nonvested units, January 1 57,736 — Granted 56,013 58,790 Forfeited — (1,054 ) Nonvested units, December 31 113,749 57,736 Performance-Contingent Restricted

Asset Retirement Obligations

Asset Retirement Obligations	12 Months Ended
	Dec. 31, 2017
Schedule of Asset Retirement Obligations [Line Items]
Asset Retirement Obligations (AROs)	ASSET RETIREMENT OBLIGATIONS Recognized AROs relate to legal obligations for the removal, closure or dismantlement of several assets including, but not limited to, ash ponds, wind farms, active ash landfills, solar generation and above ground storage tanks. Recognized AROs also include legal obligations for the management and final disposition of asbestos, polychlorinated biphenyls and lead-based paint. AROs are recorded in “Other current liabilities” and “Other liabilities” on the balance sheets. Refer to Note 2 for information regarding regulatory assets related to AROs. A reconciliation of the changes in AROs associated with long-lived assets is as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Balance, January 1 $195.7 $214.0 $124.7 $132.9 $61.4 $71.9 Revisions in estimated cash flows 4.3 (13.3 ) 7.0 (5.8 ) (2.7 ) (7.5 ) Liabilities settled (23.5 ) (14.0 ) (13.1 ) (6.8 ) (10.4 ) (7.2 ) Liabilities incurred 2.0 2.6 11.7 0.7 — 1.9 Accretion expense 6.0 6.4 3.8 3.7 2.1 2.3 Balance, December 31 $184.5 $195.7 $134.1 $124.7 $50.4 $61.4 In addition, certain AROs related to EGU assets have not been recognized. Due to an indeterminate remediation date, the fair values of the AROs for these assets cannot be currently estimated. A liability for these AROs will be recorded when fair value is determinable. Removal costs of these EGUs are being recovered in rates and are recorded in regulatory liabilities.
IPL [Member]
Schedule of Asset Retirement Obligations [Line Items]
Asset Retirement Obligations (AROs)	ASSET RETIREMENT OBLIGATIONS Recognized AROs relate to legal obligations for the removal, closure or dismantlement of several assets including, but not limited to, ash ponds, wind farms, active ash landfills, solar generation and above ground storage tanks. Recognized AROs also include legal obligations for the management and final disposition of asbestos, polychlorinated biphenyls and lead-based paint. AROs are recorded in “Other current liabilities” and “Other liabilities” on the balance sheets. Refer to Note 2 for information regarding regulatory assets related to AROs. A reconciliation of the changes in AROs associated with long-lived assets is as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Balance, January 1 $195.7 $214.0 $124.7 $132.9 $61.4 $71.9 Revisions in estimated cash flows 4.3 (13.3 ) 7.0 (5.8 ) (2.7 ) (7.5 ) Liabilities settled (23.5 ) (14.0 ) (13.1 ) (6.8 ) (10.4 ) (7.2 ) Liabilities incurred 2.0 2.6 11.7 0.7 — 1.9 Accretion expense 6.0 6.4 3.8 3.7 2.1 2.3 Balance, December 31 $184.5 $195.7 $134.1 $124.7 $50.4 $61.4 In addition, certain AROs related to EGU assets have not been recognized. Due to an indeterminate remediation date, the fair values of the AROs for these assets cannot be currently estimated. A liability for these AROs will be recorded when fair value is determinable. Removal costs of these EGUs are being recovered in rates and are recorded in regulatory liabilities.
WPL [Member]
Schedule of Asset Retirement Obligations [Line Items]
Asset Retirement Obligations (AROs)	ASSET RETIREMENT OBLIGATIONS Recognized AROs relate to legal obligations for the removal, closure or dismantlement of several assets including, but not limited to, ash ponds, wind farms, active ash landfills, solar generation and above ground storage tanks. Recognized AROs also include legal obligations for the management and final disposition of asbestos, polychlorinated biphenyls and lead-based paint. AROs are recorded in “Other current liabilities” and “Other liabilities” on the balance sheets. Refer to Note 2 for information regarding regulatory assets related to AROs. A reconciliation of the changes in AROs associated with long-lived assets is as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Balance, January 1 $195.7 $214.0 $124.7 $132.9 $61.4 $71.9 Revisions in estimated cash flows 4.3 (13.3 ) 7.0 (5.8 ) (2.7 ) (7.5 ) Liabilities settled (23.5 ) (14.0 ) (13.1 ) (6.8 ) (10.4 ) (7.2 ) Liabilities incurred 2.0 2.6 11.7 0.7 — 1.9 Accretion expense 6.0 6.4 3.8 3.7 2.1 2.3 Balance, December 31 $184.5 $195.7 $134.1 $124.7 $50.4 $61.4 In addition, certain AROs related to EGU assets have not been recognized. Due to an indeterminate remediation date, the fair values of the AROs for these assets cannot be currently estimated. A liability for these AROs will be recorded when fair value is determinable. Removal costs of these EGUs are being recovered in rates and are recorded in regulatory liabilities.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2017
Fair Value Measurements	FAIR VALUE MEASUREMENTS Valuation Hierarchy - Fair value measurement accounting establishes three levels of fair value hierarchy that prioritize the inputs to valuation techniques used to measure fair value. Level 1 pricing inputs are quoted prices available in active markets for identical assets or liabilities as of the reporting date. Level 2 pricing inputs are quoted prices for similar assets or liabilities in active markets or quoted prices for identical or similar assets or liabilities in markets that are not active as of the reporting date. Level 3 pricing inputs are unobservable inputs for assets or liabilities for which little or no market data exist and require significant management judgment or estimation. The fair value hierarchy gives the highest priority to quoted prices in active markets (Level 1) and the lowest priority to unobservable data (Level 3). In some cases, the inputs used to measure fair value might fall in different levels of the fair value hierarchy. The lowest level input that is significant to a fair value measurement in its entirety determines the applicable level in the fair value hierarchy. Assessing the significance of a particular input to the fair value measurement in its entirety requires judgment, considering factors specific to the asset or liability. Valuation Techniques - Derivative assets and derivative liabilities - Derivative instruments are used for risk management purposes to mitigate exposures to fluctuations in certain commodity prices, transmission congestion costs and rail transportation costs. Risk policies are maintained that govern the use of such derivative instruments. Derivative instruments were not designated as hedging instruments and included the following: Risk management purpose Type of instrument Mitigate pricing volatility for: Electricity purchased to supply customers Electric physical forward contracts (WPL) Fuel used to supply natural gas-fired EGUs Natural gas swap, options and physical forward contracts (IPL and WPL) Natural gas supplied to retail customers Natural gas options and physical forward contracts (IPL and WPL) Natural gas swap contracts (IPL) Fuel used at coal-fired EGUs Coal physical forward contracts (IPL and WPL) Optimize the value of natural gas pipeline capacity Natural gas physical forward contracts (IPL and WPL) Natural gas swap contracts (IPL) Manage transmission congestion costs FTRs (IPL and WPL) Manage rail transportation costs Diesel fuel swap contracts (WPL) Swap, option and physical forward commodity contracts were non-exchange-based derivative instruments and were valued using indicative price quotations from a pricing vendor that provides daily exchange forward price settlements, from broker or dealer quotations, from market publications or from on-line exchanges. The indicative price quotations reflected the average of the bid-ask mid-point prices and were obtained from sources believed to provide the most liquid market for the commodity. A portion of these indicative price quotations were corroborated using quoted prices for similar assets or liabilities in active markets and categorized derivative instruments based on such indicative price quotations as Level 2. Commodity contracts that were valued using indicative price quotations based on significant assumptions such as seasonal or monthly shaping and indicative price quotations that could not be readily corroborated were categorized as Level 3. Swap, option and physical forward commodity contracts were predominately at liquid trading points. FTRs were valued using auction prices and were categorized as Level 3. Refer to Note 15 for additional details of derivative assets and derivative liabilities. Deferred proceeds (sales of receivables) - The fair value of IPL’s deferred proceeds related to its sales of accounts receivable program was calculated each reporting date using the cost approach valuation technique. The fair value represents the carrying amount of receivables sold less the allowance for doubtful accounts associated with the receivables sold and cash amounts received from the receivables sold due to the short-term nature of the collection period. These inputs were considered unobservable and deferred proceeds were categorized as Level 3. Deferred proceeds represent IPL’s maximum exposure to loss related to the receivables sold. Refer to Note 5(b) for additional information regarding deferred proceeds. Long-term debt (including current maturities) - The fair value of long-term debt instruments was based on quoted market prices for similar liabilities at each reporting date or on a discounted cash flow methodology, which utilizes assumptions of current market pricing curves at each reporting date, and was substantially classified as Level 2. Refer to Note 9(b) for additional information regarding long-term debt. Cumulative preferred stock - The fair value of IPL’s 5.1% cumulative preferred stock was based on its closing market price quoted by the New York Stock Exchange at each reporting date, and was classified as Level 1. Refer to Note 8 for additional information regarding cumulative preferred stock. Fair Value of Financial Instruments - The carrying amounts of current assets and current liabilities approximate fair value because of the short maturity of such financial instruments. Carrying amounts and the related estimated fair values of other financial instruments at December 31 were as follows (in millions): Alliant Energy 2017 2016 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $25.1 $— $4.1 $21.0 $25.1 $41.4 $— $4.6 $36.8 $41.4 Deferred proceeds 222.1 — — 222.1 222.1 211.1 — — 211.1 211.1 Liabilities and equity: Derivatives 41.7 — 8.5 33.2 41.7 28.6 — 0.5 28.1 28.6 Long-term debt (incl. current maturities) 4,866.3 — 5,444.6 2.9 5,447.5 4,320.2 — 4,795.7 3.3 4,799.0 Cumulative preferred stock of IPL 200.0 203.8 — — 203.8 200.0 194.8 — — 194.8 IPL 2017 2016 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $17.1 $— $2.0 $15.1 $17.1 $20.8 $— $2.8 $18.0 $20.8 Deferred proceeds 222.1 — — 222.1 222.1 211.1 — — 211.1 211.1 Liabilities and equity: Derivatives 19.4 — 2.9 16.5 19.4 8.3 — 0.4 7.9 8.3 Long-term debt (incl. current maturities) 2,406.0 — 2,665.7 — 2,665.7 2,153.5 — 2,352.3 — 2,352.3 Cumulative preferred stock 200.0 203.8 — — 203.8 200.0 194.8 — — 194.8 WPL 2017 2016 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $8.0 $— $2.1 $5.9 $8.0 $20.6 $— $1.8 $18.8 $20.6 Liabilities and equity: Derivatives 22.3 — 5.6 16.7 22.3 20.3 — 0.1 20.2 20.3 Long-term debt 1,833.4 — 2,147.9 — 2,147.9 1,535.2 — 1,807.4 — 1,807.4 Information for fair value measurements using significant unobservable inputs (Level 3 inputs) was as follows (in millions): Alliant Energy Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2017 2016 2017 2016 Beginning balance, January 1 $8.7 ($32.7 ) $211.1 $172.0 Total net gains (losses) included in changes in net assets (realized/unrealized) (32.9 ) 30.7 — — Transfers into Level 3 — 0.9 — — Transfers out of Level 3 12.2 1.2 — — Purchases 28.3 22.0 — — Sales (0.3 ) (1.0 ) — — Settlements (a) (28.2 ) (12.4 ) 11.0 39.1 Ending balance, December 31 ($12.2 ) $8.7 $222.1 $211.1 The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31 ($31.0 ) $32.7 $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2017 2016 2017 2016 Beginning balance, January 1 $10.1 ($1.9 ) $211.1 $172.0 Total net gains (losses) included in changes in net assets (realized/unrealized) (14.8 ) 7.3 — — Transfers into Level 3 — 0.5 — — Transfers out of Level 3 3.1 0.2 — — Purchases 24.6 20.6 — — Sales (0.2 ) (1.0 ) — — Settlements (a) (24.2 ) (15.6 ) 11.0 39.1 Ending balance, December 31 ($1.4 ) $10.1 $222.1 $211.1 The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31 ($13.5 ) $8.5 $— $— WPL Commodity Contract Derivative Assets and (Liabilities), net 2017 2016 Beginning balance, January 1 ($1.4 ) ($30.8 ) Total net gains (losses) included in changes in net assets (realized/unrealized) (18.1 ) 23.4 Transfers into Level 3 — 0.4 Transfers out of Level 3 9.1 1.0 Purchases 3.7 1.4 Sales (0.1 ) — Settlements (4.0 ) 3.2 Ending balance, December 31 ($10.8 ) ($1.4 ) The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31 ($17.5 ) $24.2 (a) Settlements related to deferred proceeds are due to the change in the carrying amount of receivables sold less the allowance for doubtful accounts associated with the receivables sold and cash amounts received from the receivables sold. Commodity Contracts - The fair value of electric, natural gas, coal and diesel fuel commodity contracts categorized as Level 3 was recognized as net derivative assets (liabilities) at December 31 as follows (in millions): Alliant Energy IPL WPL Excluding FTRs FTRs Excluding FTRs FTRs Excluding FTRs FTRs 2017 ($23.5 ) $11.3 ($11.5 ) $10.1 ($12.0 ) $1.2 2016 (2.3 ) 11.0 0.1 10.0 (2.4 ) 1.0
IPL [Member]
Fair Value Measurements	FAIR VALUE MEASUREMENTS Valuation Hierarchy - Fair value measurement accounting establishes three levels of fair value hierarchy that prioritize the inputs to valuation techniques used to measure fair value. Level 1 pricing inputs are quoted prices available in active markets for identical assets or liabilities as of the reporting date. Level 2 pricing inputs are quoted prices for similar assets or liabilities in active markets or quoted prices for identical or similar assets or liabilities in markets that are not active as of the reporting date. Level 3 pricing inputs are unobservable inputs for assets or liabilities for which little or no market data exist and require significant management judgment or estimation. The fair value hierarchy gives the highest priority to quoted prices in active markets (Level 1) and the lowest priority to unobservable data (Level 3). In some cases, the inputs used to measure fair value might fall in different levels of the fair value hierarchy. The lowest level input that is significant to a fair value measurement in its entirety determines the applicable level in the fair value hierarchy. Assessing the significance of a particular input to the fair value measurement in its entirety requires judgment, considering factors specific to the asset or liability. Valuation Techniques - Derivative assets and derivative liabilities - Derivative instruments are used for risk management purposes to mitigate exposures to fluctuations in certain commodity prices, transmission congestion costs and rail transportation costs. Risk policies are maintained that govern the use of such derivative instruments. Derivative instruments were not designated as hedging instruments and included the following: Risk management purpose Type of instrument Mitigate pricing volatility for: Electricity purchased to supply customers Electric physical forward contracts (WPL) Fuel used to supply natural gas-fired EGUs Natural gas swap, options and physical forward contracts (IPL and WPL) Natural gas supplied to retail customers Natural gas options and physical forward contracts (IPL and WPL) Natural gas swap contracts (IPL) Fuel used at coal-fired EGUs Coal physical forward contracts (IPL and WPL) Optimize the value of natural gas pipeline capacity Natural gas physical forward contracts (IPL and WPL) Natural gas swap contracts (IPL) Manage transmission congestion costs FTRs (IPL and WPL) Manage rail transportation costs Diesel fuel swap contracts (WPL) Swap, option and physical forward commodity contracts were non-exchange-based derivative instruments and were valued using indicative price quotations from a pricing vendor that provides daily exchange forward price settlements, from broker or dealer quotations, from market publications or from on-line exchanges. The indicative price quotations reflected the average of the bid-ask mid-point prices and were obtained from sources believed to provide the most liquid market for the commodity. A portion of these indicative price quotations were corroborated using quoted prices for similar assets or liabilities in active markets and categorized derivative instruments based on such indicative price quotations as Level 2. Commodity contracts that were valued using indicative price quotations based on significant assumptions such as seasonal or monthly shaping and indicative price quotations that could not be readily corroborated were categorized as Level 3. Swap, option and physical forward commodity contracts were predominately at liquid trading points. FTRs were valued using auction prices and were categorized as Level 3. Refer to Note 15 for additional details of derivative assets and derivative liabilities. Deferred proceeds (sales of receivables) - The fair value of IPL’s deferred proceeds related to its sales of accounts receivable program was calculated each reporting date using the cost approach valuation technique. The fair value represents the carrying amount of receivables sold less the allowance for doubtful accounts associated with the receivables sold and cash amounts received from the receivables sold due to the short-term nature of the collection period. These inputs were considered unobservable and deferred proceeds were categorized as Level 3. Deferred proceeds represent IPL’s maximum exposure to loss related to the receivables sold. Refer to Note 5(b) for additional information regarding deferred proceeds. Long-term debt (including current maturities) - The fair value of long-term debt instruments was based on quoted market prices for similar liabilities at each reporting date or on a discounted cash flow methodology, which utilizes assumptions of current market pricing curves at each reporting date, and was substantially classified as Level 2. Refer to Note 9(b) for additional information regarding long-term debt. Cumulative preferred stock - The fair value of IPL’s 5.1% cumulative preferred stock was based on its closing market price quoted by the New York Stock Exchange at each reporting date, and was classified as Level 1. Refer to Note 8 for additional information regarding cumulative preferred stock. Fair Value of Financial Instruments - The carrying amounts of current assets and current liabilities approximate fair value because of the short maturity of such financial instruments. Carrying amounts and the related estimated fair values of other financial instruments at December 31 were as follows (in millions): Alliant Energy 2017 2016 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $25.1 $— $4.1 $21.0 $25.1 $41.4 $— $4.6 $36.8 $41.4 Deferred proceeds 222.1 — — 222.1 222.1 211.1 — — 211.1 211.1 Liabilities and equity: Derivatives 41.7 — 8.5 33.2 41.7 28.6 — 0.5 28.1 28.6 Long-term debt (incl. current maturities) 4,866.3 — 5,444.6 2.9 5,447.5 4,320.2 — 4,795.7 3.3 4,799.0 Cumulative preferred stock of IPL 200.0 203.8 — — 203.8 200.0 194.8 — — 194.8 IPL 2017 2016 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $17.1 $— $2.0 $15.1 $17.1 $20.8 $— $2.8 $18.0 $20.8 Deferred proceeds 222.1 — — 222.1 222.1 211.1 — — 211.1 211.1 Liabilities and equity: Derivatives 19.4 — 2.9 16.5 19.4 8.3 — 0.4 7.9 8.3 Long-term debt (incl. current maturities) 2,406.0 — 2,665.7 — 2,665.7 2,153.5 — 2,352.3 — 2,352.3 Cumulative preferred stock 200.0 203.8 — — 203.8 200.0 194.8 — — 194.8 WPL 2017 2016 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $8.0 $— $2.1 $5.9 $8.0 $20.6 $— $1.8 $18.8 $20.6 Liabilities and equity: Derivatives 22.3 — 5.6 16.7 22.3 20.3 — 0.1 20.2 20.3 Long-term debt 1,833.4 — 2,147.9 — 2,147.9 1,535.2 — 1,807.4 — 1,807.4 Information for fair value measurements using significant unobservable inputs (Level 3 inputs) was as follows (in millions): Alliant Energy Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2017 2016 2017 2016 Beginning balance, January 1 $8.7 ($32.7 ) $211.1 $172.0 Total net gains (losses) included in changes in net assets (realized/unrealized) (32.9 ) 30.7 — — Transfers into Level 3 — 0.9 — — Transfers out of Level 3 12.2 1.2 — — Purchases 28.3 22.0 — — Sales (0.3 ) (1.0 ) — — Settlements (a) (28.2 ) (12.4 ) 11.0 39.1 Ending balance, December 31 ($12.2 ) $8.7 $222.1 $211.1 The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31 ($31.0 ) $32.7 $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2017 2016 2017 2016 Beginning balance, January 1 $10.1 ($1.9 ) $211.1 $172.0 Total net gains (losses) included in changes in net assets (realized/unrealized) (14.8 ) 7.3 — — Transfers into Level 3 — 0.5 — — Transfers out of Level 3 3.1 0.2 — — Purchases 24.6 20.6 — — Sales (0.2 ) (1.0 ) — — Settlements (a) (24.2 ) (15.6 ) 11.0 39.1 Ending balance, December 31 ($1.4 ) $10.1 $222.1 $211.1 The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31 ($13.5 ) $8.5 $— $— WPL Commodity Contract Derivative Assets and (Liabilities), net 2017 2016 Beginning balance, January 1 ($1.4 ) ($30.8 ) Total net gains (losses) included in changes in net assets (realized/unrealized) (18.1 ) 23.4 Transfers into Level 3 — 0.4 Transfers out of Level 3 9.1 1.0 Purchases 3.7 1.4 Sales (0.1 ) — Settlements (4.0 ) 3.2 Ending balance, December 31 ($10.8 ) ($1.4 ) The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31 ($17.5 ) $24.2 (a) Settlements related to deferred proceeds are due to the change in the carrying amount of receivables sold less the allowance for doubtful accounts associated with the receivables sold and cash amounts received from the receivables sold. Commodity Contracts - The fair value of electric, natural gas, coal and diesel fuel commodity contracts categorized as Level 3 was recognized as net derivative assets (liabilities) at December 31 as follows (in millions): Alliant Energy IPL WPL Excluding FTRs FTRs Excluding FTRs FTRs Excluding FTRs FTRs 2017 ($23.5 ) $11.3 ($11.5 ) $10.1 ($12.0 ) $1.2 2016 (2.3 ) 11.0 0.1 10.0 (2.4 ) 1.0
WPL [Member]
Fair Value Measurements	FAIR VALUE MEASUREMENTS Valuation Hierarchy - Fair value measurement accounting establishes three levels of fair value hierarchy that prioritize the inputs to valuation techniques used to measure fair value. Level 1 pricing inputs are quoted prices available in active markets for identical assets or liabilities as of the reporting date. Level 2 pricing inputs are quoted prices for similar assets or liabilities in active markets or quoted prices for identical or similar assets or liabilities in markets that are not active as of the reporting date. Level 3 pricing inputs are unobservable inputs for assets or liabilities for which little or no market data exist and require significant management judgment or estimation. The fair value hierarchy gives the highest priority to quoted prices in active markets (Level 1) and the lowest priority to unobservable data (Level 3). In some cases, the inputs used to measure fair value might fall in different levels of the fair value hierarchy. The lowest level input that is significant to a fair value measurement in its entirety determines the applicable level in the fair value hierarchy. Assessing the significance of a particular input to the fair value measurement in its entirety requires judgment, considering factors specific to the asset or liability. Valuation Techniques - Derivative assets and derivative liabilities - Derivative instruments are used for risk management purposes to mitigate exposures to fluctuations in certain commodity prices, transmission congestion costs and rail transportation costs. Risk policies are maintained that govern the use of such derivative instruments. Derivative instruments were not designated as hedging instruments and included the following: Risk management purpose Type of instrument Mitigate pricing volatility for: Electricity purchased to supply customers Electric physical forward contracts (WPL) Fuel used to supply natural gas-fired EGUs Natural gas swap, options and physical forward contracts (IPL and WPL) Natural gas supplied to retail customers Natural gas options and physical forward contracts (IPL and WPL) Natural gas swap contracts (IPL) Fuel used at coal-fired EGUs Coal physical forward contracts (IPL and WPL) Optimize the value of natural gas pipeline capacity Natural gas physical forward contracts (IPL and WPL) Natural gas swap contracts (IPL) Manage transmission congestion costs FTRs (IPL and WPL) Manage rail transportation costs Diesel fuel swap contracts (WPL) Swap, option and physical forward commodity contracts were non-exchange-based derivative instruments and were valued using indicative price quotations from a pricing vendor that provides daily exchange forward price settlements, from broker or dealer quotations, from market publications or from on-line exchanges. The indicative price quotations reflected the average of the bid-ask mid-point prices and were obtained from sources believed to provide the most liquid market for the commodity. A portion of these indicative price quotations were corroborated using quoted prices for similar assets or liabilities in active markets and categorized derivative instruments based on such indicative price quotations as Level 2. Commodity contracts that were valued using indicative price quotations based on significant assumptions such as seasonal or monthly shaping and indicative price quotations that could not be readily corroborated were categorized as Level 3. Swap, option and physical forward commodity contracts were predominately at liquid trading points. FTRs were valued using auction prices and were categorized as Level 3. Refer to Note 15 for additional details of derivative assets and derivative liabilities. Deferred proceeds (sales of receivables) - The fair value of IPL’s deferred proceeds related to its sales of accounts receivable program was calculated each reporting date using the cost approach valuation technique. The fair value represents the carrying amount of receivables sold less the allowance for doubtful accounts associated with the receivables sold and cash amounts received from the receivables sold due to the short-term nature of the collection period. These inputs were considered unobservable and deferred proceeds were categorized as Level 3. Deferred proceeds represent IPL’s maximum exposure to loss related to the receivables sold. Refer to Note 5(b) for additional information regarding deferred proceeds. Long-term debt (including current maturities) - The fair value of long-term debt instruments was based on quoted market prices for similar liabilities at each reporting date or on a discounted cash flow methodology, which utilizes assumptions of current market pricing curves at each reporting date, and was substantially classified as Level 2. Refer to Note 9(b) for additional information regarding long-term debt. Cumulative preferred stock - The fair value of IPL’s 5.1% cumulative preferred stock was based on its closing market price quoted by the New York Stock Exchange at each reporting date, and was classified as Level 1. Refer to Note 8 for additional information regarding cumulative preferred stock. Fair Value of Financial Instruments - The carrying amounts of current assets and current liabilities approximate fair value because of the short maturity of such financial instruments. Carrying amounts and the related estimated fair values of other financial instruments at December 31 were as follows (in millions): Alliant Energy 2017 2016 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $25.1 $— $4.1 $21.0 $25.1 $41.4 $— $4.6 $36.8 $41.4 Deferred proceeds 222.1 — — 222.1 222.1 211.1 — — 211.1 211.1 Liabilities and equity: Derivatives 41.7 — 8.5 33.2 41.7 28.6 — 0.5 28.1 28.6 Long-term debt (incl. current maturities) 4,866.3 — 5,444.6 2.9 5,447.5 4,320.2 — 4,795.7 3.3 4,799.0 Cumulative preferred stock of IPL 200.0 203.8 — — 203.8 200.0 194.8 — — 194.8 IPL 2017 2016 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $17.1 $— $2.0 $15.1 $17.1 $20.8 $— $2.8 $18.0 $20.8 Deferred proceeds 222.1 — — 222.1 222.1 211.1 — — 211.1 211.1 Liabilities and equity: Derivatives 19.4 — 2.9 16.5 19.4 8.3 — 0.4 7.9 8.3 Long-term debt (incl. current maturities) 2,406.0 — 2,665.7 — 2,665.7 2,153.5 — 2,352.3 — 2,352.3 Cumulative preferred stock 200.0 203.8 — — 203.8 200.0 194.8 — — 194.8 WPL 2017 2016 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $8.0 $— $2.1 $5.9 $8.0 $20.6 $— $1.8 $18.8 $20.6 Liabilities and equity: Derivatives 22.3 — 5.6 16.7 22.3 20.3 — 0.1 20.2 20.3 Long-term debt 1,833.4 — 2,147.9 — 2,147.9 1,535.2 — 1,807.4 — 1,807.4 Information for fair value measurements using significant unobservable inputs (Level 3 inputs) was as follows (in millions): Alliant Energy Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2017 2016 2017 2016 Beginning balance, January 1 $8.7 ($32.7 ) $211.1 $172.0 Total net gains (losses) included in changes in net assets (realized/unrealized) (32.9 ) 30.7 — — Transfers into Level 3 — 0.9 — — Transfers out of Level 3 12.2 1.2 — — Purchases 28.3 22.0 — — Sales (0.3 ) (1.0 ) — — Settlements (a) (28.2 ) (12.4 ) 11.0 39.1 Ending balance, December 31 ($12.2 ) $8.7 $222.1 $211.1 The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31 ($31.0 ) $32.7 $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2017 2016 2017 2016 Beginning balance, January 1 $10.1 ($1.9 ) $211.1 $172.0 Total net gains (losses) included in changes in net assets (realized/unrealized) (14.8 ) 7.3 — — Transfers into Level 3 — 0.5 — — Transfers out of Level 3 3.1 0.2 — — Purchases 24.6 20.6 — — Sales (0.2 ) (1.0 ) — — Settlements (a) (24.2 ) (15.6 ) 11.0 39.1 Ending balance, December 31 ($1.4 ) $10.1 $222.1 $211.1 The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31 ($13.5 ) $8.5 $— $— WPL Commodity Contract Derivative Assets and (Liabilities), net 2017 2016 Beginning balance, January 1 ($1.4 ) ($30.8 ) Total net gains (losses) included in changes in net assets (realized/unrealized) (18.1 ) 23.4 Transfers into Level 3 — 0.4 Transfers out of Level 3 9.1 1.0 Purchases 3.7 1.4 Sales (0.1 ) — Settlements (4.0 ) 3.2 Ending balance, December 31 ($10.8 ) ($1.4 ) The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31 ($17.5 ) $24.2 (a) Settlements related to deferred proceeds are due to the change in the carrying amount of receivables sold less the allowance for doubtful accounts associated with the receivables sold and cash amounts received from the receivables sold. Commodity Contracts - The fair value of electric, natural gas, coal and diesel fuel commodity contracts categorized as Level 3 was recognized as net derivative assets (liabilities) at December 31 as follows (in millions): Alliant Energy IPL WPL Excluding FTRs FTRs Excluding FTRs FTRs Excluding FTRs FTRs 2017 ($23.5 ) $11.3 ($11.5 ) $10.1 ($12.0 ) $1.2 2016 (2.3 ) 11.0 0.1 10.0 (2.4 ) 1.0

Derivative Instruments

Derivative Instruments	12 Months Ended
	Dec. 31, 2017
Derivative [Line Items]
Derivative Instruments	DERIVATIVE INSTRUMENTS Commodity Derivatives - Purpose - Derivative instruments are used for risk management purposes to mitigate exposures to fluctuations in certain commodity prices, transmission congestion costs and rail transportation costs. Refer to Note 14 for detailed discussion of derivative instruments. Notional Amounts - As of December 31, 2017 , gross notional amounts and settlement/delivery years related to outstanding swap contracts, option contracts, physical forward contracts and FTRs that were accounted for as commodity derivative instruments were as follows (units in thousands): Electricity FTRs Natural Gas Coal Diesel Fuel MWhs Years MWhs Years Dths Years Tons Years Gallons Years Alliant Energy 1,314 2018 8,970 2018 170,463 2018-2026 8,177 2018-2020 6,552 2018-2019 IPL — — 5,886 2018 72,662 2018-2026 3,339 2018-2020 — — WPL 1,314 2018 3,084 2018 97,801 2018-2026 4,838 2018-2020 6,552 2018-2019 Financial Statement Presentation - Derivative instruments are recorded at fair value each reporting date on the balance sheet as assets or liabilities. At December 31, the fair values of current derivative assets are included in “Other current assets,” non-current derivative assets are included in “Deferred charges and other,” current derivative liabilities are included in “Other current liabilities” and non-current derivative liabilities are included in “Other liabilities” on the balance sheets as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Current derivative assets $21.1 $29.4 $15.8 $19.1 $5.3 $10.3 Non-current derivative assets 4.0 12.0 1.3 1.7 2.7 10.3 Current derivative liabilities 18.7 13.3 5.0 2.7 13.7 10.6 Non-current derivative liabilities 23.0 15.3 14.4 5.6 8.6 9.7 Credit Risk-related Contingent Features - Various agreements contain credit risk-related contingent features, including requirements to maintain certain credit ratings and/or limitations on liability positions under the agreements based on credit ratings. Certain of these agreements with credit risk-related contingency features are accounted for as derivative instruments. In the event of a material change in creditworthiness or if liability positions exceed certain contractual limits, credit support may need to be provided in the form of letters of credit or cash collateral up to the amount of exposure under the contracts, or the contracts may need to be unwound and underlying liability positions paid. At December 31, 2017 and 2016 , the aggregate fair value of all derivative instruments with credit risk-related contingent features in a net liability position was not materially different than amounts that would be required to be posted as credit support to counterparties by Alliant Energy, IPL or WPL if the most restrictive credit risk-related contingent features for derivative agreements in a net liability position were triggered. Balance Sheet Offsetting - The fair value amounts of derivative instruments subject to a master netting arrangement are not netted by counterparty on the balance sheets. However, if the fair value amounts of derivative instruments by counterparty were netted, amounts would not be materially different from gross amounts of derivative assets and derivative liabilities at December 31, 2017 and 2016 . Fair value amounts recognized for the right to reclaim cash collateral (receivable) or the obligation to return cash collateral (payable) are not offset against fair value amounts recognized for derivative instruments executed with the same counterparty under the same master netting arrangement.
IPL [Member]
Derivative [Line Items]
Derivative Instruments	DERIVATIVE INSTRUMENTS Commodity Derivatives - Purpose - Derivative instruments are used for risk management purposes to mitigate exposures to fluctuations in certain commodity prices, transmission congestion costs and rail transportation costs. Refer to Note 14 for detailed discussion of derivative instruments. Notional Amounts - As of December 31, 2017 , gross notional amounts and settlement/delivery years related to outstanding swap contracts, option contracts, physical forward contracts and FTRs that were accounted for as commodity derivative instruments were as follows (units in thousands): Electricity FTRs Natural Gas Coal Diesel Fuel MWhs Years MWhs Years Dths Years Tons Years Gallons Years Alliant Energy 1,314 2018 8,970 2018 170,463 2018-2026 8,177 2018-2020 6,552 2018-2019 IPL — — 5,886 2018 72,662 2018-2026 3,339 2018-2020 — — WPL 1,314 2018 3,084 2018 97,801 2018-2026 4,838 2018-2020 6,552 2018-2019 Financial Statement Presentation - Derivative instruments are recorded at fair value each reporting date on the balance sheet as assets or liabilities. At December 31, the fair values of current derivative assets are included in “Other current assets,” non-current derivative assets are included in “Deferred charges and other,” current derivative liabilities are included in “Other current liabilities” and non-current derivative liabilities are included in “Other liabilities” on the balance sheets as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Current derivative assets $21.1 $29.4 $15.8 $19.1 $5.3 $10.3 Non-current derivative assets 4.0 12.0 1.3 1.7 2.7 10.3 Current derivative liabilities 18.7 13.3 5.0 2.7 13.7 10.6 Non-current derivative liabilities 23.0 15.3 14.4 5.6 8.6 9.7 Credit Risk-related Contingent Features - Various agreements contain credit risk-related contingent features, including requirements to maintain certain credit ratings and/or limitations on liability positions under the agreements based on credit ratings. Certain of these agreements with credit risk-related contingency features are accounted for as derivative instruments. In the event of a material change in creditworthiness or if liability positions exceed certain contractual limits, credit support may need to be provided in the form of letters of credit or cash collateral up to the amount of exposure under the contracts, or the contracts may need to be unwound and underlying liability positions paid. At December 31, 2017 and 2016 , the aggregate fair value of all derivative instruments with credit risk-related contingent features in a net liability position was not materially different than amounts that would be required to be posted as credit support to counterparties by Alliant Energy, IPL or WPL if the most restrictive credit risk-related contingent features for derivative agreements in a net liability position were triggered. Balance Sheet Offsetting - The fair value amounts of derivative instruments subject to a master netting arrangement are not netted by counterparty on the balance sheets. However, if the fair value amounts of derivative instruments by counterparty were netted, amounts would not be materially different from gross amounts of derivative assets and derivative liabilities at December 31, 2017 and 2016 . Fair value amounts recognized for the right to reclaim cash collateral (receivable) or the obligation to return cash collateral (payable) are not offset against fair value amounts recognized for derivative instruments executed with the same counterparty under the same master netting arrangement.
WPL [Member]
Derivative [Line Items]
Derivative Instruments	DERIVATIVE INSTRUMENTS Commodity Derivatives - Purpose - Derivative instruments are used for risk management purposes to mitigate exposures to fluctuations in certain commodity prices, transmission congestion costs and rail transportation costs. Refer to Note 14 for detailed discussion of derivative instruments. Notional Amounts - As of December 31, 2017 , gross notional amounts and settlement/delivery years related to outstanding swap contracts, option contracts, physical forward contracts and FTRs that were accounted for as commodity derivative instruments were as follows (units in thousands): Electricity FTRs Natural Gas Coal Diesel Fuel MWhs Years MWhs Years Dths Years Tons Years Gallons Years Alliant Energy 1,314 2018 8,970 2018 170,463 2018-2026 8,177 2018-2020 6,552 2018-2019 IPL — — 5,886 2018 72,662 2018-2026 3,339 2018-2020 — — WPL 1,314 2018 3,084 2018 97,801 2018-2026 4,838 2018-2020 6,552 2018-2019 Financial Statement Presentation - Derivative instruments are recorded at fair value each reporting date on the balance sheet as assets or liabilities. At December 31, the fair values of current derivative assets are included in “Other current assets,” non-current derivative assets are included in “Deferred charges and other,” current derivative liabilities are included in “Other current liabilities” and non-current derivative liabilities are included in “Other liabilities” on the balance sheets as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Current derivative assets $21.1 $29.4 $15.8 $19.1 $5.3 $10.3 Non-current derivative assets 4.0 12.0 1.3 1.7 2.7 10.3 Current derivative liabilities 18.7 13.3 5.0 2.7 13.7 10.6 Non-current derivative liabilities 23.0 15.3 14.4 5.6 8.6 9.7 Credit Risk-related Contingent Features - Various agreements contain credit risk-related contingent features, including requirements to maintain certain credit ratings and/or limitations on liability positions under the agreements based on credit ratings. Certain of these agreements with credit risk-related contingency features are accounted for as derivative instruments. In the event of a material change in creditworthiness or if liability positions exceed certain contractual limits, credit support may need to be provided in the form of letters of credit or cash collateral up to the amount of exposure under the contracts, or the contracts may need to be unwound and underlying liability positions paid. At December 31, 2017 and 2016 , the aggregate fair value of all derivative instruments with credit risk-related contingent features in a net liability position was not materially different than amounts that would be required to be posted as credit support to counterparties by Alliant Energy, IPL or WPL if the most restrictive credit risk-related contingent features for derivative agreements in a net liability position were triggered. Balance Sheet Offsetting - The fair value amounts of derivative instruments subject to a master netting arrangement are not netted by counterparty on the balance sheets. However, if the fair value amounts of derivative instruments by counterparty were netted, amounts would not be materially different from gross amounts of derivative assets and derivative liabilities at December 31, 2017 and 2016 . Fair value amounts recognized for the right to reclaim cash collateral (receivable) or the obligation to return cash collateral (payable) are not offset against fair value amounts recognized for derivative instruments executed with the same counterparty under the same master netting arrangement.

Commitments And Contingencies

Commitments And Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments And Contingencies	COMMITMENTS AND CONTINGENCIES (a) Capital Purchase Obligations - Various contractual obligations contain minimum future commitments related to capital expenditures for certain construction projects. IPL’s projects include the expansion of wind generation and installation of an SCR system at Ottumwa Unit 1 to reduce NOx emissions at the EGU. WPL’s projects include West Riverside. At December 31, 2017 , Alliant Energy’s, IPL’s and WPL’s minimum future commitments related to certain contractual obligations for these projects were $82 million , $15 million and $67 million , respectively. (b) Other Purchase Obligations - Various commodity supply, transportation and storage contracts help meet obligations to provide electricity and natural gas to utility customers. In addition, there are various purchase obligations associated with other goods and services. At December 31, 2017 , minimum future commitments related to these purchase obligations were as follows (in millions): Alliant Energy 2018 2019 2020 2021 2022 Thereafter Total Purchased power (a) $188 $159 $135 $149 $140 $461 $1,232 Natural gas 229 140 130 110 84 253 946 Coal (b) 107 59 21 5 — — 192 Other (c) 21 6 6 3 2 2 40 $545 $364 $292 $267 $226 $716 $2,410 IPL 2018 2019 2020 2021 2022 Thereafter Total Purchased power (a) $130 $144 $135 $149 $140 $461 $1,159 Natural gas 126 58 44 32 22 102 384 Coal (b) 51 31 11 5 — — 98 Other (c) 15 3 3 3 2 2 28 $322 $236 $193 $189 $164 $565 $1,669 WPL 2018 2019 2020 2021 2022 Thereafter Total Purchased power (a) $58 $15 $— $— $— $— $73 Natural gas 103 82 86 78 62 151 562 Coal (b) 56 28 10 — — — 94 Other (c) 5 1 — — — — 6 $222 $126 $96 $78 $62 $151 $735 (a) Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. (b) Corporate Services entered into system-wide coal contracts on behalf of IPL and WPL that include minimum future commitments. These commitments were assigned to IPL and WPL based on information available as of December 31, 2017 regarding expected future usage, which is subject to change. (c) Includes individual commitments incurred during the normal course of business that exceeded $1 million at December 31, 2017 . Certain contracts are considered leases and are therefore not included here, but are included in Note 10 . (c) Legal Proceedings - Flood Damage Claims - In 2013, several plaintiffs purporting to represent a class of residential and commercial property owners filed a complaint against CRANDIC, Alliant Energy and various other defendants in the Iowa District Court for Linn County. Plaintiffs assert claims of negligence and strict liability based on their allegations that CRANDIC (along with other defendants) caused or exacerbated flooding of the Cedar River in June 2008. In February 2016, the Iowa District Court for Linn County ruled in favor of Alliant Energy and CRANDIC and dismissed all claims against them, resulting in no loss. In August 2016, the Iowa District Court for Linn County dismissed all claims against the remaining defendants. In September 2016, plaintiffs filed a notice of appeal with the Supreme Court of Iowa. Alliant Energy does not currently believe any material losses for this complaint are both probable and reasonably estimated, and therefore has not recognized any material loss contingency amounts as of December 31, 2017 . Other - Alliant Energy, IPL and WPL are involved in other legal and administrative proceedings before various courts and agencies with respect to matters arising in the ordinary course of business. Although unable to predict the outcome of these matters, Alliant Energy, IPL and WPL believe that appropriate reserves have been established and final disposition of these actions will not have a material effect on their financial condition or results of operations. (d) Guarantees and Indemnifications - Whiting Petroleum - In 2004, Alliant Energy sold its remaining interest in Whiting Petroleum. Whiting Petroleum is an independent oil and gas company. Alliant Energy Resources, LLC, as the successor to a predecessor entity that owned Whiting Petroleum, and a wholly-owned subsidiary of AEF, continues to guarantee the partnership obligations of an affiliate of Whiting Petroleum under general partnership agreements in the oil and gas industry, including with respect to the future abandonment of certain platforms off the coast of California and related onshore plant and equipment owned by the partnerships. The guarantees do not include a maximum limit. As of December 31, 2017 , the present value of the abandonment obligations is estimated at $33 million . Alliant Energy is not aware of any material liabilities related to these guarantees of which it is probable that Alliant Energy Resources, LLC will be obligated to pay and therefore has not recognized any material liabilities related to this guarantee as of December 31, 2017 . Non-utility Wind Investment in Oklahoma - In July 2017, a wholly-owned subsidiary of AEF acquired a cash equity ownership interest in a non-utility wind farm located in Oklahoma. The wind farm provides electricity to a third-party under a long-term PPA. Alliant Energy provided a parent guarantee of its subsidiary’s indemnification obligations under the related operating agreement and PPA. Alliant Energy’s obligations under the operating agreement were $98 million as of December 31, 2017 and will reduce annually until expiring in July 2047 . Alliant Energy’s obligations under the PPA are subject to a maximum limit of $17 million and expire in December 2031 , subject to potential extension. Alliant Energy is not aware of any material liabilities related to this guarantee that it is probable that it will be obligated to pay and therefore has not recognized any material liabilities related to this guarantee as of December 31, 2017 . Refer to Note 6(a) for further discussion of the non-utility wind investment. IPL’s Minnesota Electric Distribution Assets - IPL provided indemnifications associated with the July 2015 sale of its Minnesota electric distribution assets for losses resulting from potential breach of IPL’s representations, warranties and obligations under the sale agreement. Alliant Energy and IPL believe the likelihood of having to make any material cash payments under these indemnifications is remote. IPL has not recorded any material liabilities related to these indemnifications as of December 31, 2017 . The general terms of the indemnifications provided by IPL included a maximum limit of $17 million and expire in October 2020 . Refer to Note 3 for further discussion of the sale of IPL’s Minnesota electric distribution assets. (e) Environmental Matters - Alliant Energy, IPL and WPL are subject to environmental regulations as a result of their current and past operations. These regulations are designed to protect public health and the environment and have resulted in compliance, remediation, containment and monitoring obligations, which are recorded as current and non-current environmental liabilities. Substantially all of the environmental liabilities recorded on the balance sheets relate to MGP sites. MGP Sites - IPL and WPL have current or previous ownership interests in various sites that are previously associated with the production of gas for which IPL and WPL have, or may have in the future, liability for investigation, remediation and monitoring costs. IPL and WPL are working pursuant to the requirements of various federal and state agencies to investigate, mitigate, prevent and remediate, where necessary, the environmental impacts to property, including natural resources, at and around these former MGP sites in order to protect public health and the environment. At December 31, 2017 , estimated future costs expected to be incurred for the investigation, remediation and monitoring of the MGP sites, as well as environmental liabilities recorded on the balance sheets for these sites, which are not discounted, were as follows (in millions). At December 31, 2017 , such amounts for WPL were not material. Alliant Energy IPL Range of estimated future costs $11 - $30 $9 - $27 Current and non-current environmental liabilities 15 13 WPL Consent Decree - In 2013, the U.S. District Court for the Western District of Wisconsin approved a Consent Decree that WPL, along with the other owners of Edgewater and Columbia, entered into with the EPA and the Sierra Club, thereby resolving claims against WPL. Such claims included allegations that the owners of Edgewater, Nelson Dewey and Columbia violated the Prevention of Significant Deterioration program requirements, Title V Operating Permit requirements of the CAA and the Wisconsin State Implementation Plan designed to implement the CAA. WPL has completed various requirements under the Consent Decree. WPL’s remaining requirements include installing an SCR system at Columbia Unit 2 and fuel switching or retiring Edgewater Unit 4 by December 31, 2018. The Consent Decree also establishes SO2, NOx and particulate matter emission rate limits for Columbia Units 1 and 2, and Edgewater Units 4 and 5. In addition, the Consent Decree includes annual plant-wide SO2 and NOx emission caps for Columbia and Edgewater. Alliant Energy and WPL currently expect to recover material costs incurred by WPL related to compliance with the terms of the Consent Decree from WPL’s electric customers. IPL Consent Decree - In 2015, the U.S. District Court for the Northern District of Iowa approved a Consent Decree that IPL entered into with the EPA, the Sierra Club, the State of Iowa and Linn County in Iowa, thereby resolving potential CAA issues associated with emissions from IPL’s coal-fired generating facilities in Iowa. IPL has completed various requirements under the Consent Decree. IPL’s remaining requirements include installing an SCR system or equivalent NOx reduction system at Ottumwa by December 31, 2019, and fuel switching or retiring Burlington by December 31, 2021 and Prairie Creek Units 1 and 3 by December 31, 2025. The Consent Decree also establishes SO2, NOx and particulate matter emission rate limits with varying averaging times for Burlington, Lansing, M.L. Kapp, Ottumwa and Prairie Creek. In addition, the Consent Decree includes calendar-year SO2 and NOx emission caps for Prairie Creek, and calendar-year SO2 and NOx emission caps in aggregate for Burlington, Lansing, M.L. Kapp, Ottumwa and Prairie Creek. Alliant Energy and IPL currently expect to recover material costs incurred by IPL related to compliance with the terms of the Consent Decree from IPL’s electric customers. Other Environmental Contingencies - In addition to the environmental liabilities discussed above, various environmental rules are monitored that may have a significant impact on future operations. Several of these environmental rules are subject to legal challenges, reconsideration and/or other uncertainties. Given uncertainties regarding the outcome, timing and compliance plans for these environmental matters, the complete financial impact of each of these rules is not able to be determined; however future capital investments and/or modifications to EGUs to comply with certain of these rules could be significant. Specific current, proposed or potential environmental matters include, among others: CSAPR, Effluent Limitation Guidelines, CCR Rule, and various legislation and EPA regulations to monitor and regulate the emission of GHG, including carbon emissions from new (CAA Section 111(b)) and existing (CAA Section 111(d)) fossil-fueled EGUs. (f) Credit Risk - IPL provides retail electric and gas services in Iowa and wholesale electric service in Minnesota, Illinois and Iowa. WPL provides retail electric and gas services and wholesale electric service in Wisconsin. The geographic concentration of IPL’s and WPL’s customers did not contribute significantly to overall credit risk exposure. In addition, as a result of a diverse customer base, IPL and WPL did not have any significant credit risk concentration for receivables arising from the sale of electricity or gas services. Alliant Energy, IPL and WPL are subject to credit risk related to the ability of counterparties to meet their contractual payment obligations or the potential non-performance of counterparties to deliver contracted commodities and other goods or services at the contracted price. Credit policies are maintained to mitigate credit risk. These credit policies include evaluation of the financial condition of certain counterparties, use of credit risk-related contingent provisions in certain agreements that require credit support from counterparties not meeting specific criteria, diversification of counterparties to reduce concentrations of credit risk and the use of standardized agreements that facilitate the netting of cash flows associated with certain counterparties. Based on these credit policies and counterparty diversification, as well as utility cost recovery mechanisms, it is unlikely that counterparty non-performance would have a material effect on financial condition or results of operations. However, there is no assurance that these items will protect against all losses from counterparty non-performance. Refer to Notes 5(a) and 15 for details of allowances for doubtful accounts and credit risk-related contingent features, respectively.
IPL [Member]
Commitments And Contingencies	COMMITMENTS AND CONTINGENCIES (a) Capital Purchase Obligations - Various contractual obligations contain minimum future commitments related to capital expenditures for certain construction projects. IPL’s projects include the expansion of wind generation and installation of an SCR system at Ottumwa Unit 1 to reduce NOx emissions at the EGU. WPL’s projects include West Riverside. At December 31, 2017 , Alliant Energy’s, IPL’s and WPL’s minimum future commitments related to certain contractual obligations for these projects were $82 million , $15 million and $67 million , respectively. (b) Other Purchase Obligations - Various commodity supply, transportation and storage contracts help meet obligations to provide electricity and natural gas to utility customers. In addition, there are various purchase obligations associated with other goods and services. At December 31, 2017 , minimum future commitments related to these purchase obligations were as follows (in millions): Alliant Energy 2018 2019 2020 2021 2022 Thereafter Total Purchased power (a) $188 $159 $135 $149 $140 $461 $1,232 Natural gas 229 140 130 110 84 253 946 Coal (b) 107 59 21 5 — — 192 Other (c) 21 6 6 3 2 2 40 $545 $364 $292 $267 $226 $716 $2,410 IPL 2018 2019 2020 2021 2022 Thereafter Total Purchased power (a) $130 $144 $135 $149 $140 $461 $1,159 Natural gas 126 58 44 32 22 102 384 Coal (b) 51 31 11 5 — — 98 Other (c) 15 3 3 3 2 2 28 $322 $236 $193 $189 $164 $565 $1,669 WPL 2018 2019 2020 2021 2022 Thereafter Total Purchased power (a) $58 $15 $— $— $— $— $73 Natural gas 103 82 86 78 62 151 562 Coal (b) 56 28 10 — — — 94 Other (c) 5 1 — — — — 6 $222 $126 $96 $78 $62 $151 $735 (a) Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. (b) Corporate Services entered into system-wide coal contracts on behalf of IPL and WPL that include minimum future commitments. These commitments were assigned to IPL and WPL based on information available as of December 31, 2017 regarding expected future usage, which is subject to change. (c) Includes individual commitments incurred during the normal course of business that exceeded $1 million at December 31, 2017 . Certain contracts are considered leases and are therefore not included here, but are included in Note 10 . (c) Legal Proceedings - Flood Damage Claims - In 2013, several plaintiffs purporting to represent a class of residential and commercial property owners filed a complaint against CRANDIC, Alliant Energy and various other defendants in the Iowa District Court for Linn County. Plaintiffs assert claims of negligence and strict liability based on their allegations that CRANDIC (along with other defendants) caused or exacerbated flooding of the Cedar River in June 2008. In February 2016, the Iowa District Court for Linn County ruled in favor of Alliant Energy and CRANDIC and dismissed all claims against them, resulting in no loss. In August 2016, the Iowa District Court for Linn County dismissed all claims against the remaining defendants. In September 2016, plaintiffs filed a notice of appeal with the Supreme Court of Iowa. Alliant Energy does not currently believe any material losses for this complaint are both probable and reasonably estimated, and therefore has not recognized any material loss contingency amounts as of December 31, 2017 . Other - Alliant Energy, IPL and WPL are involved in other legal and administrative proceedings before various courts and agencies with respect to matters arising in the ordinary course of business. Although unable to predict the outcome of these matters, Alliant Energy, IPL and WPL believe that appropriate reserves have been established and final disposition of these actions will not have a material effect on their financial condition or results of operations. (d) Guarantees and Indemnifications - Whiting Petroleum - In 2004, Alliant Energy sold its remaining interest in Whiting Petroleum. Whiting Petroleum is an independent oil and gas company. Alliant Energy Resources, LLC, as the successor to a predecessor entity that owned Whiting Petroleum, and a wholly-owned subsidiary of AEF, continues to guarantee the partnership obligations of an affiliate of Whiting Petroleum under general partnership agreements in the oil and gas industry, including with respect to the future abandonment of certain platforms off the coast of California and related onshore plant and equipment owned by the partnerships. The guarantees do not include a maximum limit. As of December 31, 2017 , the present value of the abandonment obligations is estimated at $33 million . Alliant Energy is not aware of any material liabilities related to these guarantees of which it is probable that Alliant Energy Resources, LLC will be obligated to pay and therefore has not recognized any material liabilities related to this guarantee as of December 31, 2017 . Non-utility Wind Investment in Oklahoma - In July 2017, a wholly-owned subsidiary of AEF acquired a cash equity ownership interest in a non-utility wind farm located in Oklahoma. The wind farm provides electricity to a third-party under a long-term PPA. Alliant Energy provided a parent guarantee of its subsidiary’s indemnification obligations under the related operating agreement and PPA. Alliant Energy’s obligations under the operating agreement were $98 million as of December 31, 2017 and will reduce annually until expiring in July 2047 . Alliant Energy’s obligations under the PPA are subject to a maximum limit of $17 million and expire in December 2031 , subject to potential extension. Alliant Energy is not aware of any material liabilities related to this guarantee that it is probable that it will be obligated to pay and therefore has not recognized any material liabilities related to this guarantee as of December 31, 2017 . Refer to Note 6(a) for further discussion of the non-utility wind investment. IPL’s Minnesota Electric Distribution Assets - IPL provided indemnifications associated with the July 2015 sale of its Minnesota electric distribution assets for losses resulting from potential breach of IPL’s representations, warranties and obligations under the sale agreement. Alliant Energy and IPL believe the likelihood of having to make any material cash payments under these indemnifications is remote. IPL has not recorded any material liabilities related to these indemnifications as of December 31, 2017 . The general terms of the indemnifications provided by IPL included a maximum limit of $17 million and expire in October 2020 . Refer to Note 3 for further discussion of the sale of IPL’s Minnesota electric distribution assets. (e) Environmental Matters - Alliant Energy, IPL and WPL are subject to environmental regulations as a result of their current and past operations. These regulations are designed to protect public health and the environment and have resulted in compliance, remediation, containment and monitoring obligations, which are recorded as current and non-current environmental liabilities. Substantially all of the environmental liabilities recorded on the balance sheets relate to MGP sites. MGP Sites - IPL and WPL have current or previous ownership interests in various sites that are previously associated with the production of gas for which IPL and WPL have, or may have in the future, liability for investigation, remediation and monitoring costs. IPL and WPL are working pursuant to the requirements of various federal and state agencies to investigate, mitigate, prevent and remediate, where necessary, the environmental impacts to property, including natural resources, at and around these former MGP sites in order to protect public health and the environment. At December 31, 2017 , estimated future costs expected to be incurred for the investigation, remediation and monitoring of the MGP sites, as well as environmental liabilities recorded on the balance sheets for these sites, which are not discounted, were as follows (in millions). At December 31, 2017 , such amounts for WPL were not material. Alliant Energy IPL Range of estimated future costs $11 - $30 $9 - $27 Current and non-current environmental liabilities 15 13 WPL Consent Decree - In 2013, the U.S. District Court for the Western District of Wisconsin approved a Consent Decree that WPL, along with the other owners of Edgewater and Columbia, entered into with the EPA and the Sierra Club, thereby resolving claims against WPL. Such claims included allegations that the owners of Edgewater, Nelson Dewey and Columbia violated the Prevention of Significant Deterioration program requirements, Title V Operating Permit requirements of the CAA and the Wisconsin State Implementation Plan designed to implement the CAA. WPL has completed various requirements under the Consent Decree. WPL’s remaining requirements include installing an SCR system at Columbia Unit 2 and fuel switching or retiring Edgewater Unit 4 by December 31, 2018. The Consent Decree also establishes SO2, NOx and particulate matter emission rate limits for Columbia Units 1 and 2, and Edgewater Units 4 and 5. In addition, the Consent Decree includes annual plant-wide SO2 and NOx emission caps for Columbia and Edgewater. Alliant Energy and WPL currently expect to recover material costs incurred by WPL related to compliance with the terms of the Consent Decree from WPL’s electric customers. IPL Consent Decree - In 2015, the U.S. District Court for the Northern District of Iowa approved a Consent Decree that IPL entered into with the EPA, the Sierra Club, the State of Iowa and Linn County in Iowa, thereby resolving potential CAA issues associated with emissions from IPL’s coal-fired generating facilities in Iowa. IPL has completed various requirements under the Consent Decree. IPL’s remaining requirements include installing an SCR system or equivalent NOx reduction system at Ottumwa by December 31, 2019, and fuel switching or retiring Burlington by December 31, 2021 and Prairie Creek Units 1 and 3 by December 31, 2025. The Consent Decree also establishes SO2, NOx and particulate matter emission rate limits with varying averaging times for Burlington, Lansing, M.L. Kapp, Ottumwa and Prairie Creek. In addition, the Consent Decree includes calendar-year SO2 and NOx emission caps for Prairie Creek, and calendar-year SO2 and NOx emission caps in aggregate for Burlington, Lansing, M.L. Kapp, Ottumwa and Prairie Creek. Alliant Energy and IPL currently expect to recover material costs incurred by IPL related to compliance with the terms of the Consent Decree from IPL’s electric customers. Other Environmental Contingencies - In addition to the environmental liabilities discussed above, various environmental rules are monitored that may have a significant impact on future operations. Several of these environmental rules are subject to legal challenges, reconsideration and/or other uncertainties. Given uncertainties regarding the outcome, timing and compliance plans for these environmental matters, the complete financial impact of each of these rules is not able to be determined; however future capital investments and/or modifications to EGUs to comply with certain of these rules could be significant. Specific current, proposed or potential environmental matters include, among others: CSAPR, Effluent Limitation Guidelines, CCR Rule, and various legislation and EPA regulations to monitor and regulate the emission of GHG, including carbon emissions from new (CAA Section 111(b)) and existing (CAA Section 111(d)) fossil-fueled EGUs. (f) Credit Risk - IPL provides retail electric and gas services in Iowa and wholesale electric service in Minnesota, Illinois and Iowa. WPL provides retail electric and gas services and wholesale electric service in Wisconsin. The geographic concentration of IPL’s and WPL’s customers did not contribute significantly to overall credit risk exposure. In addition, as a result of a diverse customer base, IPL and WPL did not have any significant credit risk concentration for receivables arising from the sale of electricity or gas services. Alliant Energy, IPL and WPL are subject to credit risk related to the ability of counterparties to meet their contractual payment obligations or the potential non-performance of counterparties to deliver contracted commodities and other goods or services at the contracted price. Credit policies are maintained to mitigate credit risk. These credit policies include evaluation of the financial condition of certain counterparties, use of credit risk-related contingent provisions in certain agreements that require credit support from counterparties not meeting specific criteria, diversification of counterparties to reduce concentrations of credit risk and the use of standardized agreements that facilitate the netting of cash flows associated with certain counterparties. Based on these credit policies and counterparty diversification, as well as utility cost recovery mechanisms, it is unlikely that counterparty non-performance would have a material effect on financial condition or results of operations. However, there is no assurance that these items will protect against all losses from counterparty non-performance. Refer to Notes 5(a) and 15 for details of allowances for doubtful accounts and credit risk-related contingent features, respectively.
WPL [Member]
Commitments And Contingencies	COMMITMENTS AND CONTINGENCIES (a) Capital Purchase Obligations - Various contractual obligations contain minimum future commitments related to capital expenditures for certain construction projects. IPL’s projects include the expansion of wind generation and installation of an SCR system at Ottumwa Unit 1 to reduce NOx emissions at the EGU. WPL’s projects include West Riverside. At December 31, 2017 , Alliant Energy’s, IPL’s and WPL’s minimum future commitments related to certain contractual obligations for these projects were $82 million , $15 million and $67 million , respectively. (b) Other Purchase Obligations - Various commodity supply, transportation and storage contracts help meet obligations to provide electricity and natural gas to utility customers. In addition, there are various purchase obligations associated with other goods and services. At December 31, 2017 , minimum future commitments related to these purchase obligations were as follows (in millions): Alliant Energy 2018 2019 2020 2021 2022 Thereafter Total Purchased power (a) $188 $159 $135 $149 $140 $461 $1,232 Natural gas 229 140 130 110 84 253 946 Coal (b) 107 59 21 5 — — 192 Other (c) 21 6 6 3 2 2 40 $545 $364 $292 $267 $226 $716 $2,410 IPL 2018 2019 2020 2021 2022 Thereafter Total Purchased power (a) $130 $144 $135 $149 $140 $461 $1,159 Natural gas 126 58 44 32 22 102 384 Coal (b) 51 31 11 5 — — 98 Other (c) 15 3 3 3 2 2 28 $322 $236 $193 $189 $164 $565 $1,669 WPL 2018 2019 2020 2021 2022 Thereafter Total Purchased power (a) $58 $15 $— $— $— $— $73 Natural gas 103 82 86 78 62 151 562 Coal (b) 56 28 10 — — — 94 Other (c) 5 1 — — — — 6 $222 $126 $96 $78 $62 $151 $735 (a) Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. (b) Corporate Services entered into system-wide coal contracts on behalf of IPL and WPL that include minimum future commitments. These commitments were assigned to IPL and WPL based on information available as of December 31, 2017 regarding expected future usage, which is subject to change. (c) Includes individual commitments incurred during the normal course of business that exceeded $1 million at December 31, 2017 . Certain contracts are considered leases and are therefore not included here, but are included in Note 10 . (c) Legal Proceedings - Flood Damage Claims - In 2013, several plaintiffs purporting to represent a class of residential and commercial property owners filed a complaint against CRANDIC, Alliant Energy and various other defendants in the Iowa District Court for Linn County. Plaintiffs assert claims of negligence and strict liability based on their allegations that CRANDIC (along with other defendants) caused or exacerbated flooding of the Cedar River in June 2008. In February 2016, the Iowa District Court for Linn County ruled in favor of Alliant Energy and CRANDIC and dismissed all claims against them, resulting in no loss. In August 2016, the Iowa District Court for Linn County dismissed all claims against the remaining defendants. In September 2016, plaintiffs filed a notice of appeal with the Supreme Court of Iowa. Alliant Energy does not currently believe any material losses for this complaint are both probable and reasonably estimated, and therefore has not recognized any material loss contingency amounts as of December 31, 2017 . Other - Alliant Energy, IPL and WPL are involved in other legal and administrative proceedings before various courts and agencies with respect to matters arising in the ordinary course of business. Although unable to predict the outcome of these matters, Alliant Energy, IPL and WPL believe that appropriate reserves have been established and final disposition of these actions will not have a material effect on their financial condition or results of operations. (e) Environmental Matters - Alliant Energy, IPL and WPL are subject to environmental regulations as a result of their current and past operations. These regulations are designed to protect public health and the environment and have resulted in compliance, remediation, containment and monitoring obligations, which are recorded as current and non-current environmental liabilities. Substantially all of the environmental liabilities recorded on the balance sheets relate to MGP sites. MGP Sites - IPL and WPL have current or previous ownership interests in various sites that are previously associated with the production of gas for which IPL and WPL have, or may have in the future, liability for investigation, remediation and monitoring costs. IPL and WPL are working pursuant to the requirements of various federal and state agencies to investigate, mitigate, prevent and remediate, where necessary, the environmental impacts to property, including natural resources, at and around these former MGP sites in order to protect public health and the environment. At December 31, 2017 , estimated future costs expected to be incurred for the investigation, remediation and monitoring of the MGP sites, as well as environmental liabilities recorded on the balance sheets for these sites, which are not discounted, were as follows (in millions). At December 31, 2017 , such amounts for WPL were not material. Alliant Energy IPL Range of estimated future costs $11 - $30 $9 - $27 Current and non-current environmental liabilities 15 13 WPL Consent Decree - In 2013, the U.S. District Court for the Western District of Wisconsin approved a Consent Decree that WPL, along with the other owners of Edgewater and Columbia, entered into with the EPA and the Sierra Club, thereby resolving claims against WPL. Such claims included allegations that the owners of Edgewater, Nelson Dewey and Columbia violated the Prevention of Significant Deterioration program requirements, Title V Operating Permit requirements of the CAA and the Wisconsin State Implementation Plan designed to implement the CAA. WPL has completed various requirements under the Consent Decree. WPL’s remaining requirements include installing an SCR system at Columbia Unit 2 and fuel switching or retiring Edgewater Unit 4 by December 31, 2018. The Consent Decree also establishes SO2, NOx and particulate matter emission rate limits for Columbia Units 1 and 2, and Edgewater Units 4 and 5. In addition, the Consent Decree includes annual plant-wide SO2 and NOx emission caps for Columbia and Edgewater. Alliant Energy and WPL currently expect to recover material costs incurred by WPL related to compliance with the terms of the Consent Decree from WPL’s electric customers. IPL Consent Decree - In 2015, the U.S. District Court for the Northern District of Iowa approved a Consent Decree that IPL entered into with the EPA, the Sierra Club, the State of Iowa and Linn County in Iowa, thereby resolving potential CAA issues associated with emissions from IPL’s coal-fired generating facilities in Iowa. IPL has completed various requirements under the Consent Decree. IPL’s remaining requirements include installing an SCR system or equivalent NOx reduction system at Ottumwa by December 31, 2019, and fuel switching or retiring Burlington by December 31, 2021 and Prairie Creek Units 1 and 3 by December 31, 2025. The Consent Decree also establishes SO2, NOx and particulate matter emission rate limits with varying averaging times for Burlington, Lansing, M.L. Kapp, Ottumwa and Prairie Creek. In addition, the Consent Decree includes calendar-year SO2 and NOx emission caps for Prairie Creek, and calendar-year SO2 and NOx emission caps in aggregate for Burlington, Lansing, M.L. Kapp, Ottumwa and Prairie Creek. Alliant Energy and IPL currently expect to recover material costs incurred by IPL related to compliance with the terms of the Consent Decree from IPL’s electric customers. Other Environmental Contingencies - In addition to the environmental liabilities discussed above, various environmental rules are monitored that may have a significant impact on future operations. Several of these environmental rules are subject to legal challenges, reconsideration and/or other uncertainties. Given uncertainties regarding the outcome, timing and compliance plans for these environmental matters, the complete financial impact of each of these rules is not able to be determined; however future capital investments and/or modifications to EGUs to comply with certain of these rules could be significant. Specific current, proposed or potential environmental matters include, among others: CSAPR, Effluent Limitation Guidelines, CCR Rule, and various legislation and EPA regulations to monitor and regulate the emission of GHG, including carbon emissions from new (CAA Section 111(b)) and existing (CAA Section 111(d)) fossil-fueled EGUs. (f) Credit Risk - IPL provides retail electric and gas services in Iowa and wholesale electric service in Minnesota, Illinois and Iowa. WPL provides retail electric and gas services and wholesale electric service in Wisconsin. The geographic concentration of IPL’s and WPL’s customers did not contribute significantly to overall credit risk exposure. In addition, as a result of a diverse customer base, IPL and WPL did not have any significant credit risk concentration for receivables arising from the sale of electricity or gas services. Alliant Energy, IPL and WPL are subject to credit risk related to the ability of counterparties to meet their contractual payment obligations or the potential non-performance of counterparties to deliver contracted commodities and other goods or services at the contracted price. Credit policies are maintained to mitigate credit risk. These credit policies include evaluation of the financial condition of certain counterparties, use of credit risk-related contingent provisions in certain agreements that require credit support from counterparties not meeting specific criteria, diversification of counterparties to reduce concentrations of credit risk and the use of standardized agreements that facilitate the netting of cash flows associated with certain counterparties. Based on these credit policies and counterparty diversification, as well as utility cost recovery mechanisms, it is unlikely that counterparty non-performance would have a material effect on financial condition or results of operations. However, there is no assurance that these items will protect against all losses from counterparty non-performance. Refer to Notes 5(a) and 15 for details of allowances for doubtful accounts and credit risk-related contingent features, respectively.

Segments Of Business

Segments Of Business	12 Months Ended
	Dec. 31, 2017
Segment Reporting Information [Line Items]
Segments Of Business	SEGMENTS OF BUSINESS In the fourth quarter of 2017, Alliant Energy and WPL modified the segment reporting related to their ATC Investment, consistent with information used by their chief operating decision maker to evaluate performance and allocate resources. As of December 31, 2017, the ATC Investment is no longer included in Alliant Energy’s utility electric operations reportable segment or WPL’s electric operations reportable segment. As a result, all prior period amounts impacted by this change were reclassified to conform to the new presentation. Alliant Energy’s related amounts were reclassified from “Electric Utility” to “ATC Investment, Non-Utility, Parent and Other.” WPL’s related amounts were reclassified from “Electric” to “Other.” Alliant Energy - Alliant Energy’s principal businesses as of December 31, 2017 are: • Utility - includes the operations of IPL and WPL, which primarily serve retail customers in Iowa and Wisconsin. The utility business has three reportable segments: a) utility electric operations; b) utility gas operations; and c) utility other, which includes steam operations and the unallocated portions of the utility business. Various line items in the following tables are not allocated to the electric and gas segments for management reporting purposes, and therefore, are included only in “Total Utility.” • ATC Investment, Non-utility, Parent and Other - includes the operations of AEF and its subsidiaries, Corporate Services, the Alliant Energy parent company, and any Alliant Energy parent company consolidating adjustments. AEF is comprised of Alliant Energy’s ATC Investment, Transportation, a non-utility wind investment, the Sheboygan Falls Energy Facility and other non-utility investments, which are described in Note 1(a) . Alliant Energy’s administrative support services are directly charged to the applicable segment where practicable. In all other cases, administrative support services are allocated to the applicable segment based on services agreements. Intersegment revenues were not material to Alliant Energy’s operations and there was no single customer whose revenues were 10% or more of Alliant Energy’s consolidated revenues. All of Alliant Energy’s operations and assets are located in the U.S. Certain financial information relating to Alliant Energy’s business segments, which represent the services provided to its customers, was as follows (in millions): ATC Investment, Utility Non-utility, Alliant Energy 2017 Electric Gas Other Total Parent and Other Consolidated Operating revenues $2,894.7 $400.9 $47.5 $3,343.1 $39.1 $3,382.2 Depreciation and amortization 412.0 38.2 7.7 457.9 3.9 461.8 Operating income (loss) 586.5 45.3 (11.7 ) 620.1 33.3 653.4 Interest expense 206.2 9.4 215.6 Equity income from unconsolidated investments, net (0.7 ) — — (0.7 ) (44.1 ) (44.8 ) Income taxes 51.0 15.7 66.7 Net income attributable to Alliant Energy common shareowners 403.4 53.9 457.3 Total assets 11,396.2 1,199.8 766.5 13,362.5 825.3 14,187.8 Investments in equity method subsidiaries 8.3 — — 8.3 373.1 381.4 Construction and acquisition expenditures 1,154.9 125.2 1.7 1,281.8 185.1 1,466.9 ATC Investment, Utility Non-utility, Alliant Energy 2016 Electric Gas Other Total Parent and Other Consolidated Operating revenues $2,875.5 $355.4 $48.6 $3,279.5 $40.5 $3,320.0 Depreciation and amortization 367.0 34.2 2.1 403.3 8.3 411.6 Operating income (loss) 571.9 30.7 (4.8 ) 597.8 (60.8 ) 537.0 Interest expense 194.6 1.6 196.2 Equity income from unconsolidated investments, net (0.7 ) — — (0.7 ) (38.9 ) (39.6 ) Income tax expense (benefit) 71.4 (12.0 ) 59.4 Net income (loss) attributable to Alliant Energy common shareowners 385.2 (13.7 ) 371.5 Total assets 10,722.9 1,091.1 781.0 12,595.0 778.8 13,373.8 Investments in equity method subsidiaries 7.7 — — 7.7 318.3 326.0 Construction and acquisition expenditures 994.0 137.1 0.1 1,131.2 65.6 1,196.8 ATC Investment, Utility Non-utility, Alliant Energy 2015 Electric Gas Other Total Parent and Other Consolidated Operating revenues $2,770.5 $381.2 $57.9 $3,209.6 $44.0 $3,253.6 Depreciation and amortization 358.6 31.1 1.8 391.5 9.8 401.3 Operating income 514.1 34.6 1.9 550.6 26.4 577.0 Interest expense 189.2 (2.1 ) 187.1 Equity income from unconsolidated investments, net (0.9 ) — — (0.9 ) (32.9 ) (33.8 ) Income taxes 46.2 24.2 70.4 Net income attributable to Alliant Energy common shareowners 343.4 34.8 378.2 Total assets 9,918.0 939.3 828.9 11,686.2 809.0 12,495.2 Investments in equity method subsidiaries 8.7 — — 8.7 294.2 302.9 Construction and acquisition expenditures 852.5 106.4 1.4 960.3 74.0 1,034.3 IPL - IPL is a utility primarily serving retail customers in Iowa and includes three reportable segments: a) electric operations; b) gas operations; and c) other, which includes steam operations and the unallocated portions of the utility business. Various line items in the following tables are not allocated to the electric and gas segments for management reporting purposes, and therefore, are included only in “Total.” Intersegment revenues were not material to IPL’s operations and there was no single customer whose revenues were 10% or more of IPL’s consolidated revenues. All of IPL’s operations and assets are located in the U.S. Certain financial information relating to IPL’s business segments, which represent the services provided to its customers, was as follows (in millions): 2017 Electric Gas Other Total Operating revenues $1,598.9 $226.0 $45.4 $1,870.3 Depreciation and amortization 215.1 22.2 7.7 245.0 Operating income (loss) 281.1 20.8 (5.0 ) 296.9 Interest expense 112.4 Income tax benefit (10.9 ) Earnings available for common stock 216.8 Total assets 6,524.4 727.9 353.7 7,606.0 Construction and acquisition expenditures 594.1 80.7 1.2 676.0 2016 Electric Gas Other Total Operating revenues $1,569.7 $204.0 $46.7 $1,820.4 Depreciation and amortization 189.4 19.3 2.1 210.8 Operating income 252.0 15.5 3.3 270.8 Interest expense 103.2 Income tax benefit (5.9 ) Earnings available for common stock 215.6 Total assets 6,278.2 653.3 373.2 7,304.7 Construction and acquisition expenditures 598.1 91.5 0.1 689.7 2015 Electric Gas Other Total Operating revenues $1,503.8 $217.3 $53.4 $1,774.5 Depreciation and amortization 187.9 17.5 1.8 207.2 Operating income 218.8 17.7 5.4 241.9 Interest expense 96.8 Income tax benefit (22.7 ) Earnings available for common stock 186.0 Total assets 5,754.1 548.2 406.8 6,709.1 Construction and acquisition expenditures 561.2 56.7 1.4 619.3 WPL - WPL is a utility serving customers in Wisconsin and includes three reportable segments: a) electric operations; b) gas operations; and c) other, which includes WPL’s investment in ATC in 2015 and 2016, and the unallocated portions of the utility business. Refer to Note 6(a) for discussion of WPL’s transfer of its investment in ATC to ATI as of December 31, 2016. Various line items in the following tables are not allocated to the electric and gas segments for management reporting purposes, and therefore, are included only in “Total.” Intersegment revenues were not material to WPL’s operations and there was no single customer whose revenues were 10% or more of WPL’s consolidated revenues. All of WPL’s operations and assets are located in the U.S. Certain financial information relating to WPL’s business segments, which represent the services provided to its customers, was as follows (in millions): 2017 Electric Gas Other Total Operating revenues $1,295.8 $174.9 $2.1 $1,472.8 Depreciation and amortization 196.9 16.0 — 212.9 Operating income (loss) 305.4 24.5 (6.7 ) 323.2 Interest expense 93.8 Equity income from unconsolidated investments (0.7 ) — — (0.7 ) Income taxes 61.9 Earnings available for common stock 186.6 Total assets 4,871.8 471.9 412.8 5,756.5 Investments in equity method subsidiaries 8.3 — — 8.3 Construction and acquisition expenditures 592.4 44.5 0.5 637.4 2016 Electric Gas Other Total Operating revenues $1,305.8 $151.4 $1.9 $1,459.1 Depreciation and amortization 177.6 14.9 — 192.5 Operating income (loss) 319.9 15.2 (8.1 ) 327.0 Interest expense 91.4 Equity income from unconsolidated investments (0.7 ) — (39.1 ) (39.8 ) Income taxes 93.3 Earnings available for common stock 190.4 Total assets 4,444.7 437.8 407.8 5,290.3 Investments in equity method subsidiaries 7.7 — — 7.7 Construction and acquisition expenditures 395.9 45.6 11.5 453.0 2015 Electric Gas Other Total Operating revenues $1,266.7 $163.9 $4.5 $1,435.1 Depreciation and amortization 170.7 13.6 — 184.3 Operating income (loss) 295.3 16.9 (3.5 ) 308.7 Interest expense 92.4 Equity income from unconsolidated investments (0.9 ) — (34.2 ) (35.1 ) Income taxes 82.9 Earnings available for common stock 176.3 Total assets 4,163.9 391.1 715.4 5,270.4 Investments in equity method subsidiaries 8.7 — 293.3 302.0 Construction and acquisition expenditures 291.3 49.7 3.3 344.3
IPL [Member]
Segment Reporting Information [Line Items]
Segments Of Business	SEGMENTS OF BUSINESS In the fourth quarter of 2017, Alliant Energy and WPL modified the segment reporting related to their ATC Investment, consistent with information used by their chief operating decision maker to evaluate performance and allocate resources. As of December 31, 2017, the ATC Investment is no longer included in Alliant Energy’s utility electric operations reportable segment or WPL’s electric operations reportable segment. As a result, all prior period amounts impacted by this change were reclassified to conform to the new presentation. Alliant Energy’s related amounts were reclassified from “Electric Utility” to “ATC Investment, Non-Utility, Parent and Other.” WPL’s related amounts were reclassified from “Electric” to “Other.” Alliant Energy - Alliant Energy’s principal businesses as of December 31, 2017 are: • Utility - includes the operations of IPL and WPL, which primarily serve retail customers in Iowa and Wisconsin. The utility business has three reportable segments: a) utility electric operations; b) utility gas operations; and c) utility other, which includes steam operations and the unallocated portions of the utility business. Various line items in the following tables are not allocated to the electric and gas segments for management reporting purposes, and therefore, are included only in “Total Utility.” • ATC Investment, Non-utility, Parent and Other - includes the operations of AEF and its subsidiaries, Corporate Services, the Alliant Energy parent company, and any Alliant Energy parent company consolidating adjustments. AEF is comprised of Alliant Energy’s ATC Investment, Transportation, a non-utility wind investment, the Sheboygan Falls Energy Facility and other non-utility investments, which are described in Note 1(a) . Alliant Energy’s administrative support services are directly charged to the applicable segment where practicable. In all other cases, administrative support services are allocated to the applicable segment based on services agreements. Intersegment revenues were not material to Alliant Energy’s operations and there was no single customer whose revenues were 10% or more of Alliant Energy’s consolidated revenues. All of Alliant Energy’s operations and assets are located in the U.S. Certain financial information relating to Alliant Energy’s business segments, which represent the services provided to its customers, was as follows (in millions): ATC Investment, Utility Non-utility, Alliant Energy 2017 Electric Gas Other Total Parent and Other Consolidated Operating revenues $2,894.7 $400.9 $47.5 $3,343.1 $39.1 $3,382.2 Depreciation and amortization 412.0 38.2 7.7 457.9 3.9 461.8 Operating income (loss) 586.5 45.3 (11.7 ) 620.1 33.3 653.4 Interest expense 206.2 9.4 215.6 Equity income from unconsolidated investments, net (0.7 ) — — (0.7 ) (44.1 ) (44.8 ) Income taxes 51.0 15.7 66.7 Net income attributable to Alliant Energy common shareowners 403.4 53.9 457.3 Total assets 11,396.2 1,199.8 766.5 13,362.5 825.3 14,187.8 Investments in equity method subsidiaries 8.3 — — 8.3 373.1 381.4 Construction and acquisition expenditures 1,154.9 125.2 1.7 1,281.8 185.1 1,466.9 ATC Investment, Utility Non-utility, Alliant Energy 2016 Electric Gas Other Total Parent and Other Consolidated Operating revenues $2,875.5 $355.4 $48.6 $3,279.5 $40.5 $3,320.0 Depreciation and amortization 367.0 34.2 2.1 403.3 8.3 411.6 Operating income (loss) 571.9 30.7 (4.8 ) 597.8 (60.8 ) 537.0 Interest expense 194.6 1.6 196.2 Equity income from unconsolidated investments, net (0.7 ) — — (0.7 ) (38.9 ) (39.6 ) Income tax expense (benefit) 71.4 (12.0 ) 59.4 Net income (loss) attributable to Alliant Energy common shareowners 385.2 (13.7 ) 371.5 Total assets 10,722.9 1,091.1 781.0 12,595.0 778.8 13,373.8 Investments in equity method subsidiaries 7.7 — — 7.7 318.3 326.0 Construction and acquisition expenditures 994.0 137.1 0.1 1,131.2 65.6 1,196.8 ATC Investment, Utility Non-utility, Alliant Energy 2015 Electric Gas Other Total Parent and Other Consolidated Operating revenues $2,770.5 $381.2 $57.9 $3,209.6 $44.0 $3,253.6 Depreciation and amortization 358.6 31.1 1.8 391.5 9.8 401.3 Operating income 514.1 34.6 1.9 550.6 26.4 577.0 Interest expense 189.2 (2.1 ) 187.1 Equity income from unconsolidated investments, net (0.9 ) — — (0.9 ) (32.9 ) (33.8 ) Income taxes 46.2 24.2 70.4 Net income attributable to Alliant Energy common shareowners 343.4 34.8 378.2 Total assets 9,918.0 939.3 828.9 11,686.2 809.0 12,495.2 Investments in equity method subsidiaries 8.7 — — 8.7 294.2 302.9 Construction and acquisition expenditures 852.5 106.4 1.4 960.3 74.0 1,034.3 IPL - IPL is a utility primarily serving retail customers in Iowa and includes three reportable segments: a) electric operations; b) gas operations; and c) other, which includes steam operations and the unallocated portions of the utility business. Various line items in the following tables are not allocated to the electric and gas segments for management reporting purposes, and therefore, are included only in “Total.” Intersegment revenues were not material to IPL’s operations and there was no single customer whose revenues were 10% or more of IPL’s consolidated revenues. All of IPL’s operations and assets are located in the U.S. Certain financial information relating to IPL’s business segments, which represent the services provided to its customers, was as follows (in millions): 2017 Electric Gas Other Total Operating revenues $1,598.9 $226.0 $45.4 $1,870.3 Depreciation and amortization 215.1 22.2 7.7 245.0 Operating income (loss) 281.1 20.8 (5.0 ) 296.9 Interest expense 112.4 Income tax benefit (10.9 ) Earnings available for common stock 216.8 Total assets 6,524.4 727.9 353.7 7,606.0 Construction and acquisition expenditures 594.1 80.7 1.2 676.0 2016 Electric Gas Other Total Operating revenues $1,569.7 $204.0 $46.7 $1,820.4 Depreciation and amortization 189.4 19.3 2.1 210.8 Operating income 252.0 15.5 3.3 270.8 Interest expense 103.2 Income tax benefit (5.9 ) Earnings available for common stock 215.6 Total assets 6,278.2 653.3 373.2 7,304.7 Construction and acquisition expenditures 598.1 91.5 0.1 689.7 2015 Electric Gas Other Total Operating revenues $1,503.8 $217.3 $53.4 $1,774.5 Depreciation and amortization 187.9 17.5 1.8 207.2 Operating income 218.8 17.7 5.4 241.9 Interest expense 96.8 Income tax benefit (22.7 ) Earnings available for common stock 186.0 Total assets 5,754.1 548.2 406.8 6,709.1 Construction and acquisition expenditures 561.2 56.7 1.4 619.3 WPL - WPL is a utility serving customers in Wisconsin and includes three reportable segments: a) electric operations; b) gas operations; and c) other, which includes WPL’s investment in ATC in 2015 and 2016, and the unallocated portions of the utility business. Refer to Note 6(a) for discussion of WPL’s transfer of its investment in ATC to ATI as of December 31, 2016. Various line items in the following tables are not allocated to the electric and gas segments for management reporting purposes, and therefore, are included only in “Total.” Intersegment revenues were not material to WPL’s operations and there was no single customer whose revenues were 10% or more of WPL’s consolidated revenues. All of WPL’s operations and assets are located in the U.S. Certain financial information relating to WPL’s business segments, which represent the services provided to its customers, was as follows (in millions): 2017 Electric Gas Other Total Operating revenues $1,295.8 $174.9 $2.1 $1,472.8 Depreciation and amortization 196.9 16.0 — 212.9 Operating income (loss) 305.4 24.5 (6.7 ) 323.2 Interest expense 93.8 Equity income from unconsolidated investments (0.7 ) — — (0.7 ) Income taxes 61.9 Earnings available for common stock 186.6 Total assets 4,871.8 471.9 412.8 5,756.5 Investments in equity method subsidiaries 8.3 — — 8.3 Construction and acquisition expenditures 592.4 44.5 0.5 637.4 2016 Electric Gas Other Total Operating revenues $1,305.8 $151.4 $1.9 $1,459.1 Depreciation and amortization 177.6 14.9 — 192.5 Operating income (loss) 319.9 15.2 (8.1 ) 327.0 Interest expense 91.4 Equity income from unconsolidated investments (0.7 ) — (39.1 ) (39.8 ) Income taxes 93.3 Earnings available for common stock 190.4 Total assets 4,444.7 437.8 407.8 5,290.3 Investments in equity method subsidiaries 7.7 — — 7.7 Construction and acquisition expenditures 395.9 45.6 11.5 453.0 2015 Electric Gas Other Total Operating revenues $1,266.7 $163.9 $4.5 $1,435.1 Depreciation and amortization 170.7 13.6 — 184.3 Operating income (loss) 295.3 16.9 (3.5 ) 308.7 Interest expense 92.4 Equity income from unconsolidated investments (0.9 ) — (34.2 ) (35.1 ) Income taxes 82.9 Earnings available for common stock 176.3 Total assets 4,163.9 391.1 715.4 5,270.4 Investments in equity method subsidiaries 8.7 — 293.3 302.0 Construction and acquisition expenditures 291.3 49.7 3.3 344.3
WPL [Member]
Segment Reporting Information [Line Items]
Segments Of Business	SEGMENTS OF BUSINESS In the fourth quarter of 2017, Alliant Energy and WPL modified the segment reporting related to their ATC Investment, consistent with information used by their chief operating decision maker to evaluate performance and allocate resources. As of December 31, 2017, the ATC Investment is no longer included in Alliant Energy’s utility electric operations reportable segment or WPL’s electric operations reportable segment. As a result, all prior period amounts impacted by this change were reclassified to conform to the new presentation. Alliant Energy’s related amounts were reclassified from “Electric Utility” to “ATC Investment, Non-Utility, Parent and Other.” WPL’s related amounts were reclassified from “Electric” to “Other.” Alliant Energy - Alliant Energy’s principal businesses as of December 31, 2017 are: • Utility - includes the operations of IPL and WPL, which primarily serve retail customers in Iowa and Wisconsin. The utility business has three reportable segments: a) utility electric operations; b) utility gas operations; and c) utility other, which includes steam operations and the unallocated portions of the utility business. Various line items in the following tables are not allocated to the electric and gas segments for management reporting purposes, and therefore, are included only in “Total Utility.” • ATC Investment, Non-utility, Parent and Other - includes the operations of AEF and its subsidiaries, Corporate Services, the Alliant Energy parent company, and any Alliant Energy parent company consolidating adjustments. AEF is comprised of Alliant Energy’s ATC Investment, Transportation, a non-utility wind investment, the Sheboygan Falls Energy Facility and other non-utility investments, which are described in Note 1(a) . Alliant Energy’s administrative support services are directly charged to the applicable segment where practicable. In all other cases, administrative support services are allocated to the applicable segment based on services agreements. Intersegment revenues were not material to Alliant Energy’s operations and there was no single customer whose revenues were 10% or more of Alliant Energy’s consolidated revenues. All of Alliant Energy’s operations and assets are located in the U.S. Certain financial information relating to Alliant Energy’s business segments, which represent the services provided to its customers, was as follows (in millions): ATC Investment, Utility Non-utility, Alliant Energy 2017 Electric Gas Other Total Parent and Other Consolidated Operating revenues $2,894.7 $400.9 $47.5 $3,343.1 $39.1 $3,382.2 Depreciation and amortization 412.0 38.2 7.7 457.9 3.9 461.8 Operating income (loss) 586.5 45.3 (11.7 ) 620.1 33.3 653.4 Interest expense 206.2 9.4 215.6 Equity income from unconsolidated investments, net (0.7 ) — — (0.7 ) (44.1 ) (44.8 ) Income taxes 51.0 15.7 66.7 Net income attributable to Alliant Energy common shareowners 403.4 53.9 457.3 Total assets 11,396.2 1,199.8 766.5 13,362.5 825.3 14,187.8 Investments in equity method subsidiaries 8.3 — — 8.3 373.1 381.4 Construction and acquisition expenditures 1,154.9 125.2 1.7 1,281.8 185.1 1,466.9 ATC Investment, Utility Non-utility, Alliant Energy 2016 Electric Gas Other Total Parent and Other Consolidated Operating revenues $2,875.5 $355.4 $48.6 $3,279.5 $40.5 $3,320.0 Depreciation and amortization 367.0 34.2 2.1 403.3 8.3 411.6 Operating income (loss) 571.9 30.7 (4.8 ) 597.8 (60.8 ) 537.0 Interest expense 194.6 1.6 196.2 Equity income from unconsolidated investments, net (0.7 ) — — (0.7 ) (38.9 ) (39.6 ) Income tax expense (benefit) 71.4 (12.0 ) 59.4 Net income (loss) attributable to Alliant Energy common shareowners 385.2 (13.7 ) 371.5 Total assets 10,722.9 1,091.1 781.0 12,595.0 778.8 13,373.8 Investments in equity method subsidiaries 7.7 — — 7.7 318.3 326.0 Construction and acquisition expenditures 994.0 137.1 0.1 1,131.2 65.6 1,196.8 ATC Investment, Utility Non-utility, Alliant Energy 2015 Electric Gas Other Total Parent and Other Consolidated Operating revenues $2,770.5 $381.2 $57.9 $3,209.6 $44.0 $3,253.6 Depreciation and amortization 358.6 31.1 1.8 391.5 9.8 401.3 Operating income 514.1 34.6 1.9 550.6 26.4 577.0 Interest expense 189.2 (2.1 ) 187.1 Equity income from unconsolidated investments, net (0.9 ) — — (0.9 ) (32.9 ) (33.8 ) Income taxes 46.2 24.2 70.4 Net income attributable to Alliant Energy common shareowners 343.4 34.8 378.2 Total assets 9,918.0 939.3 828.9 11,686.2 809.0 12,495.2 Investments in equity method subsidiaries 8.7 — — 8.7 294.2 302.9 Construction and acquisition expenditures 852.5 106.4 1.4 960.3 74.0 1,034.3 IPL - IPL is a utility primarily serving retail customers in Iowa and includes three reportable segments: a) electric operations; b) gas operations; and c) other, which includes steam operations and the unallocated portions of the utility business. Various line items in the following tables are not allocated to the electric and gas segments for management reporting purposes, and therefore, are included only in “Total.” Intersegment revenues were not material to IPL’s operations and there was no single customer whose revenues were 10% or more of IPL’s consolidated revenues. All of IPL’s operations and assets are located in the U.S. Certain financial information relating to IPL’s business segments, which represent the services provided to its customers, was as follows (in millions): 2017 Electric Gas Other Total Operating revenues $1,598.9 $226.0 $45.4 $1,870.3 Depreciation and amortization 215.1 22.2 7.7 245.0 Operating income (loss) 281.1 20.8 (5.0 ) 296.9 Interest expense 112.4 Income tax benefit (10.9 ) Earnings available for common stock 216.8 Total assets 6,524.4 727.9 353.7 7,606.0 Construction and acquisition expenditures 594.1 80.7 1.2 676.0 2016 Electric Gas Other Total Operating revenues $1,569.7 $204.0 $46.7 $1,820.4 Depreciation and amortization 189.4 19.3 2.1 210.8 Operating income 252.0 15.5 3.3 270.8 Interest expense 103.2 Income tax benefit (5.9 ) Earnings available for common stock 215.6 Total assets 6,278.2 653.3 373.2 7,304.7 Construction and acquisition expenditures 598.1 91.5 0.1 689.7 2015 Electric Gas Other Total Operating revenues $1,503.8 $217.3 $53.4 $1,774.5 Depreciation and amortization 187.9 17.5 1.8 207.2 Operating income 218.8 17.7 5.4 241.9 Interest expense 96.8 Income tax benefit (22.7 ) Earnings available for common stock 186.0 Total assets 5,754.1 548.2 406.8 6,709.1 Construction and acquisition expenditures 561.2 56.7 1.4 619.3 WPL - WPL is a utility serving customers in Wisconsin and includes three reportable segments: a) electric operations; b) gas operations; and c) other, which includes WPL’s investment in ATC in 2015 and 2016, and the unallocated portions of the utility business. Refer to Note 6(a) for discussion of WPL’s transfer of its investment in ATC to ATI as of December 31, 2016. Various line items in the following tables are not allocated to the electric and gas segments for management reporting purposes, and therefore, are included only in “Total.” Intersegment revenues were not material to WPL’s operations and there was no single customer whose revenues were 10% or more of WPL’s consolidated revenues. All of WPL’s operations and assets are located in the U.S. Certain financial information relating to WPL’s business segments, which represent the services provided to its customers, was as follows (in millions): 2017 Electric Gas Other Total Operating revenues $1,295.8 $174.9 $2.1 $1,472.8 Depreciation and amortization 196.9 16.0 — 212.9 Operating income (loss) 305.4 24.5 (6.7 ) 323.2 Interest expense 93.8 Equity income from unconsolidated investments (0.7 ) — — (0.7 ) Income taxes 61.9 Earnings available for common stock 186.6 Total assets 4,871.8 471.9 412.8 5,756.5 Investments in equity method subsidiaries 8.3 — — 8.3 Construction and acquisition expenditures 592.4 44.5 0.5 637.4 2016 Electric Gas Other Total Operating revenues $1,305.8 $151.4 $1.9 $1,459.1 Depreciation and amortization 177.6 14.9 — 192.5 Operating income (loss) 319.9 15.2 (8.1 ) 327.0 Interest expense 91.4 Equity income from unconsolidated investments (0.7 ) — (39.1 ) (39.8 ) Income taxes 93.3 Earnings available for common stock 190.4 Total assets 4,444.7 437.8 407.8 5,290.3 Investments in equity method subsidiaries 7.7 — — 7.7 Construction and acquisition expenditures 395.9 45.6 11.5 453.0 2015 Electric Gas Other Total Operating revenues $1,266.7 $163.9 $4.5 $1,435.1 Depreciation and amortization 170.7 13.6 — 184.3 Operating income (loss) 295.3 16.9 (3.5 ) 308.7 Interest expense 92.4 Equity income from unconsolidated investments (0.9 ) — (34.2 ) (35.1 ) Income taxes 82.9 Earnings available for common stock 176.3 Total assets 4,163.9 391.1 715.4 5,270.4 Investments in equity method subsidiaries 8.7 — 293.3 302.0 Construction and acquisition expenditures 291.3 49.7 3.3 344.3

Related Parties

Related Parties	12 Months Ended
	Dec. 31, 2017
Related Party Transaction [Line Items]
Related Parties	RELATED PARTIES Service Agreements - Pursuant to service agreements, IPL and WPL receive various administrative and general services from an affiliate, Corporate Services. These services are billed to IPL and WPL at cost based on expenses incurred by Corporate Services for the benefit of IPL and WPL, respectively. These costs consisted primarily of employee compensation and benefits, fees associated with various professional services, depreciation and amortization of property, plant and equipment, and a return on net assets. Corporate Services also acts as agent on behalf of IPL and WPL pursuant to the service agreements. As agent, Corporate Services enters into energy, capacity, ancillary services, and transmission sale and purchase transactions within MISO. Corporate Services assigns such sales and purchases among IPL and WPL based on statements received from MISO. The amounts billed for services provided, sales credited and purchases were as follows (in millions): IPL WPL 2017 2016 2015 2017 2016 2015 Corporate Services billings $177 $161 $150 $135 $133 $121 Sales credited 23 8 10 13 7 24 Purchases billed 364 433 366 115 102 66 As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2017 2016 IPL $114 $104 WPL 61 72 ATC - Pursuant to various agreements, WPL receives a range of transmission services from ATC. WPL provides operation, maintenance, and construction services to ATC. WPL and ATC also bill each other for use of shared facilities owned by each party. The related amounts billed between the parties were as follows (in millions): 2017 2016 2015 ATC billings to WPL $105 $110 $101 WPL billings to ATC 10 13 13 As of December 31, 2017 and 2016 , WPL owed ATC net amounts of $9 million and $8 million , respectively. Refer to Note 6(a) for discussion of WPL’s transfer of its investment in ATC to ATI on December 31, 2016. WPL’s Sheboygan Falls Energy Facility Lease - Refer to Note 10(b) for discussion of WPL’s Sheboygan Falls Energy Facility lease. Franklin County Wind Farm - Refer to Note 3 for discussion of the transfer of the Franklin County wind farm from AEF to IPL in April 2017.
IPL [Member]
Related Party Transaction [Line Items]
Related Parties	RELATED PARTIES Service Agreements - Pursuant to service agreements, IPL and WPL receive various administrative and general services from an affiliate, Corporate Services. These services are billed to IPL and WPL at cost based on expenses incurred by Corporate Services for the benefit of IPL and WPL, respectively. These costs consisted primarily of employee compensation and benefits, fees associated with various professional services, depreciation and amortization of property, plant and equipment, and a return on net assets. Corporate Services also acts as agent on behalf of IPL and WPL pursuant to the service agreements. As agent, Corporate Services enters into energy, capacity, ancillary services, and transmission sale and purchase transactions within MISO. Corporate Services assigns such sales and purchases among IPL and WPL based on statements received from MISO. The amounts billed for services provided, sales credited and purchases were as follows (in millions): IPL WPL 2017 2016 2015 2017 2016 2015 Corporate Services billings $177 $161 $150 $135 $133 $121 Sales credited 23 8 10 13 7 24 Purchases billed 364 433 366 115 102 66 As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2017 2016 IPL $114 $104 WPL 61 72 ATC - Pursuant to various agreements, WPL receives a range of transmission services from ATC. WPL provides operation, maintenance, and construction services to ATC. WPL and ATC also bill each other for use of shared facilities owned by each party. The related amounts billed between the parties were as follows (in millions): 2017 2016 2015 ATC billings to WPL $105 $110 $101 WPL billings to ATC 10 13 13 As of December 31, 2017 and 2016 , WPL owed ATC net amounts of $9 million and $8 million , respectively. Refer to Note 6(a) for discussion of WPL’s transfer of its investment in ATC to ATI on December 31, 2016. WPL’s Sheboygan Falls Energy Facility Lease - Refer to Note 10(b) for discussion of WPL’s Sheboygan Falls Energy Facility lease. Franklin County Wind Farm - Refer to Note 3 for discussion of the transfer of the Franklin County wind farm from AEF to IPL in April 2017.
WPL [Member]
Related Party Transaction [Line Items]
Related Parties	RELATED PARTIES Service Agreements - Pursuant to service agreements, IPL and WPL receive various administrative and general services from an affiliate, Corporate Services. These services are billed to IPL and WPL at cost based on expenses incurred by Corporate Services for the benefit of IPL and WPL, respectively. These costs consisted primarily of employee compensation and benefits, fees associated with various professional services, depreciation and amortization of property, plant and equipment, and a return on net assets. Corporate Services also acts as agent on behalf of IPL and WPL pursuant to the service agreements. As agent, Corporate Services enters into energy, capacity, ancillary services, and transmission sale and purchase transactions within MISO. Corporate Services assigns such sales and purchases among IPL and WPL based on statements received from MISO. The amounts billed for services provided, sales credited and purchases were as follows (in millions): IPL WPL 2017 2016 2015 2017 2016 2015 Corporate Services billings $177 $161 $150 $135 $133 $121 Sales credited 23 8 10 13 7 24 Purchases billed 364 433 366 115 102 66 As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2017 2016 IPL $114 $104 WPL 61 72 ATC - Pursuant to various agreements, WPL receives a range of transmission services from ATC. WPL provides operation, maintenance, and construction services to ATC. WPL and ATC also bill each other for use of shared facilities owned by each party. The related amounts billed between the parties were as follows (in millions): 2017 2016 2015 ATC billings to WPL $105 $110 $101 WPL billings to ATC 10 13 13 As of December 31, 2017 and 2016 , WPL owed ATC net amounts of $9 million and $8 million , respectively. Refer to Note 6(a) for discussion of WPL’s transfer of its investment in ATC to ATI on December 31, 2016. WPL’s Sheboygan Falls Energy Facility Lease - Refer to Note 10(b) for discussion of WPL’s Sheboygan Falls Energy Facility lease. Franklin County Wind Farm - Refer to Note 3 for discussion of the transfer of the Franklin County wind farm from AEF to IPL in April 2017.

Selected Consolidated Quarterly

Selected Consolidated Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2017
Selected Consolidated Quarterly Financial Data (Unaudited)	SELECTED CONSOLIDATED QUARTERLY FINANCIAL DATA (UNAUDITED) Alliant Energy - All “per share” references refer to earnings per diluted share. Summation of the individual quarters may not equal annual totals due to rounding. 2017 2016 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions, except per share data) Operating revenues $853.9 $765.3 $906.9 $856.1 $843.8 $754.6 $924.6 $797.0 Operating income 142.9 149.3 231.5 129.7 145.9 128.6 162.6 99.9 Amounts attributable to Alliant Energy common shareowners: Income from continuing operations, net of tax 99.0 94.3 168.8 93.8 97.6 84.4 128.8 63.0 Income (loss) from discontinued operations, net of tax 1.4 — — — (1.1 ) (0.5 ) (0.4 ) (0.3 ) Net income 100.4 94.3 168.8 93.8 96.5 83.9 128.4 62.7 Earnings per weighted average common share attributable to Alliant Energy common shareowners: Income from continuing operations, net of tax 0.43 0.41 0.73 0.41 0.43 0.37 0.57 0.28 Income from discontinued operations, net of tax 0.01 — — — — — — — Net income 0.44 0.41 0.73 0.41 0.43 0.37 0.57 0.28 IPL - Earnings per share data is not disclosed for IPL given Alliant Energy is the sole shareowner of all shares of IPL’s common stock outstanding during the periods presented. 2017 2016 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Operating revenues $450.5 $420.2 $527.4 $472.2 $458.7 $411.0 $516.2 $434.5 Operating income 49.6 66.3 131.8 49.2 62.0 48.0 125.9 34.9 Net income 39.8 45.3 123.0 18.9 48.2 34.4 116.7 26.5 Earnings available for common stock 37.2 42.8 120.4 16.4 45.6 31.9 114.1 24.0 WPL - Earnings per share data is not disclosed for WPL given Alliant Energy is the sole shareowner of all shares of WPL’s common stock outstanding during the periods presented. 2017 2016 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Operating revenues $393.1 $334.8 $370.2 $374.7 $375.6 $334.3 $397.0 $352.2 Operating income 86.0 73.7 90.7 72.8 78.8 75.0 115.0 58.2 Net income 45.5 38.1 49.8 53.2 47.0 43.7 69.6 32.5 Earnings available for common stock 45.5 38.1 49.8 53.2 46.5 43.2 69.0 31.7
IPL [Member]
Selected Consolidated Quarterly Financial Data (Unaudited)	SELECTED CONSOLIDATED QUARTERLY FINANCIAL DATA (UNAUDITED) Alliant Energy - All “per share” references refer to earnings per diluted share. Summation of the individual quarters may not equal annual totals due to rounding. 2017 2016 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions, except per share data) Operating revenues $853.9 $765.3 $906.9 $856.1 $843.8 $754.6 $924.6 $797.0 Operating income 142.9 149.3 231.5 129.7 145.9 128.6 162.6 99.9 Amounts attributable to Alliant Energy common shareowners: Income from continuing operations, net of tax 99.0 94.3 168.8 93.8 97.6 84.4 128.8 63.0 Income (loss) from discontinued operations, net of tax 1.4 — — — (1.1 ) (0.5 ) (0.4 ) (0.3 ) Net income 100.4 94.3 168.8 93.8 96.5 83.9 128.4 62.7 Earnings per weighted average common share attributable to Alliant Energy common shareowners: Income from continuing operations, net of tax 0.43 0.41 0.73 0.41 0.43 0.37 0.57 0.28 Income from discontinued operations, net of tax 0.01 — — — — — — — Net income 0.44 0.41 0.73 0.41 0.43 0.37 0.57 0.28 IPL - Earnings per share data is not disclosed for IPL given Alliant Energy is the sole shareowner of all shares of IPL’s common stock outstanding during the periods presented. 2017 2016 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Operating revenues $450.5 $420.2 $527.4 $472.2 $458.7 $411.0 $516.2 $434.5 Operating income 49.6 66.3 131.8 49.2 62.0 48.0 125.9 34.9 Net income 39.8 45.3 123.0 18.9 48.2 34.4 116.7 26.5 Earnings available for common stock 37.2 42.8 120.4 16.4 45.6 31.9 114.1 24.0 WPL - Earnings per share data is not disclosed for WPL given Alliant Energy is the sole shareowner of all shares of WPL’s common stock outstanding during the periods presented. 2017 2016 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Operating revenues $393.1 $334.8 $370.2 $374.7 $375.6 $334.3 $397.0 $352.2 Operating income 86.0 73.7 90.7 72.8 78.8 75.0 115.0 58.2 Net income 45.5 38.1 49.8 53.2 47.0 43.7 69.6 32.5 Earnings available for common stock 45.5 38.1 49.8 53.2 46.5 43.2 69.0 31.7
WPL [Member]
Selected Consolidated Quarterly Financial Data (Unaudited)	SELECTED CONSOLIDATED QUARTERLY FINANCIAL DATA (UNAUDITED) Alliant Energy - All “per share” references refer to earnings per diluted share. Summation of the individual quarters may not equal annual totals due to rounding. 2017 2016 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions, except per share data) Operating revenues $853.9 $765.3 $906.9 $856.1 $843.8 $754.6 $924.6 $797.0 Operating income 142.9 149.3 231.5 129.7 145.9 128.6 162.6 99.9 Amounts attributable to Alliant Energy common shareowners: Income from continuing operations, net of tax 99.0 94.3 168.8 93.8 97.6 84.4 128.8 63.0 Income (loss) from discontinued operations, net of tax 1.4 — — — (1.1 ) (0.5 ) (0.4 ) (0.3 ) Net income 100.4 94.3 168.8 93.8 96.5 83.9 128.4 62.7 Earnings per weighted average common share attributable to Alliant Energy common shareowners: Income from continuing operations, net of tax 0.43 0.41 0.73 0.41 0.43 0.37 0.57 0.28 Income from discontinued operations, net of tax 0.01 — — — — — — — Net income 0.44 0.41 0.73 0.41 0.43 0.37 0.57 0.28 IPL - Earnings per share data is not disclosed for IPL given Alliant Energy is the sole shareowner of all shares of IPL’s common stock outstanding during the periods presented. 2017 2016 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Operating revenues $450.5 $420.2 $527.4 $472.2 $458.7 $411.0 $516.2 $434.5 Operating income 49.6 66.3 131.8 49.2 62.0 48.0 125.9 34.9 Net income 39.8 45.3 123.0 18.9 48.2 34.4 116.7 26.5 Earnings available for common stock 37.2 42.8 120.4 16.4 45.6 31.9 114.1 24.0 WPL - Earnings per share data is not disclosed for WPL given Alliant Energy is the sole shareowner of all shares of WPL’s common stock outstanding during the periods presented. 2017 2016 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Operating revenues $393.1 $334.8 $370.2 $374.7 $375.6 $334.3 $397.0 $352.2 Operating income 86.0 73.7 90.7 72.8 78.8 75.0 115.0 58.2 Net income 45.5 38.1 49.8 53.2 47.0 43.7 69.6 32.5 Earnings available for common stock 45.5 38.1 49.8 53.2 46.5 43.2 69.0 31.7

Condensed Parent Company Financ

Condensed Parent Company Financial Statements	12 Months Ended
	Dec. 31, 2017
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Condensed Parent Company Financial Statements	SCHEDULE I - CONDENSED PARENT COMPANY FINANCIAL STATEMENTS ALLIANT ENERGY CORPORATION (Parent Company Only) CONDENSED STATEMENTS OF INCOME Year Ended December 31, 2017 2016 2015 (in millions) Operating revenues $— $1 $2 Operating expenses 2 3 3 Operating loss (2 ) (2 ) (1 ) Interest expense and other: Equity earnings from consolidated subsidiaries (457 ) (374 ) (379 ) Interest expense 3 3 3 Interest income — (2 ) (3 ) Total interest expense and other (454 ) (373 ) (379 ) Income before income taxes 452 371 378 Income tax benefit (6 ) (1 ) (1 ) Net income $458 $372 $379 The accompanying Notes to Condensed Financial Statements are an integral part of these statements. ALLIANT ENERGY CORPORATION (Parent Company Only) CONDENSED BALANCE SHEETS December 31, 2017 2016 (in millions) ASSETS Current assets: Notes receivable from affiliated companies $50 $74 Other 7 5 Total current assets 57 79 Investments: Investments in consolidated subsidiaries 4,676 4,211 Other 2 2 Total investments 4,678 4,213 Other assets 78 64 Total assets $4,813 $4,356 LIABILITIES AND EQUITY Current liabilities: Commercial paper $295 $192 Notes payable to affiliated companies 305 275 Other 12 12 Total current liabilities 612 479 Other liabilities 20 18 Common equity: Common stock and additional paid-in capital 1,848 1,695 Retained earnings 2,344 2,174 Shares in deferred compensation trust (11 ) (10 ) Total common equity 4,181 3,859 Total liabilities and equity $4,813 $4,356 The accompanying Notes to Condensed Financial Statements are an integral part of these statements. ALLIANT ENERGY CORPORATION (Parent Company Only) CONDENSED STATEMENTS OF CASH FLOWS Year Ended December 31, 2017 2016 2015 (in millions) Net cash flows from operating activities $273 $254 $262 Cash flows from (used for) investing activities: Capital contributions to consolidated subsidiaries (290 ) (250 ) (165 ) Capital repayments from consolidated subsidiaries — 130 — Net change in notes receivable from and payable to affiliates 54 294 2 Other — 10 — Net cash flows from (used for) investing activities (236 ) 184 (163 ) Cash flows used for financing activities: Common stock dividends (288 ) (267 ) (247 ) Proceeds from issuance of common stock, net 150 27 151 Payments to retire long-term debt — (250 ) — Net change in commercial paper 103 52 (1 ) Other (2 ) — (2 ) Net cash flows used for financing activities (37 ) (438 ) (99 ) Net increase (decrease) in cash and cash equivalents — — — Cash and cash equivalents at beginning of period — — — Cash and cash equivalents at end of period $— $— $— Supplemental cash flows information: Cash paid during the period for: Interest, net of capitalized interest ($3 ) ($3 ) ($3 ) Income taxes, net — (37 ) (9 ) The accompanying Notes to Condensed Financial Statements are an integral part of these statements. ALLIANT ENERGY CORPORATION (Parent Company Only) NOTES TO CONDENSED FINANCIAL STATEMENTS Pursuant to rules and regulations of the SEC, the Condensed Financial Statements of Alliant Energy Corporation (Parent Company Only) do not reflect all of the information and notes normally included with financial statements prepared in accordance with GAAP. Therefore, these Condensed Financial Statements should be read in conjunction with the Financial Statements and related Notes included in the combined 2017 Form 10-K, Part II, Item 8 , which is incorporated herein by reference. In the Condensed Financial Statements of Alliant Energy Corporation (Parent Company Only), investments in subsidiaries are accounted for using the equity method.

Valuation and Qualifying Accoun

Valuation and Qualifying Accounts And Reserves	12 Months Ended
	Dec. 31, 2017
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation and Qualifying Accounts And Reserves	SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS AND RESERVES Additions Balance, Charged to Charged to Other Balance, Description January 1 Expense Accounts (a) Deductions (b) December 31 (in millions) Valuation and Qualifying Accounts Which are Deducted in the Balance Sheet From the Assets to Which They Apply: Accumulated Provision for Uncollectible Accounts: Alliant Energy (c) Year ended December 31, 2017 $8.7 $15.1 $5.4 $17.2 $12.0 Year ended December 31, 2016 4.8 17.4 8.8 22.3 8.7 Year ended December 31, 2015 5.1 8.1 3.0 11.4 4.8 IPL (c) Year ended December 31, 2017 $1.1 $14.9 $— $14.7 $1.3 Year ended December 31, 2016 0.6 17.2 — 16.7 1.1 Year ended December 31, 2015 0.4 8.1 — 7.9 0.6 WPL Year ended December 31, 2017 $7.1 $0.2 $5.4 $2.0 $10.7 Year ended December 31, 2016 3.7 0.1 8.8 5.5 7.1 Year ended December 31, 2015 4.2 — 3.0 3.5 3.7 Note: The above provisions relate to various customer, notes and other receivable balances included in various line items on the respective balance sheets. Other Reserves: Accumulated Provision for Other Reserves (d): Alliant Energy Year ended December 31, 2017 $25.1 $3.3 $5.1 $10.5 $23.0 Year ended December 31, 2016 27.1 6.1 — 8.1 25.1 Year ended December 31, 2015 32.6 6.5 — 12.0 27.1 IPL Year ended December 31, 2017 $8.7 $0.3 $— $1.4 $7.6 Year ended December 31, 2016 9.4 1.0 — 1.7 8.7 Year ended December 31, 2015 10.6 2.1 — 3.3 9.4 WPL Year ended December 31, 2017 $8.1 $0.1 $— $1.8 $6.4 Year ended December 31, 2016 11.4 1.8 — 5.1 8.1 Year ended December 31, 2015 16.3 0.7 — 5.6 11.4 (a) Accumulated provision for uncollectible accounts: In accordance with its regulatory treatment, certain amounts provided by WPL are recorded in regulatory assets. WPL expenses these amounts when an uncollectible account is written-off. Accumulated provision for other reserves: In 2017, Alliant Energy recorded amounts to deferred tax liabilities related to the impacts of Tax Reform, which is discussed in Note 11 . (b) Deductions are of the nature for which the reserves were created. In the case of the accumulated provision for uncollectible accounts, deductions from this reserve are reduced by recoveries of amounts previously written off. (c) Refer to Note 5(b) for discussion of IPL’s sales of accounts receivable program. (d) Other reserves are largely related to injury and damage claims arising in the ordinary course of business, and the impacts of Tax Reform.
IPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation and Qualifying Accounts And Reserves	SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS AND RESERVES Additions Balance, Charged to Charged to Other Balance, Description January 1 Expense Accounts (a) Deductions (b) December 31 (in millions) Valuation and Qualifying Accounts Which are Deducted in the Balance Sheet From the Assets to Which They Apply: Accumulated Provision for Uncollectible Accounts: Alliant Energy (c) Year ended December 31, 2017 $8.7 $15.1 $5.4 $17.2 $12.0 Year ended December 31, 2016 4.8 17.4 8.8 22.3 8.7 Year ended December 31, 2015 5.1 8.1 3.0 11.4 4.8 IPL (c) Year ended December 31, 2017 $1.1 $14.9 $— $14.7 $1.3 Year ended December 31, 2016 0.6 17.2 — 16.7 1.1 Year ended December 31, 2015 0.4 8.1 — 7.9 0.6 WPL Year ended December 31, 2017 $7.1 $0.2 $5.4 $2.0 $10.7 Year ended December 31, 2016 3.7 0.1 8.8 5.5 7.1 Year ended December 31, 2015 4.2 — 3.0 3.5 3.7 Note: The above provisions relate to various customer, notes and other receivable balances included in various line items on the respective balance sheets. Other Reserves: Accumulated Provision for Other Reserves (d): Alliant Energy Year ended December 31, 2017 $25.1 $3.3 $5.1 $10.5 $23.0 Year ended December 31, 2016 27.1 6.1 — 8.1 25.1 Year ended December 31, 2015 32.6 6.5 — 12.0 27.1 IPL Year ended December 31, 2017 $8.7 $0.3 $— $1.4 $7.6 Year ended December 31, 2016 9.4 1.0 — 1.7 8.7 Year ended December 31, 2015 10.6 2.1 — 3.3 9.4 WPL Year ended December 31, 2017 $8.1 $0.1 $— $1.8 $6.4 Year ended December 31, 2016 11.4 1.8 — 5.1 8.1 Year ended December 31, 2015 16.3 0.7 — 5.6 11.4 (a) Accumulated provision for uncollectible accounts: In accordance with its regulatory treatment, certain amounts provided by WPL are recorded in regulatory assets. WPL expenses these amounts when an uncollectible account is written-off. Accumulated provision for other reserves: In 2017, Alliant Energy recorded amounts to deferred tax liabilities related to the impacts of Tax Reform, which is discussed in Note 11 . (b) Deductions are of the nature for which the reserves were created. In the case of the accumulated provision for uncollectible accounts, deductions from this reserve are reduced by recoveries of amounts previously written off. (c) Refer to Note 5(b) for discussion of IPL’s sales of accounts receivable program. (d) Other reserves are largely related to injury and damage claims arising in the ordinary course of business, and the impacts of Tax Reform.
WPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation and Qualifying Accounts And Reserves	SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS AND RESERVES Additions Balance, Charged to Charged to Other Balance, Description January 1 Expense Accounts (a) Deductions (b) December 31 (in millions) Valuation and Qualifying Accounts Which are Deducted in the Balance Sheet From the Assets to Which They Apply: Accumulated Provision for Uncollectible Accounts: Alliant Energy (c) Year ended December 31, 2017 $8.7 $15.1 $5.4 $17.2 $12.0 Year ended December 31, 2016 4.8 17.4 8.8 22.3 8.7 Year ended December 31, 2015 5.1 8.1 3.0 11.4 4.8 IPL (c) Year ended December 31, 2017 $1.1 $14.9 $— $14.7 $1.3 Year ended December 31, 2016 0.6 17.2 — 16.7 1.1 Year ended December 31, 2015 0.4 8.1 — 7.9 0.6 WPL Year ended December 31, 2017 $7.1 $0.2 $5.4 $2.0 $10.7 Year ended December 31, 2016 3.7 0.1 8.8 5.5 7.1 Year ended December 31, 2015 4.2 — 3.0 3.5 3.7 Note: The above provisions relate to various customer, notes and other receivable balances included in various line items on the respective balance sheets. Other Reserves: Accumulated Provision for Other Reserves (d): Alliant Energy Year ended December 31, 2017 $25.1 $3.3 $5.1 $10.5 $23.0 Year ended December 31, 2016 27.1 6.1 — 8.1 25.1 Year ended December 31, 2015 32.6 6.5 — 12.0 27.1 IPL Year ended December 31, 2017 $8.7 $0.3 $— $1.4 $7.6 Year ended December 31, 2016 9.4 1.0 — 1.7 8.7 Year ended December 31, 2015 10.6 2.1 — 3.3 9.4 WPL Year ended December 31, 2017 $8.1 $0.1 $— $1.8 $6.4 Year ended December 31, 2016 11.4 1.8 — 5.1 8.1 Year ended December 31, 2015 16.3 0.7 — 5.6 11.4 (a) Accumulated provision for uncollectible accounts: In accordance with its regulatory treatment, certain amounts provided by WPL are recorded in regulatory assets. WPL expenses these amounts when an uncollectible account is written-off. Accumulated provision for other reserves: In 2017, Alliant Energy recorded amounts to deferred tax liabilities related to the impacts of Tax Reform, which is discussed in Note 11 . (b) Deductions are of the nature for which the reserves were created. In the case of the accumulated provision for uncollectible accounts, deductions from this reserve are reduced by recoveries of amounts previously written off. (c) Refer to Note 5(b) for discussion of IPL’s sales of accounts receivable program. (d) Other reserves are largely related to injury and damage claims arising in the ordinary course of business, and the impacts of Tax Reform.

Summary Of Significant Accoun29

Summary Of Significant Accounting Policies (Policy)	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
General, Basis of Presentation	The financial statements reflect investments in controlled subsidiaries on a consolidated basis and Alliant Energy’s, IPL’s and WPL’s proportionate shares of jointly-owned utility EGUs. Unconsolidated investments, which Alliant Energy and WPL do not control, but have the ability to exercise significant influence over operating and financial policies, are accounted for under the equity method of accounting. Under the equity method of accounting, Alliant Energy and WPL initially record the investment at cost, and adjust the carrying amount of the investment to recognize their respective share of the earnings or losses of the investee. Dividends received from an investee reduce the carrying amount of the equity investment. Investments that do not meet the criteria for consolidation or the equity method of accounting are accounted for under the cost method. Refer to Notes 1(m) and 6(a) for further discussion of VIEs and equity method investments, respectively. All intercompany balances and transactions, other than certain transactions affecting the rate-making process at IPL and WPL, have been eliminated from the financial statements. Such transactions not eliminated include costs that are recoverable from customers through rate-making processes. Discontinued operations reported in Alliant Energy’s income statements is related to various warranty claims associated with the sale of RMT, Inc. in 2013, which have resulted in operating expenses and income subsequent to the sale.
General, Basis of Accounting	The financial statements are prepared in conformity with GAAP, which give recognition to the rate-making and accounting practices of FERC and state commissions having regulatory jurisdiction.
General, Cash Flows Presentation	Alliant Energy presents cash flows from continuing operations together with cash flows from discontinued operations in its cash flows statements.
General, Reclassification	Certain prior period amounts in the Financial Statements and Notes have been reclassified to conform to the current period presentation for comparative purposes, including reclassifications resulting from modifications to segment reporting as discussed in Note 17 .
General, Use of Estimates	The preparation of the financial statements requires management to make estimates and assumptions that affect: (a) the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the financial statements; and (b) the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Regulatory Assets and Regulatory Liabilities	Alliant Energy, IPL and WPL are subject to regulation by FERC and various state regulatory commissions. As a result, Alliant Energy, IPL and WPL are subject to GAAP provisions for regulated operations, which provide that rate-regulated public utilities record certain costs and credits allowed in the rate-making process in different periods than for non-utility entities. Regulatory assets generally represent incurred costs that have been deferred as such costs are probable of recovery in future customer rates. Regulatory liabilities generally represent obligations to make refunds to customers or amounts collected in rates for which the related costs have not yet been incurred. Amounts recorded as regulatory assets or regulatory liabilities are generally recognized in the income statements at the time they are reflected in rates.
Income Taxes	The liability method of accounting is followed for deferred taxes, which requires the establishment of deferred tax assets and liabilities, as appropriate, for temporary differences between the tax basis of assets and liabilities and the amounts reported in the financial statements. Deferred taxes are recorded using currently enacted tax rates, including impacts from Tax Reform and estimates of state apportionment. Changes in deferred tax assets and liabilities associated with certain property-related differences at IPL are accounted for differently than other subsidiaries of Alliant Energy due to rate-making practices in Iowa. Rate-making practices in Iowa do not include the impact of certain deferred tax expenses (benefits) in the determination of retail rates. Based on these rate-making practices, deferred tax expense (benefit) related to these property-related differences at IPL is not recorded in the income statement but instead recorded to regulatory assets or regulatory liabilities until these temporary differences reverse. Refer to Note 2 for further discussion of these tax-related regulatory assets and regulatory liabilities associated with property-related differences at IPL. In Wisconsin, the PSCW allows rate recovery of deferred tax expense on all temporary differences. Investment tax credits are deferred and amortized to income over the average lives of the related property. Tax Reform repealed corporate federal AMT and allows unutilized AMT credits to be refunded over the next four tax years beginning with the U.S. federal tax return for calendar year 2018. Other tax credits reduce income tax expense in the year claimed. Alliant Energy files a consolidated federal income tax return and a combined return in Wisconsin, which include Alliant Energy and its subsidiaries. Alliant Energy subsidiaries with a presence in Iowa file as part of a consolidated return in Iowa. Alliant Energy allocates consolidated income tax expense to its subsidiaries that are members of the group that file a consolidated or combined income tax return. IPL and WPL use the modified separate return approach for calculating their income tax provisions and related deferred tax assets and liabilities. IPL and WPL are assumed to file separate tax returns with the federal and state taxing authorities, except that net operating losses (and other current or deferred tax attributes) are characterized as realized (or realizable) by IPL and WPL when those tax attributes are realized (or realizable) by the consolidated tax return group of Alliant Energy (even if IPL and WPL would not otherwise have realized the attributes on a stand-alone basis). The difference in the income taxes recorded for IPL and WPL under the modified separate return method compared to the income taxes recorded on a separate return basis was not material in 2017 , 2016 and 2015 .
Cash and Cash Equivalents	Cash and cash equivalents include short-term liquid investments that have original maturities of less than 90 days.
Property, Plant and Equipment	Utility Plant - General - Utility plant is recorded at the original cost of acquisition or construction, which includes material, labor, contractor services, AFUDC and allocable overheads, such as supervision, engineering, benefits, certain taxes and transportation. Repairs, replacements and renewals of items of property determined to be less than a unit of property or that do not increase the property’s life or functionality are charged to maintenance expense. Generally, ordinary retirements of utility plant and salvage value are netted and charged to accumulated depreciation upon removal from utility plant accounts and no gain or loss is recognized consistent with rate-making principles. However, if regulators have approved recovery of the remaining net book value of property, plant and equipment that is retired early, or such approval by regulators is probable, the remaining net book value is reclassified from property, plant and equipment to regulatory assets upon retirement. Property, plant and equipment that is probable of being retired early is classified as plant anticipated to be retired early. Non-utility Property - General - Non-utility property is recorded at the original cost of acquisition or construction, which includes material, labor and contractor services. Repairs, replacements and renewals of items of property determined to be less than a unit of property or that do not increase the property’s life or functionality are charged to maintenance expense. Upon retirement or sale of non-utility property, the original cost and related accumulated depreciation are removed from the accounts and any gain or loss is included in the income statements. Costs related to software developed or obtained for internal use are capitalized and amortized on a straight-line basis over the estimated useful life of the related software. If software is retired prior to being fully amortized, the difference is recorded as a loss in the income statements.
Depreciation	IPL and WPL use a combination of remaining life and straight-line depreciation methods as approved by their respective regulatory commissions. The composite or group method of depreciation is used, in which a single depreciation rate is applied to the gross investment in a particular class of property. This method pools similar assets and then depreciates each group as a whole. Periodic depreciation studies are performed to determine the appropriate group lives, net salvage, estimated cost of removal and group depreciation rates. These depreciation studies are subject to review and approval by IPL’s and WPL’s respective regulatory commissions. Depreciation expense is included within the recoverable cost of service component of rates charged to customers. The average rates of depreciation for electric, gas and other properties, consistent with current rate-making practices, were as follows: IPL WPL 2017 2016 2015 2017 2016 2015 Electric - generation 3.5% 3.5% 3.6% 3.5% 3.1% 3.2% Electric - distribution 2.4% 2.4% 2.4% 2.6% 2.6% 2.7% Electric - other 4.5% 4.2% 4.0% 6.9% 4.7% 4.5% Gas 3.4% 3.3% 3.2% 2.5% 2.5% 2.5% Other 4.0% 3.9% 3.9% 6.0% 5.9% 6.0% In September 2016, the PSCW issued an order approving the implementation of updated depreciation rates for WPL effective January 1, 2017 as a result of an updated depreciation study. In February 2018, the IUB issued an order approving the implementation of updated depreciation rates for IPL, which are currently expected to be effective in the first half of 2018, as a result of an updated depreciation study. IPL estimates the new average rates of depreciation for its electric generation and electric distribution properties will be approximately 3.8% and 2.8% , respectively, during 2018.
AFUDC	AFUDC represents costs to finance construction additions, including a return on equity component and cost of debt component as required by regulatory accounting. AFUDC for IPL’s construction projects is calculated in accordance with FERC guidelines. AFUDC for WPL’s retail and wholesale jurisdiction construction projects is calculated in accordance with PSCW and FERC guidelines, respectively. The AFUDC rates, computed in accordance with the prescribed regulatory formula, were as follows: 2017 2016 2015 IPL (Marshalltown CWIP) (a) 7.8% 7.9% 7.9% IPL (Wind generation CWIP) (b) 7.6% N/A N/A IPL (other CWIP) 7.6% 7.7% 7.7% WPL (retail jurisdiction) 7.6% 8.2% 8.2% WPL (wholesale jurisdiction) 6.0% 6.7% 7.9% (a) In 2013, the IUB issued an order establishing rate-making principles that require a 10.3% return on common equity for the calculation of AFUDC related to the construction of Marshalltown. (b) In 2016, the IUB issued an order establishing rate-making principles that require a return on common equity for the calculation of AFUDC related to the construction of up to 500 MW of new wind generation equal to the greater of 10.0% or whatever percentage the IUB finds reasonable during IPL’s most recent retail electric rate proceeding. In accordance with their most recent rate orders, IPL applies its AFUDC rates to 100% of applicable CWIP balances and WPL generally applies its AFUDC rates to 50% of applicable CWIP balances. WPL may apply its AFUDC rates to 100% of the retail portion of the CWIP balances for construction projects requiring a CA or CPCN that were approved by the PSCW after its most recent rate order, including West Riverside.
Operating Revenues	Utility - Revenues from Alliant Energy’s utility business are primarily from electricity and natural gas sales and are recognized on an accrual basis as services are rendered or commodities are delivered to customers. Energy sales to individual customers are based on the reading of customers’ meters, which occurs on a systematic basis throughout each reporting period. Amounts of energy delivered to customers since the date of the last meter reading are estimated at the end of each reporting period and the corresponding estimated unbilled revenue is recorded in such reporting period. The unbilled revenue estimate is based on daily system demand volumes, estimated customer usage by class, temperature impacts, line losses and the most recent customer rates. IPL and WPL accrue revenues from their wholesale customers to the extent that the actual net revenue requirements calculated in accordance with FERC-approved formula rates for the reporting period are higher or lower than the amounts billed to wholesale customers during such period. Regulatory assets or regulatory liabilities are recorded as the offset for these accrued revenues under formulaic rate-making programs. IPL’s estimated recovery amount is recorded in the current period of service and is reflected in customer bills within two years under the provisions of approved formula rates. WPL’s estimated recovery amount is recorded in the current period of service and subject to final adjustments after a customer audit period in the subsequent year. Final settled recovery amounts are reflected in WPL’s customer bills within two years under the provisions of approved formula rates. IPL and WPL participate in bid/offer-based wholesale energy and ancillary services markets operated by MISO. MISO requires that all load serving entities and generation owners, including IPL and WPL, submit hourly bids and offers for energy and ancillary services. The MISO transactions are grouped together, resulting in a net supply to or net purchase from MISO for each hour of each day. The net supply to MISO is recorded in “Electric utility operating revenues” and the net purchase from MISO is recorded in “Electric production fuel and purchased power” in the income statements. Non-utility - Revenues from Alliant Energy’s non-utility businesses are primarily from its Transportation business and are recognized on an accrual basis as services are rendered or goods are delivered to customers. Taxes Collected from Customers - Sales or various other taxes collected by certain of Alliant Energy’s subsidiaries on behalf of other agencies are recorded on a net basis and are not included in operating revenues.
Utility Cost Recovery Mechanisms	Electric Production Fuel and Purchased Power (Fuel-related Costs) - Fuel-related costs are incurred to generate and purchase electricity to meet the demand of IPL’s and WPL’s electric customers. These fuel-related costs include the cost of fossil fuels (primarily natural gas and coal) used to produce electricity at their EGUs and electricity purchased from MISO wholesale energy markets and under PPAs. These fuel-related costs are recorded in “Electric production fuel and purchased power” in the income statements. IPL Retail - The cost recovery mechanisms for IPL’s retail electric customers provide for monthly adjustments to their electric rates for changes in fuel-related costs. Changes in the under-/over-collection of these costs are recognized in “Electric production fuel and purchased power” in Alliant Energy’s and IPL’s income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and IPL’s balance sheets until they are reflected in future billings to customers. WPL Retail - The cost recovery mechanism for WPL’s retail electric customers is based on forecasts of certain fuel-related costs expected to be incurred during forward-looking test year periods and fuel monitoring ranges determined by the PSCW during each retail electric rate proceeding or in a separate fuel cost plan approval proceeding. If WPL’s actual fuel-related costs fall outside these fuel monitoring ranges, WPL is authorized to defer the incremental under-/over-collection of fuel-related costs that are outside the approved ranges. Deferral of under-collections are reduced to the extent actual return on common equity earned by WPL during the fuel cost plan year exceeds the most recently authorized return on common equity. Deferred amounts for fuel-related costs outside the approved fuel monitoring ranges are recognized in “Electric production fuel and purchased power” in Alliant Energy’s and WPL’s income statements. The cumulative effects of these deferred amounts are recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and WPL’s balance sheets until they are reflected in future billings to customers. IPL and WPL Wholesale - The cost recovery mechanisms for IPL’s and WPL’s wholesale electric customers provide for subsequent adjustments to their electric rates for changes in fuel-related costs. Changes in the under-/over-collection of these costs are recognized in “Electric production fuel and purchased power” in the income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on the balance sheets until they are reflected in future billings to customers. Purchased Electric Capacity - PPAs help meet the electricity demand of IPL’s and WPL’s customers. Certain of these PPAs include minimum payments for IPL’s and WPL’s rights to electric generating capacity, which are charged each period to “Electric production fuel and purchased power” in the income statements. Purchased electric capacity expenses are recovered from IPL’s and WPL’s retail electric customers through changes in base rates determined during periodic rate proceedings. Purchased electric capacity expenses are recovered from IPL’s and WPL’s wholesale electric customers through annual changes in base rates determined by a formula rate structure. Electric Transmission Service - Costs incurred for the transmission of electricity to meet the demands of IPL’s and WPL’s customers are charged to “Electric transmission service” in the income statements. IPL Retail - Electric transmission service expense is recovered from IPL’s retail electric customers through a transmission cost rider. This cost recovery mechanism provides for annual adjustments to electric rates charged to retail electric customers for changes in electric transmission service expense. Changes in the under-/over-collection of these costs are recognized in “Electric transmission service” in Alliant Energy’s and IPL’s income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and IPL’s balance sheets until they are reflected in future billings to customers. WPL Retail - Electric transmission service expense is recovered from WPL’s retail electric customers through changes in base rates determined during periodic rate proceedings. Pursuant to escrow accounting treatment approved by the PSCW, the difference between actual electric transmission service expense incurred and the amount of electric transmission service costs collected from customers as electric revenues is recognized in “Electric transmission service” in Alliant Energy’s and WPL’s income statements. An offsetting amount is recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and WPL’s balance sheets until reflected in future billings to customers. The PSCW’s December 2016 order for WPL’s retail electric rate review (2017/2018 Test Period) extends this escrow accounting treatment through 2018. IPL and WPL Wholesale - IPL and WPL arrange transmission service for the majority of their respective wholesale electric customers. Electric transmission service expense is allocated to and recovered from these customers based on a load ratio share computation. Cost of Gas Sold - Costs are incurred for the purchase, transportation and storage of natural gas to serve IPL’s and WPL’s gas customers and the costs associated with the natural gas delivered to customers are charged to “Cost of gas sold” in the income statements. The tariffs for IPL’s and WPL’s retail gas customers provide for subsequent adjustments to their rates each month for changes in the cost of gas sold. Changes in the under-/over-collection of these costs are also recognized in “Cost of gas sold” in the income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on the balance sheets until they are reflected in future billings to customers. Energy Efficiency Costs - Costs are incurred to fund energy efficiency programs and initiatives that help customers reduce their energy usage. The costs incurred for these programs and initiatives are charged to “Other operation and maintenance” in the income statements. Energy efficiency costs incurred by IPL are recovered from its retail electric and gas customers through an additional tariff called an energy efficiency cost recovery factor, which is revised annually and includes a reconciliation to eliminate any under-/over-collection of energy efficiency costs from prior periods. Energy efficiency costs incurred by WPL are recovered from retail electric and gas customers through changes in base rates determined during periodic rate proceedings. Reconciliations of any under-/over-collection of energy efficiency costs from prior periods are also addressed in WPL’s periodic rate proceedings. Changes in the under-/over-collection of energy efficiency costs for IPL and WPL are recognized in “Other operation and maintenance” in the income statements. The cumulative effects of the under-/over-collection of these costs for IPL and WPL are recorded in regulatory assets or regulatory liabilities on the balance sheets until they are reflected in future billings to customers.
Financial Instruments	Financial instruments are periodically used for risk management purposes to mitigate exposures to fluctuations in certain commodity prices and transmission congestion costs. The fair value of those financial instruments that are determined to be derivatives are recorded as assets or liabilities on the balance sheets. Certain commodity purchase and sales contracts qualify for and have been designated under the normal purchase and sale exception and are accounted for on the accrual basis of accounting. Alliant Energy, IPL and WPL have elected to not net the fair value amounts of derivatives subject to a master netting arrangement by counterparty. Alliant Energy, IPL and WPL do not offset fair value amounts recognized for the right to reclaim cash collateral (receivable) or the obligation to return cash collateral (payable) against fair value amounts recognized for derivative instruments that are executed with the same counterparty under the same master netting arrangement.
Asset Impairments	Property, Plant and Equipment of Regulated Operations - Property, plant and equipment of regulated operations are reviewed for possible impairment whenever events or changes in circumstances indicate all or a portion of the carrying value of the assets may be disallowed for rate-making purposes. If IPL or WPL are disallowed recovery of any portion of the carrying value of their regulated property, plant and equipment that is under construction, has been recently completed or is probable of abandonment, or conclude it is probable recovery will be disallowed, an impairment charge is recognized equal to the amount of the carrying value that was disallowed or is probable of being disallowed. If IPL or WPL are only allowed a partial return on the carrying value of their regulated property, plant and equipment that is under construction, has been recently completed or is probable of abandonment, or conclude it is probable a full return will not be allowed, an impairment charge is recognized equal to the difference between the carrying value and the present value of the future revenues expected from their regulated property, plant and equipment. Property, Plant and Equipment of Non-utility Operations - Property, plant and equipment of non-utility operations are reviewed for possible impairment whenever events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Impairment is indicated if the carrying value of an asset exceeds its undiscounted future cash flows. If an impairment is indicated, a charge is recognized equal to the amount the carrying value exceeds the asset’s fair value. Refer to Note 3 for discussion of Alliant Energy’s impairment analysis of the Franklin County wind farm assets and resulting asset valuation charges recorded by Alliant Energy in 2016. Unconsolidated Equity Investments - If events or circumstances indicate the carrying value of investments accounted for under the equity method of accounting exceeds fair value and the decline in value is other than temporary, potential impairment is assessed. If an impairment is indicated, a charge is recognized equal to the amount the carrying value exceeds the investment’s fair value.
Asset Retirement Obligations	The fair value of a legal obligation associated with the retirement of an asset is recorded as a liability when an asset is placed in service, when a legal obligation is subsequently identified or when sufficient information becomes available to determine a reasonable estimate of the fair value of future retirement costs. When an ARO is recorded as a liability, an equivalent amount is added to the asset cost. The fair value of AROs at inception is determined using discounted cash flows analyses. The liability is accreted to its present value and the capitalized cost is depreciated over the useful life of the related asset. Accretion and depreciation expenses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory assets on the balance sheets. Upon regulatory approval to recover IPL’s AROs expenditures, its regulatory assets are amortized to depreciation and amortization expenses in Alliant Energy’s and IPL’s income statements over the same time period the ARO expenditures are recovered from IPL’s customers. WPL’s regulatory assets related to AROs are being recovered as a component of depreciation rates pursuant to PSCW and FERC orders. Accretion and depreciation expenses related to AROs for Alliant Energy’s non-utility operations are recorded to depreciation and amortization expenses in Alliant Energy’s income statements. Upon settlement of the ARO liability, an entity settles the obligation for its recorded amount or incurs a gain or loss. Any gains or losses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory liabilities or regulatory assets on the balance sheets.
Debt Issuance and Retirement Costs	Debt issuance costs and debt premiums or discounts are presented on the balance sheet as a direct deduction from the carrying amount of the related debt liability, and are deferred and amortized over the expected life of each debt issue, considering maturity dates and, if applicable, redemption rights held by others. Alliant Energy’s non-utility businesses and Corporate Services expense in the period of retirement any unamortized debt issuance costs and debt premiums or discounts on debt retired early.
Allowance for Doubtful Accounts	Allowances for doubtful accounts are recorded for estimated losses resulting from the inability of customers to make required payments. Allowances for doubtful accounts are estimated based on historical write-offs, customer arrears and other economic factors within IPL’s and WPL’s service territories.
Variable Interest Entities	An entity is considered a VIE if its equity investors do not have sufficient equity at risk for the entity to finance its activities without additional subordinated financial support from other parties, or its equity investors lack any of the following characteristics: (1) power, through voting rights or similar rights, to direct the activities of the entity that most significantly impact the entity’s economic performance; (2) the obligation to absorb expected losses of the entity; or (3) the right to receive expected benefits of the entity. The primary beneficiary of a VIE is required to consolidate the VIE. The financial statements do not reflect any consolidated VIEs.
New Accounting Standards	Revenue Recognition - In May 2014, the Financial Accounting Standards Board issued an accounting standard providing principles for recognizing revenue for the transfer of promised goods or services to customers with the consideration to which the entity expects to be entitled in exchange for those goods or services. This standard also requires disclosures about the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers. Alliant Energy, IPL and WPL adopted this standard on January 1, 2018 and used the modified retrospective method of adoption and there will be no cumulative effect adjustments made to the opening retained earnings balance on January 1, 2018 upon adoption. The majority of Alliant Energy’s, IPL’s and WPL’s revenues are from retail electric and gas sales from tariff offerings that provide electricity or natural gas without a defined contractual term. For such arrangements, revenues from contracts with customers will be equivalent to the electricity or natural gas supplied and billed, or estimated to be billed, and there will be no significant shift in the timing or pattern of revenue recognition for such sales. Alliant Energy, IPL and WPL did not have a material change in revenue recognition as a result of the adoption of this standard, and the most significant impact to their financial statements is in the form of additional disclosures. The incremental disclosures in future filings will include a separate footnote disclosure, including disaggregation of revenue by location and customer class. Leases - In February 2016, the Financial Accounting Standards Board issued an accounting standard requiring lease assets and lease liabilities, including operating leases, to be recognized on the balance sheet for all leases with terms longer than 12 months. The standard also requires disclosure of key information about leasing arrangements. Alliant Energy, IPL and WPL currently expect to adopt this standard on January 1, 2019 and are evaluating the impact of this standard on their financial condition and results of operations and expect an increase in assets and liabilities from recognizing operating leases on their balance sheets. Presentation of Net Periodic Pension and Postretirement Benefit Costs - In March 2017, the Financial Accounting Standards Board issued an accounting standard amending the income statement presentation of the components of net periodic benefit costs for defined benefit pension and other postretirement plans. The standard requires entities to (1) disaggregate the current service cost component from the other components of net periodic benefit costs and present it with other employee compensation costs in the income statement; and (2) include the other components in the income statement outside of operating income. This new presentation will shift the majority of the net periodic benefit costs from “Other operation and maintenance” expenses to “Interest expense and other” expenses in the income statements in future filings. In addition, only the service cost component of net periodic benefit costs is eligible for capitalization into property, plant and equipment, when applicable. IPL and WPL, as rate-regulated entities, will capitalize the other components of net periodic benefit costs into regulatory assets or regulatory liabilities. Alliant Energy, IPL and WPL adopted this standard on January 1, 2018 and used the retrospective method of adoption for the presentation requirements and the prospective method of adoption for the capitalization requirements. Share-based Compensation Award Payments - In March 2016, the Financial Accounting Standards Board issued an accounting standard intended to simplify certain aspects of the accounting for share-based compensation award payments and the associated income taxes. This standard changes the accounting for excess tax benefits, whereby such benefits are recognized in the income statement instead of additional paid-in capital on the balance sheet. Alliant Energy adopted this standard on January 1, 2016, which resulted in a cumulative effect of a decrease to its deferred tax liabilities and an increase to its January 1, 2016 retained earnings balance of $3.1 million , which is included in “Other” in Alliant Energy’s common equity statement in 2016.
IPL [Member]
Property, Plant and Equipment [Line Items]
General, Basis of Presentation	The financial statements reflect investments in controlled subsidiaries on a consolidated basis and Alliant Energy’s, IPL’s and WPL’s proportionate shares of jointly-owned utility EGUs. Unconsolidated investments, which Alliant Energy and WPL do not control, but have the ability to exercise significant influence over operating and financial policies, are accounted for under the equity method of accounting. Under the equity method of accounting, Alliant Energy and WPL initially record the investment at cost, and adjust the carrying amount of the investment to recognize their respective share of the earnings or losses of the investee. Dividends received from an investee reduce the carrying amount of the equity investment. Investments that do not meet the criteria for consolidation or the equity method of accounting are accounted for under the cost method. Refer to Notes 1(m) and 6(a) for further discussion of VIEs and equity method investments, respectively. All intercompany balances and transactions, other than certain transactions affecting the rate-making process at IPL and WPL, have been eliminated from the financial statements. Such transactions not eliminated include costs that are recoverable from customers through rate-making processes.
General, Basis of Accounting	The financial statements are prepared in conformity with GAAP, which give recognition to the rate-making and accounting practices of FERC and state commissions having regulatory jurisdiction.
General, Reclassification	Certain prior period amounts in the Financial Statements and Notes have been reclassified to conform to the current period presentation for comparative purposes, including reclassifications resulting from modifications to segment reporting as discussed in Note 17 .
General, Use of Estimates	The preparation of the financial statements requires management to make estimates and assumptions that affect: (a) the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the financial statements; and (b) the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Regulatory Assets and Regulatory Liabilities	Alliant Energy, IPL and WPL are subject to regulation by FERC and various state regulatory commissions. As a result, Alliant Energy, IPL and WPL are subject to GAAP provisions for regulated operations, which provide that rate-regulated public utilities record certain costs and credits allowed in the rate-making process in different periods than for non-utility entities. Regulatory assets generally represent incurred costs that have been deferred as such costs are probable of recovery in future customer rates. Regulatory liabilities generally represent obligations to make refunds to customers or amounts collected in rates for which the related costs have not yet been incurred. Amounts recorded as regulatory assets or regulatory liabilities are generally recognized in the income statements at the time they are reflected in rates.
Income Taxes	The liability method of accounting is followed for deferred taxes, which requires the establishment of deferred tax assets and liabilities, as appropriate, for temporary differences between the tax basis of assets and liabilities and the amounts reported in the financial statements. Deferred taxes are recorded using currently enacted tax rates, including impacts from Tax Reform and estimates of state apportionment. Changes in deferred tax assets and liabilities associated with certain property-related differences at IPL are accounted for differently than other subsidiaries of Alliant Energy due to rate-making practices in Iowa. Rate-making practices in Iowa do not include the impact of certain deferred tax expenses (benefits) in the determination of retail rates. Based on these rate-making practices, deferred tax expense (benefit) related to these property-related differences at IPL is not recorded in the income statement but instead recorded to regulatory assets or regulatory liabilities until these temporary differences reverse. Refer to Note 2 for further discussion of these tax-related regulatory assets and regulatory liabilities associated with property-related differences at IPL. In Wisconsin, the PSCW allows rate recovery of deferred tax expense on all temporary differences. Investment tax credits are deferred and amortized to income over the average lives of the related property. Tax Reform repealed corporate federal AMT and allows unutilized AMT credits to be refunded over the next four tax years beginning with the U.S. federal tax return for calendar year 2018. Other tax credits reduce income tax expense in the year claimed. Alliant Energy files a consolidated federal income tax return and a combined return in Wisconsin, which include Alliant Energy and its subsidiaries. Alliant Energy subsidiaries with a presence in Iowa file as part of a consolidated return in Iowa. Alliant Energy allocates consolidated income tax expense to its subsidiaries that are members of the group that file a consolidated or combined income tax return. IPL and WPL use the modified separate return approach for calculating their income tax provisions and related deferred tax assets and liabilities. IPL and WPL are assumed to file separate tax returns with the federal and state taxing authorities, except that net operating losses (and other current or deferred tax attributes) are characterized as realized (or realizable) by IPL and WPL when those tax attributes are realized (or realizable) by the consolidated tax return group of Alliant Energy (even if IPL and WPL would not otherwise have realized the attributes on a stand-alone basis). The difference in the income taxes recorded for IPL and WPL under the modified separate return method compared to the income taxes recorded on a separate return basis was not material in 2017 , 2016 and 2015 .
Cash and Cash Equivalents	Cash and cash equivalents include short-term liquid investments that have original maturities of less than 90 days.
Property, Plant and Equipment	Utility Plant - General - Utility plant is recorded at the original cost of acquisition or construction, which includes material, labor, contractor services, AFUDC and allocable overheads, such as supervision, engineering, benefits, certain taxes and transportation. Repairs, replacements and renewals of items of property determined to be less than a unit of property or that do not increase the property’s life or functionality are charged to maintenance expense. Generally, ordinary retirements of utility plant and salvage value are netted and charged to accumulated depreciation upon removal from utility plant accounts and no gain or loss is recognized consistent with rate-making principles. However, if regulators have approved recovery of the remaining net book value of property, plant and equipment that is retired early, or such approval by regulators is probable, the remaining net book value is reclassified from property, plant and equipment to regulatory assets upon retirement. Property, plant and equipment that is probable of being retired early is classified as plant anticipated to be retired early.
Depreciation	IPL and WPL use a combination of remaining life and straight-line depreciation methods as approved by their respective regulatory commissions. The composite or group method of depreciation is used, in which a single depreciation rate is applied to the gross investment in a particular class of property. This method pools similar assets and then depreciates each group as a whole. Periodic depreciation studies are performed to determine the appropriate group lives, net salvage, estimated cost of removal and group depreciation rates. These depreciation studies are subject to review and approval by IPL’s and WPL’s respective regulatory commissions. Depreciation expense is included within the recoverable cost of service component of rates charged to customers. The average rates of depreciation for electric, gas and other properties, consistent with current rate-making practices, were as follows: IPL WPL 2017 2016 2015 2017 2016 2015 Electric - generation 3.5% 3.5% 3.6% 3.5% 3.1% 3.2% Electric - distribution 2.4% 2.4% 2.4% 2.6% 2.6% 2.7% Electric - other 4.5% 4.2% 4.0% 6.9% 4.7% 4.5% Gas 3.4% 3.3% 3.2% 2.5% 2.5% 2.5% Other 4.0% 3.9% 3.9% 6.0% 5.9% 6.0% In September 2016, the PSCW issued an order approving the implementation of updated depreciation rates for WPL effective January 1, 2017 as a result of an updated depreciation study. In February 2018, the IUB issued an order approving the implementation of updated depreciation rates for IPL, which are currently expected to be effective in the first half of 2018, as a result of an updated depreciation study. IPL estimates the new average rates of depreciation for its electric generation and electric distribution properties will be approximately 3.8% and 2.8% , respectively, during 2018.
AFUDC	AFUDC represents costs to finance construction additions, including a return on equity component and cost of debt component as required by regulatory accounting. AFUDC for IPL’s construction projects is calculated in accordance with FERC guidelines. AFUDC for WPL’s retail and wholesale jurisdiction construction projects is calculated in accordance with PSCW and FERC guidelines, respectively. The AFUDC rates, computed in accordance with the prescribed regulatory formula, were as follows: 2017 2016 2015 IPL (Marshalltown CWIP) (a) 7.8% 7.9% 7.9% IPL (Wind generation CWIP) (b) 7.6% N/A N/A IPL (other CWIP) 7.6% 7.7% 7.7% WPL (retail jurisdiction) 7.6% 8.2% 8.2% WPL (wholesale jurisdiction) 6.0% 6.7% 7.9% (a) In 2013, the IUB issued an order establishing rate-making principles that require a 10.3% return on common equity for the calculation of AFUDC related to the construction of Marshalltown. (b) In 2016, the IUB issued an order establishing rate-making principles that require a return on common equity for the calculation of AFUDC related to the construction of up to 500 MW of new wind generation equal to the greater of 10.0% or whatever percentage the IUB finds reasonable during IPL’s most recent retail electric rate proceeding. In accordance with their most recent rate orders, IPL applies its AFUDC rates to 100% of applicable CWIP balances and WPL generally applies its AFUDC rates to 50% of applicable CWIP balances. WPL may apply its AFUDC rates to 100% of the retail portion of the CWIP balances for construction projects requiring a CA or CPCN that were approved by the PSCW after its most recent rate order, including West Riverside.
Operating Revenues	Utility - Revenues from Alliant Energy’s utility business are primarily from electricity and natural gas sales and are recognized on an accrual basis as services are rendered or commodities are delivered to customers. Energy sales to individual customers are based on the reading of customers’ meters, which occurs on a systematic basis throughout each reporting period. Amounts of energy delivered to customers since the date of the last meter reading are estimated at the end of each reporting period and the corresponding estimated unbilled revenue is recorded in such reporting period. The unbilled revenue estimate is based on daily system demand volumes, estimated customer usage by class, temperature impacts, line losses and the most recent customer rates. IPL and WPL accrue revenues from their wholesale customers to the extent that the actual net revenue requirements calculated in accordance with FERC-approved formula rates for the reporting period are higher or lower than the amounts billed to wholesale customers during such period. Regulatory assets or regulatory liabilities are recorded as the offset for these accrued revenues under formulaic rate-making programs. IPL’s estimated recovery amount is recorded in the current period of service and is reflected in customer bills within two years under the provisions of approved formula rates. WPL’s estimated recovery amount is recorded in the current period of service and subject to final adjustments after a customer audit period in the subsequent year. Final settled recovery amounts are reflected in WPL’s customer bills within two years under the provisions of approved formula rates. IPL and WPL participate in bid/offer-based wholesale energy and ancillary services markets operated by MISO. MISO requires that all load serving entities and generation owners, including IPL and WPL, submit hourly bids and offers for energy and ancillary services. The MISO transactions are grouped together, resulting in a net supply to or net purchase from MISO for each hour of each day. The net supply to MISO is recorded in “Electric utility operating revenues” and the net purchase from MISO is recorded in “Electric production fuel and purchased power” in the income statements. Non-utility - Revenues from Alliant Energy’s non-utility businesses are primarily from its Transportation business and are recognized on an accrual basis as services are rendered or goods are delivered to customers. Taxes Collected from Customers - Sales or various other taxes collected by certain of Alliant Energy’s subsidiaries on behalf of other agencies are recorded on a net basis and are not included in operating revenues.
Utility Cost Recovery Mechanisms	Electric Production Fuel and Purchased Power (Fuel-related Costs) - Fuel-related costs are incurred to generate and purchase electricity to meet the demand of IPL’s and WPL’s electric customers. These fuel-related costs include the cost of fossil fuels (primarily natural gas and coal) used to produce electricity at their EGUs and electricity purchased from MISO wholesale energy markets and under PPAs. These fuel-related costs are recorded in “Electric production fuel and purchased power” in the income statements. IPL Retail - The cost recovery mechanisms for IPL’s retail electric customers provide for monthly adjustments to their electric rates for changes in fuel-related costs. Changes in the under-/over-collection of these costs are recognized in “Electric production fuel and purchased power” in Alliant Energy’s and IPL’s income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and IPL’s balance sheets until they are reflected in future billings to customers. WPL Retail - The cost recovery mechanism for WPL’s retail electric customers is based on forecasts of certain fuel-related costs expected to be incurred during forward-looking test year periods and fuel monitoring ranges determined by the PSCW during each retail electric rate proceeding or in a separate fuel cost plan approval proceeding. If WPL’s actual fuel-related costs fall outside these fuel monitoring ranges, WPL is authorized to defer the incremental under-/over-collection of fuel-related costs that are outside the approved ranges. Deferral of under-collections are reduced to the extent actual return on common equity earned by WPL during the fuel cost plan year exceeds the most recently authorized return on common equity. Deferred amounts for fuel-related costs outside the approved fuel monitoring ranges are recognized in “Electric production fuel and purchased power” in Alliant Energy’s and WPL’s income statements. The cumulative effects of these deferred amounts are recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and WPL’s balance sheets until they are reflected in future billings to customers. IPL and WPL Wholesale - The cost recovery mechanisms for IPL’s and WPL’s wholesale electric customers provide for subsequent adjustments to their electric rates for changes in fuel-related costs. Changes in the under-/over-collection of these costs are recognized in “Electric production fuel and purchased power” in the income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on the balance sheets until they are reflected in future billings to customers. Purchased Electric Capacity - PPAs help meet the electricity demand of IPL’s and WPL’s customers. Certain of these PPAs include minimum payments for IPL’s and WPL’s rights to electric generating capacity, which are charged each period to “Electric production fuel and purchased power” in the income statements. Purchased electric capacity expenses are recovered from IPL’s and WPL’s retail electric customers through changes in base rates determined during periodic rate proceedings. Purchased electric capacity expenses are recovered from IPL’s and WPL’s wholesale electric customers through annual changes in base rates determined by a formula rate structure. Electric Transmission Service - Costs incurred for the transmission of electricity to meet the demands of IPL’s and WPL’s customers are charged to “Electric transmission service” in the income statements. IPL Retail - Electric transmission service expense is recovered from IPL’s retail electric customers through a transmission cost rider. This cost recovery mechanism provides for annual adjustments to electric rates charged to retail electric customers for changes in electric transmission service expense. Changes in the under-/over-collection of these costs are recognized in “Electric transmission service” in Alliant Energy’s and IPL’s income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and IPL’s balance sheets until they are reflected in future billings to customers. WPL Retail - Electric transmission service expense is recovered from WPL’s retail electric customers through changes in base rates determined during periodic rate proceedings. Pursuant to escrow accounting treatment approved by the PSCW, the difference between actual electric transmission service expense incurred and the amount of electric transmission service costs collected from customers as electric revenues is recognized in “Electric transmission service” in Alliant Energy’s and WPL’s income statements. An offsetting amount is recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and WPL’s balance sheets until reflected in future billings to customers. The PSCW’s December 2016 order for WPL’s retail electric rate review (2017/2018 Test Period) extends this escrow accounting treatment through 2018. IPL and WPL Wholesale - IPL and WPL arrange transmission service for the majority of their respective wholesale electric customers. Electric transmission service expense is allocated to and recovered from these customers based on a load ratio share computation. Cost of Gas Sold - Costs are incurred for the purchase, transportation and storage of natural gas to serve IPL’s and WPL’s gas customers and the costs associated with the natural gas delivered to customers are charged to “Cost of gas sold” in the income statements. The tariffs for IPL’s and WPL’s retail gas customers provide for subsequent adjustments to their rates each month for changes in the cost of gas sold. Changes in the under-/over-collection of these costs are also recognized in “Cost of gas sold” in the income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on the balance sheets until they are reflected in future billings to customers. Energy Efficiency Costs - Costs are incurred to fund energy efficiency programs and initiatives that help customers reduce their energy usage. The costs incurred for these programs and initiatives are charged to “Other operation and maintenance” in the income statements. Energy efficiency costs incurred by IPL are recovered from its retail electric and gas customers through an additional tariff called an energy efficiency cost recovery factor, which is revised annually and includes a reconciliation to eliminate any under-/over-collection of energy efficiency costs from prior periods. Energy efficiency costs incurred by WPL are recovered from retail electric and gas customers through changes in base rates determined during periodic rate proceedings. Reconciliations of any under-/over-collection of energy efficiency costs from prior periods are also addressed in WPL’s periodic rate proceedings. Changes in the under-/over-collection of energy efficiency costs for IPL and WPL are recognized in “Other operation and maintenance” in the income statements. The cumulative effects of the under-/over-collection of these costs for IPL and WPL are recorded in regulatory assets or regulatory liabilities on the balance sheets until they are reflected in future billings to customers.
Financial Instruments	Financial instruments are periodically used for risk management purposes to mitigate exposures to fluctuations in certain commodity prices and transmission congestion costs. The fair value of those financial instruments that are determined to be derivatives are recorded as assets or liabilities on the balance sheets. Certain commodity purchase and sales contracts qualify for and have been designated under the normal purchase and sale exception and are accounted for on the accrual basis of accounting. Alliant Energy, IPL and WPL have elected to not net the fair value amounts of derivatives subject to a master netting arrangement by counterparty. Alliant Energy, IPL and WPL do not offset fair value amounts recognized for the right to reclaim cash collateral (receivable) or the obligation to return cash collateral (payable) against fair value amounts recognized for derivative instruments that are executed with the same counterparty under the same master netting arrangement.
Asset Impairments	Property, Plant and Equipment of Regulated Operations - Property, plant and equipment of regulated operations are reviewed for possible impairment whenever events or changes in circumstances indicate all or a portion of the carrying value of the assets may be disallowed for rate-making purposes. If IPL or WPL are disallowed recovery of any portion of the carrying value of their regulated property, plant and equipment that is under construction, has been recently completed or is probable of abandonment, or conclude it is probable recovery will be disallowed, an impairment charge is recognized equal to the amount of the carrying value that was disallowed or is probable of being disallowed. If IPL or WPL are only allowed a partial return on the carrying value of their regulated property, plant and equipment that is under construction, has been recently completed or is probable of abandonment, or conclude it is probable a full return will not be allowed, an impairment charge is recognized equal to the difference between the carrying value and the present value of the future revenues expected from their regulated property, plant and equipment. Property, Plant and Equipment of Non-utility Operations - Property, plant and equipment of non-utility operations are reviewed for possible impairment whenever events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Impairment is indicated if the carrying value of an asset exceeds its undiscounted future cash flows. If an impairment is indicated, a charge is recognized equal to the amount the carrying value exceeds the asset’s fair value. Refer to Note 3 for discussion of Alliant Energy’s impairment analysis of the Franklin County wind farm assets and resulting asset valuation charges recorded by Alliant Energy in 2016. Unconsolidated Equity Investments - If events or circumstances indicate the carrying value of investments accounted for under the equity method of accounting exceeds fair value and the decline in value is other than temporary, potential impairment is assessed. If an impairment is indicated, a charge is recognized equal to the amount the carrying value exceeds the investment’s fair value.
Asset Retirement Obligations	The fair value of a legal obligation associated with the retirement of an asset is recorded as a liability when an asset is placed in service, when a legal obligation is subsequently identified or when sufficient information becomes available to determine a reasonable estimate of the fair value of future retirement costs. When an ARO is recorded as a liability, an equivalent amount is added to the asset cost. The fair value of AROs at inception is determined using discounted cash flows analyses. The liability is accreted to its present value and the capitalized cost is depreciated over the useful life of the related asset. Accretion and depreciation expenses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory assets on the balance sheets. Upon regulatory approval to recover IPL’s AROs expenditures, its regulatory assets are amortized to depreciation and amortization expenses in Alliant Energy’s and IPL’s income statements over the same time period the ARO expenditures are recovered from IPL’s customers. WPL’s regulatory assets related to AROs are being recovered as a component of depreciation rates pursuant to PSCW and FERC orders. Accretion and depreciation expenses related to AROs for Alliant Energy’s non-utility operations are recorded to depreciation and amortization expenses in Alliant Energy’s income statements. Upon settlement of the ARO liability, an entity settles the obligation for its recorded amount or incurs a gain or loss. Any gains or losses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory liabilities or regulatory assets on the balance sheets.
Debt Issuance and Retirement Costs	Debt issuance costs and debt premiums or discounts are presented on the balance sheet as a direct deduction from the carrying amount of the related debt liability, and are deferred and amortized over the expected life of each debt issue, considering maturity dates and, if applicable, redemption rights held by others. Alliant Energy’s non-utility businesses and Corporate Services expense in the period of retirement any unamortized debt issuance costs and debt premiums or discounts on debt retired early.
Allowance for Doubtful Accounts	Allowances for doubtful accounts are recorded for estimated losses resulting from the inability of customers to make required payments. Allowances for doubtful accounts are estimated based on historical write-offs, customer arrears and other economic factors within IPL’s and WPL’s service territories.
Variable Interest Entities	An entity is considered a VIE if its equity investors do not have sufficient equity at risk for the entity to finance its activities without additional subordinated financial support from other parties, or its equity investors lack any of the following characteristics: (1) power, through voting rights or similar rights, to direct the activities of the entity that most significantly impact the entity’s economic performance; (2) the obligation to absorb expected losses of the entity; or (3) the right to receive expected benefits of the entity. The primary beneficiary of a VIE is required to consolidate the VIE. The financial statements do not reflect any consolidated VIEs.
New Accounting Standards	Revenue Recognition - In May 2014, the Financial Accounting Standards Board issued an accounting standard providing principles for recognizing revenue for the transfer of promised goods or services to customers with the consideration to which the entity expects to be entitled in exchange for those goods or services. This standard also requires disclosures about the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers. Alliant Energy, IPL and WPL adopted this standard on January 1, 2018 and used the modified retrospective method of adoption and there will be no cumulative effect adjustments made to the opening retained earnings balance on January 1, 2018 upon adoption. The majority of Alliant Energy’s, IPL’s and WPL’s revenues are from retail electric and gas sales from tariff offerings that provide electricity or natural gas without a defined contractual term. For such arrangements, revenues from contracts with customers will be equivalent to the electricity or natural gas supplied and billed, or estimated to be billed, and there will be no significant shift in the timing or pattern of revenue recognition for such sales. Alliant Energy, IPL and WPL did not have a material change in revenue recognition as a result of the adoption of this standard, and the most significant impact to their financial statements is in the form of additional disclosures. The incremental disclosures in future filings will include a separate footnote disclosure, including disaggregation of revenue by location and customer class. Leases - In February 2016, the Financial Accounting Standards Board issued an accounting standard requiring lease assets and lease liabilities, including operating leases, to be recognized on the balance sheet for all leases with terms longer than 12 months. The standard also requires disclosure of key information about leasing arrangements. Alliant Energy, IPL and WPL currently expect to adopt this standard on January 1, 2019 and are evaluating the impact of this standard on their financial condition and results of operations and expect an increase in assets and liabilities from recognizing operating leases on their balance sheets. Presentation of Net Periodic Pension and Postretirement Benefit Costs - In March 2017, the Financial Accounting Standards Board issued an accounting standard amending the income statement presentation of the components of net periodic benefit costs for defined benefit pension and other postretirement plans. The standard requires entities to (1) disaggregate the current service cost component from the other components of net periodic benefit costs and present it with other employee compensation costs in the income statement; and (2) include the other components in the income statement outside of operating income. This new presentation will shift the majority of the net periodic benefit costs from “Other operation and maintenance” expenses to “Interest expense and other” expenses in the income statements in future filings. In addition, only the service cost component of net periodic benefit costs is eligible for capitalization into property, plant and equipment, when applicable. IPL and WPL, as rate-regulated entities, will capitalize the other components of net periodic benefit costs into regulatory assets or regulatory liabilities. Alliant Energy, IPL and WPL adopted this standard on January 1, 2018 and used the retrospective method of adoption for the presentation requirements and the prospective method of adoption for the capitalization requirements.
WPL [Member]
Property, Plant and Equipment [Line Items]
General, Basis of Presentation	The financial statements reflect investments in controlled subsidiaries on a consolidated basis and Alliant Energy’s, IPL’s and WPL’s proportionate shares of jointly-owned utility EGUs. Unconsolidated investments, which Alliant Energy and WPL do not control, but have the ability to exercise significant influence over operating and financial policies, are accounted for under the equity method of accounting. Under the equity method of accounting, Alliant Energy and WPL initially record the investment at cost, and adjust the carrying amount of the investment to recognize their respective share of the earnings or losses of the investee. Dividends received from an investee reduce the carrying amount of the equity investment. Investments that do not meet the criteria for consolidation or the equity method of accounting are accounted for under the cost method. Refer to Notes 1(m) and 6(a) for further discussion of VIEs and equity method investments, respectively. All intercompany balances and transactions, other than certain transactions affecting the rate-making process at IPL and WPL, have been eliminated from the financial statements. Such transactions not eliminated include costs that are recoverable from customers through rate-making processes.
General, Basis of Accounting	The financial statements are prepared in conformity with GAAP, which give recognition to the rate-making and accounting practices of FERC and state commissions having regulatory jurisdiction.
General, Reclassification	Certain prior period amounts in the Financial Statements and Notes have been reclassified to conform to the current period presentation for comparative purposes, including reclassifications resulting from modifications to segment reporting as discussed in Note 17 .
General, Use of Estimates	The preparation of the financial statements requires management to make estimates and assumptions that affect: (a) the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the financial statements; and (b) the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Regulatory Assets and Regulatory Liabilities	Alliant Energy, IPL and WPL are subject to regulation by FERC and various state regulatory commissions. As a result, Alliant Energy, IPL and WPL are subject to GAAP provisions for regulated operations, which provide that rate-regulated public utilities record certain costs and credits allowed in the rate-making process in different periods than for non-utility entities. Regulatory assets generally represent incurred costs that have been deferred as such costs are probable of recovery in future customer rates. Regulatory liabilities generally represent obligations to make refunds to customers or amounts collected in rates for which the related costs have not yet been incurred. Amounts recorded as regulatory assets or regulatory liabilities are generally recognized in the income statements at the time they are reflected in rates.
Income Taxes	The liability method of accounting is followed for deferred taxes, which requires the establishment of deferred tax assets and liabilities, as appropriate, for temporary differences between the tax basis of assets and liabilities and the amounts reported in the financial statements. Deferred taxes are recorded using currently enacted tax rates, including impacts from Tax Reform and estimates of state apportionment. Changes in deferred tax assets and liabilities associated with certain property-related differences at IPL are accounted for differently than other subsidiaries of Alliant Energy due to rate-making practices in Iowa. Rate-making practices in Iowa do not include the impact of certain deferred tax expenses (benefits) in the determination of retail rates. Based on these rate-making practices, deferred tax expense (benefit) related to these property-related differences at IPL is not recorded in the income statement but instead recorded to regulatory assets or regulatory liabilities until these temporary differences reverse. Refer to Note 2 for further discussion of these tax-related regulatory assets and regulatory liabilities associated with property-related differences at IPL. In Wisconsin, the PSCW allows rate recovery of deferred tax expense on all temporary differences. Investment tax credits are deferred and amortized to income over the average lives of the related property. Tax Reform repealed corporate federal AMT and allows unutilized AMT credits to be refunded over the next four tax years beginning with the U.S. federal tax return for calendar year 2018. Other tax credits reduce income tax expense in the year claimed. Alliant Energy files a consolidated federal income tax return and a combined return in Wisconsin, which include Alliant Energy and its subsidiaries. Alliant Energy subsidiaries with a presence in Iowa file as part of a consolidated return in Iowa. Alliant Energy allocates consolidated income tax expense to its subsidiaries that are members of the group that file a consolidated or combined income tax return. IPL and WPL use the modified separate return approach for calculating their income tax provisions and related deferred tax assets and liabilities. IPL and WPL are assumed to file separate tax returns with the federal and state taxing authorities, except that net operating losses (and other current or deferred tax attributes) are characterized as realized (or realizable) by IPL and WPL when those tax attributes are realized (or realizable) by the consolidated tax return group of Alliant Energy (even if IPL and WPL would not otherwise have realized the attributes on a stand-alone basis). The difference in the income taxes recorded for IPL and WPL under the modified separate return method compared to the income taxes recorded on a separate return basis was not material in 2017 , 2016 and 2015 .
Cash and Cash Equivalents	Cash and cash equivalents include short-term liquid investments that have original maturities of less than 90 days.
Property, Plant and Equipment	Utility Plant - General - Utility plant is recorded at the original cost of acquisition or construction, which includes material, labor, contractor services, AFUDC and allocable overheads, such as supervision, engineering, benefits, certain taxes and transportation. Repairs, replacements and renewals of items of property determined to be less than a unit of property or that do not increase the property’s life or functionality are charged to maintenance expense. Generally, ordinary retirements of utility plant and salvage value are netted and charged to accumulated depreciation upon removal from utility plant accounts and no gain or loss is recognized consistent with rate-making principles. However, if regulators have approved recovery of the remaining net book value of property, plant and equipment that is retired early, or such approval by regulators is probable, the remaining net book value is reclassified from property, plant and equipment to regulatory assets upon retirement. Property, plant and equipment that is probable of being retired early is classified as plant anticipated to be retired early.
Depreciation	IPL and WPL use a combination of remaining life and straight-line depreciation methods as approved by their respective regulatory commissions. The composite or group method of depreciation is used, in which a single depreciation rate is applied to the gross investment in a particular class of property. This method pools similar assets and then depreciates each group as a whole. Periodic depreciation studies are performed to determine the appropriate group lives, net salvage, estimated cost of removal and group depreciation rates. These depreciation studies are subject to review and approval by IPL’s and WPL’s respective regulatory commissions. Depreciation expense is included within the recoverable cost of service component of rates charged to customers. The average rates of depreciation for electric, gas and other properties, consistent with current rate-making practices, were as follows: IPL WPL 2017 2016 2015 2017 2016 2015 Electric - generation 3.5% 3.5% 3.6% 3.5% 3.1% 3.2% Electric - distribution 2.4% 2.4% 2.4% 2.6% 2.6% 2.7% Electric - other 4.5% 4.2% 4.0% 6.9% 4.7% 4.5% Gas 3.4% 3.3% 3.2% 2.5% 2.5% 2.5% Other 4.0% 3.9% 3.9% 6.0% 5.9% 6.0% In September 2016, the PSCW issued an order approving the implementation of updated depreciation rates for WPL effective January 1, 2017 as a result of an updated depreciation study. In February 2018, the IUB issued an order approving the implementation of updated depreciation rates for IPL, which are currently expected to be effective in the first half of 2018, as a result of an updated depreciation study. IPL estimates the new average rates of depreciation for its electric generation and electric distribution properties will be approximately 3.8% and 2.8% , respectively, during 2018.
AFUDC	AFUDC represents costs to finance construction additions, including a return on equity component and cost of debt component as required by regulatory accounting. AFUDC for IPL’s construction projects is calculated in accordance with FERC guidelines. AFUDC for WPL’s retail and wholesale jurisdiction construction projects is calculated in accordance with PSCW and FERC guidelines, respectively. The AFUDC rates, computed in accordance with the prescribed regulatory formula, were as follows: 2017 2016 2015 IPL (Marshalltown CWIP) (a) 7.8% 7.9% 7.9% IPL (Wind generation CWIP) (b) 7.6% N/A N/A IPL (other CWIP) 7.6% 7.7% 7.7% WPL (retail jurisdiction) 7.6% 8.2% 8.2% WPL (wholesale jurisdiction) 6.0% 6.7% 7.9% (a) In 2013, the IUB issued an order establishing rate-making principles that require a 10.3% return on common equity for the calculation of AFUDC related to the construction of Marshalltown. (b) In 2016, the IUB issued an order establishing rate-making principles that require a return on common equity for the calculation of AFUDC related to the construction of up to 500 MW of new wind generation equal to the greater of 10.0% or whatever percentage the IUB finds reasonable during IPL’s most recent retail electric rate proceeding. In accordance with their most recent rate orders, IPL applies its AFUDC rates to 100% of applicable CWIP balances and WPL generally applies its AFUDC rates to 50% of applicable CWIP balances. WPL may apply its AFUDC rates to 100% of the retail portion of the CWIP balances for construction projects requiring a CA or CPCN that were approved by the PSCW after its most recent rate order, including West Riverside.
Operating Revenues	Utility - Revenues from Alliant Energy’s utility business are primarily from electricity and natural gas sales and are recognized on an accrual basis as services are rendered or commodities are delivered to customers. Energy sales to individual customers are based on the reading of customers’ meters, which occurs on a systematic basis throughout each reporting period. Amounts of energy delivered to customers since the date of the last meter reading are estimated at the end of each reporting period and the corresponding estimated unbilled revenue is recorded in such reporting period. The unbilled revenue estimate is based on daily system demand volumes, estimated customer usage by class, temperature impacts, line losses and the most recent customer rates. IPL and WPL accrue revenues from their wholesale customers to the extent that the actual net revenue requirements calculated in accordance with FERC-approved formula rates for the reporting period are higher or lower than the amounts billed to wholesale customers during such period. Regulatory assets or regulatory liabilities are recorded as the offset for these accrued revenues under formulaic rate-making programs. IPL’s estimated recovery amount is recorded in the current period of service and is reflected in customer bills within two years under the provisions of approved formula rates. WPL’s estimated recovery amount is recorded in the current period of service and subject to final adjustments after a customer audit period in the subsequent year. Final settled recovery amounts are reflected in WPL’s customer bills within two years under the provisions of approved formula rates. IPL and WPL participate in bid/offer-based wholesale energy and ancillary services markets operated by MISO. MISO requires that all load serving entities and generation owners, including IPL and WPL, submit hourly bids and offers for energy and ancillary services. The MISO transactions are grouped together, resulting in a net supply to or net purchase from MISO for each hour of each day. The net supply to MISO is recorded in “Electric utility operating revenues” and the net purchase from MISO is recorded in “Electric production fuel and purchased power” in the income statements. Non-utility - Revenues from Alliant Energy’s non-utility businesses are primarily from its Transportation business and are recognized on an accrual basis as services are rendered or goods are delivered to customers. Taxes Collected from Customers - Sales or various other taxes collected by certain of Alliant Energy’s subsidiaries on behalf of other agencies are recorded on a net basis and are not included in operating revenues.
Utility Cost Recovery Mechanisms	Electric Production Fuel and Purchased Power (Fuel-related Costs) - Fuel-related costs are incurred to generate and purchase electricity to meet the demand of IPL’s and WPL’s electric customers. These fuel-related costs include the cost of fossil fuels (primarily natural gas and coal) used to produce electricity at their EGUs and electricity purchased from MISO wholesale energy markets and under PPAs. These fuel-related costs are recorded in “Electric production fuel and purchased power” in the income statements. IPL Retail - The cost recovery mechanisms for IPL’s retail electric customers provide for monthly adjustments to their electric rates for changes in fuel-related costs. Changes in the under-/over-collection of these costs are recognized in “Electric production fuel and purchased power” in Alliant Energy’s and IPL’s income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and IPL’s balance sheets until they are reflected in future billings to customers. WPL Retail - The cost recovery mechanism for WPL’s retail electric customers is based on forecasts of certain fuel-related costs expected to be incurred during forward-looking test year periods and fuel monitoring ranges determined by the PSCW during each retail electric rate proceeding or in a separate fuel cost plan approval proceeding. If WPL’s actual fuel-related costs fall outside these fuel monitoring ranges, WPL is authorized to defer the incremental under-/over-collection of fuel-related costs that are outside the approved ranges. Deferral of under-collections are reduced to the extent actual return on common equity earned by WPL during the fuel cost plan year exceeds the most recently authorized return on common equity. Deferred amounts for fuel-related costs outside the approved fuel monitoring ranges are recognized in “Electric production fuel and purchased power” in Alliant Energy’s and WPL’s income statements. The cumulative effects of these deferred amounts are recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and WPL’s balance sheets until they are reflected in future billings to customers. IPL and WPL Wholesale - The cost recovery mechanisms for IPL’s and WPL’s wholesale electric customers provide for subsequent adjustments to their electric rates for changes in fuel-related costs. Changes in the under-/over-collection of these costs are recognized in “Electric production fuel and purchased power” in the income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on the balance sheets until they are reflected in future billings to customers. Purchased Electric Capacity - PPAs help meet the electricity demand of IPL’s and WPL’s customers. Certain of these PPAs include minimum payments for IPL’s and WPL’s rights to electric generating capacity, which are charged each period to “Electric production fuel and purchased power” in the income statements. Purchased electric capacity expenses are recovered from IPL’s and WPL’s retail electric customers through changes in base rates determined during periodic rate proceedings. Purchased electric capacity expenses are recovered from IPL’s and WPL’s wholesale electric customers through annual changes in base rates determined by a formula rate structure. Electric Transmission Service - Costs incurred for the transmission of electricity to meet the demands of IPL’s and WPL’s customers are charged to “Electric transmission service” in the income statements. IPL Retail - Electric transmission service expense is recovered from IPL’s retail electric customers through a transmission cost rider. This cost recovery mechanism provides for annual adjustments to electric rates charged to retail electric customers for changes in electric transmission service expense. Changes in the under-/over-collection of these costs are recognized in “Electric transmission service” in Alliant Energy’s and IPL’s income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and IPL’s balance sheets until they are reflected in future billings to customers. WPL Retail - Electric transmission service expense is recovered from WPL’s retail electric customers through changes in base rates determined during periodic rate proceedings. Pursuant to escrow accounting treatment approved by the PSCW, the difference between actual electric transmission service expense incurred and the amount of electric transmission service costs collected from customers as electric revenues is recognized in “Electric transmission service” in Alliant Energy’s and WPL’s income statements. An offsetting amount is recorded in regulatory assets or regulatory liabilities on Alliant Energy’s and WPL’s balance sheets until reflected in future billings to customers. The PSCW’s December 2016 order for WPL’s retail electric rate review (2017/2018 Test Period) extends this escrow accounting treatment through 2018. IPL and WPL Wholesale - IPL and WPL arrange transmission service for the majority of their respective wholesale electric customers. Electric transmission service expense is allocated to and recovered from these customers based on a load ratio share computation. Cost of Gas Sold - Costs are incurred for the purchase, transportation and storage of natural gas to serve IPL’s and WPL’s gas customers and the costs associated with the natural gas delivered to customers are charged to “Cost of gas sold” in the income statements. The tariffs for IPL’s and WPL’s retail gas customers provide for subsequent adjustments to their rates each month for changes in the cost of gas sold. Changes in the under-/over-collection of these costs are also recognized in “Cost of gas sold” in the income statements. The cumulative effects of the under-/over-collection of these costs are recorded in regulatory assets or regulatory liabilities on the balance sheets until they are reflected in future billings to customers. Energy Efficiency Costs - Costs are incurred to fund energy efficiency programs and initiatives that help customers reduce their energy usage. The costs incurred for these programs and initiatives are charged to “Other operation and maintenance” in the income statements. Energy efficiency costs incurred by IPL are recovered from its retail electric and gas customers through an additional tariff called an energy efficiency cost recovery factor, which is revised annually and includes a reconciliation to eliminate any under-/over-collection of energy efficiency costs from prior periods. Energy efficiency costs incurred by WPL are recovered from retail electric and gas customers through changes in base rates determined during periodic rate proceedings. Reconciliations of any under-/over-collection of energy efficiency costs from prior periods are also addressed in WPL’s periodic rate proceedings. Changes in the under-/over-collection of energy efficiency costs for IPL and WPL are recognized in “Other operation and maintenance” in the income statements. The cumulative effects of the under-/over-collection of these costs for IPL and WPL are recorded in regulatory assets or regulatory liabilities on the balance sheets until they are reflected in future billings to customers.
Financial Instruments	Financial instruments are periodically used for risk management purposes to mitigate exposures to fluctuations in certain commodity prices and transmission congestion costs. The fair value of those financial instruments that are determined to be derivatives are recorded as assets or liabilities on the balance sheets. Certain commodity purchase and sales contracts qualify for and have been designated under the normal purchase and sale exception and are accounted for on the accrual basis of accounting. Alliant Energy, IPL and WPL have elected to not net the fair value amounts of derivatives subject to a master netting arrangement by counterparty. Alliant Energy, IPL and WPL do not offset fair value amounts recognized for the right to reclaim cash collateral (receivable) or the obligation to return cash collateral (payable) against fair value amounts recognized for derivative instruments that are executed with the same counterparty under the same master netting arrangement.
Asset Impairments	Property, Plant and Equipment of Regulated Operations - Property, plant and equipment of regulated operations are reviewed for possible impairment whenever events or changes in circumstances indicate all or a portion of the carrying value of the assets may be disallowed for rate-making purposes. If IPL or WPL are disallowed recovery of any portion of the carrying value of their regulated property, plant and equipment that is under construction, has been recently completed or is probable of abandonment, or conclude it is probable recovery will be disallowed, an impairment charge is recognized equal to the amount of the carrying value that was disallowed or is probable of being disallowed. If IPL or WPL are only allowed a partial return on the carrying value of their regulated property, plant and equipment that is under construction, has been recently completed or is probable of abandonment, or conclude it is probable a full return will not be allowed, an impairment charge is recognized equal to the difference between the carrying value and the present value of the future revenues expected from their regulated property, plant and equipment. Property, Plant and Equipment of Non-utility Operations - Property, plant and equipment of non-utility operations are reviewed for possible impairment whenever events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Impairment is indicated if the carrying value of an asset exceeds its undiscounted future cash flows. If an impairment is indicated, a charge is recognized equal to the amount the carrying value exceeds the asset’s fair value. Refer to Note 3 for discussion of Alliant Energy’s impairment analysis of the Franklin County wind farm assets and resulting asset valuation charges recorded by Alliant Energy in 2016. Unconsolidated Equity Investments - If events or circumstances indicate the carrying value of investments accounted for under the equity method of accounting exceeds fair value and the decline in value is other than temporary, potential impairment is assessed. If an impairment is indicated, a charge is recognized equal to the amount the carrying value exceeds the investment’s fair value.
Asset Retirement Obligations	The fair value of a legal obligation associated with the retirement of an asset is recorded as a liability when an asset is placed in service, when a legal obligation is subsequently identified or when sufficient information becomes available to determine a reasonable estimate of the fair value of future retirement costs. When an ARO is recorded as a liability, an equivalent amount is added to the asset cost. The fair value of AROs at inception is determined using discounted cash flows analyses. The liability is accreted to its present value and the capitalized cost is depreciated over the useful life of the related asset. Accretion and depreciation expenses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory assets on the balance sheets. Upon regulatory approval to recover IPL’s AROs expenditures, its regulatory assets are amortized to depreciation and amortization expenses in Alliant Energy’s and IPL’s income statements over the same time period the ARO expenditures are recovered from IPL’s customers. WPL’s regulatory assets related to AROs are being recovered as a component of depreciation rates pursuant to PSCW and FERC orders. Accretion and depreciation expenses related to AROs for Alliant Energy’s non-utility operations are recorded to depreciation and amortization expenses in Alliant Energy’s income statements. Upon settlement of the ARO liability, an entity settles the obligation for its recorded amount or incurs a gain or loss. Any gains or losses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory liabilities or regulatory assets on the balance sheets.
Debt Issuance and Retirement Costs	Debt issuance costs and debt premiums or discounts are presented on the balance sheet as a direct deduction from the carrying amount of the related debt liability, and are deferred and amortized over the expected life of each debt issue, considering maturity dates and, if applicable, redemption rights held by others. Alliant Energy’s non-utility businesses and Corporate Services expense in the period of retirement any unamortized debt issuance costs and debt premiums or discounts on debt retired early.
Allowance for Doubtful Accounts	Allowances for doubtful accounts are recorded for estimated losses resulting from the inability of customers to make required payments. Allowances for doubtful accounts are estimated based on historical write-offs, customer arrears and other economic factors within IPL’s and WPL’s service territories.
Variable Interest Entities	An entity is considered a VIE if its equity investors do not have sufficient equity at risk for the entity to finance its activities without additional subordinated financial support from other parties, or its equity investors lack any of the following characteristics: (1) power, through voting rights or similar rights, to direct the activities of the entity that most significantly impact the entity’s economic performance; (2) the obligation to absorb expected losses of the entity; or (3) the right to receive expected benefits of the entity. The primary beneficiary of a VIE is required to consolidate the VIE. The financial statements do not reflect any consolidated VIEs.
New Accounting Standards	Revenue Recognition - In May 2014, the Financial Accounting Standards Board issued an accounting standard providing principles for recognizing revenue for the transfer of promised goods or services to customers with the consideration to which the entity expects to be entitled in exchange for those goods or services. This standard also requires disclosures about the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers. Alliant Energy, IPL and WPL adopted this standard on January 1, 2018 and used the modified retrospective method of adoption and there will be no cumulative effect adjustments made to the opening retained earnings balance on January 1, 2018 upon adoption. The majority of Alliant Energy’s, IPL’s and WPL’s revenues are from retail electric and gas sales from tariff offerings that provide electricity or natural gas without a defined contractual term. For such arrangements, revenues from contracts with customers will be equivalent to the electricity or natural gas supplied and billed, or estimated to be billed, and there will be no significant shift in the timing or pattern of revenue recognition for such sales. Alliant Energy, IPL and WPL did not have a material change in revenue recognition as a result of the adoption of this standard, and the most significant impact to their financial statements is in the form of additional disclosures. The incremental disclosures in future filings will include a separate footnote disclosure, including disaggregation of revenue by location and customer class. Leases - In February 2016, the Financial Accounting Standards Board issued an accounting standard requiring lease assets and lease liabilities, including operating leases, to be recognized on the balance sheet for all leases with terms longer than 12 months. The standard also requires disclosure of key information about leasing arrangements. Alliant Energy, IPL and WPL currently expect to adopt this standard on January 1, 2019 and are evaluating the impact of this standard on their financial condition and results of operations and expect an increase in assets and liabilities from recognizing operating leases on their balance sheets. Presentation of Net Periodic Pension and Postretirement Benefit Costs - In March 2017, the Financial Accounting Standards Board issued an accounting standard amending the income statement presentation of the components of net periodic benefit costs for defined benefit pension and other postretirement plans. The standard requires entities to (1) disaggregate the current service cost component from the other components of net periodic benefit costs and present it with other employee compensation costs in the income statement; and (2) include the other components in the income statement outside of operating income. This new presentation will shift the majority of the net periodic benefit costs from “Other operation and maintenance” expenses to “Interest expense and other” expenses in the income statements in future filings. In addition, only the service cost component of net periodic benefit costs is eligible for capitalization into property, plant and equipment, when applicable. IPL and WPL, as rate-regulated entities, will capitalize the other components of net periodic benefit costs into regulatory assets or regulatory liabilities. Alliant Energy, IPL and WPL adopted this standard on January 1, 2018 and used the retrospective method of adoption for the presentation requirements and the prospective method of adoption for the capitalization requirements.

Summary Of Significant Accoun30

Summary Of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Schedule of Average Rates of Depreciation	The average rates of depreciation for electric, gas and other properties, consistent with current rate-making practices, were as follows: IPL WPL 2017 2016 2015 2017 2016 2015 Electric - generation 3.5% 3.5% 3.6% 3.5% 3.1% 3.2% Electric - distribution 2.4% 2.4% 2.4% 2.6% 2.6% 2.7% Electric - other 4.5% 4.2% 4.0% 6.9% 4.7% 4.5% Gas 3.4% 3.3% 3.2% 2.5% 2.5% 2.5% Other 4.0% 3.9% 3.9% 6.0% 5.9% 6.0%
Schedule of Allowance for Funds Used During Construction Rate	The AFUDC rates, computed in accordance with the prescribed regulatory formula, were as follows: 2017 2016 2015 IPL (Marshalltown CWIP) (a) 7.8% 7.9% 7.9% IPL (Wind generation CWIP) (b) 7.6% N/A N/A IPL (other CWIP) 7.6% 7.7% 7.7% WPL (retail jurisdiction) 7.6% 8.2% 8.2% WPL (wholesale jurisdiction) 6.0% 6.7% 7.9% (a) In 2013, the IUB issued an order establishing rate-making principles that require a 10.3% return on common equity for the calculation of AFUDC related to the construction of Marshalltown. (b) In 2016, the IUB issued an order establishing rate-making principles that require a return on common equity for the calculation of AFUDC related to the construction of up to 500 MW of new wind generation equal to the greater of 10.0% or whatever percentage the IUB finds reasonable during IPL’s most recent retail electric rate proceeding.
IPL [Member]
Schedule of Average Rates of Depreciation	The average rates of depreciation for electric, gas and other properties, consistent with current rate-making practices, were as follows: IPL WPL 2017 2016 2015 2017 2016 2015 Electric - generation 3.5% 3.5% 3.6% 3.5% 3.1% 3.2% Electric - distribution 2.4% 2.4% 2.4% 2.6% 2.6% 2.7% Electric - other 4.5% 4.2% 4.0% 6.9% 4.7% 4.5% Gas 3.4% 3.3% 3.2% 2.5% 2.5% 2.5% Other 4.0% 3.9% 3.9% 6.0% 5.9% 6.0%
Schedule of Allowance for Funds Used During Construction Rate	The AFUDC rates, computed in accordance with the prescribed regulatory formula, were as follows: 2017 2016 2015 IPL (Marshalltown CWIP) (a) 7.8% 7.9% 7.9% IPL (Wind generation CWIP) (b) 7.6% N/A N/A IPL (other CWIP) 7.6% 7.7% 7.7% WPL (retail jurisdiction) 7.6% 8.2% 8.2% WPL (wholesale jurisdiction) 6.0% 6.7% 7.9% (a) In 2013, the IUB issued an order establishing rate-making principles that require a 10.3% return on common equity for the calculation of AFUDC related to the construction of Marshalltown. (b) In 2016, the IUB issued an order establishing rate-making principles that require a return on common equity for the calculation of AFUDC related to the construction of up to 500 MW of new wind generation equal to the greater of 10.0% or whatever percentage the IUB finds reasonable during IPL’s most recent retail electric rate proceeding.
WPL [Member]
Schedule of Average Rates of Depreciation	The average rates of depreciation for electric, gas and other properties, consistent with current rate-making practices, were as follows: IPL WPL 2017 2016 2015 2017 2016 2015 Electric - generation 3.5% 3.5% 3.6% 3.5% 3.1% 3.2% Electric - distribution 2.4% 2.4% 2.4% 2.6% 2.6% 2.7% Electric - other 4.5% 4.2% 4.0% 6.9% 4.7% 4.5% Gas 3.4% 3.3% 3.2% 2.5% 2.5% 2.5% Other 4.0% 3.9% 3.9% 6.0% 5.9% 6.0%
Schedule of Allowance for Funds Used During Construction Rate	The AFUDC rates, computed in accordance with the prescribed regulatory formula, were as follows: 2017 2016 2015 IPL (Marshalltown CWIP) (a) 7.8% 7.9% 7.9% IPL (Wind generation CWIP) (b) 7.6% N/A N/A IPL (other CWIP) 7.6% 7.7% 7.7% WPL (retail jurisdiction) 7.6% 8.2% 8.2% WPL (wholesale jurisdiction) 6.0% 6.7% 7.9% (a) In 2013, the IUB issued an order establishing rate-making principles that require a 10.3% return on common equity for the calculation of AFUDC related to the construction of Marshalltown. (b) In 2016, the IUB issued an order establishing rate-making principles that require a return on common equity for the calculation of AFUDC related to the construction of up to 500 MW of new wind generation equal to the greater of 10.0% or whatever percentage the IUB finds reasonable during IPL’s most recent retail electric rate proceeding.

Regulatory Matters (Tables)

Regulatory Matters (Tables)	12 Months Ended
	Dec. 31, 2017
Regulatory Assets [Line Items]
Schedule of Regulatory Assets	At December 31, regulatory assets were comprised of the following items (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Tax-related $778.2 $1,055.6 $750.5 $1,022.4 $27.7 $33.2 Pension and OPEB costs 548.0 578.7 274.4 294.0 273.6 284.7 AROs 109.3 105.9 72.5 64.3 36.8 41.6 EGUs retired early 63.8 41.4 31.6 — 32.2 41.4 Derivatives 45.3 30.7 21.8 10.0 23.5 20.7 Emission allowances 25.5 26.2 25.5 26.2 — — Other 96.6 76.6 55.3 41.9 41.3 34.7 $1,666.7 $1,915.1 $1,231.6 $1,458.8 $435.1 $456.3
Schedule of Regulatory Liabilities	At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Tax-related $899.4 $7.7 $399.5 $1.9 $499.9 $5.8 Cost of removal obligations 410.0 411.6 274.5 269.4 135.5 142.2 Electric transmission cost recovery 90.4 72.0 26.4 35.7 64.0 36.3 IPL’s tax benefit riders 25.0 83.5 25.0 83.5 — — Commodity cost recovery 21.0 30.8 14.6 17.8 6.4 13.0 Energy efficiency cost recovery 19.9 20.5 — — 19.9 20.5 Other 31.5 54.9 15.4 22.5 16.1 32.4 $1,497.2 $681.0 $755.4 $430.8 $741.8 $250.2
Tax Benefit Riders	In 2017 , Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities increased (decreased) by ($59) million as follows (in millions): Electric tax benefit rider credits ($65 ) Gas tax benefit rider credits (6 ) Rate-making accounting change for capitalized interest 17 Tax Reform adjustment (Refer to Note 11 ) (5 ) ($59 )
Electric Tax Benefit Rider	Details for IPL’s electric tax benefit rider are as follows (in millions): 2017 2016 2015 Credit to IPL’s Iowa retail electric customers’ bills with reduction to electric revenues (based on customers’ KWh usage) $65 $64 $72 Income tax benefit resulting from decreased taxable income caused by credits 27 27 30 Income tax benefit representing tax benefits realized from electric tax benefit rider 38 37 42
Revenue Requirement Adjustment	The revenue requirement adjustment resulted in increases to electric revenues in Alliant Energy’s and IPL’s income statements and was recognized through the energy adjustment clause as a reduction of the credits on IPL’s Iowa retail electric customers’ bills from the electric tax benefit rider as follows (in millions): 2016 2015 Revenue requirement adjustment $14 $14
Gas Tax Benefit Rider	Details for IPL’s gas tax benefit rider are as follows (in millions): 2017 2016 2015 Credit to IPL’s Iowa retail gas customers’ bills with reduction to gas revenues (based on a fixed amount per day) $6 $12 $12 Income tax benefit resulting from decreased taxable income caused by credits 3 5 5 Income tax benefit representing tax benefits realized from gas tax benefit rider 3 7 7
Customer Billing Credits	The IUB approved a settlement agreement in 2014 related to rates charged to IPL’s Iowa retail electric customers. The settlement agreement extended IPL’s Iowa retail electric base rates authorized in its 2009 Test Year rate review through 2016 and provided targeted retail electric customer billing credits of $105 million in aggregate. IPL recorded such billing credits as follows (in millions): 2016 2015 2014 Billing credits to reduce retail electric customers’ bills $9 $24 $72
IPL [Member]
Regulatory Assets [Line Items]
Schedule of Regulatory Assets	At December 31, regulatory assets were comprised of the following items (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Tax-related $778.2 $1,055.6 $750.5 $1,022.4 $27.7 $33.2 Pension and OPEB costs 548.0 578.7 274.4 294.0 273.6 284.7 AROs 109.3 105.9 72.5 64.3 36.8 41.6 EGUs retired early 63.8 41.4 31.6 — 32.2 41.4 Derivatives 45.3 30.7 21.8 10.0 23.5 20.7 Emission allowances 25.5 26.2 25.5 26.2 — — Other 96.6 76.6 55.3 41.9 41.3 34.7 $1,666.7 $1,915.1 $1,231.6 $1,458.8 $435.1 $456.3
Schedule of Regulatory Liabilities	At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Tax-related $899.4 $7.7 $399.5 $1.9 $499.9 $5.8 Cost of removal obligations 410.0 411.6 274.5 269.4 135.5 142.2 Electric transmission cost recovery 90.4 72.0 26.4 35.7 64.0 36.3 IPL’s tax benefit riders 25.0 83.5 25.0 83.5 — — Commodity cost recovery 21.0 30.8 14.6 17.8 6.4 13.0 Energy efficiency cost recovery 19.9 20.5 — — 19.9 20.5 Other 31.5 54.9 15.4 22.5 16.1 32.4 $1,497.2 $681.0 $755.4 $430.8 $741.8 $250.2
Tax Benefit Riders	In 2017 , Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities increased (decreased) by ($59) million as follows (in millions): Electric tax benefit rider credits ($65 ) Gas tax benefit rider credits (6 ) Rate-making accounting change for capitalized interest 17 Tax Reform adjustment (Refer to Note 11 ) (5 ) ($59 )
Electric Tax Benefit Rider	Details for IPL’s electric tax benefit rider are as follows (in millions): 2017 2016 2015 Credit to IPL’s Iowa retail electric customers’ bills with reduction to electric revenues (based on customers’ KWh usage) $65 $64 $72 Income tax benefit resulting from decreased taxable income caused by credits 27 27 30 Income tax benefit representing tax benefits realized from electric tax benefit rider 38 37 42
Revenue Requirement Adjustment	The revenue requirement adjustment resulted in increases to electric revenues in Alliant Energy’s and IPL’s income statements and was recognized through the energy adjustment clause as a reduction of the credits on IPL’s Iowa retail electric customers’ bills from the electric tax benefit rider as follows (in millions): 2016 2015 Revenue requirement adjustment $14 $14
Gas Tax Benefit Rider	Details for IPL’s gas tax benefit rider are as follows (in millions): 2017 2016 2015 Credit to IPL’s Iowa retail gas customers’ bills with reduction to gas revenues (based on a fixed amount per day) $6 $12 $12 Income tax benefit resulting from decreased taxable income caused by credits 3 5 5 Income tax benefit representing tax benefits realized from gas tax benefit rider 3 7 7
Customer Billing Credits	The IUB approved a settlement agreement in 2014 related to rates charged to IPL’s Iowa retail electric customers. The settlement agreement extended IPL’s Iowa retail electric base rates authorized in its 2009 Test Year rate review through 2016 and provided targeted retail electric customer billing credits of $105 million in aggregate. IPL recorded such billing credits as follows (in millions): 2016 2015 2014 Billing credits to reduce retail electric customers’ bills $9 $24 $72
WPL [Member]
Regulatory Assets [Line Items]
Schedule of Regulatory Assets	At December 31, regulatory assets were comprised of the following items (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Tax-related $778.2 $1,055.6 $750.5 $1,022.4 $27.7 $33.2 Pension and OPEB costs 548.0 578.7 274.4 294.0 273.6 284.7 AROs 109.3 105.9 72.5 64.3 36.8 41.6 EGUs retired early 63.8 41.4 31.6 — 32.2 41.4 Derivatives 45.3 30.7 21.8 10.0 23.5 20.7 Emission allowances 25.5 26.2 25.5 26.2 — — Other 96.6 76.6 55.3 41.9 41.3 34.7 $1,666.7 $1,915.1 $1,231.6 $1,458.8 $435.1 $456.3
Schedule of Regulatory Liabilities	At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Tax-related $899.4 $7.7 $399.5 $1.9 $499.9 $5.8 Cost of removal obligations 410.0 411.6 274.5 269.4 135.5 142.2 Electric transmission cost recovery 90.4 72.0 26.4 35.7 64.0 36.3 IPL’s tax benefit riders 25.0 83.5 25.0 83.5 — — Commodity cost recovery 21.0 30.8 14.6 17.8 6.4 13.0 Energy efficiency cost recovery 19.9 20.5 — — 19.9 20.5 Other 31.5 54.9 15.4 22.5 16.1 32.4 $1,497.2 $681.0 $755.4 $430.8 $741.8 $250.2

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	At December 31, details of property, plant and equipment on the balance sheets were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Utility: Electric plant: Generation in service $6,655.3 $5,866.9 $3,715.9 $2,916.8 $2,939.4 $2,950.1 Distribution in service 5,123.5 4,739.2 2,820.9 2,589.3 2,302.6 2,149.9 Other in service 425.1 329.1 282.3 223.5 142.8 105.6 Anticipated to be retired early (a)(b) 93.0 108.3 — 108.3 93.0 — Total electric plant 12,296.9 11,043.5 6,819.1 5,837.9 5,477.8 5,205.6 Gas plant in service 1,244.0 1,107.6 654.8 556.7 589.2 550.9 Other plant in service 571.9 549.3 333.4 313.0 238.5 236.3 Accumulated depreciation (4,283.1 ) (4,135.7 ) (2,311.0 ) (2,258.3 ) (1,972.1 ) (1,877.4 ) Net plant 9,829.7 8,564.7 5,496.3 4,449.3 4,333.4 4,115.4 Leased Sheboygan Falls Energy Facility, net (c) — — — — 46.2 52.4 Construction work in progress 962.2 1,226.8 424.4 968.1 537.8 258.7 Other, net 6.0 18.4 5.5 18.2 0.5 0.2 Total utility 10,797.9 9,809.9 5,926.2 5,435.6 4,917.9 4,426.7 Non-utility and other: Non-utility Generation, net (d) 90.9 135.0 — — — — Corporate Services and other, net (e) 345.7 334.3 — — — — Total non-utility and other 436.6 469.3 — — — — Total property, plant and equipment $11,234.5 $10,279.2 $5,926.2 $5,435.6 $4,917.9 $4,426.7 (a) In 2017, IPL retired Sutherland Unit 3 and reclassified the remaining net book value of this EGU from property, plant and equipment to a regulatory asset on Alliant Energy’s and IPL’s balance sheets. Refer to Note 2 for further discussion, including recovery of the remaining net book value of Sutherland Unit 3. (b) In 2017, WPL received approval from MISO to retire Edgewater Unit 4 and currently anticipates retiring this EGU by September 30, 2018. In 2016, the PSCW authorized WPL to recover the remaining net book value of Edgewater Unit 4 over a 10 -year period beginning the later of the retirement date of the EGU or January 1, 2019. (c) Less accumulated amortization of $77.6 million and $71.4 million for WPL as of December 31, 2017 and 2016 , respectively. The Sheboygan Falls Energy Facility is eliminated from WPL upon consolidation and is included in the “Non-utility Generation, net” line within Alliant Energy’s consolidated property, plant and equipment. (d) Less accumulated depreciation of $50.5 million and $46.5 million for Alliant Energy as of December 31, 2017 and 2016 , respectively. Refer to “Franklin County Wind Farm” below for discussion of the April 2017 transfer of the Franklin County wind farm from AEF to IPL pursuant to a February 2017 FERC order. (e) Less accumulated depreciation of $285.6 million and $272.0 million for Alliant Energy as of December 31, 2017 and 2016 , respectively.
Assets Purchased and Liabilities Assumed	As of the closing date, the estimated fair values of the assets purchased and liabilities assumed by IPL were as follows (in millions): Electric plant in service $40 Current assets 2 Total assets acquired 42 Other liabilities 10 Net assets acquired $32
Allowance For Funds Used During Construction	The amount of AFUDC generated by equity and debt components was as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Equity $33.6 $42.3 $24.4 $21.1 $35.2 $18.6 $12.5 $7.1 $5.8 Debt 16.1 20.2 12.5 10.3 16.8 9.6 5.8 3.4 2.9 $49.7 $62.5 $36.9 $31.4 $52.0 $28.2 $18.3 $10.5 $8.7
IPL [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	At December 31, details of property, plant and equipment on the balance sheets were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Utility: Electric plant: Generation in service $6,655.3 $5,866.9 $3,715.9 $2,916.8 $2,939.4 $2,950.1 Distribution in service 5,123.5 4,739.2 2,820.9 2,589.3 2,302.6 2,149.9 Other in service 425.1 329.1 282.3 223.5 142.8 105.6 Anticipated to be retired early (a)(b) 93.0 108.3 — 108.3 93.0 — Total electric plant 12,296.9 11,043.5 6,819.1 5,837.9 5,477.8 5,205.6 Gas plant in service 1,244.0 1,107.6 654.8 556.7 589.2 550.9 Other plant in service 571.9 549.3 333.4 313.0 238.5 236.3 Accumulated depreciation (4,283.1 ) (4,135.7 ) (2,311.0 ) (2,258.3 ) (1,972.1 ) (1,877.4 ) Net plant 9,829.7 8,564.7 5,496.3 4,449.3 4,333.4 4,115.4 Leased Sheboygan Falls Energy Facility, net (c) — — — — 46.2 52.4 Construction work in progress 962.2 1,226.8 424.4 968.1 537.8 258.7 Other, net 6.0 18.4 5.5 18.2 0.5 0.2 Total utility 10,797.9 9,809.9 5,926.2 5,435.6 4,917.9 4,426.7 Non-utility and other: Non-utility Generation, net (d) 90.9 135.0 — — — — Corporate Services and other, net (e) 345.7 334.3 — — — — Total non-utility and other 436.6 469.3 — — — — Total property, plant and equipment $11,234.5 $10,279.2 $5,926.2 $5,435.6 $4,917.9 $4,426.7 (a) In 2017, IPL retired Sutherland Unit 3 and reclassified the remaining net book value of this EGU from property, plant and equipment to a regulatory asset on Alliant Energy’s and IPL’s balance sheets. Refer to Note 2 for further discussion, including recovery of the remaining net book value of Sutherland Unit 3. (b) In 2017, WPL received approval from MISO to retire Edgewater Unit 4 and currently anticipates retiring this EGU by September 30, 2018. In 2016, the PSCW authorized WPL to recover the remaining net book value of Edgewater Unit 4 over a 10 -year period beginning the later of the retirement date of the EGU or January 1, 2019. (c) Less accumulated amortization of $77.6 million and $71.4 million for WPL as of December 31, 2017 and 2016 , respectively. The Sheboygan Falls Energy Facility is eliminated from WPL upon consolidation and is included in the “Non-utility Generation, net” line within Alliant Energy’s consolidated property, plant and equipment. (d) Less accumulated depreciation of $50.5 million and $46.5 million for Alliant Energy as of December 31, 2017 and 2016 , respectively. Refer to “Franklin County Wind Farm” below for discussion of the April 2017 transfer of the Franklin County wind farm from AEF to IPL pursuant to a February 2017 FERC order. (e) Less accumulated depreciation of $285.6 million and $272.0 million for Alliant Energy as of December 31, 2017 and 2016 , respectively.
Assets Purchased and Liabilities Assumed	As of the closing date, the estimated fair values of the assets purchased and liabilities assumed by IPL were as follows (in millions): Electric plant in service $40 Current assets 2 Total assets acquired 42 Other liabilities 10 Net assets acquired $32
Allowance For Funds Used During Construction	The amount of AFUDC generated by equity and debt components was as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Equity $33.6 $42.3 $24.4 $21.1 $35.2 $18.6 $12.5 $7.1 $5.8 Debt 16.1 20.2 12.5 10.3 16.8 9.6 5.8 3.4 2.9 $49.7 $62.5 $36.9 $31.4 $52.0 $28.2 $18.3 $10.5 $8.7
WPL [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	At December 31, details of property, plant and equipment on the balance sheets were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Utility: Electric plant: Generation in service $6,655.3 $5,866.9 $3,715.9 $2,916.8 $2,939.4 $2,950.1 Distribution in service 5,123.5 4,739.2 2,820.9 2,589.3 2,302.6 2,149.9 Other in service 425.1 329.1 282.3 223.5 142.8 105.6 Anticipated to be retired early (a)(b) 93.0 108.3 — 108.3 93.0 — Total electric plant 12,296.9 11,043.5 6,819.1 5,837.9 5,477.8 5,205.6 Gas plant in service 1,244.0 1,107.6 654.8 556.7 589.2 550.9 Other plant in service 571.9 549.3 333.4 313.0 238.5 236.3 Accumulated depreciation (4,283.1 ) (4,135.7 ) (2,311.0 ) (2,258.3 ) (1,972.1 ) (1,877.4 ) Net plant 9,829.7 8,564.7 5,496.3 4,449.3 4,333.4 4,115.4 Leased Sheboygan Falls Energy Facility, net (c) — — — — 46.2 52.4 Construction work in progress 962.2 1,226.8 424.4 968.1 537.8 258.7 Other, net 6.0 18.4 5.5 18.2 0.5 0.2 Total utility 10,797.9 9,809.9 5,926.2 5,435.6 4,917.9 4,426.7 Non-utility and other: Non-utility Generation, net (d) 90.9 135.0 — — — — Corporate Services and other, net (e) 345.7 334.3 — — — — Total non-utility and other 436.6 469.3 — — — — Total property, plant and equipment $11,234.5 $10,279.2 $5,926.2 $5,435.6 $4,917.9 $4,426.7 (a) In 2017, IPL retired Sutherland Unit 3 and reclassified the remaining net book value of this EGU from property, plant and equipment to a regulatory asset on Alliant Energy’s and IPL’s balance sheets. Refer to Note 2 for further discussion, including recovery of the remaining net book value of Sutherland Unit 3. (b) In 2017, WPL received approval from MISO to retire Edgewater Unit 4 and currently anticipates retiring this EGU by September 30, 2018. In 2016, the PSCW authorized WPL to recover the remaining net book value of Edgewater Unit 4 over a 10 -year period beginning the later of the retirement date of the EGU or January 1, 2019. (c) Less accumulated amortization of $77.6 million and $71.4 million for WPL as of December 31, 2017 and 2016 , respectively. The Sheboygan Falls Energy Facility is eliminated from WPL upon consolidation and is included in the “Non-utility Generation, net” line within Alliant Energy’s consolidated property, plant and equipment. (d) Less accumulated depreciation of $50.5 million and $46.5 million for Alliant Energy as of December 31, 2017 and 2016 , respectively. Refer to “Franklin County Wind Farm” below for discussion of the April 2017 transfer of the Franklin County wind farm from AEF to IPL pursuant to a February 2017 FERC order. (e) Less accumulated depreciation of $285.6 million and $272.0 million for Alliant Energy as of December 31, 2017 and 2016 , respectively.
Allowance For Funds Used During Construction	The amount of AFUDC generated by equity and debt components was as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Equity $33.6 $42.3 $24.4 $21.1 $35.2 $18.6 $12.5 $7.1 $5.8 Debt 16.1 20.2 12.5 10.3 16.8 9.6 5.8 3.4 2.9 $49.7 $62.5 $36.9 $31.4 $52.0 $28.2 $18.3 $10.5 $8.7

Jointly-Owned Electric Utilit33

Jointly-Owned Electric Utility Plant (Tables)	12 Months Ended
	Dec. 31, 2017
Jointly Owned Electric Utility Plant [Line Items]
Components of Jointly-Owned Electric Utility Plants	Information relative to IPL’s and WPL’s ownership interest in these jointly-owned coal-fired EGUs at December 31, 2017 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0 % $501.8 $144.9 $37.3 George Neal Unit 4 25.7 % 187.2 85.2 0.8 George Neal Unit 3 28.0 % 150.6 53.0 2.5 Louisa Unit 1 4.0 % 37.8 22.4 1.4 877.4 305.5 42.0 WPL Columbia Units 1-2 50.1 % 714.7 216.8 46.9 Edgewater Unit 4 68.2 % 99.9 60.4 0.1 814.6 277.2 47.0 Alliant Energy $1,692.0 $582.7 $89.0
IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Components of Jointly-Owned Electric Utility Plants	Information relative to IPL’s and WPL’s ownership interest in these jointly-owned coal-fired EGUs at December 31, 2017 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0 % $501.8 $144.9 $37.3 George Neal Unit 4 25.7 % 187.2 85.2 0.8 George Neal Unit 3 28.0 % 150.6 53.0 2.5 Louisa Unit 1 4.0 % 37.8 22.4 1.4 877.4 305.5 42.0 WPL Columbia Units 1-2 50.1 % 714.7 216.8 46.9 Edgewater Unit 4 68.2 % 99.9 60.4 0.1 814.6 277.2 47.0 Alliant Energy $1,692.0 $582.7 $89.0
WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Components of Jointly-Owned Electric Utility Plants	Information relative to IPL’s and WPL’s ownership interest in these jointly-owned coal-fired EGUs at December 31, 2017 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0 % $501.8 $144.9 $37.3 George Neal Unit 4 25.7 % 187.2 85.2 0.8 George Neal Unit 3 28.0 % 150.6 53.0 2.5 Louisa Unit 1 4.0 % 37.8 22.4 1.4 877.4 305.5 42.0 WPL Columbia Units 1-2 50.1 % 714.7 216.8 46.9 Edgewater Unit 4 68.2 % 99.9 60.4 0.1 814.6 277.2 47.0 Alliant Energy $1,692.0 $582.7 $89.0

Receivables (Tables)

Receivables (Tables)	12 Months Ended
	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable Details	Details for accounts receivable included on the balance sheets as of December 31 were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Customer $103.3 $111.7 $— $— $97.7 $104.4 Unbilled utility revenues 85.1 90.2 — — 85.1 90.2 Deferred proceeds 222.1 211.1 222.1 211.1 — — Other 84.3 89.0 44.1 30.7 40.1 38.8 Allowance for doubtful accounts (12.0 ) (8.7 ) (1.3 ) (1.1 ) (10.7 ) (7.1 ) $482.8 $493.3 $264.9 $240.7 $212.2 $226.3
Maximum And Average Outstanding Cash Proceeds	IPL’s maximum and average outstanding aggregate cash proceeds (based on daily outstanding balances) related to the sales of accounts receivable program were as follows (in millions): Maximum Average 2017 2016 2015 2017 2016 2015 Outstanding aggregate cash proceeds $112.0 $172.0 $137.0 $62.2 $73.2 $46.7
Receivables Sold Under The Agreement	As of December 31, the attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2017 2016 Customer accounts receivable $133.8 $157.6 Unbilled utility revenues 112.7 90.4 Other receivables 0.3 0.1 Receivables sold to third party 246.8 248.1 Less: cash proceeds (a) 12.0 21.0 Deferred proceeds 234.8 227.1 Less: allowance for doubtful accounts 12.7 16.0 Fair value of deferred proceeds $222.1 $211.1 Outstanding receivables past due $44.7 $68.0 (a) Changes in cash proceeds are presented in “Sales of accounts receivable” in operating activities in Alliant Energy’s and IPL’s cash flows statements.
Additional Attributes Of Receivables Sold Under The Agreement	Additional attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2017 2016 2015 Collections reinvested in receivables $1,647.1 $1,818.1 $1,812.9 Write-offs, net of recoveries 17.7 4.8 8.8
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable Details	Details for accounts receivable included on the balance sheets as of December 31 were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Customer $103.3 $111.7 $— $— $97.7 $104.4 Unbilled utility revenues 85.1 90.2 — — 85.1 90.2 Deferred proceeds 222.1 211.1 222.1 211.1 — — Other 84.3 89.0 44.1 30.7 40.1 38.8 Allowance for doubtful accounts (12.0 ) (8.7 ) (1.3 ) (1.1 ) (10.7 ) (7.1 ) $482.8 $493.3 $264.9 $240.7 $212.2 $226.3
Maximum And Average Outstanding Cash Proceeds	IPL’s maximum and average outstanding aggregate cash proceeds (based on daily outstanding balances) related to the sales of accounts receivable program were as follows (in millions): Maximum Average 2017 2016 2015 2017 2016 2015 Outstanding aggregate cash proceeds $112.0 $172.0 $137.0 $62.2 $73.2 $46.7
Receivables Sold Under The Agreement	As of December 31, the attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2017 2016 Customer accounts receivable $133.8 $157.6 Unbilled utility revenues 112.7 90.4 Other receivables 0.3 0.1 Receivables sold to third party 246.8 248.1 Less: cash proceeds (a) 12.0 21.0 Deferred proceeds 234.8 227.1 Less: allowance for doubtful accounts 12.7 16.0 Fair value of deferred proceeds $222.1 $211.1 Outstanding receivables past due $44.7 $68.0 (a) Changes in cash proceeds are presented in “Sales of accounts receivable” in operating activities in Alliant Energy’s and IPL’s cash flows statements.
Additional Attributes Of Receivables Sold Under The Agreement	Additional attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2017 2016 2015 Collections reinvested in receivables $1,647.1 $1,818.1 $1,812.9 Write-offs, net of recoveries 17.7 4.8 8.8
WPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable Details	Details for accounts receivable included on the balance sheets as of December 31 were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Customer $103.3 $111.7 $— $— $97.7 $104.4 Unbilled utility revenues 85.1 90.2 — — 85.1 90.2 Deferred proceeds 222.1 211.1 222.1 211.1 — — Other 84.3 89.0 44.1 30.7 40.1 38.8 Allowance for doubtful accounts (12.0 ) (8.7 ) (1.3 ) (1.1 ) (10.7 ) (7.1 ) $482.8 $493.3 $264.9 $240.7 $212.2 $226.3

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2017
Investment [Line Items]
Unconsolidated Equity Investments	Unconsolidated investments accounted for under the equity method of accounting are as follows (in millions): Ownership Interest at Carrying Value at December 31, Equity (Income) / Loss December 31, 2017 2017 2016 2017 2016 2015 Alliant Energy ATC Investment (a) 16%-20% $274.2 $317.6 ($42.4 ) ($39.1 ) ($34.2 ) Wind Investment in Oklahoma 50% 98.3 — (1.8 ) — — Other Various 8.9 8.4 (0.6 ) (0.5 ) 0.4 $381.4 $326.0 ($44.8 ) ($39.6 ) ($33.8 ) WPL ATC —% $— $— $— ($39.1 ) ($34.2 ) Wisconsin River Power Company 50% 8.3 7.7 (0.7 ) (0.7 ) (0.9 ) $8.3 $7.7 ($0.7 ) ($39.8 ) ($35.1 ) (a) As of December 31, 2017 , Alliant Energy’s ATC Investment is comprised of a 16% ownership interest in ATC and a 20% ownership interest in ATC Holdco LLC, which are described below. In 2017, Alliant Energy’s investment in ATC decreased due to the impacts of Tax Reform. Refer to Note 11 for further discussion. Alliant Energy currently has the ability to exercise significant influence over ATC’s financial and operating policies through its participation on ATC’s Board of Directors. Refer to Note 18 for information regarding related party transactions with ATC.
Summary Aggregate Financial Information	Summary aggregate financial information from the financial statements of these investments is as follows (in millions): Alliant Energy WPL 2017 2016 2015 2017 2016 2015 Operating revenues $741 $658 $624 $8 $658 $624 Operating income 374 331 299 4 331 299 Net income 267 232 186 2 234 202 As of December 31: Current assets 104 82 7 6 Non-current assets 5,041 4,340 20 19 Current liabilities 770 498 2 3 Non-current liabilities 2,038 2,144 8 7 Minority interest 255 — — —
Cash Surrender Value of Life Insurance Policies	At December 31, the cash surrender value of these investments was as follows (in millions): Alliant Energy WPL 2017 2016 2017 2016 Cash surrender value $10.4 $10.6 $5.6 $5.8
WPL [Member]
Investment [Line Items]
Unconsolidated Equity Investments	Unconsolidated investments accounted for under the equity method of accounting are as follows (in millions): Ownership Interest at Carrying Value at December 31, Equity (Income) / Loss December 31, 2017 2017 2016 2017 2016 2015 Alliant Energy ATC Investment (a) 16%-20% $274.2 $317.6 ($42.4 ) ($39.1 ) ($34.2 ) Wind Investment in Oklahoma 50% 98.3 — (1.8 ) — — Other Various 8.9 8.4 (0.6 ) (0.5 ) 0.4 $381.4 $326.0 ($44.8 ) ($39.6 ) ($33.8 ) WPL ATC —% $— $— $— ($39.1 ) ($34.2 ) Wisconsin River Power Company 50% 8.3 7.7 (0.7 ) (0.7 ) (0.9 ) $8.3 $7.7 ($0.7 ) ($39.8 ) ($35.1 ) (a) As of December 31, 2017 , Alliant Energy’s ATC Investment is comprised of a 16% ownership interest in ATC and a 20% ownership interest in ATC Holdco LLC, which are described below. In 2017, Alliant Energy’s investment in ATC decreased due to the impacts of Tax Reform. Refer to Note 11 for further discussion. Alliant Energy currently has the ability to exercise significant influence over ATC’s financial and operating policies through its participation on ATC’s Board of Directors. Refer to Note 18 for information regarding related party transactions with ATC.
Summary Aggregate Financial Information	Summary aggregate financial information from the financial statements of these investments is as follows (in millions): Alliant Energy WPL 2017 2016 2015 2017 2016 2015 Operating revenues $741 $658 $624 $8 $658 $624 Operating income 374 331 299 4 331 299 Net income 267 232 186 2 234 202 As of December 31: Current assets 104 82 7 6 Non-current assets 5,041 4,340 20 19 Current liabilities 770 498 2 3 Non-current liabilities 2,038 2,144 8 7 Minority interest 255 — — —
Cash Surrender Value of Life Insurance Policies	At December 31, the cash surrender value of these investments was as follows (in millions): Alliant Energy WPL 2017 2016 2017 2016 Cash surrender value $10.4 $10.6 $5.6 $5.8

Common Equity (Tables)

Common Equity (Tables)	12 Months Ended
	Dec. 31, 2017
Schedule of Common Equity [Line Items]
Common Stock Activity	A summary of Alliant Energy’s common stock activity was as follows: 2017 2016 2015 Shares outstanding, January 1 227,673,654 226,918,432 221,871,360 At-the-market offering programs 3,074,931 — 4,373,234 Shareowner Direct Plan issuances 640,723 732,814 606,010 Equity-based compensation plans ( Note 12(b) ) 5,185 22,408 112,756 Other (45,847 ) — (44,928 ) Shares outstanding, December 31 231,348,646 227,673,654 226,918,432

Redeemable Preferred Stock (Tab

Redeemable Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2017
Class of Stock [Line Items]
Schedule of Carrying Value of Cumulative Preferred Stock	Information related to the carrying value of IPL’s cumulative preferred stock at December 31 was as follows: Series Liquidation Preference/Stated Value Shares Authorized Shares Outstanding 2017 2016 (in millions) 5.1% $25 8,000,000 8,000,000 $200.0 $200.0
IPL [Member]
Class of Stock [Line Items]
Schedule of Carrying Value of Cumulative Preferred Stock	Information related to the carrying value of IPL’s cumulative preferred stock at December 31 was as follows: Series Liquidation Preference/Stated Value Shares Authorized Shares Outstanding 2017 2016 (in millions) 5.1% $25 8,000,000 8,000,000 $200.0 $200.0

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Line Items]
Other Short-Term Borrowings	Information regarding commercial paper classified as short-term debt and back-stopped by the credit facility was as follows (dollars in millions): Alliant Energy IPL WPL December 31 2017 2016 2017 2016 2017 2016 Commercial paper outstanding $320.2 $244.1 $— $— $25.0 $52.3 Commercial paper weighted average interest rates 2.0% 0.9% N/A N/A 1.5% 0.7% Available credit facility capacity $679.8 $755.9 $250.0 $300.0 $325.0 $347.7 Alliant Energy IPL WPL For the year ended 2017 2016 2017 2016 2017 2016 Maximum amount outstanding (based on daily outstanding balances) $424.4 $251.8 $20.0 $3.1 $271.2 $118.3 Average amount outstanding (based on daily outstanding balances) $294.3 $179.0 $0.5 $— $118.2 $38.1 Weighted average interest rates 1.2% 0.6% 1.3% 0.7% 1.0% 0.4%
Schedule of Debt-To-Capital Ratios	The required debt-to-capital ratios compared to the actual debt-to-capital ratios at December 31, 2017 were as follows: Alliant Energy IPL WPL Requirement, not to exceed 65% 65% 65% Actual 54% 47% 51%
Schedule of Long-term Debt	Long-term debt, net as of December 31 was as follows (dollars in millions): 2017 2016 Alliant Energy IPL WPL Alliant Energy IPL WPL Senior Debentures (a): 5.875%, due 2018 $100.0 $100.0 $— $100.0 $100.0 $— 7.25%, due 2018 250.0 250.0 — 250.0 250.0 — 3.65%, due 2020 200.0 200.0 — 200.0 200.0 — 3.25%, due 2024 (b) 500.0 500.0 — 250.0 250.0 — 3.4%, due 2025 250.0 250.0 — 250.0 250.0 — 5.5%, due 2025 50.0 50.0 — 50.0 50.0 — 6.45%, due 2033 100.0 100.0 — 100.0 100.0 — 6.3%, due 2034 125.0 125.0 — 125.0 125.0 — 6.25%, due 2039 300.0 300.0 — 300.0 300.0 — 4.7%, due 2043 250.0 250.0 — 250.0 250.0 — 3.7%, due 2046 300.0 300.0 — 300.0 300.0 — 2,425.0 2,425.0 — 2,175.0 2,175.0 — Debentures (a): 5%, due 2019 250.0 — 250.0 250.0 — 250.0 4.6%, due 2020 150.0 — 150.0 150.0 — 150.0 2.25%, due 2022 250.0 — 250.0 250.0 — 250.0 3.05%, due 2027 (c) 300.0 — 300.0 — — — 6.25%, due 2034 100.0 — 100.0 100.0 — 100.0 6.375%, due 2037 300.0 — 300.0 300.0 — 300.0 7.6%, due 2038 250.0 — 250.0 250.0 — 250.0 4.1%, due 2044 250.0 — 250.0 250.0 — 250.0 1,850.0 — 1,850.0 1,550.0 — 1,550.0 Other: AEF term loan credit agreement through 2018, 2% at December 31, 2017 (with Alliant Energy as guarantor) (d) 500.0 — — 500.0 — — Corporate Services 3.45% senior notes, due 2022 (a) 75.0 — — 75.0 — — Sheboygan Power, LLC 5.06% senior secured notes, due 2018 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 49.6 — — 53.8 — — Other, 1% at December 31, 2017, due 2018 to 2025 2.9 — — 3.3 — — 627.5 — — 632.1 — — Subtotal 4,902.5 2,425.0 1,850.0 4,357.1 2,175.0 1,550.0 Current maturities (855.7 ) (350.0 ) — (4.6 ) — — Unamortized debt issuance costs (25.4 ) (14.3 ) (10.5 ) (23.4 ) (13.7 ) (9.1 ) Unamortized debt (discount) and premium, net (10.8 ) (4.7 ) (6.1 ) (13.5 ) (7.8 ) (5.7 ) Long-term debt, net (e) $4,010.6 $2,056.0 $1,833.4 $4,315.6 $2,153.5 $1,535.2 (a) Contains optional redemption provisions which, if elected by the issuer at its sole discretion, could require material redemption premium payments by the issuer. The redemption premium payments under these optional redemption provisions are variable and dependent on applicable U.S. Treasury rates at the time of redemption. (b) In 2017, IPL issued $250 million of 3.25% senior debentures due 2024. The proceeds from the issuance were used by IPL to reduce commercial paper classified as long-term debt, reduce cash amounts received from its sales of accounts receivable program and for general corporate purposes. (c) In 2017, WPL issued $300 million of 3.05% debentures due 2027. The proceeds from the issuance were used by WPL to reduce commercial paper and for general corporate purposes. (d) Refer to Note 9(a) for discussion of a financial covenant contained in AEF’s term loan credit agreement. (e) There were no significant sinking fund requirements related to the outstanding long-term debt.
Schedule of Debt Maturities	At December 31, 2017 , long-term debt maturities for 2018 through 2022 were as follows (in millions): 2018 2019 2020 2021 2022 IPL $350 $— $200 $— $— WPL — 250 150 — 250 Corporate Services — — — — 75 AEF 506 6 7 8 8 Alliant Energy $856 $256 $357 $8 $333
IPL [Member]
Debt Disclosure [Line Items]
Other Short-Term Borrowings	Information regarding commercial paper classified as short-term debt and back-stopped by the credit facility was as follows (dollars in millions): Alliant Energy IPL WPL December 31 2017 2016 2017 2016 2017 2016 Commercial paper outstanding $320.2 $244.1 $— $— $25.0 $52.3 Commercial paper weighted average interest rates 2.0% 0.9% N/A N/A 1.5% 0.7% Available credit facility capacity $679.8 $755.9 $250.0 $300.0 $325.0 $347.7 Alliant Energy IPL WPL For the year ended 2017 2016 2017 2016 2017 2016 Maximum amount outstanding (based on daily outstanding balances) $424.4 $251.8 $20.0 $3.1 $271.2 $118.3 Average amount outstanding (based on daily outstanding balances) $294.3 $179.0 $0.5 $— $118.2 $38.1 Weighted average interest rates 1.2% 0.6% 1.3% 0.7% 1.0% 0.4%
Schedule of Debt-To-Capital Ratios	The required debt-to-capital ratios compared to the actual debt-to-capital ratios at December 31, 2017 were as follows: Alliant Energy IPL WPL Requirement, not to exceed 65% 65% 65% Actual 54% 47% 51%
Schedule of Long-term Debt	Long-term debt, net as of December 31 was as follows (dollars in millions): 2017 2016 Alliant Energy IPL WPL Alliant Energy IPL WPL Senior Debentures (a): 5.875%, due 2018 $100.0 $100.0 $— $100.0 $100.0 $— 7.25%, due 2018 250.0 250.0 — 250.0 250.0 — 3.65%, due 2020 200.0 200.0 — 200.0 200.0 — 3.25%, due 2024 (b) 500.0 500.0 — 250.0 250.0 — 3.4%, due 2025 250.0 250.0 — 250.0 250.0 — 5.5%, due 2025 50.0 50.0 — 50.0 50.0 — 6.45%, due 2033 100.0 100.0 — 100.0 100.0 — 6.3%, due 2034 125.0 125.0 — 125.0 125.0 — 6.25%, due 2039 300.0 300.0 — 300.0 300.0 — 4.7%, due 2043 250.0 250.0 — 250.0 250.0 — 3.7%, due 2046 300.0 300.0 — 300.0 300.0 — 2,425.0 2,425.0 — 2,175.0 2,175.0 — Debentures (a): 5%, due 2019 250.0 — 250.0 250.0 — 250.0 4.6%, due 2020 150.0 — 150.0 150.0 — 150.0 2.25%, due 2022 250.0 — 250.0 250.0 — 250.0 3.05%, due 2027 (c) 300.0 — 300.0 — — — 6.25%, due 2034 100.0 — 100.0 100.0 — 100.0 6.375%, due 2037 300.0 — 300.0 300.0 — 300.0 7.6%, due 2038 250.0 — 250.0 250.0 — 250.0 4.1%, due 2044 250.0 — 250.0 250.0 — 250.0 1,850.0 — 1,850.0 1,550.0 — 1,550.0 Other: AEF term loan credit agreement through 2018, 2% at December 31, 2017 (with Alliant Energy as guarantor) (d) 500.0 — — 500.0 — — Corporate Services 3.45% senior notes, due 2022 (a) 75.0 — — 75.0 — — Sheboygan Power, LLC 5.06% senior secured notes, due 2018 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 49.6 — — 53.8 — — Other, 1% at December 31, 2017, due 2018 to 2025 2.9 — — 3.3 — — 627.5 — — 632.1 — — Subtotal 4,902.5 2,425.0 1,850.0 4,357.1 2,175.0 1,550.0 Current maturities (855.7 ) (350.0 ) — (4.6 ) — — Unamortized debt issuance costs (25.4 ) (14.3 ) (10.5 ) (23.4 ) (13.7 ) (9.1 ) Unamortized debt (discount) and premium, net (10.8 ) (4.7 ) (6.1 ) (13.5 ) (7.8 ) (5.7 ) Long-term debt, net (e) $4,010.6 $2,056.0 $1,833.4 $4,315.6 $2,153.5 $1,535.2 (a) Contains optional redemption provisions which, if elected by the issuer at its sole discretion, could require material redemption premium payments by the issuer. The redemption premium payments under these optional redemption provisions are variable and dependent on applicable U.S. Treasury rates at the time of redemption. (b) In 2017, IPL issued $250 million of 3.25% senior debentures due 2024. The proceeds from the issuance were used by IPL to reduce commercial paper classified as long-term debt, reduce cash amounts received from its sales of accounts receivable program and for general corporate purposes. (c) In 2017, WPL issued $300 million of 3.05% debentures due 2027. The proceeds from the issuance were used by WPL to reduce commercial paper and for general corporate purposes. (d) Refer to Note 9(a) for discussion of a financial covenant contained in AEF’s term loan credit agreement. (e) There were no significant sinking fund requirements related to the outstanding long-term debt.
Schedule of Debt Maturities	At December 31, 2017 , long-term debt maturities for 2018 through 2022 were as follows (in millions): 2018 2019 2020 2021 2022 IPL $350 $— $200 $— $— WPL — 250 150 — 250 Corporate Services — — — — 75 AEF 506 6 7 8 8 Alliant Energy $856 $256 $357 $8 $333
WPL [Member]
Debt Disclosure [Line Items]
Other Short-Term Borrowings	Information regarding commercial paper classified as short-term debt and back-stopped by the credit facility was as follows (dollars in millions): Alliant Energy IPL WPL December 31 2017 2016 2017 2016 2017 2016 Commercial paper outstanding $320.2 $244.1 $— $— $25.0 $52.3 Commercial paper weighted average interest rates 2.0% 0.9% N/A N/A 1.5% 0.7% Available credit facility capacity $679.8 $755.9 $250.0 $300.0 $325.0 $347.7 Alliant Energy IPL WPL For the year ended 2017 2016 2017 2016 2017 2016 Maximum amount outstanding (based on daily outstanding balances) $424.4 $251.8 $20.0 $3.1 $271.2 $118.3 Average amount outstanding (based on daily outstanding balances) $294.3 $179.0 $0.5 $— $118.2 $38.1 Weighted average interest rates 1.2% 0.6% 1.3% 0.7% 1.0% 0.4%
Schedule of Debt-To-Capital Ratios	The required debt-to-capital ratios compared to the actual debt-to-capital ratios at December 31, 2017 were as follows: Alliant Energy IPL WPL Requirement, not to exceed 65% 65% 65% Actual 54% 47% 51%
Schedule of Long-term Debt	Long-term debt, net as of December 31 was as follows (dollars in millions): 2017 2016 Alliant Energy IPL WPL Alliant Energy IPL WPL Senior Debentures (a): 5.875%, due 2018 $100.0 $100.0 $— $100.0 $100.0 $— 7.25%, due 2018 250.0 250.0 — 250.0 250.0 — 3.65%, due 2020 200.0 200.0 — 200.0 200.0 — 3.25%, due 2024 (b) 500.0 500.0 — 250.0 250.0 — 3.4%, due 2025 250.0 250.0 — 250.0 250.0 — 5.5%, due 2025 50.0 50.0 — 50.0 50.0 — 6.45%, due 2033 100.0 100.0 — 100.0 100.0 — 6.3%, due 2034 125.0 125.0 — 125.0 125.0 — 6.25%, due 2039 300.0 300.0 — 300.0 300.0 — 4.7%, due 2043 250.0 250.0 — 250.0 250.0 — 3.7%, due 2046 300.0 300.0 — 300.0 300.0 — 2,425.0 2,425.0 — 2,175.0 2,175.0 — Debentures (a): 5%, due 2019 250.0 — 250.0 250.0 — 250.0 4.6%, due 2020 150.0 — 150.0 150.0 — 150.0 2.25%, due 2022 250.0 — 250.0 250.0 — 250.0 3.05%, due 2027 (c) 300.0 — 300.0 — — — 6.25%, due 2034 100.0 — 100.0 100.0 — 100.0 6.375%, due 2037 300.0 — 300.0 300.0 — 300.0 7.6%, due 2038 250.0 — 250.0 250.0 — 250.0 4.1%, due 2044 250.0 — 250.0 250.0 — 250.0 1,850.0 — 1,850.0 1,550.0 — 1,550.0 Other: AEF term loan credit agreement through 2018, 2% at December 31, 2017 (with Alliant Energy as guarantor) (d) 500.0 — — 500.0 — — Corporate Services 3.45% senior notes, due 2022 (a) 75.0 — — 75.0 — — Sheboygan Power, LLC 5.06% senior secured notes, due 2018 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 49.6 — — 53.8 — — Other, 1% at December 31, 2017, due 2018 to 2025 2.9 — — 3.3 — — 627.5 — — 632.1 — — Subtotal 4,902.5 2,425.0 1,850.0 4,357.1 2,175.0 1,550.0 Current maturities (855.7 ) (350.0 ) — (4.6 ) — — Unamortized debt issuance costs (25.4 ) (14.3 ) (10.5 ) (23.4 ) (13.7 ) (9.1 ) Unamortized debt (discount) and premium, net (10.8 ) (4.7 ) (6.1 ) (13.5 ) (7.8 ) (5.7 ) Long-term debt, net (e) $4,010.6 $2,056.0 $1,833.4 $4,315.6 $2,153.5 $1,535.2 (a) Contains optional redemption provisions which, if elected by the issuer at its sole discretion, could require material redemption premium payments by the issuer. The redemption premium payments under these optional redemption provisions are variable and dependent on applicable U.S. Treasury rates at the time of redemption. (b) In 2017, IPL issued $250 million of 3.25% senior debentures due 2024. The proceeds from the issuance were used by IPL to reduce commercial paper classified as long-term debt, reduce cash amounts received from its sales of accounts receivable program and for general corporate purposes. (c) In 2017, WPL issued $300 million of 3.05% debentures due 2027. The proceeds from the issuance were used by WPL to reduce commercial paper and for general corporate purposes. (d) Refer to Note 9(a) for discussion of a financial covenant contained in AEF’s term loan credit agreement. (e) There were no significant sinking fund requirements related to the outstanding long-term debt.
Schedule of Debt Maturities	At December 31, 2017 , long-term debt maturities for 2018 through 2022 were as follows (in millions): 2018 2019 2020 2021 2022 IPL $350 $— $200 $— $— WPL — 250 150 — 250 Corporate Services — — — — 75 AEF 506 6 7 8 8 Alliant Energy $856 $256 $357 $8 $333

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2017
Schedule of Future Minimum Rental Payments for Operating Leases	At December 31, 2017 , future minimum operating lease payments, excluding contingent rentals, were as follows (in millions): 2018 2019 2020 2021 2022 Thereafter Total Alliant Energy $6 $5 $2 $2 $1 $13 $29 IPL 3 2 1 1 1 13 21 WPL 3 3 1 — — — 7
IPL [Member]
Schedule of Future Minimum Rental Payments for Operating Leases	At December 31, 2017 , future minimum operating lease payments, excluding contingent rentals, were as follows (in millions): 2018 2019 2020 2021 2022 Thereafter Total Alliant Energy $6 $5 $2 $2 $1 $13 $29 IPL 3 2 1 1 1 13 21 WPL 3 3 1 — — — 7
WPL [Member]
Schedule of Future Minimum Rental Payments for Operating Leases	At December 31, 2017 , future minimum operating lease payments, excluding contingent rentals, were as follows (in millions): 2018 2019 2020 2021 2022 Thereafter Total Alliant Energy $6 $5 $2 $2 $1 $13 $29 IPL 3 2 1 1 1 13 21 WPL 3 3 1 — — — 7
WPL [Member] | Sheboygan Falls Energy Facility [Member]
Schedule of Capital Lease Expense	The Sheboygan Falls Energy Facility lease expenses were included in WPL’s income statements as follows (in millions): 2017 2016 2015 Interest expense $8.7 $9.3 $9.9 Depreciation and amortization 6.2 6.2 6.2 $14.9 $15.5 $16.1
Schedule of Future Minimum Lease Payments for Capital Leases	At December 31, 2017 , WPL’s estimated future minimum capital lease payments for the Sheboygan Falls Energy Facility were as follows (in millions): 2018 2019 2020 2021 2022 Thereafter Total Less: amount representing interest Present value of minimum capital lease payments Sheboygan Falls Energy Facility $15 $15 $15 $15 $15 $35 $110 $33 $77

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax [Line Items]
Schedule Of Impacts Of the Tax Cuts And Jobs Act of 2017	Alliant Energy, IPL and WPL have completed their assessment of the applicable provisions of Tax Reform, and the impacts to their 2017 balance sheets and income statements are as follows (in millions): Alliant Energy IPL WPL Balance Sheet Other current assets $— $4.7 ($1.4 ) Investments: ATC Investment (75.0 ) — — Other assets: Regulatory assets (387.6 ) (375.0 ) (12.7 ) Deferred charges and other 41.0 — — Total Tax Reform impact on assets ($421.6 ) ($370.3 ) ($14.1 ) Other liabilities: Deferred tax liabilities ($1,331.9 ) ($757.2 ) ($523.8 ) Regulatory liabilities 885.9 390.7 495.2 Other 6.3 — — Total Tax Reform impact on liabilities ($439.7 ) ($366.5 ) ($28.6 ) Income Statement Income tax expense (benefit) ($18.1 ) $3.8 ($14.5 )
Schedule of Components of Income Tax Expense (Benefit)	The components of “Income tax expense (benefit)” in the income statements were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Current tax expense (benefit): Federal ($41.0 ) $1.8 $2.0 ($27.9 ) ($12.8 ) ($14.1 ) $5.5 ($22.3 ) $4.7 State 8.5 17.2 3.2 1.6 15.5 11.5 2.5 1.1 0.6 IPL’s tax benefit riders (40.4 ) (44.2 ) (49.0 ) (40.4 ) (44.2 ) (49.0 ) — — — Deferred tax expense (benefit): Federal 159.5 112.8 120.8 72.5 59.1 40.7 55.0 112.3 76.8 State 12.3 4.9 27.9 (2.2 ) (9.0 ) 3.3 16.6 20.8 20.2 Production tax credits (31.1 ) (31.8 ) (33.1 ) (14.1 ) (14.0 ) (14.5 ) (17.0 ) (17.8 ) (18.6 ) Investment tax credits (1.1 ) (1.3 ) (1.4 ) (0.4 ) (0.5 ) (0.6 ) (0.7 ) (0.8 ) (0.8 ) $66.7 $59.4 $70.4 ($10.9 ) ($5.9 ) ($22.7 ) $61.9 $93.3 $82.9
Schedule Of Effective Income Tax Rates	The overall income tax rates shown in the following table were computed by dividing income tax expense (benefit) by income from continuing operations before income taxes. Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Statutory federal income tax rate 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % State income taxes, net of federal benefits 5.5 5.4 5.2 6.5 6.4 6.2 5.1 5.1 5.1 Effect of rate-making on property-related differences (8.5 ) (8.5 ) (6.8 ) (19.1 ) (16.2 ) (17.2 ) (1.7 ) (0.7 ) (0.5 ) IPL’s tax benefit riders (7.6 ) (10.0 ) (10.6 ) (18.7 ) (20.1 ) (28.3 ) — — — Production tax credits (6.1 ) (7.2 ) (7.2 ) (6.7 ) (6.3 ) (8.3 ) (7.1 ) (6.2 ) (7.1 ) Impact of Tax Reform (3.4 ) — — 1.7 — — (5.8 ) — — Other items, net (2.4 ) (1.3 ) (0.3 ) (3.7 ) (1.5 ) (0.5 ) (0.6 ) (0.6 ) (0.7 ) Overall income tax rate 12.5 % 13.4 % 15.3 % (5.0 %) (2.7 %) (13.1 %) 24.9 % 32.6 % 31.8 %
Schedule of Deferred Tax Assets and Liabilities	The deferred tax assets and liabilities included on the balance sheets at December 31 arise from the following temporary differences (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Deferred tax liabilities: Property $1,852.7 $2,919.0 $1,102.6 $1,677.0 $674.2 $1,124.5 ATC Investment 86.4 153.1 — — — — Other 75.9 95.1 63.4 71.4 36.5 58.8 Total deferred tax liabilities 2,015.0 3,167.2 1,166.0 1,748.4 710.7 1,183.3 Deferred tax assets: Federal credit carryforwards 260.7 268.4 113.1 95.9 131.0 112.9 Net operating losses carryforwards - federal 174.4 173.3 107.4 69.6 43.7 75.4 Regulatory liability - IPL’s tax benefit riders 7.4 34.7 7.4 34.7 — — Net operating losses carryforwards - state 41.3 32.9 0.9 0.6 0.2 0.1 Other 61.8 87.9 27.1 35.8 14.7 23.6 Subtotal deferred tax assets 545.6 597.2 255.9 236.6 189.6 212.0 Valuation allowance (9.0 ) (0.2 ) (0.6 ) — (1.3 ) (0.3 ) Total deferred tax assets 536.6 597.0 255.3 236.6 188.3 211.7 Total deferred tax liabilities, net $1,478.4 $2,570.2 $910.7 $1,511.8 $522.4 $971.6
Summary Of Tax Credit Carryforwards	At December 31, 2017 , carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2030-2037 $852 $537 $208 State net operating losses 2018-2037 700 13 2 Federal tax credits 2022-2037 267 119 131
Schedule Of Open Tax Years	Tax years that remain subject to the statute of limitations in the major jurisdictions for each of Alliant Energy, IPL and WPL are as follows: Consolidated federal income tax returns (a) 2014 - 2016 Consolidated Iowa income tax returns (b) 2014 - 2016 Wisconsin combined tax returns (c) 2013 - 2016 (a) These federal tax returns are effectively settled as a result of participation in the IRS Compliance Assurance Program, which allows Alliant Energy and the IRS to work together to resolve issues related to Alliant Energy’s current tax year before filing its federal income tax return. The statute of limitations for these federal tax returns expires three years from each filing date. (b) The statute of limitations for these Iowa tax returns expires three years from each filing date. (c) The statute of limitations for these Wisconsin combined tax returns expires four years from each filing date.
IPL [Member]
Income Tax [Line Items]
Schedule Of Impacts Of the Tax Cuts And Jobs Act of 2017	Alliant Energy, IPL and WPL have completed their assessment of the applicable provisions of Tax Reform, and the impacts to their 2017 balance sheets and income statements are as follows (in millions): Alliant Energy IPL WPL Balance Sheet Other current assets $— $4.7 ($1.4 ) Investments: ATC Investment (75.0 ) — — Other assets: Regulatory assets (387.6 ) (375.0 ) (12.7 ) Deferred charges and other 41.0 — — Total Tax Reform impact on assets ($421.6 ) ($370.3 ) ($14.1 ) Other liabilities: Deferred tax liabilities ($1,331.9 ) ($757.2 ) ($523.8 ) Regulatory liabilities 885.9 390.7 495.2 Other 6.3 — — Total Tax Reform impact on liabilities ($439.7 ) ($366.5 ) ($28.6 ) Income Statement Income tax expense (benefit) ($18.1 ) $3.8 ($14.5 )
Schedule of Components of Income Tax Expense (Benefit)	The components of “Income tax expense (benefit)” in the income statements were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Current tax expense (benefit): Federal ($41.0 ) $1.8 $2.0 ($27.9 ) ($12.8 ) ($14.1 ) $5.5 ($22.3 ) $4.7 State 8.5 17.2 3.2 1.6 15.5 11.5 2.5 1.1 0.6 IPL’s tax benefit riders (40.4 ) (44.2 ) (49.0 ) (40.4 ) (44.2 ) (49.0 ) — — — Deferred tax expense (benefit): Federal 159.5 112.8 120.8 72.5 59.1 40.7 55.0 112.3 76.8 State 12.3 4.9 27.9 (2.2 ) (9.0 ) 3.3 16.6 20.8 20.2 Production tax credits (31.1 ) (31.8 ) (33.1 ) (14.1 ) (14.0 ) (14.5 ) (17.0 ) (17.8 ) (18.6 ) Investment tax credits (1.1 ) (1.3 ) (1.4 ) (0.4 ) (0.5 ) (0.6 ) (0.7 ) (0.8 ) (0.8 ) $66.7 $59.4 $70.4 ($10.9 ) ($5.9 ) ($22.7 ) $61.9 $93.3 $82.9
Schedule Of Effective Income Tax Rates	The overall income tax rates shown in the following table were computed by dividing income tax expense (benefit) by income from continuing operations before income taxes. Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Statutory federal income tax rate 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % State income taxes, net of federal benefits 5.5 5.4 5.2 6.5 6.4 6.2 5.1 5.1 5.1 Effect of rate-making on property-related differences (8.5 ) (8.5 ) (6.8 ) (19.1 ) (16.2 ) (17.2 ) (1.7 ) (0.7 ) (0.5 ) IPL’s tax benefit riders (7.6 ) (10.0 ) (10.6 ) (18.7 ) (20.1 ) (28.3 ) — — — Production tax credits (6.1 ) (7.2 ) (7.2 ) (6.7 ) (6.3 ) (8.3 ) (7.1 ) (6.2 ) (7.1 ) Impact of Tax Reform (3.4 ) — — 1.7 — — (5.8 ) — — Other items, net (2.4 ) (1.3 ) (0.3 ) (3.7 ) (1.5 ) (0.5 ) (0.6 ) (0.6 ) (0.7 ) Overall income tax rate 12.5 % 13.4 % 15.3 % (5.0 %) (2.7 %) (13.1 %) 24.9 % 32.6 % 31.8 %
Schedule of Deferred Tax Assets and Liabilities	The deferred tax assets and liabilities included on the balance sheets at December 31 arise from the following temporary differences (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Deferred tax liabilities: Property $1,852.7 $2,919.0 $1,102.6 $1,677.0 $674.2 $1,124.5 ATC Investment 86.4 153.1 — — — — Other 75.9 95.1 63.4 71.4 36.5 58.8 Total deferred tax liabilities 2,015.0 3,167.2 1,166.0 1,748.4 710.7 1,183.3 Deferred tax assets: Federal credit carryforwards 260.7 268.4 113.1 95.9 131.0 112.9 Net operating losses carryforwards - federal 174.4 173.3 107.4 69.6 43.7 75.4 Regulatory liability - IPL’s tax benefit riders 7.4 34.7 7.4 34.7 — — Net operating losses carryforwards - state 41.3 32.9 0.9 0.6 0.2 0.1 Other 61.8 87.9 27.1 35.8 14.7 23.6 Subtotal deferred tax assets 545.6 597.2 255.9 236.6 189.6 212.0 Valuation allowance (9.0 ) (0.2 ) (0.6 ) — (1.3 ) (0.3 ) Total deferred tax assets 536.6 597.0 255.3 236.6 188.3 211.7 Total deferred tax liabilities, net $1,478.4 $2,570.2 $910.7 $1,511.8 $522.4 $971.6
Summary Of Tax Credit Carryforwards	At December 31, 2017 , carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2030-2037 $852 $537 $208 State net operating losses 2018-2037 700 13 2 Federal tax credits 2022-2037 267 119 131
Schedule Of Open Tax Years	Tax years that remain subject to the statute of limitations in the major jurisdictions for each of Alliant Energy, IPL and WPL are as follows: Consolidated federal income tax returns (a) 2014 - 2016 Consolidated Iowa income tax returns (b) 2014 - 2016 Wisconsin combined tax returns (c) 2013 - 2016 (a) These federal tax returns are effectively settled as a result of participation in the IRS Compliance Assurance Program, which allows Alliant Energy and the IRS to work together to resolve issues related to Alliant Energy’s current tax year before filing its federal income tax return. The statute of limitations for these federal tax returns expires three years from each filing date. (b) The statute of limitations for these Iowa tax returns expires three years from each filing date. (c) The statute of limitations for these Wisconsin combined tax returns expires four years from each filing date.
WPL [Member]
Income Tax [Line Items]
Schedule Of Impacts Of the Tax Cuts And Jobs Act of 2017	Alliant Energy, IPL and WPL have completed their assessment of the applicable provisions of Tax Reform, and the impacts to their 2017 balance sheets and income statements are as follows (in millions): Alliant Energy IPL WPL Balance Sheet Other current assets $— $4.7 ($1.4 ) Investments: ATC Investment (75.0 ) — — Other assets: Regulatory assets (387.6 ) (375.0 ) (12.7 ) Deferred charges and other 41.0 — — Total Tax Reform impact on assets ($421.6 ) ($370.3 ) ($14.1 ) Other liabilities: Deferred tax liabilities ($1,331.9 ) ($757.2 ) ($523.8 ) Regulatory liabilities 885.9 390.7 495.2 Other 6.3 — — Total Tax Reform impact on liabilities ($439.7 ) ($366.5 ) ($28.6 ) Income Statement Income tax expense (benefit) ($18.1 ) $3.8 ($14.5 )
Schedule of Components of Income Tax Expense (Benefit)	The components of “Income tax expense (benefit)” in the income statements were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Current tax expense (benefit): Federal ($41.0 ) $1.8 $2.0 ($27.9 ) ($12.8 ) ($14.1 ) $5.5 ($22.3 ) $4.7 State 8.5 17.2 3.2 1.6 15.5 11.5 2.5 1.1 0.6 IPL’s tax benefit riders (40.4 ) (44.2 ) (49.0 ) (40.4 ) (44.2 ) (49.0 ) — — — Deferred tax expense (benefit): Federal 159.5 112.8 120.8 72.5 59.1 40.7 55.0 112.3 76.8 State 12.3 4.9 27.9 (2.2 ) (9.0 ) 3.3 16.6 20.8 20.2 Production tax credits (31.1 ) (31.8 ) (33.1 ) (14.1 ) (14.0 ) (14.5 ) (17.0 ) (17.8 ) (18.6 ) Investment tax credits (1.1 ) (1.3 ) (1.4 ) (0.4 ) (0.5 ) (0.6 ) (0.7 ) (0.8 ) (0.8 ) $66.7 $59.4 $70.4 ($10.9 ) ($5.9 ) ($22.7 ) $61.9 $93.3 $82.9
Schedule Of Effective Income Tax Rates	The overall income tax rates shown in the following table were computed by dividing income tax expense (benefit) by income from continuing operations before income taxes. Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Statutory federal income tax rate 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % 35.0 % State income taxes, net of federal benefits 5.5 5.4 5.2 6.5 6.4 6.2 5.1 5.1 5.1 Effect of rate-making on property-related differences (8.5 ) (8.5 ) (6.8 ) (19.1 ) (16.2 ) (17.2 ) (1.7 ) (0.7 ) (0.5 ) IPL’s tax benefit riders (7.6 ) (10.0 ) (10.6 ) (18.7 ) (20.1 ) (28.3 ) — — — Production tax credits (6.1 ) (7.2 ) (7.2 ) (6.7 ) (6.3 ) (8.3 ) (7.1 ) (6.2 ) (7.1 ) Impact of Tax Reform (3.4 ) — — 1.7 — — (5.8 ) — — Other items, net (2.4 ) (1.3 ) (0.3 ) (3.7 ) (1.5 ) (0.5 ) (0.6 ) (0.6 ) (0.7 ) Overall income tax rate 12.5 % 13.4 % 15.3 % (5.0 %) (2.7 %) (13.1 %) 24.9 % 32.6 % 31.8 %
Schedule of Deferred Tax Assets and Liabilities	The deferred tax assets and liabilities included on the balance sheets at December 31 arise from the following temporary differences (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Deferred tax liabilities: Property $1,852.7 $2,919.0 $1,102.6 $1,677.0 $674.2 $1,124.5 ATC Investment 86.4 153.1 — — — — Other 75.9 95.1 63.4 71.4 36.5 58.8 Total deferred tax liabilities 2,015.0 3,167.2 1,166.0 1,748.4 710.7 1,183.3 Deferred tax assets: Federal credit carryforwards 260.7 268.4 113.1 95.9 131.0 112.9 Net operating losses carryforwards - federal 174.4 173.3 107.4 69.6 43.7 75.4 Regulatory liability - IPL’s tax benefit riders 7.4 34.7 7.4 34.7 — — Net operating losses carryforwards - state 41.3 32.9 0.9 0.6 0.2 0.1 Other 61.8 87.9 27.1 35.8 14.7 23.6 Subtotal deferred tax assets 545.6 597.2 255.9 236.6 189.6 212.0 Valuation allowance (9.0 ) (0.2 ) (0.6 ) — (1.3 ) (0.3 ) Total deferred tax assets 536.6 597.0 255.3 236.6 188.3 211.7 Total deferred tax liabilities, net $1,478.4 $2,570.2 $910.7 $1,511.8 $522.4 $971.6
Summary Of Tax Credit Carryforwards	At December 31, 2017 , carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2030-2037 $852 $537 $208 State net operating losses 2018-2037 700 13 2 Federal tax credits 2022-2037 267 119 131
Schedule Of Open Tax Years	Tax years that remain subject to the statute of limitations in the major jurisdictions for each of Alliant Energy, IPL and WPL are as follows: Consolidated federal income tax returns (a) 2014 - 2016 Consolidated Iowa income tax returns (b) 2014 - 2016 Wisconsin combined tax returns (c) 2013 - 2016 (a) These federal tax returns are effectively settled as a result of participation in the IRS Compliance Assurance Program, which allows Alliant Energy and the IRS to work together to resolve issues related to Alliant Energy’s current tax year before filing its federal income tax return. The statute of limitations for these federal tax returns expires three years from each filing date. (b) The statute of limitations for these Iowa tax returns expires three years from each filing date. (c) The statute of limitations for these Wisconsin combined tax returns expires four years from each filing date.

Benefit Plans (Tables)

Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Assumptions Used To Measure Benefit Plans	The assumptions for defined benefit pension and OPEB plans at the measurement date of December 31 were as follows: Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2015 2017 2016 2015 Discount rate for benefit obligations 3.66% 4.19% 4.47% 3.53% 3.98% 4.30% Discount rate for net periodic cost 4.19% 4.47% 4.18% 3.98% 4.30% 3.97% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.80% 6.30% 6.20% Rate of compensation increase 3.65 % - 4.50% 3.65 % - 4.50% 3.65 % - 4.50% N/A N/A N/A Medical cost trend on covered charges: Initial trend rate (end of year) N/A N/A N/A 6.75% 7.00% 7.25% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans IPL 2017 2016 2015 2017 2016 2015 Discount rate for benefit obligations 3.68% 4.22% 4.50% 3.51% 3.95% 4.28% Discount rate for net periodic cost 4.22% 4.50% 4.20% 3.95% 4.28% 3.94% Expected rate of return on plan assets 7.60% 7.60% 7.60% 6.20% 6.60% 6.60% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Medical cost trend on covered charges: Initial trend rate (end of year) N/A N/A N/A 6.75% 7.00% 7.25% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans WPL 2017 2016 2015 2017 2016 2015 Discount rate for benefit obligations 3.69% 4.23% 4.51% 3.51% 3.96% 4.28% Discount rate for net periodic cost 4.23% 4.51% 4.20% 3.96% 4.28% 3.96% Expected rate of return on plan assets 7.60% 7.60% 7.60% 3.50% 4.70% 4.60% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Medical cost trend on covered charges: Initial trend rate (end of year) N/A N/A N/A 6.75% 7.00% 7.25% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00%
Net Periodic Benefit Costs (Credits)	The components of net periodic benefit costs (credits) for sponsored defined benefit pension and OPEB plans are included in the tables below (in millions). Net periodic benefit costs are primarily included in “Other operation and maintenance” in the income statements. Alliant Energy Defined Benefit Pension Plans OPEB Plans 2017 2016 2015 2017 2016 2015 Service cost $12.5 $12.6 $15.9 $5.0 $5.3 $5.5 Interest cost 51.0 53.0 53.6 8.6 9.4 9.1 Expected return on plan assets (a) (65.5 ) (65.5 ) (75.0 ) (6.1 ) (6.1 ) (8.4 ) Amortization of prior service credit (b) (0.4 ) (0.3 ) (0.2 ) (0.2 ) (4.1 ) (11.3 ) Amortization of actuarial loss (c) 37.6 37.4 35.4 3.8 4.7 4.8 Additional benefit costs — — 0.5 — — — Settlement losses (d) 0.9 — — — — — $36.1 $37.2 $30.2 $11.1 $9.2 ($0.3 ) IPL Defined Benefit Pension Plans OPEB Plans 2017 2016 2015 2017 2016 2015 Service cost $7.3 $7.5 $8.8 $2.1 $2.3 $2.4 Interest cost 23.5 24.5 25.0 3.5 3.8 3.8 Expected return on plan assets (a) (30.8 ) (30.9 ) (35.8 ) (4.3 ) (4.3 ) (5.7 ) Amortization of prior service credit (b) (0.2 ) (0.2 ) (0.1 ) — (2.6 ) (6.1 ) Amortization of actuarial loss (c) 16.1 16.5 15.3 2.0 2.6 2.3 $15.9 $17.4 $13.2 $3.3 $1.8 ($3.3 ) WPL Defined Benefit Pension Plans OPEB Plans 2017 2016 2015 2017 2016 2015 Service cost $4.9 $4.9 $5.8 $1.9 $2.0 $2.1 Interest cost 21.8 22.3 22.6 3.4 3.8 3.7 Expected return on plan assets (a) (28.5 ) (28.3 ) (32.4 ) (0.8 ) (0.8 ) (1.5 ) Amortization of prior service cost (credit) (b) 0.1 0.2 0.2 (0.2 ) (0.9 ) (3.5 ) Amortization of actuarial loss (c) 18.5 17.6 16.8 1.6 1.8 2.2 Additional benefit costs — — 0.5 — — — $16.8 $16.7 $13.5 $5.9 $5.9 $3.0 (a) The expected return on plan assets is based on the expected rate of return on plan assets and the fair value approach to the market-related value of plan assets. (b) Unrecognized prior service costs (credits) for the OPEB plans are amortized over the average future service period to full eligibility of the participants of each plan. (c) Unrecognized net actuarial gains or losses in excess of 10% of the greater of the plans’ benefit obligations or assets are amortized over the average future service lives of plan participants, except for the Alliant Energy Cash Balance Pension Plan where gains or losses outside the 10% threshold are amortized over the time period the participants are expected to receive benefits. (d) Settlement losses related to payments made to retired executives of Alliant Energy.
Estimated Amortization From Regulatory Assets, Regulatory Liabilities And Accumulated Other Comprehensive Loss Into Net Periodic Benefit Cost	The estimated amortization from “Regulatory assets” and “Regulatory liabilities” on the balance sheets and AOCL on Alliant Energy’s balance sheet into net periodic benefit cost in 2018 is as follows (in millions): Alliant Energy IPL WPL Defined Benefit Defined Benefit Defined Benefit Pension Plans OPEB Plans Pension Plans OPEB Plans Pension Plans OPEB Plans Actuarial loss $35.2 $3.4 $15.0 $1.2 $17.2 $2.0 Prior service credit (0.7 ) (0.2 ) (0.2 ) — (0.2 ) (0.2 ) $34.5 $3.2 $14.8 $1.2 $17.0 $1.8
Funded Status Of Benefit Plans	A reconciliation of the funded status of qualified and non-qualified defined benefit pension and OPEB plans to the amounts recognized on the balance sheets at December 31 was as follows (in millions): Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2017 2016 Change in benefit obligation: Net benefit obligation at January 1 $1,244.3 $1,206.3 $220.1 $221.4 Service cost 12.5 12.6 5.0 5.3 Interest cost 51.0 53.0 8.6 9.4 Plan participants’ contributions — — 2.9 2.4 Actuarial (gain) loss 83.6 48.3 5.4 (0.3 ) Gross benefits paid (88.3 ) (75.9 ) (19.7 ) (18.1 ) Net benefit obligation at December 31 1,303.1 1,244.3 222.3 220.1 Change in plan assets: Fair value of plan assets at January 1 895.7 895.0 105.8 106.9 Actual return on plan assets 136.7 74.3 12.9 8.2 Employer contributions 6.6 2.3 9.2 6.4 Plan participants’ contributions — — 2.9 2.4 Gross benefits paid (88.3 ) (75.9 ) (19.7 ) (18.1 ) Fair value of plan assets at December 31 950.7 895.7 111.1 105.8 Under funded status at December 31 ($352.4 ) ($348.6 ) ($111.2 ) ($114.3 ) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2017 2016 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $8.8 $3.2 Current liabilities (2.2 ) (6.5 ) (9.1 ) (8.6 ) Pension and other benefit obligations (350.2 ) (342.1 ) (110.9 ) (108.9 ) Net amounts recognized at December 31 ($352.4 ) ($348.6 ) ($111.2 ) ($114.3 ) Amounts recognized in Regulatory Assets (refer to Note 2 for details) and AOCL (refer to Alliant Energy’s common equity statements for details) consist of: Net actuarial loss $509.1 $535.1 $47.4 $52.6 Prior service credit (6.5 ) (6.9 ) (1.3 ) (1.5 ) $502.6 $528.2 $46.1 $51.1 Defined Benefit Pension Plans OPEB Plans IPL 2017 2016 2017 2016 Change in benefit obligation: Net benefit obligation at January 1 $570.4 $556.1 $90.1 $91.3 Service cost 7.3 7.5 2.1 2.3 Interest cost 23.5 24.5 3.5 3.8 Plan participants’ contributions — — 1.0 0.9 Actuarial (gain) loss 34.9 19.1 (0.1 ) (0.7 ) Gross benefits paid (43.2 ) (36.8 ) (7.2 ) (7.5 ) Net benefit obligation at December 31 592.9 570.4 89.4 90.1 Change in plan assets: Fair value of plan assets at January 1 422.0 422.7 68.2 69.2 Actual return on plan assets 64.3 35.3 8.9 5.3 Employer contributions 0.6 0.8 2.0 0.3 Plan participants’ contributions — — 1.0 0.9 Gross benefits paid (43.2 ) (36.8 ) (7.2 ) (7.5 ) Fair value of plan assets at December 31 443.7 422.0 72.9 68.2 Under funded status at December 31 ($149.2 ) ($148.4 ) ($16.5 ) ($21.9 ) Defined Benefit Pension Plans OPEB Plans IPL 2017 2016 2017 2016 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $5.9 $0.4 Current liabilities (0.5 ) (0.7 ) (2.0 ) (1.9 ) Pension and other benefit obligations (148.7 ) (147.7 ) (20.4 ) (20.4 ) Net amounts recognized at December 31 ($149.2 ) ($148.4 ) ($16.5 ) ($21.9 ) Amounts recognized in Regulatory Assets consist of (refer to Note 2 for details): Net actuarial loss $218.9 $233.6 $18.7 $25.4 Prior service credit (2.1 ) (2.3 ) — — $216.8 $231.3 $18.7 $25.4 Defined Benefit Pension Plans OPEB Plans WPL 2017 2016 2017 2016 Change in benefit obligation: Net benefit obligation at January 1 $529.2 $505.9 $88.9 $89.7 Service cost 4.9 4.9 1.9 2.0 Interest cost 21.8 22.3 3.4 3.8 Plan participants’ contributions — — 1.4 1.2 Actuarial loss 38.3 25.7 4.1 0.5 Gross benefits paid (34.4 ) (29.6 ) (9.3 ) (8.3 ) Net benefit obligation at December 31 559.8 529.2 90.4 88.9 Change in plan assets: Fair value of plan assets at January 1 389.7 386.8 18.6 18.7 Actual return on plan assets 59.6 32.4 1.2 1.2 Employer contributions 0.1 0.1 6.8 5.8 Plan participants’ contributions — — 1.4 1.2 Gross benefits paid (34.4 ) (29.6 ) (9.3 ) (8.3 ) Fair value of plan assets at December 31 415.0 389.7 18.7 18.6 Under funded status at December 31 ($144.8 ) ($139.5 ) ($71.7 ) ($70.3 ) Defined Benefit Pension Plans OPEB Plans WPL 2017 2016 2017 2016 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $2.9 $2.7 Current liabilities (0.1 ) (0.1 ) (6.8 ) (6.4 ) Pension and other benefit obligations (144.7 ) (139.4 ) (67.8 ) (66.6 ) Net amounts recognized at December 31 ($144.8 ) ($139.5 ) ($71.7 ) ($70.3 ) Amounts recognized in Regulatory Assets consist of (refer to Note 2 for details): Net actuarial loss $224.7 $236.1 $23.6 $21.5 Prior service credit (1.5 ) (1.4 ) (1.3 ) (1.5 ) $223.2 $234.7 $22.3 $20.0
Accumulated Benefit Obligations	Included in the following tables are accumulated benefit obligations, aggregate amounts applicable to defined benefit pension and OPEB plans with accumulated benefit obligations in excess of plan assets, as well as defined benefit pension plans with projected benefit obligations in excess of plan assets as of the December 31 measurement date (in millions): Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2017 2016 Accumulated benefit obligations $1,269.0 $1,201.5 $222.3 $220.1 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,269.0 1,201.5 222.3 220.1 Fair value of plan assets 950.7 895.7 111.1 105.8 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,303.1 1,244.3 N/A N/A Fair value of plan assets 950.7 895.7 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2017 2016 2017 2016 Accumulated benefit obligations $573.1 $546.7 $89.4 $90.1 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 573.1 546.7 89.4 90.1 Fair value of plan assets 443.7 422.0 72.9 68.2 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 592.9 570.4 N/A N/A Fair value of plan assets 443.7 422.0 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2017 2016 2017 2016 Accumulated benefit obligations $548.1 $513.2 $90.4 $88.9 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 548.1 513.2 90.4 88.9 Fair value of plan assets 415.0 389.7 18.7 18.6 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 559.8 529.2 N/A N/A Fair value of plan assets 415.0 389.7 N/A N/A
Regulatory Assets	In addition to the amounts recognized in regulatory assets in the above tables for IPL and WPL, regulatory assets were recognized for amounts associated with Corporate Services employees participating in other Alliant Energy sponsored benefit plans that were allocated to IPL and WPL at December 31 as follows (in millions): IPL WPL 2017 2016 2017 2016 Regulatory assets $38.9 $37.3 $28.1 $30.0
Estimated Future Employer Contributions	Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2018 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $6.3 $4.4 $0.3 OPEB plans 9.0 2.0 6.8 (a) Alliant Energy sponsors several non-qualified defined benefit pension plans that cover certain current and former key employees of IPL and WPL. Alliant Energy allocates pension costs to IPL and WPL for these plans. In addition, IPL and WPL amounts reflect funding for their non-bargaining employees who are participants in the Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans.
Expected Benefit Payments	Expected benefit payments for the qualified and non-qualified defined benefit plans, which reflect expected future service, as appropriate, are as follows (in millions): Alliant Energy 2018 2019 2020 2021 2022 2023 - 2027 Defined benefit pension benefits $72.2 $73.9 $76.4 $77.1 $92.9 $398.2 OPEB 18.2 18.4 17.7 17.5 17.2 80.1 $90.4 $92.3 $94.1 $94.6 $110.1 $478.3 IPL 2018 2019 2020 2021 2022 2023 - 2027 Defined benefit pension benefits $33.9 $33.8 $36.5 $36.6 $37.6 $188.4 OPEB 7.1 7.1 7.2 7.1 7.0 32.6 $41.0 $40.9 $43.7 $43.7 $44.6 $221.0 WPL 2018 2019 2020 2021 2022 2023 - 2027 Defined benefit pension benefits $31.4 $32.0 $32.4 $32.4 $32.7 $167.8 OPEB 8.3 8.3 7.5 7.3 7.0 31.8 $39.7 $40.3 $39.9 $39.7 $39.7 $199.6
Recognized Compensation Expense And Income Tax Benefits	A summary of compensation expense, including amounts allocated to IPL and WPL, and the related income tax benefits recognized for share-based compensation awards was as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Compensation expense $15.1 $18.0 $10.7 $8.3 $9.5 $5.7 $6.4 $7.9 $4.7 Income tax benefits 6.2 7.4 4.4 3.4 4.0 2.4 2.6 3.2 1.9
Schedule of Carrying Value and Fair Market Value of the Deferred Compensation Obligation	At December 31, the carrying value of the deferred compensation obligation for the company stock account and the shares in the deferred compensation trust based on the historical value of the shares of Alliant Energy common stock contributed to the rabbi trust, and the fair market value of the shares held in the rabbi trust, were as follows (in millions): 2017 2016 Carrying value $11.1 $10.0 Fair market value 19.7 16.7
IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumptions Used To Measure Benefit Plans	The assumptions for defined benefit pension and OPEB plans at the measurement date of December 31 were as follows: Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2015 2017 2016 2015 Discount rate for benefit obligations 3.66% 4.19% 4.47% 3.53% 3.98% 4.30% Discount rate for net periodic cost 4.19% 4.47% 4.18% 3.98% 4.30% 3.97% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.80% 6.30% 6.20% Rate of compensation increase 3.65 % - 4.50% 3.65 % - 4.50% 3.65 % - 4.50% N/A N/A N/A Medical cost trend on covered charges: Initial trend rate (end of year) N/A N/A N/A 6.75% 7.00% 7.25% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans IPL 2017 2016 2015 2017 2016 2015 Discount rate for benefit obligations 3.68% 4.22% 4.50% 3.51% 3.95% 4.28% Discount rate for net periodic cost 4.22% 4.50% 4.20% 3.95% 4.28% 3.94% Expected rate of return on plan assets 7.60% 7.60% 7.60% 6.20% 6.60% 6.60% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Medical cost trend on covered charges: Initial trend rate (end of year) N/A N/A N/A 6.75% 7.00% 7.25% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans WPL 2017 2016 2015 2017 2016 2015 Discount rate for benefit obligations 3.69% 4.23% 4.51% 3.51% 3.96% 4.28% Discount rate for net periodic cost 4.23% 4.51% 4.20% 3.96% 4.28% 3.96% Expected rate of return on plan assets 7.60% 7.60% 7.60% 3.50% 4.70% 4.60% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Medical cost trend on covered charges: Initial trend rate (end of year) N/A N/A N/A 6.75% 7.00% 7.25% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00%
Net Periodic Benefit Costs (Credits)	The components of net periodic benefit costs (credits) for sponsored defined benefit pension and OPEB plans are included in the tables below (in millions). Net periodic benefit costs are primarily included in “Other operation and maintenance” in the income statements. Alliant Energy Defined Benefit Pension Plans OPEB Plans 2017 2016 2015 2017 2016 2015 Service cost $12.5 $12.6 $15.9 $5.0 $5.3 $5.5 Interest cost 51.0 53.0 53.6 8.6 9.4 9.1 Expected return on plan assets (a) (65.5 ) (65.5 ) (75.0 ) (6.1 ) (6.1 ) (8.4 ) Amortization of prior service credit (b) (0.4 ) (0.3 ) (0.2 ) (0.2 ) (4.1 ) (11.3 ) Amortization of actuarial loss (c) 37.6 37.4 35.4 3.8 4.7 4.8 Additional benefit costs — — 0.5 — — — Settlement losses (d) 0.9 — — — — — $36.1 $37.2 $30.2 $11.1 $9.2 ($0.3 ) IPL Defined Benefit Pension Plans OPEB Plans 2017 2016 2015 2017 2016 2015 Service cost $7.3 $7.5 $8.8 $2.1 $2.3 $2.4 Interest cost 23.5 24.5 25.0 3.5 3.8 3.8 Expected return on plan assets (a) (30.8 ) (30.9 ) (35.8 ) (4.3 ) (4.3 ) (5.7 ) Amortization of prior service credit (b) (0.2 ) (0.2 ) (0.1 ) — (2.6 ) (6.1 ) Amortization of actuarial loss (c) 16.1 16.5 15.3 2.0 2.6 2.3 $15.9 $17.4 $13.2 $3.3 $1.8 ($3.3 ) WPL Defined Benefit Pension Plans OPEB Plans 2017 2016 2015 2017 2016 2015 Service cost $4.9 $4.9 $5.8 $1.9 $2.0 $2.1 Interest cost 21.8 22.3 22.6 3.4 3.8 3.7 Expected return on plan assets (a) (28.5 ) (28.3 ) (32.4 ) (0.8 ) (0.8 ) (1.5 ) Amortization of prior service cost (credit) (b) 0.1 0.2 0.2 (0.2 ) (0.9 ) (3.5 ) Amortization of actuarial loss (c) 18.5 17.6 16.8 1.6 1.8 2.2 Additional benefit costs — — 0.5 — — — $16.8 $16.7 $13.5 $5.9 $5.9 $3.0 (a) The expected return on plan assets is based on the expected rate of return on plan assets and the fair value approach to the market-related value of plan assets. (b) Unrecognized prior service costs (credits) for the OPEB plans are amortized over the average future service period to full eligibility of the participants of each plan. (c) Unrecognized net actuarial gains or losses in excess of 10% of the greater of the plans’ benefit obligations or assets are amortized over the average future service lives of plan participants, except for the Alliant Energy Cash Balance Pension Plan where gains or losses outside the 10% threshold are amortized over the time period the participants are expected to receive benefits. (d) Settlement losses related to payments made to retired executives of Alliant Energy.
Estimated Amortization From Regulatory Assets, Regulatory Liabilities And Accumulated Other Comprehensive Loss Into Net Periodic Benefit Cost	The estimated amortization from “Regulatory assets” and “Regulatory liabilities” on the balance sheets and AOCL on Alliant Energy’s balance sheet into net periodic benefit cost in 2018 is as follows (in millions): Alliant Energy IPL WPL Defined Benefit Defined Benefit Defined Benefit Pension Plans OPEB Plans Pension Plans OPEB Plans Pension Plans OPEB Plans Actuarial loss $35.2 $3.4 $15.0 $1.2 $17.2 $2.0 Prior service credit (0.7 ) (0.2 ) (0.2 ) — (0.2 ) (0.2 ) $34.5 $3.2 $14.8 $1.2 $17.0 $1.8
Funded Status Of Benefit Plans	A reconciliation of the funded status of qualified and non-qualified defined benefit pension and OPEB plans to the amounts recognized on the balance sheets at December 31 was as follows (in millions): Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2017 2016 Change in benefit obligation: Net benefit obligation at January 1 $1,244.3 $1,206.3 $220.1 $221.4 Service cost 12.5 12.6 5.0 5.3 Interest cost 51.0 53.0 8.6 9.4 Plan participants’ contributions — — 2.9 2.4 Actuarial (gain) loss 83.6 48.3 5.4 (0.3 ) Gross benefits paid (88.3 ) (75.9 ) (19.7 ) (18.1 ) Net benefit obligation at December 31 1,303.1 1,244.3 222.3 220.1 Change in plan assets: Fair value of plan assets at January 1 895.7 895.0 105.8 106.9 Actual return on plan assets 136.7 74.3 12.9 8.2 Employer contributions 6.6 2.3 9.2 6.4 Plan participants’ contributions — — 2.9 2.4 Gross benefits paid (88.3 ) (75.9 ) (19.7 ) (18.1 ) Fair value of plan assets at December 31 950.7 895.7 111.1 105.8 Under funded status at December 31 ($352.4 ) ($348.6 ) ($111.2 ) ($114.3 ) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2017 2016 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $8.8 $3.2 Current liabilities (2.2 ) (6.5 ) (9.1 ) (8.6 ) Pension and other benefit obligations (350.2 ) (342.1 ) (110.9 ) (108.9 ) Net amounts recognized at December 31 ($352.4 ) ($348.6 ) ($111.2 ) ($114.3 ) Amounts recognized in Regulatory Assets (refer to Note 2 for details) and AOCL (refer to Alliant Energy’s common equity statements for details) consist of: Net actuarial loss $509.1 $535.1 $47.4 $52.6 Prior service credit (6.5 ) (6.9 ) (1.3 ) (1.5 ) $502.6 $528.2 $46.1 $51.1 Defined Benefit Pension Plans OPEB Plans IPL 2017 2016 2017 2016 Change in benefit obligation: Net benefit obligation at January 1 $570.4 $556.1 $90.1 $91.3 Service cost 7.3 7.5 2.1 2.3 Interest cost 23.5 24.5 3.5 3.8 Plan participants’ contributions — — 1.0 0.9 Actuarial (gain) loss 34.9 19.1 (0.1 ) (0.7 ) Gross benefits paid (43.2 ) (36.8 ) (7.2 ) (7.5 ) Net benefit obligation at December 31 592.9 570.4 89.4 90.1 Change in plan assets: Fair value of plan assets at January 1 422.0 422.7 68.2 69.2 Actual return on plan assets 64.3 35.3 8.9 5.3 Employer contributions 0.6 0.8 2.0 0.3 Plan participants’ contributions — — 1.0 0.9 Gross benefits paid (43.2 ) (36.8 ) (7.2 ) (7.5 ) Fair value of plan assets at December 31 443.7 422.0 72.9 68.2 Under funded status at December 31 ($149.2 ) ($148.4 ) ($16.5 ) ($21.9 ) Defined Benefit Pension Plans OPEB Plans IPL 2017 2016 2017 2016 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $5.9 $0.4 Current liabilities (0.5 ) (0.7 ) (2.0 ) (1.9 ) Pension and other benefit obligations (148.7 ) (147.7 ) (20.4 ) (20.4 ) Net amounts recognized at December 31 ($149.2 ) ($148.4 ) ($16.5 ) ($21.9 ) Amounts recognized in Regulatory Assets consist of (refer to Note 2 for details): Net actuarial loss $218.9 $233.6 $18.7 $25.4 Prior service credit (2.1 ) (2.3 ) — — $216.8 $231.3 $18.7 $25.4 Defined Benefit Pension Plans OPEB Plans WPL 2017 2016 2017 2016 Change in benefit obligation: Net benefit obligation at January 1 $529.2 $505.9 $88.9 $89.7 Service cost 4.9 4.9 1.9 2.0 Interest cost 21.8 22.3 3.4 3.8 Plan participants’ contributions — — 1.4 1.2 Actuarial loss 38.3 25.7 4.1 0.5 Gross benefits paid (34.4 ) (29.6 ) (9.3 ) (8.3 ) Net benefit obligation at December 31 559.8 529.2 90.4 88.9 Change in plan assets: Fair value of plan assets at January 1 389.7 386.8 18.6 18.7 Actual return on plan assets 59.6 32.4 1.2 1.2 Employer contributions 0.1 0.1 6.8 5.8 Plan participants’ contributions — — 1.4 1.2 Gross benefits paid (34.4 ) (29.6 ) (9.3 ) (8.3 ) Fair value of plan assets at December 31 415.0 389.7 18.7 18.6 Under funded status at December 31 ($144.8 ) ($139.5 ) ($71.7 ) ($70.3 ) Defined Benefit Pension Plans OPEB Plans WPL 2017 2016 2017 2016 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $2.9 $2.7 Current liabilities (0.1 ) (0.1 ) (6.8 ) (6.4 ) Pension and other benefit obligations (144.7 ) (139.4 ) (67.8 ) (66.6 ) Net amounts recognized at December 31 ($144.8 ) ($139.5 ) ($71.7 ) ($70.3 ) Amounts recognized in Regulatory Assets consist of (refer to Note 2 for details): Net actuarial loss $224.7 $236.1 $23.6 $21.5 Prior service credit (1.5 ) (1.4 ) (1.3 ) (1.5 ) $223.2 $234.7 $22.3 $20.0
Accumulated Benefit Obligations	Included in the following tables are accumulated benefit obligations, aggregate amounts applicable to defined benefit pension and OPEB plans with accumulated benefit obligations in excess of plan assets, as well as defined benefit pension plans with projected benefit obligations in excess of plan assets as of the December 31 measurement date (in millions): Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2017 2016 Accumulated benefit obligations $1,269.0 $1,201.5 $222.3 $220.1 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,269.0 1,201.5 222.3 220.1 Fair value of plan assets 950.7 895.7 111.1 105.8 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,303.1 1,244.3 N/A N/A Fair value of plan assets 950.7 895.7 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2017 2016 2017 2016 Accumulated benefit obligations $573.1 $546.7 $89.4 $90.1 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 573.1 546.7 89.4 90.1 Fair value of plan assets 443.7 422.0 72.9 68.2 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 592.9 570.4 N/A N/A Fair value of plan assets 443.7 422.0 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2017 2016 2017 2016 Accumulated benefit obligations $548.1 $513.2 $90.4 $88.9 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 548.1 513.2 90.4 88.9 Fair value of plan assets 415.0 389.7 18.7 18.6 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 559.8 529.2 N/A N/A Fair value of plan assets 415.0 389.7 N/A N/A
Regulatory Assets	In addition to the amounts recognized in regulatory assets in the above tables for IPL and WPL, regulatory assets were recognized for amounts associated with Corporate Services employees participating in other Alliant Energy sponsored benefit plans that were allocated to IPL and WPL at December 31 as follows (in millions): IPL WPL 2017 2016 2017 2016 Regulatory assets $38.9 $37.3 $28.1 $30.0
Estimated Future Employer Contributions	Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2018 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $6.3 $4.4 $0.3 OPEB plans 9.0 2.0 6.8 (a) Alliant Energy sponsors several non-qualified defined benefit pension plans that cover certain current and former key employees of IPL and WPL. Alliant Energy allocates pension costs to IPL and WPL for these plans. In addition, IPL and WPL amounts reflect funding for their non-bargaining employees who are participants in the Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans.
Expected Benefit Payments	Expected benefit payments for the qualified and non-qualified defined benefit plans, which reflect expected future service, as appropriate, are as follows (in millions): Alliant Energy 2018 2019 2020 2021 2022 2023 - 2027 Defined benefit pension benefits $72.2 $73.9 $76.4 $77.1 $92.9 $398.2 OPEB 18.2 18.4 17.7 17.5 17.2 80.1 $90.4 $92.3 $94.1 $94.6 $110.1 $478.3 IPL 2018 2019 2020 2021 2022 2023 - 2027 Defined benefit pension benefits $33.9 $33.8 $36.5 $36.6 $37.6 $188.4 OPEB 7.1 7.1 7.2 7.1 7.0 32.6 $41.0 $40.9 $43.7 $43.7 $44.6 $221.0 WPL 2018 2019 2020 2021 2022 2023 - 2027 Defined benefit pension benefits $31.4 $32.0 $32.4 $32.4 $32.7 $167.8 OPEB 8.3 8.3 7.5 7.3 7.0 31.8 $39.7 $40.3 $39.9 $39.7 $39.7 $199.6
Recognized Compensation Expense And Income Tax Benefits	A summary of compensation expense, including amounts allocated to IPL and WPL, and the related income tax benefits recognized for share-based compensation awards was as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Compensation expense $15.1 $18.0 $10.7 $8.3 $9.5 $5.7 $6.4 $7.9 $4.7 Income tax benefits 6.2 7.4 4.4 3.4 4.0 2.4 2.6 3.2 1.9
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumptions Used To Measure Benefit Plans	The assumptions for defined benefit pension and OPEB plans at the measurement date of December 31 were as follows: Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2015 2017 2016 2015 Discount rate for benefit obligations 3.66% 4.19% 4.47% 3.53% 3.98% 4.30% Discount rate for net periodic cost 4.19% 4.47% 4.18% 3.98% 4.30% 3.97% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.80% 6.30% 6.20% Rate of compensation increase 3.65 % - 4.50% 3.65 % - 4.50% 3.65 % - 4.50% N/A N/A N/A Medical cost trend on covered charges: Initial trend rate (end of year) N/A N/A N/A 6.75% 7.00% 7.25% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans IPL 2017 2016 2015 2017 2016 2015 Discount rate for benefit obligations 3.68% 4.22% 4.50% 3.51% 3.95% 4.28% Discount rate for net periodic cost 4.22% 4.50% 4.20% 3.95% 4.28% 3.94% Expected rate of return on plan assets 7.60% 7.60% 7.60% 6.20% 6.60% 6.60% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Medical cost trend on covered charges: Initial trend rate (end of year) N/A N/A N/A 6.75% 7.00% 7.25% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans WPL 2017 2016 2015 2017 2016 2015 Discount rate for benefit obligations 3.69% 4.23% 4.51% 3.51% 3.96% 4.28% Discount rate for net periodic cost 4.23% 4.51% 4.20% 3.96% 4.28% 3.96% Expected rate of return on plan assets 7.60% 7.60% 7.60% 3.50% 4.70% 4.60% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Medical cost trend on covered charges: Initial trend rate (end of year) N/A N/A N/A 6.75% 7.00% 7.25% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00%
Net Periodic Benefit Costs (Credits)	The components of net periodic benefit costs (credits) for sponsored defined benefit pension and OPEB plans are included in the tables below (in millions). Net periodic benefit costs are primarily included in “Other operation and maintenance” in the income statements. Alliant Energy Defined Benefit Pension Plans OPEB Plans 2017 2016 2015 2017 2016 2015 Service cost $12.5 $12.6 $15.9 $5.0 $5.3 $5.5 Interest cost 51.0 53.0 53.6 8.6 9.4 9.1 Expected return on plan assets (a) (65.5 ) (65.5 ) (75.0 ) (6.1 ) (6.1 ) (8.4 ) Amortization of prior service credit (b) (0.4 ) (0.3 ) (0.2 ) (0.2 ) (4.1 ) (11.3 ) Amortization of actuarial loss (c) 37.6 37.4 35.4 3.8 4.7 4.8 Additional benefit costs — — 0.5 — — — Settlement losses (d) 0.9 — — — — — $36.1 $37.2 $30.2 $11.1 $9.2 ($0.3 ) IPL Defined Benefit Pension Plans OPEB Plans 2017 2016 2015 2017 2016 2015 Service cost $7.3 $7.5 $8.8 $2.1 $2.3 $2.4 Interest cost 23.5 24.5 25.0 3.5 3.8 3.8 Expected return on plan assets (a) (30.8 ) (30.9 ) (35.8 ) (4.3 ) (4.3 ) (5.7 ) Amortization of prior service credit (b) (0.2 ) (0.2 ) (0.1 ) — (2.6 ) (6.1 ) Amortization of actuarial loss (c) 16.1 16.5 15.3 2.0 2.6 2.3 $15.9 $17.4 $13.2 $3.3 $1.8 ($3.3 ) WPL Defined Benefit Pension Plans OPEB Plans 2017 2016 2015 2017 2016 2015 Service cost $4.9 $4.9 $5.8 $1.9 $2.0 $2.1 Interest cost 21.8 22.3 22.6 3.4 3.8 3.7 Expected return on plan assets (a) (28.5 ) (28.3 ) (32.4 ) (0.8 ) (0.8 ) (1.5 ) Amortization of prior service cost (credit) (b) 0.1 0.2 0.2 (0.2 ) (0.9 ) (3.5 ) Amortization of actuarial loss (c) 18.5 17.6 16.8 1.6 1.8 2.2 Additional benefit costs — — 0.5 — — — $16.8 $16.7 $13.5 $5.9 $5.9 $3.0 (a) The expected return on plan assets is based on the expected rate of return on plan assets and the fair value approach to the market-related value of plan assets. (b) Unrecognized prior service costs (credits) for the OPEB plans are amortized over the average future service period to full eligibility of the participants of each plan. (c) Unrecognized net actuarial gains or losses in excess of 10% of the greater of the plans’ benefit obligations or assets are amortized over the average future service lives of plan participants, except for the Alliant Energy Cash Balance Pension Plan where gains or losses outside the 10% threshold are amortized over the time period the participants are expected to receive benefits. (d) Settlement losses related to payments made to retired executives of Alliant Energy.
Estimated Amortization From Regulatory Assets, Regulatory Liabilities And Accumulated Other Comprehensive Loss Into Net Periodic Benefit Cost	The estimated amortization from “Regulatory assets” and “Regulatory liabilities” on the balance sheets and AOCL on Alliant Energy’s balance sheet into net periodic benefit cost in 2018 is as follows (in millions): Alliant Energy IPL WPL Defined Benefit Defined Benefit Defined Benefit Pension Plans OPEB Plans Pension Plans OPEB Plans Pension Plans OPEB Plans Actuarial loss $35.2 $3.4 $15.0 $1.2 $17.2 $2.0 Prior service credit (0.7 ) (0.2 ) (0.2 ) — (0.2 ) (0.2 ) $34.5 $3.2 $14.8 $1.2 $17.0 $1.8
Funded Status Of Benefit Plans	A reconciliation of the funded status of qualified and non-qualified defined benefit pension and OPEB plans to the amounts recognized on the balance sheets at December 31 was as follows (in millions): Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2017 2016 Change in benefit obligation: Net benefit obligation at January 1 $1,244.3 $1,206.3 $220.1 $221.4 Service cost 12.5 12.6 5.0 5.3 Interest cost 51.0 53.0 8.6 9.4 Plan participants’ contributions — — 2.9 2.4 Actuarial (gain) loss 83.6 48.3 5.4 (0.3 ) Gross benefits paid (88.3 ) (75.9 ) (19.7 ) (18.1 ) Net benefit obligation at December 31 1,303.1 1,244.3 222.3 220.1 Change in plan assets: Fair value of plan assets at January 1 895.7 895.0 105.8 106.9 Actual return on plan assets 136.7 74.3 12.9 8.2 Employer contributions 6.6 2.3 9.2 6.4 Plan participants’ contributions — — 2.9 2.4 Gross benefits paid (88.3 ) (75.9 ) (19.7 ) (18.1 ) Fair value of plan assets at December 31 950.7 895.7 111.1 105.8 Under funded status at December 31 ($352.4 ) ($348.6 ) ($111.2 ) ($114.3 ) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2017 2016 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $8.8 $3.2 Current liabilities (2.2 ) (6.5 ) (9.1 ) (8.6 ) Pension and other benefit obligations (350.2 ) (342.1 ) (110.9 ) (108.9 ) Net amounts recognized at December 31 ($352.4 ) ($348.6 ) ($111.2 ) ($114.3 ) Amounts recognized in Regulatory Assets (refer to Note 2 for details) and AOCL (refer to Alliant Energy’s common equity statements for details) consist of: Net actuarial loss $509.1 $535.1 $47.4 $52.6 Prior service credit (6.5 ) (6.9 ) (1.3 ) (1.5 ) $502.6 $528.2 $46.1 $51.1 Defined Benefit Pension Plans OPEB Plans IPL 2017 2016 2017 2016 Change in benefit obligation: Net benefit obligation at January 1 $570.4 $556.1 $90.1 $91.3 Service cost 7.3 7.5 2.1 2.3 Interest cost 23.5 24.5 3.5 3.8 Plan participants’ contributions — — 1.0 0.9 Actuarial (gain) loss 34.9 19.1 (0.1 ) (0.7 ) Gross benefits paid (43.2 ) (36.8 ) (7.2 ) (7.5 ) Net benefit obligation at December 31 592.9 570.4 89.4 90.1 Change in plan assets: Fair value of plan assets at January 1 422.0 422.7 68.2 69.2 Actual return on plan assets 64.3 35.3 8.9 5.3 Employer contributions 0.6 0.8 2.0 0.3 Plan participants’ contributions — — 1.0 0.9 Gross benefits paid (43.2 ) (36.8 ) (7.2 ) (7.5 ) Fair value of plan assets at December 31 443.7 422.0 72.9 68.2 Under funded status at December 31 ($149.2 ) ($148.4 ) ($16.5 ) ($21.9 ) Defined Benefit Pension Plans OPEB Plans IPL 2017 2016 2017 2016 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $5.9 $0.4 Current liabilities (0.5 ) (0.7 ) (2.0 ) (1.9 ) Pension and other benefit obligations (148.7 ) (147.7 ) (20.4 ) (20.4 ) Net amounts recognized at December 31 ($149.2 ) ($148.4 ) ($16.5 ) ($21.9 ) Amounts recognized in Regulatory Assets consist of (refer to Note 2 for details): Net actuarial loss $218.9 $233.6 $18.7 $25.4 Prior service credit (2.1 ) (2.3 ) — — $216.8 $231.3 $18.7 $25.4 Defined Benefit Pension Plans OPEB Plans WPL 2017 2016 2017 2016 Change in benefit obligation: Net benefit obligation at January 1 $529.2 $505.9 $88.9 $89.7 Service cost 4.9 4.9 1.9 2.0 Interest cost 21.8 22.3 3.4 3.8 Plan participants’ contributions — — 1.4 1.2 Actuarial loss 38.3 25.7 4.1 0.5 Gross benefits paid (34.4 ) (29.6 ) (9.3 ) (8.3 ) Net benefit obligation at December 31 559.8 529.2 90.4 88.9 Change in plan assets: Fair value of plan assets at January 1 389.7 386.8 18.6 18.7 Actual return on plan assets 59.6 32.4 1.2 1.2 Employer contributions 0.1 0.1 6.8 5.8 Plan participants’ contributions — — 1.4 1.2 Gross benefits paid (34.4 ) (29.6 ) (9.3 ) (8.3 ) Fair value of plan assets at December 31 415.0 389.7 18.7 18.6 Under funded status at December 31 ($144.8 ) ($139.5 ) ($71.7 ) ($70.3 ) Defined Benefit Pension Plans OPEB Plans WPL 2017 2016 2017 2016 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $2.9 $2.7 Current liabilities (0.1 ) (0.1 ) (6.8 ) (6.4 ) Pension and other benefit obligations (144.7 ) (139.4 ) (67.8 ) (66.6 ) Net amounts recognized at December 31 ($144.8 ) ($139.5 ) ($71.7 ) ($70.3 ) Amounts recognized in Regulatory Assets consist of (refer to Note 2 for details): Net actuarial loss $224.7 $236.1 $23.6 $21.5 Prior service credit (1.5 ) (1.4 ) (1.3 ) (1.5 ) $223.2 $234.7 $22.3 $20.0
Accumulated Benefit Obligations	Included in the following tables are accumulated benefit obligations, aggregate amounts applicable to defined benefit pension and OPEB plans with accumulated benefit obligations in excess of plan assets, as well as defined benefit pension plans with projected benefit obligations in excess of plan assets as of the December 31 measurement date (in millions): Defined Benefit Pension Plans OPEB Plans Alliant Energy 2017 2016 2017 2016 Accumulated benefit obligations $1,269.0 $1,201.5 $222.3 $220.1 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,269.0 1,201.5 222.3 220.1 Fair value of plan assets 950.7 895.7 111.1 105.8 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,303.1 1,244.3 N/A N/A Fair value of plan assets 950.7 895.7 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2017 2016 2017 2016 Accumulated benefit obligations $573.1 $546.7 $89.4 $90.1 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 573.1 546.7 89.4 90.1 Fair value of plan assets 443.7 422.0 72.9 68.2 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 592.9 570.4 N/A N/A Fair value of plan assets 443.7 422.0 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2017 2016 2017 2016 Accumulated benefit obligations $548.1 $513.2 $90.4 $88.9 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 548.1 513.2 90.4 88.9 Fair value of plan assets 415.0 389.7 18.7 18.6 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 559.8 529.2 N/A N/A Fair value of plan assets 415.0 389.7 N/A N/A
Regulatory Assets	In addition to the amounts recognized in regulatory assets in the above tables for IPL and WPL, regulatory assets were recognized for amounts associated with Corporate Services employees participating in other Alliant Energy sponsored benefit plans that were allocated to IPL and WPL at December 31 as follows (in millions): IPL WPL 2017 2016 2017 2016 Regulatory assets $38.9 $37.3 $28.1 $30.0
Estimated Future Employer Contributions	Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2018 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $6.3 $4.4 $0.3 OPEB plans 9.0 2.0 6.8 (a) Alliant Energy sponsors several non-qualified defined benefit pension plans that cover certain current and former key employees of IPL and WPL. Alliant Energy allocates pension costs to IPL and WPL for these plans. In addition, IPL and WPL amounts reflect funding for their non-bargaining employees who are participants in the Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans.
Expected Benefit Payments	Expected benefit payments for the qualified and non-qualified defined benefit plans, which reflect expected future service, as appropriate, are as follows (in millions): Alliant Energy 2018 2019 2020 2021 2022 2023 - 2027 Defined benefit pension benefits $72.2 $73.9 $76.4 $77.1 $92.9 $398.2 OPEB 18.2 18.4 17.7 17.5 17.2 80.1 $90.4 $92.3 $94.1 $94.6 $110.1 $478.3 IPL 2018 2019 2020 2021 2022 2023 - 2027 Defined benefit pension benefits $33.9 $33.8 $36.5 $36.6 $37.6 $188.4 OPEB 7.1 7.1 7.2 7.1 7.0 32.6 $41.0 $40.9 $43.7 $43.7 $44.6 $221.0 WPL 2018 2019 2020 2021 2022 2023 - 2027 Defined benefit pension benefits $31.4 $32.0 $32.4 $32.4 $32.7 $167.8 OPEB 8.3 8.3 7.5 7.3 7.0 31.8 $39.7 $40.3 $39.9 $39.7 $39.7 $199.6
Recognized Compensation Expense And Income Tax Benefits	A summary of compensation expense, including amounts allocated to IPL and WPL, and the related income tax benefits recognized for share-based compensation awards was as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 Compensation expense $15.1 $18.0 $10.7 $8.3 $9.5 $5.7 $6.4 $7.9 $4.7 Income tax benefits 6.2 7.4 4.4 3.4 4.0 2.4 2.6 3.2 1.9
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value Of Plan Assets By Asset Category, Fair Value Hierarchy Level and Allocations	At December 31, 2017 , the current target ranges and actual allocations for the defined benefit pension plan assets were as follows: Target Range Actual Allocation Allocation Cash and equivalents 0 % - 5% 3% Equity securities - U.S. 11 % - 41% 24% Equity securities - international 14 % - 34% 23% Global asset securities 5 % - 15% 10% Risk parity securities 5 % - 15% 10% Fixed income securities 20 % - 40% 30% At December 31, the fair values of qualified and non-qualified defined benefit pension plan assets were as follows (in millions): 2017 2016 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $28.2 $4.5 $23.7 $— $30.4 $5.0 $25.4 $— Equity securities - U.S. 158.3 158.3 — — 183.6 183.6 — — Equity securities - international 137.5 137.5 — — 97.4 97.4 — — Global asset securities 49.4 49.4 — — 53.0 53.0 — — Fixed income securities 135.9 55.8 80.1 — 125.4 53.6 71.8 — Total assets in fair value hierarchy 509.3 $405.5 $103.8 $— 489.8 $392.6 $97.2 $— Assets measured at net asset value 441.1 405.9 Accrued investment income 1.0 1.1 Due to brokers, net (pending trades with brokers) (0.7 ) (1.1 ) Total pension plan assets $950.7 $895.7 2017 2016 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $13.2 $2.2 $11.0 $— $14.4 $2.4 $12.0 $— Equity securities - U.S. 73.9 73.9 — — 86.5 86.5 — — Equity securities - international 64.2 64.2 — — 45.9 45.9 — — Global asset securities 23.0 23.0 — — 24.9 24.9 — — Fixed income securities 63.4 26.0 37.4 — 59.1 25.3 33.8 — Total assets in fair value hierarchy 237.7 $189.3 $48.4 $— 230.8 $185.0 $45.8 $— Assets measured at net asset value 205.8 191.2 Accrued investment income 0.5 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.5 ) Total pension plan assets $443.7 $422.0 2017 2016 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $12.3 $2.0 $10.3 $— $13.3 $2.2 $11.1 $— Equity securities - U.S. 69.1 69.1 — — 79.9 79.9 — — Equity securities - international 60.0 60.0 — — 42.4 42.4 — — Global asset securities 21.6 21.6 — — 23.0 23.0 — — Fixed income securities 59.3 24.3 35.0 — 54.5 23.3 31.2 — Total assets in fair value hierarchy 222.3 $177.0 $45.3 $— 213.1 $170.8 $42.3 $— Assets measured at net asset value 192.5 176.6 Accrued investment income 0.5 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.5 ) Total pension plan assets $415.0 $389.7
Pension Plans, Defined Benefit [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value Of Plan Assets By Asset Category, Fair Value Hierarchy Level and Allocations	At December 31, the fair values of qualified and non-qualified defined benefit pension plan assets were as follows (in millions): 2017 2016 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $28.2 $4.5 $23.7 $— $30.4 $5.0 $25.4 $— Equity securities - U.S. 158.3 158.3 — — 183.6 183.6 — — Equity securities - international 137.5 137.5 — — 97.4 97.4 — — Global asset securities 49.4 49.4 — — 53.0 53.0 — — Fixed income securities 135.9 55.8 80.1 — 125.4 53.6 71.8 — Total assets in fair value hierarchy 509.3 $405.5 $103.8 $— 489.8 $392.6 $97.2 $— Assets measured at net asset value 441.1 405.9 Accrued investment income 1.0 1.1 Due to brokers, net (pending trades with brokers) (0.7 ) (1.1 ) Total pension plan assets $950.7 $895.7 2017 2016 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $13.2 $2.2 $11.0 $— $14.4 $2.4 $12.0 $— Equity securities - U.S. 73.9 73.9 — — 86.5 86.5 — — Equity securities - international 64.2 64.2 — — 45.9 45.9 — — Global asset securities 23.0 23.0 — — 24.9 24.9 — — Fixed income securities 63.4 26.0 37.4 — 59.1 25.3 33.8 — Total assets in fair value hierarchy 237.7 $189.3 $48.4 $— 230.8 $185.0 $45.8 $— Assets measured at net asset value 205.8 191.2 Accrued investment income 0.5 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.5 ) Total pension plan assets $443.7 $422.0 2017 2016 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $12.3 $2.0 $10.3 $— $13.3 $2.2 $11.1 $— Equity securities - U.S. 69.1 69.1 — — 79.9 79.9 — — Equity securities - international 60.0 60.0 — — 42.4 42.4 — — Global asset securities 21.6 21.6 — — 23.0 23.0 — — Fixed income securities 59.3 24.3 35.0 — 54.5 23.3 31.2 — Total assets in fair value hierarchy 222.3 $177.0 $45.3 $— 213.1 $170.8 $42.3 $— Assets measured at net asset value 192.5 176.6 Accrued investment income 0.5 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.5 ) Total pension plan assets $415.0 $389.7 At December 31, 2017 , the current target ranges and actual allocations for the defined benefit pension plan assets were as follows: Target Range Actual Allocation Allocation Cash and equivalents 0 % - 5% 3% Equity securities - U.S. 11 % - 41% 24% Equity securities - international 14 % - 34% 23% Global asset securities 5 % - 15% 10% Risk parity securities 5 % - 15% 10% Fixed income securities 20 % - 40% 30%
Pension Plans, Defined Benefit [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value Of Plan Assets By Asset Category, Fair Value Hierarchy Level and Allocations	At December 31, 2017 , the current target ranges and actual allocations for the defined benefit pension plan assets were as follows: Target Range Actual Allocation Allocation Cash and equivalents 0 % - 5% 3% Equity securities - U.S. 11 % - 41% 24% Equity securities - international 14 % - 34% 23% Global asset securities 5 % - 15% 10% Risk parity securities 5 % - 15% 10% Fixed income securities 20 % - 40% 30% At December 31, the fair values of qualified and non-qualified defined benefit pension plan assets were as follows (in millions): 2017 2016 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $28.2 $4.5 $23.7 $— $30.4 $5.0 $25.4 $— Equity securities - U.S. 158.3 158.3 — — 183.6 183.6 — — Equity securities - international 137.5 137.5 — — 97.4 97.4 — — Global asset securities 49.4 49.4 — — 53.0 53.0 — — Fixed income securities 135.9 55.8 80.1 — 125.4 53.6 71.8 — Total assets in fair value hierarchy 509.3 $405.5 $103.8 $— 489.8 $392.6 $97.2 $— Assets measured at net asset value 441.1 405.9 Accrued investment income 1.0 1.1 Due to brokers, net (pending trades with brokers) (0.7 ) (1.1 ) Total pension plan assets $950.7 $895.7 2017 2016 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $13.2 $2.2 $11.0 $— $14.4 $2.4 $12.0 $— Equity securities - U.S. 73.9 73.9 — — 86.5 86.5 — — Equity securities - international 64.2 64.2 — — 45.9 45.9 — — Global asset securities 23.0 23.0 — — 24.9 24.9 — — Fixed income securities 63.4 26.0 37.4 — 59.1 25.3 33.8 — Total assets in fair value hierarchy 237.7 $189.3 $48.4 $— 230.8 $185.0 $45.8 $— Assets measured at net asset value 205.8 191.2 Accrued investment income 0.5 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.5 ) Total pension plan assets $443.7 $422.0 2017 2016 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $12.3 $2.0 $10.3 $— $13.3 $2.2 $11.1 $— Equity securities - U.S. 69.1 69.1 — — 79.9 79.9 — — Equity securities - international 60.0 60.0 — — 42.4 42.4 — — Global asset securities 21.6 21.6 — — 23.0 23.0 — — Fixed income securities 59.3 24.3 35.0 — 54.5 23.3 31.2 — Total assets in fair value hierarchy 222.3 $177.0 $45.3 $— 213.1 $170.8 $42.3 $— Assets measured at net asset value 192.5 176.6 Accrued investment income 0.5 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.5 ) Total pension plan assets $415.0 $389.7
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value Of Plan Assets By Asset Category, Fair Value Hierarchy Level and Allocations	At December 31, 2017 , the current target ranges and actual allocations for VEBA trusts with assets greater than $5 million and the WPL 401(h) assets were as follows: Target Range Actual Allocation Allocation Cash and equivalents 0 % - 5% 1% Equity securities - U.S. 0 % - 50% 26% Equity securities - international 0 % - 34% 10% Fixed income securities 20 % - 100% 63% At December 31, the fair values of OPEB plan assets were as follows (in millions): 2017 2016 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $1.2 $0.7 $0.5 $— $3.5 $2.0 $1.5 $— Equity securities - U.S. 27.9 27.9 — — 22.5 22.5 — — Equity securities - international 11.4 11.4 — — 13.5 13.5 — — Global asset securities 0.4 0.4 — — 16.5 16.5 — — Fixed income securities 66.6 66.0 0.6 — 46.8 46.2 0.6 — Total assets in fair value hierarchy 107.5 $106.4 $1.1 $— 102.8 $100.7 $2.1 $— Assets measured at net asset value 3.6 3.0 Total OPEB plan assets $111.1 $105.8 2017 2016 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.3 $0.3 $— $— $0.8 $0.8 $— $— Equity securities - U.S. 22.3 22.3 — — 17.0 17.0 — — Equity securities - international 7.5 7.5 — — 11.0 11.0 — — Global asset securities — — — — 7.0 7.0 — — Fixed income securities 42.8 42.8 — — 32.4 32.4 — — Total OPEB plan assets $72.9 $72.9 $— $— $68.2 $68.2 $— $— 2017 2016 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.6 $0.3 $0.3 $— $2.0 $0.7 $1.3 $— Global asset securities — — — — 5.5 5.5 — — Fixed income securities 18.1 18.1 — — 11.1 11.1 — — Total OPEB plan assets $18.7 $18.4 $0.3 $— $18.6 $17.3 $1.3 $—
Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value Of Plan Assets By Asset Category, Fair Value Hierarchy Level and Allocations	At December 31, 2017 , the current target ranges and actual allocations for VEBA trusts with assets greater than $5 million and the WPL 401(h) assets were as follows: Target Range Actual Allocation Allocation Cash and equivalents 0 % - 5% 1% Equity securities - U.S. 0 % - 50% 26% Equity securities - international 0 % - 34% 10% Fixed income securities 20 % - 100% 63% At December 31, the fair values of OPEB plan assets were as follows (in millions): 2017 2016 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $1.2 $0.7 $0.5 $— $3.5 $2.0 $1.5 $— Equity securities - U.S. 27.9 27.9 — — 22.5 22.5 — — Equity securities - international 11.4 11.4 — — 13.5 13.5 — — Global asset securities 0.4 0.4 — — 16.5 16.5 — — Fixed income securities 66.6 66.0 0.6 — 46.8 46.2 0.6 — Total assets in fair value hierarchy 107.5 $106.4 $1.1 $— 102.8 $100.7 $2.1 $— Assets measured at net asset value 3.6 3.0 Total OPEB plan assets $111.1 $105.8 2017 2016 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.3 $0.3 $— $— $0.8 $0.8 $— $— Equity securities - U.S. 22.3 22.3 — — 17.0 17.0 — — Equity securities - international 7.5 7.5 — — 11.0 11.0 — — Global asset securities — — — — 7.0 7.0 — — Fixed income securities 42.8 42.8 — — 32.4 32.4 — — Total OPEB plan assets $72.9 $72.9 $— $— $68.2 $68.2 $— $— 2017 2016 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.6 $0.3 $0.3 $— $2.0 $0.7 $1.3 $— Global asset securities — — — — 5.5 5.5 — — Fixed income securities 18.1 18.1 — — 11.1 11.1 — — Total OPEB plan assets $18.7 $18.4 $0.3 $— $18.6 $17.3 $1.3 $—
Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value Of Plan Assets By Asset Category, Fair Value Hierarchy Level and Allocations	At December 31, the fair values of OPEB plan assets were as follows (in millions): 2017 2016 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $1.2 $0.7 $0.5 $— $3.5 $2.0 $1.5 $— Equity securities - U.S. 27.9 27.9 — — 22.5 22.5 — — Equity securities - international 11.4 11.4 — — 13.5 13.5 — — Global asset securities 0.4 0.4 — — 16.5 16.5 — — Fixed income securities 66.6 66.0 0.6 — 46.8 46.2 0.6 — Total assets in fair value hierarchy 107.5 $106.4 $1.1 $— 102.8 $100.7 $2.1 $— Assets measured at net asset value 3.6 3.0 Total OPEB plan assets $111.1 $105.8 2017 2016 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.3 $0.3 $— $— $0.8 $0.8 $— $— Equity securities - U.S. 22.3 22.3 — — 17.0 17.0 — — Equity securities - international 7.5 7.5 — — 11.0 11.0 — — Global asset securities — — — — 7.0 7.0 — — Fixed income securities 42.8 42.8 — — 32.4 32.4 — — Total OPEB plan assets $72.9 $72.9 $— $— $68.2 $68.2 $— $— 2017 2016 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.6 $0.3 $0.3 $— $2.0 $0.7 $1.3 $— Global asset securities — — — — 5.5 5.5 — — Fixed income securities 18.1 18.1 — — 11.1 11.1 — — Total OPEB plan assets $18.7 $18.4 $0.3 $— $18.6 $17.3 $1.3 $— At December 31, 2017 , the current target ranges and actual allocations for VEBA trusts with assets greater than $5 million and the WPL 401(h) assets were as follows: Target Range Actual Allocation Allocation Cash and equivalents 0 % - 5% 1% Equity securities - U.S. 0 % - 50% 26% Equity securities - international 0 % - 34% 10% Fixed income securities 20 % - 100% 63%
401(k) Savings Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employees Participate In Defined Contribution Retirement Plans	Costs related to the 401(k) savings plans, which are partially based on the participants’ contributions and include allocated costs associated with Corporate Services employees for IPL and WPL, were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 401(k) costs $24.8 $23.6 $24.9 $12.8 $12.0 $12.7 $11.1 $10.7 $11.2
401(k) Savings Plan [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Employees Participate In Defined Contribution Retirement Plans	Costs related to the 401(k) savings plans, which are partially based on the participants’ contributions and include allocated costs associated with Corporate Services employees for IPL and WPL, were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 401(k) costs $24.8 $23.6 $24.9 $12.8 $12.0 $12.7 $11.1 $10.7 $11.2
401(k) Savings Plan [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Employees Participate In Defined Contribution Retirement Plans	Costs related to the 401(k) savings plans, which are partially based on the participants’ contributions and include allocated costs associated with Corporate Services employees for IPL and WPL, were as follows (in millions): Alliant Energy IPL WPL 2017 2016 2015 2017 2016 2015 2017 2016 2015 401(k) costs $24.8 $23.6 $24.9 $12.8 $12.0 $12.7 $11.1 $10.7 $11.2
Performance Shares and Performance Units [Member]
Defined Benefit Plan Disclosure [Line Items]
Schedule Of Equity-based Compensation Plans Activity	A summary of the performance shares and performance units activity, with amounts representing the target number of awards, was as follows: Performance Shares Performance Units 2017 2016 2015 2017 2016 2015 Nonvested awards, January 1 257,599 288,430 288,848 93,320 116,412 127,330 Granted 65,350 68,585 90,806 21,558 23,918 35,674 Vested (99,438 ) (98,186 ) (91,224 ) (37,395 ) (42,760 ) (45,690 ) Forfeited — (1,230 ) — (5,746 ) (4,250 ) (902 ) Nonvested awards, December 31 223,511 257,599 288,430 71,737 93,320 116,412 Granted Awards - Each performance share’s value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the performance period. For performance units granted in 2017 and 2016, the value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the performance period. For performance units granted in 2015, the value is based on the closing market price of one share of Alliant Energy’s common stock on the grant date of the award. The actual payout for performance shares and performance units is dependent upon actual performance and may range from zero to 200% of the target number of awards. Compensation expense for performance shares and performance units is recorded ratably over the performance period based on the fair value of the awards at each reporting period. Vested Awards - Certain performance shares and performance units vested, resulting in payouts (a combination of cash and common stock for the performance shares and cash only for the performance units) as follows: Performance Shares Performance Units 2017 2016 2015 2017 2016 2015 2014 Grant 2013 Grant 2012 Grant 2014 Grant 2013 Grant 2012 Grant Performance awards vested 99,438 98,186 91,224 37,395 42,760 45,690 Percentage of target number of performance awards 147.5% 165.0% 167.5% 147.5% 165.0% 167.5% Aggregate payout value (in millions) $5.6 $5.1 $5.1 $1.5 $1.7 $1.6 Payout - cash (in millions) $5.1 $2.9 $3.2 $1.5 $1.7 $1.6 Payout - common stock shares issued 5,185 22,408 21,950 N/A N/A N/A Fair Value of Awards - Information related to fair values of nonvested performance shares and performance units at December 31, 2017 , by year of grant, were as follows: Performance Shares Performance Units 2017 Grant 2016 Grant 2015 Grant 2017 Grant 2016 Grant 2015 Grant Nonvested awards at target 65,350 67,355 90,806 18,600 21,227 31,910 Alliant Energy common stock closing price on December 29, 2017 $42.61 $42.61 $42.61 $42.61 $42.61 N/A Alliant Energy common stock closing price on grant date N/A N/A N/A N/A N/A $32.55 Estimated payout percentage based on performance criteria 105 % 150 % 138 % 105 % 150 % 138 % Fair values of each nonvested award $44.74 $63.92 $58.80 $44.74 $63.92 $44.92
Performance Restricted Stock Unit [Member]
Defined Benefit Plan Disclosure [Line Items]
Schedule Of Equity-based Compensation Plans Activity	A summary of the performance restricted stock units activity, with amounts representing the target number of units, was as follows: 2017 2016 Units Weighted Average Grant Date Fair Value Units Weighted Average Grant Date Fair Value Nonvested units, January 1 67,355 $33.96 — $— Granted 65,350 39.12 68,585 33.96 Forfeited — — (1,230 ) 33.90 Nonvested units, December 31 132,705 36.50 67,355 33.96
Restricted Stock Units [Member]
Defined Benefit Plan Disclosure [Line Items]
Schedule Of Equity-based Compensation Plans Activity	A summary of the restricted stock units activity was as follows: 2017 2016 Nonvested units, January 1 57,736 — Granted 56,013 58,790 Forfeited — (1,054 ) Nonvested units, December 31 113,749 57,736

Asset Retirement Obligations (T

Asset Retirement Obligations (Tables)	12 Months Ended
	Dec. 31, 2017
Schedule of Asset Retirement Obligations [Line Items]
Reconciliation Of Changes In Asset Retirement Obligations (AROs)	A reconciliation of the changes in AROs associated with long-lived assets is as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Balance, January 1 $195.7 $214.0 $124.7 $132.9 $61.4 $71.9 Revisions in estimated cash flows 4.3 (13.3 ) 7.0 (5.8 ) (2.7 ) (7.5 ) Liabilities settled (23.5 ) (14.0 ) (13.1 ) (6.8 ) (10.4 ) (7.2 ) Liabilities incurred 2.0 2.6 11.7 0.7 — 1.9 Accretion expense 6.0 6.4 3.8 3.7 2.1 2.3 Balance, December 31 $184.5 $195.7 $134.1 $124.7 $50.4 $61.4
IPL [Member]
Schedule of Asset Retirement Obligations [Line Items]
Reconciliation Of Changes In Asset Retirement Obligations (AROs)	A reconciliation of the changes in AROs associated with long-lived assets is as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Balance, January 1 $195.7 $214.0 $124.7 $132.9 $61.4 $71.9 Revisions in estimated cash flows 4.3 (13.3 ) 7.0 (5.8 ) (2.7 ) (7.5 ) Liabilities settled (23.5 ) (14.0 ) (13.1 ) (6.8 ) (10.4 ) (7.2 ) Liabilities incurred 2.0 2.6 11.7 0.7 — 1.9 Accretion expense 6.0 6.4 3.8 3.7 2.1 2.3 Balance, December 31 $184.5 $195.7 $134.1 $124.7 $50.4 $61.4
WPL [Member]
Schedule of Asset Retirement Obligations [Line Items]
Reconciliation Of Changes In Asset Retirement Obligations (AROs)	A reconciliation of the changes in AROs associated with long-lived assets is as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Balance, January 1 $195.7 $214.0 $124.7 $132.9 $61.4 $71.9 Revisions in estimated cash flows 4.3 (13.3 ) 7.0 (5.8 ) (2.7 ) (7.5 ) Liabilities settled (23.5 ) (14.0 ) (13.1 ) (6.8 ) (10.4 ) (7.2 ) Liabilities incurred 2.0 2.6 11.7 0.7 — 1.9 Accretion expense 6.0 6.4 3.8 3.7 2.1 2.3 Balance, December 31 $184.5 $195.7 $134.1 $124.7 $50.4 $61.4

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Recurring Fair Value Measurements	Carrying amounts and the related estimated fair values of other financial instruments at December 31 were as follows (in millions): Alliant Energy 2017 2016 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $25.1 $— $4.1 $21.0 $25.1 $41.4 $— $4.6 $36.8 $41.4 Deferred proceeds 222.1 — — 222.1 222.1 211.1 — — 211.1 211.1 Liabilities and equity: Derivatives 41.7 — 8.5 33.2 41.7 28.6 — 0.5 28.1 28.6 Long-term debt (incl. current maturities) 4,866.3 — 5,444.6 2.9 5,447.5 4,320.2 — 4,795.7 3.3 4,799.0 Cumulative preferred stock of IPL 200.0 203.8 — — 203.8 200.0 194.8 — — 194.8 IPL 2017 2016 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $17.1 $— $2.0 $15.1 $17.1 $20.8 $— $2.8 $18.0 $20.8 Deferred proceeds 222.1 — — 222.1 222.1 211.1 — — 211.1 211.1 Liabilities and equity: Derivatives 19.4 — 2.9 16.5 19.4 8.3 — 0.4 7.9 8.3 Long-term debt (incl. current maturities) 2,406.0 — 2,665.7 — 2,665.7 2,153.5 — 2,352.3 — 2,352.3 Cumulative preferred stock 200.0 203.8 — — 203.8 200.0 194.8 — — 194.8 WPL 2017 2016 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $8.0 $— $2.1 $5.9 $8.0 $20.6 $— $1.8 $18.8 $20.6 Liabilities and equity: Derivatives 22.3 — 5.6 16.7 22.3 20.3 — 0.1 20.2 20.3 Long-term debt 1,833.4 — 2,147.9 — 2,147.9 1,535.2 — 1,807.4 — 1,807.4
Fair Value Measurements Using Significant Unobservable Inputs	Information for fair value measurements using significant unobservable inputs (Level 3 inputs) was as follows (in millions): Alliant Energy Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2017 2016 2017 2016 Beginning balance, January 1 $8.7 ($32.7 ) $211.1 $172.0 Total net gains (losses) included in changes in net assets (realized/unrealized) (32.9 ) 30.7 — — Transfers into Level 3 — 0.9 — — Transfers out of Level 3 12.2 1.2 — — Purchases 28.3 22.0 — — Sales (0.3 ) (1.0 ) — — Settlements (a) (28.2 ) (12.4 ) 11.0 39.1 Ending balance, December 31 ($12.2 ) $8.7 $222.1 $211.1 The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31 ($31.0 ) $32.7 $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2017 2016 2017 2016 Beginning balance, January 1 $10.1 ($1.9 ) $211.1 $172.0 Total net gains (losses) included in changes in net assets (realized/unrealized) (14.8 ) 7.3 — — Transfers into Level 3 — 0.5 — — Transfers out of Level 3 3.1 0.2 — — Purchases 24.6 20.6 — — Sales (0.2 ) (1.0 ) — — Settlements (a) (24.2 ) (15.6 ) 11.0 39.1 Ending balance, December 31 ($1.4 ) $10.1 $222.1 $211.1 The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31 ($13.5 ) $8.5 $— $— WPL Commodity Contract Derivative Assets and (Liabilities), net 2017 2016 Beginning balance, January 1 ($1.4 ) ($30.8 ) Total net gains (losses) included in changes in net assets (realized/unrealized) (18.1 ) 23.4 Transfers into Level 3 — 0.4 Transfers out of Level 3 9.1 1.0 Purchases 3.7 1.4 Sales (0.1 ) — Settlements (4.0 ) 3.2 Ending balance, December 31 ($10.8 ) ($1.4 ) The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31 ($17.5 ) $24.2 (a) Settlements related to deferred proceeds are due to the change in the carrying amount of receivables sold less the allowance for doubtful accounts associated with the receivables sold and cash amounts received from the receivables sold.
Fair Value Of Net Derivative Assets (Liabilities)	The fair value of electric, natural gas, coal and diesel fuel commodity contracts categorized as Level 3 was recognized as net derivative assets (liabilities) at December 31 as follows (in millions): Alliant Energy IPL WPL Excluding FTRs FTRs Excluding FTRs FTRs Excluding FTRs FTRs 2017 ($23.5 ) $11.3 ($11.5 ) $10.1 ($12.0 ) $1.2 2016 (2.3 ) 11.0 0.1 10.0 (2.4 ) 1.0
IPL [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Recurring Fair Value Measurements	Carrying amounts and the related estimated fair values of other financial instruments at December 31 were as follows (in millions): Alliant Energy 2017 2016 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $25.1 $— $4.1 $21.0 $25.1 $41.4 $— $4.6 $36.8 $41.4 Deferred proceeds 222.1 — — 222.1 222.1 211.1 — — 211.1 211.1 Liabilities and equity: Derivatives 41.7 — 8.5 33.2 41.7 28.6 — 0.5 28.1 28.6 Long-term debt (incl. current maturities) 4,866.3 — 5,444.6 2.9 5,447.5 4,320.2 — 4,795.7 3.3 4,799.0 Cumulative preferred stock of IPL 200.0 203.8 — — 203.8 200.0 194.8 — — 194.8 IPL 2017 2016 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $17.1 $— $2.0 $15.1 $17.1 $20.8 $— $2.8 $18.0 $20.8 Deferred proceeds 222.1 — — 222.1 222.1 211.1 — — 211.1 211.1 Liabilities and equity: Derivatives 19.4 — 2.9 16.5 19.4 8.3 — 0.4 7.9 8.3 Long-term debt (incl. current maturities) 2,406.0 — 2,665.7 — 2,665.7 2,153.5 — 2,352.3 — 2,352.3 Cumulative preferred stock 200.0 203.8 — — 203.8 200.0 194.8 — — 194.8 WPL 2017 2016 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $8.0 $— $2.1 $5.9 $8.0 $20.6 $— $1.8 $18.8 $20.6 Liabilities and equity: Derivatives 22.3 — 5.6 16.7 22.3 20.3 — 0.1 20.2 20.3 Long-term debt 1,833.4 — 2,147.9 — 2,147.9 1,535.2 — 1,807.4 — 1,807.4
Fair Value Measurements Using Significant Unobservable Inputs	Information for fair value measurements using significant unobservable inputs (Level 3 inputs) was as follows (in millions): Alliant Energy Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2017 2016 2017 2016 Beginning balance, January 1 $8.7 ($32.7 ) $211.1 $172.0 Total net gains (losses) included in changes in net assets (realized/unrealized) (32.9 ) 30.7 — — Transfers into Level 3 — 0.9 — — Transfers out of Level 3 12.2 1.2 — — Purchases 28.3 22.0 — — Sales (0.3 ) (1.0 ) — — Settlements (a) (28.2 ) (12.4 ) 11.0 39.1 Ending balance, December 31 ($12.2 ) $8.7 $222.1 $211.1 The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31 ($31.0 ) $32.7 $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2017 2016 2017 2016 Beginning balance, January 1 $10.1 ($1.9 ) $211.1 $172.0 Total net gains (losses) included in changes in net assets (realized/unrealized) (14.8 ) 7.3 — — Transfers into Level 3 — 0.5 — — Transfers out of Level 3 3.1 0.2 — — Purchases 24.6 20.6 — — Sales (0.2 ) (1.0 ) — — Settlements (a) (24.2 ) (15.6 ) 11.0 39.1 Ending balance, December 31 ($1.4 ) $10.1 $222.1 $211.1 The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31 ($13.5 ) $8.5 $— $— WPL Commodity Contract Derivative Assets and (Liabilities), net 2017 2016 Beginning balance, January 1 ($1.4 ) ($30.8 ) Total net gains (losses) included in changes in net assets (realized/unrealized) (18.1 ) 23.4 Transfers into Level 3 — 0.4 Transfers out of Level 3 9.1 1.0 Purchases 3.7 1.4 Sales (0.1 ) — Settlements (4.0 ) 3.2 Ending balance, December 31 ($10.8 ) ($1.4 ) The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31 ($17.5 ) $24.2 (a) Settlements related to deferred proceeds are due to the change in the carrying amount of receivables sold less the allowance for doubtful accounts associated with the receivables sold and cash amounts received from the receivables sold.
Fair Value Of Net Derivative Assets (Liabilities)	The fair value of electric, natural gas, coal and diesel fuel commodity contracts categorized as Level 3 was recognized as net derivative assets (liabilities) at December 31 as follows (in millions): Alliant Energy IPL WPL Excluding FTRs FTRs Excluding FTRs FTRs Excluding FTRs FTRs 2017 ($23.5 ) $11.3 ($11.5 ) $10.1 ($12.0 ) $1.2 2016 (2.3 ) 11.0 0.1 10.0 (2.4 ) 1.0
WPL [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Recurring Fair Value Measurements	Carrying amounts and the related estimated fair values of other financial instruments at December 31 were as follows (in millions): Alliant Energy 2017 2016 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $25.1 $— $4.1 $21.0 $25.1 $41.4 $— $4.6 $36.8 $41.4 Deferred proceeds 222.1 — — 222.1 222.1 211.1 — — 211.1 211.1 Liabilities and equity: Derivatives 41.7 — 8.5 33.2 41.7 28.6 — 0.5 28.1 28.6 Long-term debt (incl. current maturities) 4,866.3 — 5,444.6 2.9 5,447.5 4,320.2 — 4,795.7 3.3 4,799.0 Cumulative preferred stock of IPL 200.0 203.8 — — 203.8 200.0 194.8 — — 194.8 IPL 2017 2016 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $17.1 $— $2.0 $15.1 $17.1 $20.8 $— $2.8 $18.0 $20.8 Deferred proceeds 222.1 — — 222.1 222.1 211.1 — — 211.1 211.1 Liabilities and equity: Derivatives 19.4 — 2.9 16.5 19.4 8.3 — 0.4 7.9 8.3 Long-term debt (incl. current maturities) 2,406.0 — 2,665.7 — 2,665.7 2,153.5 — 2,352.3 — 2,352.3 Cumulative preferred stock 200.0 203.8 — — 203.8 200.0 194.8 — — 194.8 WPL 2017 2016 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $8.0 $— $2.1 $5.9 $8.0 $20.6 $— $1.8 $18.8 $20.6 Liabilities and equity: Derivatives 22.3 — 5.6 16.7 22.3 20.3 — 0.1 20.2 20.3 Long-term debt 1,833.4 — 2,147.9 — 2,147.9 1,535.2 — 1,807.4 — 1,807.4
Fair Value Measurements Using Significant Unobservable Inputs	Information for fair value measurements using significant unobservable inputs (Level 3 inputs) was as follows (in millions): Alliant Energy Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2017 2016 2017 2016 Beginning balance, January 1 $8.7 ($32.7 ) $211.1 $172.0 Total net gains (losses) included in changes in net assets (realized/unrealized) (32.9 ) 30.7 — — Transfers into Level 3 — 0.9 — — Transfers out of Level 3 12.2 1.2 — — Purchases 28.3 22.0 — — Sales (0.3 ) (1.0 ) — — Settlements (a) (28.2 ) (12.4 ) 11.0 39.1 Ending balance, December 31 ($12.2 ) $8.7 $222.1 $211.1 The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31 ($31.0 ) $32.7 $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2017 2016 2017 2016 Beginning balance, January 1 $10.1 ($1.9 ) $211.1 $172.0 Total net gains (losses) included in changes in net assets (realized/unrealized) (14.8 ) 7.3 — — Transfers into Level 3 — 0.5 — — Transfers out of Level 3 3.1 0.2 — — Purchases 24.6 20.6 — — Sales (0.2 ) (1.0 ) — — Settlements (a) (24.2 ) (15.6 ) 11.0 39.1 Ending balance, December 31 ($1.4 ) $10.1 $222.1 $211.1 The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31 ($13.5 ) $8.5 $— $— WPL Commodity Contract Derivative Assets and (Liabilities), net 2017 2016 Beginning balance, January 1 ($1.4 ) ($30.8 ) Total net gains (losses) included in changes in net assets (realized/unrealized) (18.1 ) 23.4 Transfers into Level 3 — 0.4 Transfers out of Level 3 9.1 1.0 Purchases 3.7 1.4 Sales (0.1 ) — Settlements (4.0 ) 3.2 Ending balance, December 31 ($10.8 ) ($1.4 ) The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31 ($17.5 ) $24.2 (a) Settlements related to deferred proceeds are due to the change in the carrying amount of receivables sold less the allowance for doubtful accounts associated with the receivables sold and cash amounts received from the receivables sold.
Fair Value Of Net Derivative Assets (Liabilities)	The fair value of electric, natural gas, coal and diesel fuel commodity contracts categorized as Level 3 was recognized as net derivative assets (liabilities) at December 31 as follows (in millions): Alliant Energy IPL WPL Excluding FTRs FTRs Excluding FTRs FTRs Excluding FTRs FTRs 2017 ($23.5 ) $11.3 ($11.5 ) $10.1 ($12.0 ) $1.2 2016 (2.3 ) 11.0 0.1 10.0 (2.4 ) 1.0

Derivative Instruments (Tables)

Derivative Instruments (Tables)	12 Months Ended
	Dec. 31, 2017
Derivative [Line Items]
Notional Amounts of Derivative Instruments	As of December 31, 2017 , gross notional amounts and settlement/delivery years related to outstanding swap contracts, option contracts, physical forward contracts and FTRs that were accounted for as commodity derivative instruments were as follows (units in thousands): Electricity FTRs Natural Gas Coal Diesel Fuel MWhs Years MWhs Years Dths Years Tons Years Gallons Years Alliant Energy 1,314 2018 8,970 2018 170,463 2018-2026 8,177 2018-2020 6,552 2018-2019 IPL — — 5,886 2018 72,662 2018-2026 3,339 2018-2020 — — WPL 1,314 2018 3,084 2018 97,801 2018-2026 4,838 2018-2020 6,552 2018-2019
Fair Value of Financial Instruments	At December 31, the fair values of current derivative assets are included in “Other current assets,” non-current derivative assets are included in “Deferred charges and other,” current derivative liabilities are included in “Other current liabilities” and non-current derivative liabilities are included in “Other liabilities” on the balance sheets as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Current derivative assets $21.1 $29.4 $15.8 $19.1 $5.3 $10.3 Non-current derivative assets 4.0 12.0 1.3 1.7 2.7 10.3 Current derivative liabilities 18.7 13.3 5.0 2.7 13.7 10.6 Non-current derivative liabilities 23.0 15.3 14.4 5.6 8.6 9.7
IPL [Member]
Derivative [Line Items]
Notional Amounts of Derivative Instruments	As of December 31, 2017 , gross notional amounts and settlement/delivery years related to outstanding swap contracts, option contracts, physical forward contracts and FTRs that were accounted for as commodity derivative instruments were as follows (units in thousands): Electricity FTRs Natural Gas Coal Diesel Fuel MWhs Years MWhs Years Dths Years Tons Years Gallons Years Alliant Energy 1,314 2018 8,970 2018 170,463 2018-2026 8,177 2018-2020 6,552 2018-2019 IPL — — 5,886 2018 72,662 2018-2026 3,339 2018-2020 — — WPL 1,314 2018 3,084 2018 97,801 2018-2026 4,838 2018-2020 6,552 2018-2019
Fair Value of Financial Instruments	At December 31, the fair values of current derivative assets are included in “Other current assets,” non-current derivative assets are included in “Deferred charges and other,” current derivative liabilities are included in “Other current liabilities” and non-current derivative liabilities are included in “Other liabilities” on the balance sheets as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Current derivative assets $21.1 $29.4 $15.8 $19.1 $5.3 $10.3 Non-current derivative assets 4.0 12.0 1.3 1.7 2.7 10.3 Current derivative liabilities 18.7 13.3 5.0 2.7 13.7 10.6 Non-current derivative liabilities 23.0 15.3 14.4 5.6 8.6 9.7
WPL [Member]
Derivative [Line Items]
Notional Amounts of Derivative Instruments	As of December 31, 2017 , gross notional amounts and settlement/delivery years related to outstanding swap contracts, option contracts, physical forward contracts and FTRs that were accounted for as commodity derivative instruments were as follows (units in thousands): Electricity FTRs Natural Gas Coal Diesel Fuel MWhs Years MWhs Years Dths Years Tons Years Gallons Years Alliant Energy 1,314 2018 8,970 2018 170,463 2018-2026 8,177 2018-2020 6,552 2018-2019 IPL — — 5,886 2018 72,662 2018-2026 3,339 2018-2020 — — WPL 1,314 2018 3,084 2018 97,801 2018-2026 4,838 2018-2020 6,552 2018-2019
Fair Value of Financial Instruments	At December 31, the fair values of current derivative assets are included in “Other current assets,” non-current derivative assets are included in “Deferred charges and other,” current derivative liabilities are included in “Other current liabilities” and non-current derivative liabilities are included in “Other liabilities” on the balance sheets as follows (in millions): Alliant Energy IPL WPL 2017 2016 2017 2016 2017 2016 Current derivative assets $21.1 $29.4 $15.8 $19.1 $5.3 $10.3 Non-current derivative assets 4.0 12.0 1.3 1.7 2.7 10.3 Current derivative liabilities 18.7 13.3 5.0 2.7 13.7 10.6 Non-current derivative liabilities 23.0 15.3 14.4 5.6 8.6 9.7

Commitments And Contingencies (

Commitments And Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Long-term Purchase Commitment [Line Items]
Other Purchase Obligations	At December 31, 2017 , minimum future commitments related to these purchase obligations were as follows (in millions): Alliant Energy 2018 2019 2020 2021 2022 Thereafter Total Purchased power (a) $188 $159 $135 $149 $140 $461 $1,232 Natural gas 229 140 130 110 84 253 946 Coal (b) 107 59 21 5 — — 192 Other (c) 21 6 6 3 2 2 40 $545 $364 $292 $267 $226 $716 $2,410 IPL 2018 2019 2020 2021 2022 Thereafter Total Purchased power (a) $130 $144 $135 $149 $140 $461 $1,159 Natural gas 126 58 44 32 22 102 384 Coal (b) 51 31 11 5 — — 98 Other (c) 15 3 3 3 2 2 28 $322 $236 $193 $189 $164 $565 $1,669 WPL 2018 2019 2020 2021 2022 Thereafter Total Purchased power (a) $58 $15 $— $— $— $— $73 Natural gas 103 82 86 78 62 151 562 Coal (b) 56 28 10 — — — 94 Other (c) 5 1 — — — — 6 $222 $126 $96 $78 $62 $151 $735 (a) Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. (b) Corporate Services entered into system-wide coal contracts on behalf of IPL and WPL that include minimum future commitments. These commitments were assigned to IPL and WPL based on information available as of December 31, 2017 regarding expected future usage, which is subject to change. (c) Includes individual commitments incurred during the normal course of business that exceeded $1 million at December 31, 2017 .
Schedule of Environmental Liabilities	At December 31, 2017 , estimated future costs expected to be incurred for the investigation, remediation and monitoring of the MGP sites, as well as environmental liabilities recorded on the balance sheets for these sites, which are not discounted, were as follows (in millions). At December 31, 2017 , such amounts for WPL were not material. Alliant Energy IPL Range of estimated future costs $11 - $30 $9 - $27 Current and non-current environmental liabilities 15 13
IPL [Member]
Long-term Purchase Commitment [Line Items]
Other Purchase Obligations	At December 31, 2017 , minimum future commitments related to these purchase obligations were as follows (in millions): Alliant Energy 2018 2019 2020 2021 2022 Thereafter Total Purchased power (a) $188 $159 $135 $149 $140 $461 $1,232 Natural gas 229 140 130 110 84 253 946 Coal (b) 107 59 21 5 — — 192 Other (c) 21 6 6 3 2 2 40 $545 $364 $292 $267 $226 $716 $2,410 IPL 2018 2019 2020 2021 2022 Thereafter Total Purchased power (a) $130 $144 $135 $149 $140 $461 $1,159 Natural gas 126 58 44 32 22 102 384 Coal (b) 51 31 11 5 — — 98 Other (c) 15 3 3 3 2 2 28 $322 $236 $193 $189 $164 $565 $1,669 WPL 2018 2019 2020 2021 2022 Thereafter Total Purchased power (a) $58 $15 $— $— $— $— $73 Natural gas 103 82 86 78 62 151 562 Coal (b) 56 28 10 — — — 94 Other (c) 5 1 — — — — 6 $222 $126 $96 $78 $62 $151 $735 (a) Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. (b) Corporate Services entered into system-wide coal contracts on behalf of IPL and WPL that include minimum future commitments. These commitments were assigned to IPL and WPL based on information available as of December 31, 2017 regarding expected future usage, which is subject to change. (c) Includes individual commitments incurred during the normal course of business that exceeded $1 million at December 31, 2017 .
Schedule of Environmental Liabilities	At December 31, 2017 , estimated future costs expected to be incurred for the investigation, remediation and monitoring of the MGP sites, as well as environmental liabilities recorded on the balance sheets for these sites, which are not discounted, were as follows (in millions). At December 31, 2017 , such amounts for WPL were not material. Alliant Energy IPL Range of estimated future costs $11 - $30 $9 - $27 Current and non-current environmental liabilities 15 13
WPL [Member]
Long-term Purchase Commitment [Line Items]
Other Purchase Obligations	At December 31, 2017 , minimum future commitments related to these purchase obligations were as follows (in millions): Alliant Energy 2018 2019 2020 2021 2022 Thereafter Total Purchased power (a) $188 $159 $135 $149 $140 $461 $1,232 Natural gas 229 140 130 110 84 253 946 Coal (b) 107 59 21 5 — — 192 Other (c) 21 6 6 3 2 2 40 $545 $364 $292 $267 $226 $716 $2,410 IPL 2018 2019 2020 2021 2022 Thereafter Total Purchased power (a) $130 $144 $135 $149 $140 $461 $1,159 Natural gas 126 58 44 32 22 102 384 Coal (b) 51 31 11 5 — — 98 Other (c) 15 3 3 3 2 2 28 $322 $236 $193 $189 $164 $565 $1,669 WPL 2018 2019 2020 2021 2022 Thereafter Total Purchased power (a) $58 $15 $— $— $— $— $73 Natural gas 103 82 86 78 62 151 562 Coal (b) 56 28 10 — — — 94 Other (c) 5 1 — — — — 6 $222 $126 $96 $78 $62 $151 $735 (a) Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. (b) Corporate Services entered into system-wide coal contracts on behalf of IPL and WPL that include minimum future commitments. These commitments were assigned to IPL and WPL based on information available as of December 31, 2017 regarding expected future usage, which is subject to change. (c) Includes individual commitments incurred during the normal course of business that exceeded $1 million at December 31, 2017 .

Segments Of Business (Tables)

Segments Of Business (Tables)	12 Months Ended
	Dec. 31, 2017
Segment Reporting Information [Line Items]
Schedule of Segments of Business	Certain financial information relating to Alliant Energy’s business segments, which represent the services provided to its customers, was as follows (in millions): ATC Investment, Utility Non-utility, Alliant Energy 2017 Electric Gas Other Total Parent and Other Consolidated Operating revenues $2,894.7 $400.9 $47.5 $3,343.1 $39.1 $3,382.2 Depreciation and amortization 412.0 38.2 7.7 457.9 3.9 461.8 Operating income (loss) 586.5 45.3 (11.7 ) 620.1 33.3 653.4 Interest expense 206.2 9.4 215.6 Equity income from unconsolidated investments, net (0.7 ) — — (0.7 ) (44.1 ) (44.8 ) Income taxes 51.0 15.7 66.7 Net income attributable to Alliant Energy common shareowners 403.4 53.9 457.3 Total assets 11,396.2 1,199.8 766.5 13,362.5 825.3 14,187.8 Investments in equity method subsidiaries 8.3 — — 8.3 373.1 381.4 Construction and acquisition expenditures 1,154.9 125.2 1.7 1,281.8 185.1 1,466.9 ATC Investment, Utility Non-utility, Alliant Energy 2016 Electric Gas Other Total Parent and Other Consolidated Operating revenues $2,875.5 $355.4 $48.6 $3,279.5 $40.5 $3,320.0 Depreciation and amortization 367.0 34.2 2.1 403.3 8.3 411.6 Operating income (loss) 571.9 30.7 (4.8 ) 597.8 (60.8 ) 537.0 Interest expense 194.6 1.6 196.2 Equity income from unconsolidated investments, net (0.7 ) — — (0.7 ) (38.9 ) (39.6 ) Income tax expense (benefit) 71.4 (12.0 ) 59.4 Net income (loss) attributable to Alliant Energy common shareowners 385.2 (13.7 ) 371.5 Total assets 10,722.9 1,091.1 781.0 12,595.0 778.8 13,373.8 Investments in equity method subsidiaries 7.7 — — 7.7 318.3 326.0 Construction and acquisition expenditures 994.0 137.1 0.1 1,131.2 65.6 1,196.8 ATC Investment, Utility Non-utility, Alliant Energy 2015 Electric Gas Other Total Parent and Other Consolidated Operating revenues $2,770.5 $381.2 $57.9 $3,209.6 $44.0 $3,253.6 Depreciation and amortization 358.6 31.1 1.8 391.5 9.8 401.3 Operating income 514.1 34.6 1.9 550.6 26.4 577.0 Interest expense 189.2 (2.1 ) 187.1 Equity income from unconsolidated investments, net (0.9 ) — — (0.9 ) (32.9 ) (33.8 ) Income taxes 46.2 24.2 70.4 Net income attributable to Alliant Energy common shareowners 343.4 34.8 378.2 Total assets 9,918.0 939.3 828.9 11,686.2 809.0 12,495.2 Investments in equity method subsidiaries 8.7 — — 8.7 294.2 302.9 Construction and acquisition expenditures 852.5 106.4 1.4 960.3 74.0 1,034.3
IPL [Member]
Segment Reporting Information [Line Items]
Schedule of Segments of Business	Certain financial information relating to IPL’s business segments, which represent the services provided to its customers, was as follows (in millions): 2017 Electric Gas Other Total Operating revenues $1,598.9 $226.0 $45.4 $1,870.3 Depreciation and amortization 215.1 22.2 7.7 245.0 Operating income (loss) 281.1 20.8 (5.0 ) 296.9 Interest expense 112.4 Income tax benefit (10.9 ) Earnings available for common stock 216.8 Total assets 6,524.4 727.9 353.7 7,606.0 Construction and acquisition expenditures 594.1 80.7 1.2 676.0 2016 Electric Gas Other Total Operating revenues $1,569.7 $204.0 $46.7 $1,820.4 Depreciation and amortization 189.4 19.3 2.1 210.8 Operating income 252.0 15.5 3.3 270.8 Interest expense 103.2 Income tax benefit (5.9 ) Earnings available for common stock 215.6 Total assets 6,278.2 653.3 373.2 7,304.7 Construction and acquisition expenditures 598.1 91.5 0.1 689.7 2015 Electric Gas Other Total Operating revenues $1,503.8 $217.3 $53.4 $1,774.5 Depreciation and amortization 187.9 17.5 1.8 207.2 Operating income 218.8 17.7 5.4 241.9 Interest expense 96.8 Income tax benefit (22.7 ) Earnings available for common stock 186.0 Total assets 5,754.1 548.2 406.8 6,709.1 Construction and acquisition expenditures 561.2 56.7 1.4 619.3
WPL [Member]
Segment Reporting Information [Line Items]
Schedule of Segments of Business	Certain financial information relating to WPL’s business segments, which represent the services provided to its customers, was as follows (in millions): 2017 Electric Gas Other Total Operating revenues $1,295.8 $174.9 $2.1 $1,472.8 Depreciation and amortization 196.9 16.0 — 212.9 Operating income (loss) 305.4 24.5 (6.7 ) 323.2 Interest expense 93.8 Equity income from unconsolidated investments (0.7 ) — — (0.7 ) Income taxes 61.9 Earnings available for common stock 186.6 Total assets 4,871.8 471.9 412.8 5,756.5 Investments in equity method subsidiaries 8.3 — — 8.3 Construction and acquisition expenditures 592.4 44.5 0.5 637.4 2016 Electric Gas Other Total Operating revenues $1,305.8 $151.4 $1.9 $1,459.1 Depreciation and amortization 177.6 14.9 — 192.5 Operating income (loss) 319.9 15.2 (8.1 ) 327.0 Interest expense 91.4 Equity income from unconsolidated investments (0.7 ) — (39.1 ) (39.8 ) Income taxes 93.3 Earnings available for common stock 190.4 Total assets 4,444.7 437.8 407.8 5,290.3 Investments in equity method subsidiaries 7.7 — — 7.7 Construction and acquisition expenditures 395.9 45.6 11.5 453.0 2015 Electric Gas Other Total Operating revenues $1,266.7 $163.9 $4.5 $1,435.1 Depreciation and amortization 170.7 13.6 — 184.3 Operating income (loss) 295.3 16.9 (3.5 ) 308.7 Interest expense 92.4 Equity income from unconsolidated investments (0.9 ) — (34.2 ) (35.1 ) Income taxes 82.9 Earnings available for common stock 176.3 Total assets 4,163.9 391.1 715.4 5,270.4 Investments in equity method subsidiaries 8.7 — 293.3 302.0 Construction and acquisition expenditures 291.3 49.7 3.3 344.3

Related Parties (Tables)

Related Parties (Tables)	12 Months Ended
	Dec. 31, 2017
Related Party Transaction [Line Items]
Services Provided, Sales Credited And Purchases Billed	The amounts billed for services provided, sales credited and purchases were as follows (in millions): IPL WPL 2017 2016 2015 2017 2016 2015 Corporate Services billings $177 $161 $150 $135 $133 $121 Sales credited 23 8 10 13 7 24 Purchases billed 364 433 366 115 102 66
Net Intercompany Payables	As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2017 2016 IPL $114 $104 WPL 61 72
Related Amounts Billed Between Parties	The related amounts billed between the parties were as follows (in millions): 2017 2016 2015 ATC billings to WPL $105 $110 $101 WPL billings to ATC 10 13 13
IPL [Member]
Related Party Transaction [Line Items]
Services Provided, Sales Credited And Purchases Billed	The amounts billed for services provided, sales credited and purchases were as follows (in millions): IPL WPL 2017 2016 2015 2017 2016 2015 Corporate Services billings $177 $161 $150 $135 $133 $121 Sales credited 23 8 10 13 7 24 Purchases billed 364 433 366 115 102 66
Net Intercompany Payables	As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2017 2016 IPL $114 $104 WPL 61 72
WPL [Member]
Related Party Transaction [Line Items]
Services Provided, Sales Credited And Purchases Billed	The amounts billed for services provided, sales credited and purchases were as follows (in millions): IPL WPL 2017 2016 2015 2017 2016 2015 Corporate Services billings $177 $161 $150 $135 $133 $121 Sales credited 23 8 10 13 7 24 Purchases billed 364 433 366 115 102 66
Net Intercompany Payables	As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2017 2016 IPL $114 $104 WPL 61 72
Related Amounts Billed Between Parties	The related amounts billed between the parties were as follows (in millions): 2017 2016 2015 ATC billings to WPL $105 $110 $101 WPL billings to ATC 10 13 13

Selected Consolidated Quarter48

Selected Consolidated Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2017
Components Of Selected Consolidated Quarterly Financial Data	All “per share” references refer to earnings per diluted share. Summation of the individual quarters may not equal annual totals due to rounding. 2017 2016 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions, except per share data) Operating revenues $853.9 $765.3 $906.9 $856.1 $843.8 $754.6 $924.6 $797.0 Operating income 142.9 149.3 231.5 129.7 145.9 128.6 162.6 99.9 Amounts attributable to Alliant Energy common shareowners: Income from continuing operations, net of tax 99.0 94.3 168.8 93.8 97.6 84.4 128.8 63.0 Income (loss) from discontinued operations, net of tax 1.4 — — — (1.1 ) (0.5 ) (0.4 ) (0.3 ) Net income 100.4 94.3 168.8 93.8 96.5 83.9 128.4 62.7 Earnings per weighted average common share attributable to Alliant Energy common shareowners: Income from continuing operations, net of tax 0.43 0.41 0.73 0.41 0.43 0.37 0.57 0.28 Income from discontinued operations, net of tax 0.01 — — — — — — — Net income 0.44 0.41 0.73 0.41 0.43 0.37 0.57 0.28
IPL [Member]
Components Of Selected Consolidated Quarterly Financial Data	Earnings per share data is not disclosed for IPL given Alliant Energy is the sole shareowner of all shares of IPL’s common stock outstanding during the periods presented. 2017 2016 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Operating revenues $450.5 $420.2 $527.4 $472.2 $458.7 $411.0 $516.2 $434.5 Operating income 49.6 66.3 131.8 49.2 62.0 48.0 125.9 34.9 Net income 39.8 45.3 123.0 18.9 48.2 34.4 116.7 26.5 Earnings available for common stock 37.2 42.8 120.4 16.4 45.6 31.9 114.1 24.0
WPL [Member]
Components Of Selected Consolidated Quarterly Financial Data	Earnings per share data is not disclosed for WPL given Alliant Energy is the sole shareowner of all shares of WPL’s common stock outstanding during the periods presented. 2017 2016 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Operating revenues $393.1 $334.8 $370.2 $374.7 $375.6 $334.3 $397.0 $352.2 Operating income 86.0 73.7 90.7 72.8 78.8 75.0 115.0 58.2 Net income 45.5 38.1 49.8 53.2 47.0 43.7 69.6 32.5 Earnings available for common stock 45.5 38.1 49.8 53.2 46.5 43.2 69.0 31.7

Condensed Parent Company Fina49

Condensed Parent Company Financial Statements (Tables)	12 Months Ended
	Dec. 31, 2017
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Condensed Statements of Income	ALLIANT ENERGY CORPORATION (Parent Company Only) CONDENSED STATEMENTS OF INCOME Year Ended December 31, 2017 2016 2015 (in millions) Operating revenues $— $1 $2 Operating expenses 2 3 3 Operating loss (2 ) (2 ) (1 ) Interest expense and other: Equity earnings from consolidated subsidiaries (457 ) (374 ) (379 ) Interest expense 3 3 3 Interest income — (2 ) (3 ) Total interest expense and other (454 ) (373 ) (379 ) Income before income taxes 452 371 378 Income tax benefit (6 ) (1 ) (1 ) Net income $458 $372 $379 The accompanying Notes to Condensed Financial Statements are an integral part of these statements.
Condensed Balance Sheets	ALLIANT ENERGY CORPORATION (Parent Company Only) CONDENSED BALANCE SHEETS December 31, 2017 2016 (in millions) ASSETS Current assets: Notes receivable from affiliated companies $50 $74 Other 7 5 Total current assets 57 79 Investments: Investments in consolidated subsidiaries 4,676 4,211 Other 2 2 Total investments 4,678 4,213 Other assets 78 64 Total assets $4,813 $4,356 LIABILITIES AND EQUITY Current liabilities: Commercial paper $295 $192 Notes payable to affiliated companies 305 275 Other 12 12 Total current liabilities 612 479 Other liabilities 20 18 Common equity: Common stock and additional paid-in capital 1,848 1,695 Retained earnings 2,344 2,174 Shares in deferred compensation trust (11 ) (10 ) Total common equity 4,181 3,859 Total liabilities and equity $4,813 $4,356 The accompanying Notes to Condensed Financial Statements are an integral part of these statements.
Condensed Statements of Cash Flows	ALLIANT ENERGY CORPORATION (Parent Company Only) CONDENSED STATEMENTS OF CASH FLOWS Year Ended December 31, 2017 2016 2015 (in millions) Net cash flows from operating activities $273 $254 $262 Cash flows from (used for) investing activities: Capital contributions to consolidated subsidiaries (290 ) (250 ) (165 ) Capital repayments from consolidated subsidiaries — 130 — Net change in notes receivable from and payable to affiliates 54 294 2 Other — 10 — Net cash flows from (used for) investing activities (236 ) 184 (163 ) Cash flows used for financing activities: Common stock dividends (288 ) (267 ) (247 ) Proceeds from issuance of common stock, net 150 27 151 Payments to retire long-term debt — (250 ) — Net change in commercial paper 103 52 (1 ) Other (2 ) — (2 ) Net cash flows used for financing activities (37 ) (438 ) (99 ) Net increase (decrease) in cash and cash equivalents — — — Cash and cash equivalents at beginning of period — — — Cash and cash equivalents at end of period $— $— $— Supplemental cash flows information: Cash paid during the period for: Interest, net of capitalized interest ($3 ) ($3 ) ($3 ) Income taxes, net — (37 ) (9 ) The accompanying Notes to Condensed Financial Statements are an integral part of these statements.

Summary Of Significant Accoun50

Summary Of Significant Accounting Policies (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017USD ($)customerMW	Dec. 31, 2016	Dec. 31, 2015
Adjustments for New Accounting Principle, Early Adoption [Member]
New Accounting Standards:
Decrease to deferred tax liabilities due to adoption of new accounting standard | $		$ (3.1)
Increase to retained earnings due to adoption of new accounting standard | $		$ 3.1
IPL [Member]
General:
Generation and distribution of steam, number of customers served (in customers) | customer		2
Property, Plant and Equipment:
AFUDC accrual recorded, percentage of estimated CWIP		100.00%
Operating Revenues:
Estimated recovery of overaccrued operating revenue, reflected on customer billing, maximum period		2 years
WPL [Member]
Property, Plant and Equipment:
AFUDC accrual recorded, percentage of estimated CWIP		50.00%
AFUDC rates, projects with approval		100.00%
Operating Revenues:
Final settlement recovery of overaccrued operating revenue, reflected on customer billing, maximum period		2 years
Alliant Energy Finance, LLC [Member]
General:
Ownership interest		50.00%
Electric capacity of wind farm (in megawatts) | MW		225
Sheboygan Falls Energy Facility [Member]
General:
Fossil-fueled EGU capacity (in megawatts) | MW		347
Capital lease, lease term		20 years
Electric - generation [Member] | IPL [Member]
Property, Plant and Equipment:
Average rate of depreciation		3.50%	3.50%	3.60%
Electric - generation [Member] | WPL [Member]
Property, Plant and Equipment:
Average rate of depreciation		3.50%	3.10%	3.20%
Electric - distribution [Member] | IPL [Member]
Property, Plant and Equipment:
Average rate of depreciation		2.40%	2.40%	2.40%
Electric - distribution [Member] | WPL [Member]
Property, Plant and Equipment:
Average rate of depreciation		2.60%	2.60%	2.70%
Scenario, Forecast [Member] | Electric - generation [Member] | IPL [Member]
Property, Plant and Equipment:
Average rate of depreciation	3.80%
Scenario, Forecast [Member] | Electric - distribution [Member] | IPL [Member]
Property, Plant and Equipment:
Average rate of depreciation	2.80%

Summary Of Significant Accoun51

Summary Of Significant Accounting Policies (Schedule Of Average Rates Of Depreciation) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Electric - generation [Member] | IPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	3.50%	3.50%	3.60%
Electric - generation [Member] | WPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	3.50%	3.10%	3.20%
Electric - distribution [Member] | IPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	2.40%	2.40%	2.40%
Electric - distribution [Member] | WPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	2.60%	2.60%	2.70%
Electric - other [Member] | IPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	4.50%	4.20%	4.00%
Electric - other [Member] | WPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	6.90%	4.70%	4.50%
Gas [Member] | IPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	3.40%	3.30%	3.20%
Gas [Member] | WPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	2.50%	2.50%	2.50%
Other [Member] | IPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	4.00%	3.90%	3.90%
Other [Member] | WPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	6.00%	5.90%	6.00%

Summary Of Significant Accoun52

Summary Of Significant Accounting Policies (Schedule Of Allowance For Funds Used During Construction Recovery Rate) (Details) - MW		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
IPL [Member] | IPL (Marshalltown CWIP) [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Recovery rates	[1]	7.80%	7.90%	7.90%
IPL [Member] | IPL (Wind generation CWIP) [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Recovery rates	[2]	7.60%
IPL [Member] | IPL (other CWIP) [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Recovery rates		7.60%	7.70%	7.70%
IPL [Member] | Marshalltown Generating Station [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Return on common equity for calculation of AFUDC		10.30%
IPL [Member] | Expansion of Wind Generation [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Return on common equity for calculation of AFUDC		10.00%
Electric capacity of wind farm (in megawatts)		500
WPL [Member] | WPL (retail jurisdiction) [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Recovery rates		7.60%	8.20%	8.20%
WPL [Member] | WPL (wholesale jurisdiction) [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Recovery rates		6.00%	6.70%	7.90%
WPL [Member] | Expansion of Wind Generation [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Electric capacity of wind farm (in megawatts)		129
[1]	In 2013, the IUB issued an order establishing rate-making principles that require a 10.3% return on common equity for the calculation of AFUDC related to the construction of Marshalltown.
[2]	In 2016, the IUB issued an order establishing rate-making principles that require a return on common equity for the calculation of AFUDC related to the construction of up to 500 MW of new wind generation equal to the greater of 10.0% or whatever percentage the IUB finds reasonable during IPL’s most recent retail electric rate proceeding.

Regulatory Matters (Narrative)

Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Regulatory Matters [Line Items]
Increase (decrease) in regulatory liabilities		$ 83.8	$ 63	$ 67.8
Depreciation and amortization		461.8	411.6	401.3
Regulatory liabilities		1,497.2	681
Regulatory assets		1,666.7	1,915.1
Commodity cost recovery [Member]
Regulatory Matters [Line Items]
Regulatory liabilities		21	30.8
IPL [Member]
Regulatory Matters [Line Items]
Regulatory assets not earning a return		56
Increase (decrease) in regulatory liabilities		71.2	67.3	75.5
Other operation and maintenance		403.8	383.7	389.9
Depreciation and amortization		245	210.8	207.2
Regulatory liabilities		755.4	430.8
Regulatory assets		1,231.6	1,458.8
IPL [Member] | Commodity cost recovery [Member]
Regulatory Matters [Line Items]
Regulatory liabilities		14.6	17.8
WPL [Member]
Regulatory Matters [Line Items]
Regulatory assets not earning a return		5
Other operation and maintenance		249	219.8	235.4
Depreciation and amortization		212.9	192.5	$ 184.3
Regulatory liabilities		$ 741.8	250.2
Annual bandwidth for fuel-related costs		2.00%
Regulatory assets		$ 435.1	456.3
WPL [Member] | Commodity cost recovery [Member]
Regulatory Matters [Line Items]
Regulatory liabilities		6.4	13
Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Matters [Line Items]
Increase (decrease) in regulatory liabilities		$ 17
Scenario, Forecast [Member] | IPL [Member]
Regulatory Matters [Line Items]
Inflation-adjusted average of actual and estimated costs recovery period	5 years
Nelson Dewey Units 1 and 2 and Edgewater Unit 3 [Member] | WPL [Member]
Regulatory Matters [Line Items]
Expenditure collection period after new rate implementation		10 years
2016 Test Year Retail Electric [Member] | IPL [Member]
Regulatory Matters [Line Items]
Interim rate increase (decrease), amount		$ 102
Interim rate increase (decrease), percentage		7.00%
Write-down of regulatory assets		$ 9
2016 Test Year Retail Electric [Member] | Alliant Energy and IPL [Member]
Regulatory Matters [Line Items]
Increase in base rates		77
2016 Test Year Retail Electric [Member] | Scenario, Forecast [Member] | IPL [Member]
Regulatory Matters [Line Items]
Settlement agreement rate increase, amount	$ 130
Settlement agreement rate increase, percentage	9.00%
2017/2018 Test Period Retail Electric [Member] | WPL [Member]
Regulatory Matters [Line Items]
Increase in base rates		60
Authorized increase (decrease) in final rates, amount		9
Reduction in fuel-related costs		51
2017/2018 Test Period Retail Electric [Member] | Alliant Energy and WPL [Member]
Regulatory Matters [Line Items]
Increase in base rates		63
2017/2018 Test Period Retail Gas [Member] | WPL [Member]
Regulatory Matters [Line Items]
Authorized increase (decrease) in final rates, amount		9
2017/2018 Test Period Retail Gas [Member] | Alliant Energy and WPL [Member]
Regulatory Matters [Line Items]
Increase in base rates		9
2015/2016 Test Period Retail Electric And Gas [Member] | WPL [Member]
Regulatory Matters [Line Items]
Authorized increase (decrease) in final rates, amount		(5)
2015/2016 Test Period Retail Electric And Gas [Member] | Alliant Energy and WPL [Member] | Deferral [Member]
Regulatory Matters [Line Items]
Regulatory liabilities		5
2016 Test Year Retail Electric Fuel Related [Member] | WPL [Member] | Commodity cost recovery [Member]
Regulatory Matters [Line Items]
Regulatory liabilities		3
Regulatory assets		$ 6
2018 Test Year Retail Electric Fuel Related [Member] | Scenario, Forecast [Member] | WPL [Member]
Regulatory Matters [Line Items]
Authorized increase (decrease) in final rates, amount	$ 6
Autorized increase in final rates, percentage	1.00%
FERC [Member] | MISO Transmission Owner Complaints [Member]
Regulatory Matters [Line Items]
Base return on equity for refund period from November 12, 2013 through February 11, 2015, percent		10.32%
Refund received, after final true-up		$ 50
FERC [Member] | MISO Transmission Owner Complaints [Member] | IPL [Member]
Regulatory Matters [Line Items]
Refund received, after final true-up		39
FERC [Member] | MISO Transmission Owner Complaints [Member] | WPL [Member]
Regulatory Matters [Line Items]
Refund received, after final true-up		11
Forward Contracts [Member] | IUB [Member] | Scenario, Forecast [Member] | IPL [Member]
Regulatory Matters [Line Items]
Recovery period, authorized	10 years
Sutherland Units One And Three [Member] | 2016 Test Year Retail Electric [Member] | IPL [Member]
Regulatory Matters [Line Items]
Other operation and maintenance		$ 4
Sutherland Units One And Three [Member] | IUB [Member] | Scenario, Forecast [Member] | IPL [Member]
Regulatory Matters [Line Items]
Recovery period, authorized	10 years
Sutherland Units One And Three [Member] | FERC [Member] | IPL [Member]
Regulatory Matters [Line Items]
Recovery period, authorized		10 years
AROs [Member]
Regulatory Matters [Line Items]
Regulatory assets		$ 109.3	105.9
AROs [Member] | IPL [Member]
Regulatory Matters [Line Items]
Regulatory assets		72.5	64.3
AROs [Member] | WPL [Member]
Regulatory Matters [Line Items]
Regulatory assets		36.8	41.6
AROs [Member] | 2016 Test Year Retail Electric [Member] | IPL [Member]
Regulatory Matters [Line Items]
Depreciation and amortization		5
AROs [Member] | IUB [Member] | Scenario, Forecast [Member] | IPL [Member]
Regulatory Matters [Line Items]
Recovery period, authorized	4 years
Other [Member]
Regulatory Matters [Line Items]
Regulatory assets		96.6	76.6
Other [Member] | IPL [Member]
Regulatory Matters [Line Items]
Regulatory assets		55.3	41.9
Other [Member] | WPL [Member]
Regulatory Matters [Line Items]
Regulatory assets		41.3	$ 34.7
Other [Member] | 2017 Test Year Retail Electric Fuel Related [Member] | Alliant Energy and WPL [Member]
Regulatory Matters [Line Items]
Regulatory assets		$ 5

Regulatory Matters (Regulatory

Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Regulatory assets	$ 1,666.7	$ 1,915.1
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	778.2	1,055.6
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	548	578.7
AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	109.3	105.9
EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	63.8	41.4
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	45.3	30.7
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	25.5	26.2
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	96.6	76.6
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,231.6	1,458.8
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	750.5	1,022.4
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	274.4	294
IPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	72.5	64.3
IPL [Member] | EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	31.6	0
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	21.8	10
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	25.5	26.2
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	55.3	41.9
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	435.1	456.3
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	27.7	33.2
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	273.6	284.7
WPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	36.8	41.6
WPL [Member] | EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	32.2	41.4
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	23.5	20.7
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 41.3	$ 34.7

Regulatory Matters (Regulator55

Regulatory Matters (Regulatory Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 1,497.2	$ 681
Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	899.4	7.7
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	410	411.6
Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	90.4	72
IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	25	83.5
Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	21	30.8
Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	19.9	20.5
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	31.5	54.9
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	755.4	430.8
IPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	399.5	1.9
IPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	274.5	269.4
IPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	26.4	35.7
IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	25	83.5
IPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	14.6	17.8
IPL [Member] | Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	0	0
IPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	15.4	22.5
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	741.8	250.2
WPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	499.9	5.8
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	135.5	142.2
WPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	64	36.3
WPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	6.4	13
WPL [Member] | Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	19.9	20.5
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 16.1	$ 32.4

Regulatory Matters (IPL's Tax B

Regulatory Matters (IPL's Tax Benefit Riders) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	$ 83.8	$ 63	$ 67.8
Alliant Energy and IPL [Member] | Electric tax benefit rider credits [Member]
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	(65)
Alliant Energy and IPL [Member] | Gas tax benefit rider credits [Member]
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	(6)
Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	17
Alliant Energy and IPL [Member] | Tax Reform adjustment [Member]
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	(5)
Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	$ (59)

Regulatory Matters (Electric Ta

Regulatory Matters (Electric Tax Benefit Rider) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	$ (83.8)	$ (63)	$ (67.8)
Income tax expense (benefit)	66.7	59.4	70.4
Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	(17)
Alliant Energy and IPL [Member] | Electric [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	(65)	(64)	(72)
Income tax expense (benefit)	(38)	(37)	(42)
Alliant Energy and IPL [Member] | Electric [Member] | Decrease In taxable income from IPL's tax benefit rider [Member]
Regulatory Liabilities [Line Items]
Income tax expense (benefit)	$ (27)	$ (27)	$ (30)

Regulatory Matters (Revenue Req

Regulatory Matters (Revenue Requirement Adjustment) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Regulatory Liabilities [Line Items]
Electric revenues	$ 2,894.7	$ 2,875.5	$ 2,770.5
Alliant Energy and IPL [Member] | Revenue Requirement Adjustment [Member]
Regulatory Liabilities [Line Items]
Electric revenues		$ 14	$ 14

Regulatory Matters (Gas Tax Ben

Regulatory Matters (Gas Tax Benefit Rider) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	$ (83.8)	$ (63)	$ (67.8)
Income tax expense (benefit)	66.7	59.4	70.4
Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	(17)
Alliant Energy and IPL [Member] | Gas [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	(6)	(12)	(12)
Income tax expense (benefit)	(3)	(7)	(7)
Alliant Energy and IPL [Member] | Gas [Member] | Decrease In taxable income from IPL's tax benefit rider [Member]
Regulatory Liabilities [Line Items]
Income tax expense (benefit)	$ (3)	$ (5)	$ (5)

Regulatory Matters (Customer Bi

Regulatory Matters (Customer Billing Credits) (Details) - USD ($) $ in Millions	12 Months Ended			36 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016
IPL [Member]
Regulatory Liabilities [Line Items]
Billing credits to reduce retail electric customers' bills	$ 9	$ 24	$ 72	$ 105

Property, Plant and Equipment61

Property, Plant and Equipment (Narrative) (Details) $ / shares in Units, $ in Millions		12 Months Ended
		Dec. 31, 2020MW	Dec. 31, 2018USD ($)MW	Dec. 31, 2017USD ($)MW	Dec. 31, 2016USD ($)$ / shares	Dec. 31, 2015USD ($)	Apr. 30, 2017USD ($)
Property, Plant and Equipment [Line Items]
Construction work in progress				$ 962.2	$ 1,226.8
Allowance for funds used during construction				49.7	62.5	$ 36.9
Asset valuation charges, pre-tax				0	86.4	0
Non-utility Generation, net	[1]			90.9	135
IPL [Member]
Property, Plant and Equipment [Line Items]
Construction work in progress				424.4	968.1
Allowance for funds used during construction				$ 31.4	52	28.2
Net assets acquired							$ 32
Wholesale power supply agreement termination notice term				5 years
Wholesale power supply agreement term				10 years
WPL [Member]
Property, Plant and Equipment [Line Items]
Construction work in progress				$ 537.8	258.7
Allowance for funds used during construction				$ 18.3	10.5	8.7
Corporate Services [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life				5 years
Corporate Services [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life				30 years
Marshalltown Generating Station [Member] | IPL [Member]
Property, Plant and Equipment [Line Items]
Fossil-fueled EGU capacity (in megawatts) | MW				706
Marshalltown Generating Station [Member] | Alliant Energy and IPL [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service				$ 645
Construction work in progress					612
Allowance for funds used during construction				$ 81	68
West Riverside Energy Center [Member] | WPL [Member]
Property, Plant and Equipment [Line Items]
Fossil-fueled EGU capacity (in megawatts) | MW				730
West Riverside Energy Center [Member] | Alliant Energy and WPL [Member]
Property, Plant and Equipment [Line Items]
Construction work in progress				$ 339	81
Allowance for funds used during construction				$ 14	2
Expansion of Wind Generation [Member] | IPL [Member]
Property, Plant and Equipment [Line Items]
Electric capacity of wind farm (in megawatts) | MW				500
Expansion of Wind Generation [Member] | WPL [Member]
Property, Plant and Equipment [Line Items]
Electric capacity of wind farm (in megawatts) | MW				129
Expansion of Wind Generation [Member] | Alliant Energy and IPL [Member]
Property, Plant and Equipment [Line Items]
Construction work in progress				$ 264	102
Allowance for funds used during construction				$ 11	1
IPL (FERC formula) [Member] | IPL [Member]
Property, Plant and Equipment [Line Items]
Authorized return on common equity				10.97%
Franklin County Wind Farm [Member]
Property, Plant and Equipment [Line Items]
Asset valuation charges, pre-tax					86
Asset valuation charges, after-tax					$ 51
Asset valuation charges, after-tax (in dollars per share) | $ / shares					$ 0.23
Gas - distribution [Member] | IPL [Member]
Property, Plant and Equipment [Line Items]
Proceeds from asset sales						11
Receipt of promissory note						2
Gas - distribution [Member] | Alliant Energy and IPL [Member]
Property, Plant and Equipment [Line Items]
Loss on sale of Minnesota distribution assets						3
Electric - distribution [Member] | IPL [Member]
Property, Plant and Equipment [Line Items]
Proceeds from asset sales						129
Electric - distribution [Member] | Alliant Energy and IPL [Member]
Property, Plant and Equipment [Line Items]
Loss on sale of Minnesota distribution assets						$ 11
Sheboygan Falls Energy Facility [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life				35 years
Non-utility Generation, net				$ 91
Customer Billing And Information System [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life				12 years
Software [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life				5 years
Scenario, Forecast [Member] | Expansion of Wind Generation [Member] | IPL [Member]
Property, Plant and Equipment [Line Items]
Electric capacity of wind farm (in megawatts) | MW		1,000
Scenario, Forecast [Member] | Expansion of Wind Generation [Member] | WPL [Member]
Property, Plant and Equipment [Line Items]
Electric capacity of wind farm (in megawatts) | MW			55
Net assets acquired			$ 74
[1]	Less accumulated depreciation of $50.5 million and $46.5 million for Alliant Energy as of December 31, 2017 and 2016, respectively. Refer to “Franklin County Wind Farm” below for discussion of the April 2017 transfer of the Franklin County wind farm from AEF to IPL pursuant to a February 2017 FERC order.

Property, Plant and Equipment62

Property, Plant and Equipment (Property, Plant and Equipment) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Electric plant anticipated to be retired early	[1],[2]		$ 93	$ 108.3
Total electric plant			12,296.9	11,043.5
Gas plant in service			1,244	1,107.6
Other plant in service			571.9	549.3
Accumulated depreciation			(4,283.1)	(4,135.7)
Net plant			9,829.7	8,564.7
Construction work in progress			962.2	1,226.8
Other, net			6	18.4
Total utility			10,797.9	9,809.9
Non-utility Generation, net	[3]		90.9	135
Corporate Services and other, net	[4]		345.7	334.3
Total non-utility and other			436.6	469.3
Total property, plant and equipment			11,234.5	10,279.2
Non-utility Generation, accumulated depreciation			50.5	46.5
Corporate Services and other, accumulated depreciation			285.6	272
Generation [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service			6,655.3	5,866.9
Distribution [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service			5,123.5	4,739.2
Other [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service			425.1	329.1
IPL [Member]
Property, Plant and Equipment [Line Items]
Electric plant anticipated to be retired early	[1]		0	108.3
Total electric plant			6,819.1	5,837.9
Gas plant in service			654.8	556.7
Other plant in service			333.4	313
Accumulated depreciation			(2,311)	(2,258.3)
Net plant			5,496.3	4,449.3
Construction work in progress			424.4	968.1
Other, net			5.5	18.2
Total utility			5,926.2	5,435.6
Total property, plant and equipment			5,926.2	5,435.6
IPL [Member] | Generation [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service			3,715.9	2,916.8
IPL [Member] | Distribution [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service			2,820.9	2,589.3
IPL [Member] | Other [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service			282.3	223.5
WPL [Member]
Property, Plant and Equipment [Line Items]
Electric plant anticipated to be retired early	[2]		93	0
Total electric plant			5,477.8	5,205.6
Gas plant in service			589.2	550.9
Other plant in service			238.5	236.3
Accumulated depreciation			(1,972.1)	(1,877.4)
Net plant			4,333.4	4,115.4
Leased Sheboygan Falls Energy Facility, net	[5]		46.2	52.4
Construction work in progress			537.8	258.7
Other, net			0.5	0.2
Total utility			4,917.9	4,426.7
Total property, plant and equipment			4,917.9	4,426.7
Leased Sheboygan Falls Energy Facility, accumulated depreciation			77.6	71.4
WPL [Member] | Generation [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service			2,939.4	2,950.1
WPL [Member] | Distribution [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service			2,302.6	2,149.9
WPL [Member] | Other [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service			$ 142.8	$ 105.6
PSCW [Member] | Scenario, Forecast [Member] | WPL [Member] | Edgewater Unit 4 [Member]
Property, Plant and Equipment [Line Items]
Recovery period, authorized		10 years
[1]	In 2017, IPL retired Sutherland Unit 3 and reclassified the remaining net book value of this EGU from property, plant and equipment to a regulatory asset on Alliant Energy’s and IPL’s balance sheets. Refer to Note 2 for further discussion, including recovery of the remaining net book value of Sutherland Unit 3.
[2]	In 2017, WPL received approval from MISO to retire Edgewater Unit 4 and currently anticipates retiring this EGU by September 30, 2018. In 2016, the PSCW authorized WPL to recover the remaining net book value of Edgewater Unit 4 over a 10-year period beginning the later of the retirement date of the EGU or January 1, 2019.
[3]	Less accumulated depreciation of $50.5 million and $46.5 million for Alliant Energy as of December 31, 2017 and 2016, respectively. Refer to “Franklin County Wind Farm” below for discussion of the April 2017 transfer of the Franklin County wind farm from AEF to IPL pursuant to a February 2017 FERC order.
[4]	Less accumulated depreciation of $285.6 million and $272.0 million for Alliant Energy as of December 31, 2017 and 2016, respectively.
[5]	Less accumulated amortization of $77.6 million and $71.4 million for WPL as of December 31, 2017 and 2016, respectively. The Sheboygan Falls Energy Facility is eliminated from WPL upon consolidation and is included in the “Non-utility Generation, net” line within Alliant Energy’s consolidated property, plant and equipment.

Property, Plant and Equipment E

Property, Plant and Equipment Estimated Fair Value of Franklin County Wind Farm Assets and Liabilities Transferred (Details) - IPL [Member] $ in Millions	Apr. 30, 2017USD ($)
Property, Plant and Equipment [Line Items]
Electric plant in service	$ 40
Current assets	2
Total assets acquired	42
Other liabilities	10
Net assets acquired	$ 32

Property, Plant and Equipment64

Property, Plant and Equipment (Equity and Debt AFUDC) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 49.7	$ 62.5	$ 36.9
Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	16.1	20.2	12.5
Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	33.6	42.3	24.4
IPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	31.4	52	28.2
IPL [Member] | Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	10.3	16.8	9.6
IPL [Member] | Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	21.1	35.2	18.6
WPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	18.3	10.5	8.7
WPL [Member] | Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	5.8	3.4	2.9
WPL [Member] | Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 12.5	$ 7.1	$ 5.8

Jointly-Owned Electric Utilit65

Jointly-Owned Electric Utility Plant (Details) $ in Millions	Dec. 31, 2017USD ($)
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	$ 1,692
Accumulated Provision for Depreciation	582.7
Construction Work in Progress	89
IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	877.4
Accumulated Provision for Depreciation	305.5
Construction Work in Progress	42
WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	814.6
Accumulated Provision for Depreciation	277.2
Construction Work in Progress	$ 47
Ottumwa Unit 1 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	48.00%
Electric Plant	$ 501.8
Accumulated Provision for Depreciation	144.9
Construction Work in Progress	$ 37.3
George Neal Unit 4 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	25.70%
Electric Plant	$ 187.2
Accumulated Provision for Depreciation	85.2
Construction Work in Progress	$ 0.8
George Neal Unit 3 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	28.00%
Electric Plant	$ 150.6
Accumulated Provision for Depreciation	53
Construction Work in Progress	$ 2.5
Louisa Unit 1 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	4.00%
Electric Plant	$ 37.8
Accumulated Provision for Depreciation	22.4
Construction Work in Progress	$ 1.4
Columbia Units 1-2 [Member] | WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	50.10%
Electric Plant	$ 714.7
Accumulated Provision for Depreciation	216.8
Construction Work in Progress	$ 46.9
Edgewater Unit 4 [Member] | WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	68.20%
Electric Plant	$ 99.9
Accumulated Provision for Depreciation	60.4
Construction Work in Progress	$ 0.1

Receivables (Narrative) (Detail

Receivables (Narrative) (Details) - IPL [Member] $ in Millions	Dec. 31, 2017USD ($)
Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Limit on cash proceeds to be received from third-party	$ 125
Receivables Sold [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Available capacity	$ 91.7

Receivables (Details of Account

Receivables (Details of Accounts Receivable) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer	$ 103.3	$ 111.7
Unbilled utility revenues	85.1	90.2
Deferred proceeds	222.1	211.1
Other	84.3	89
Allowance for doubtful accounts	(12)	(8.7)
Accounts receivable, less allowance for doubtful accounts	482.8	493.3
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer	0	0
Unbilled utility revenues	0	0
Deferred proceeds	222.1	211.1
Other	44.1	30.7
Allowance for doubtful accounts	(1.3)	(1.1)
Accounts receivable, less allowance for doubtful accounts	264.9	240.7
WPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer	97.7	104.4
Unbilled utility revenues	85.1	90.2
Other	40.1	38.8
Allowance for doubtful accounts	(10.7)	(7.1)
Accounts receivable, less allowance for doubtful accounts	$ 212.2	$ 226.3

Receivables (Maximum And Averag

Receivables (Maximum And Average Outstanding Cash Proceeds) (Details) - IPL [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding aggregate cash proceeds (based on daily outstanding balances)	$ 112	$ 172	$ 137
Average [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding aggregate cash proceeds (based on daily outstanding balances)	$ 62.2	$ 73.2	$ 46.7

Receivables (Receivables Sold U

Receivables (Receivables Sold Under The Agreement) (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair value of deferred proceeds		$ 222.1	$ 211.1
IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair value of deferred proceeds		222.1	211.1
Receivables Sold [Member] | IPL [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer accounts receivable		133.8	157.6
Unbilled utility revenues		112.7	90.4
Other receivables		0.3	0.1
Receivables sold to third party		246.8	248.1
Less: cash proceeds	[1]	12	21
Deferred proceeds		234.8	227.1
Less: allowance for doubtful accounts		12.7	16
Fair value of deferred proceeds		222.1	211.1
Outstanding receivables past due		$ 44.7	$ 68
[1]	Changes in cash proceeds are presented in “Sales of accounts receivable” in operating activities in Alliant Energy’s and IPL’s cash flows statements.

Receivables (Additional Attribu

Receivables (Additional Attributes Of Receivables Sold Under The Agreement) (Details) - IPL [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collections reinvested in receivables	$ 1,647.1	$ 1,818.1	$ 1,812.9
Write-offs, net of recoveries	$ 17.7	$ 4.8	$ 8.8

Investments (Narrative) (Detail

Investments (Narrative) (Details) $ in Millions	1 Months Ended	12 Months Ended
	Jul. 31, 2017USD ($)	Dec. 31, 2017USD ($)MW	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2013
Schedule of Equity Method Investments [Line Items]
Carrying value		$ 381.4	$ 326	$ 302.9
Proceeds from liquidation of company-owned life insurance policies			31
WPL [Member]
Schedule of Equity Method Investments [Line Items]
WPL's ownership interest in WPL Transco					100.00%
Net reduction in total equity due to transfer of ATC investment to ATI			163.6
IPL [Member]
Schedule of Equity Method Investments [Line Items]
Proceeds from liquidation of company-owned life insurance policies			$ 19
Alliant Energy Finance, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest		50.00%
Electric capacity of wind farm (in megawatts) | MW		225
Carrying value		$ 98
Term Loan Credit Agreement [Member] | Alliant Energy Finance, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Proceeds from the issuance of debt	$ 95
Variable-rate term loan credit agreement, period	364 days

Investments (Unconsolidated Equ

Investments (Unconsolidated Equity Investments) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Carrying value		$ 381.4	$ 326	$ 302.9
Equity (income) / loss		(44.8)	(39.6)	(33.8)
WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity (income) / loss		(0.7)	(39.8)	(35.1)
ATC Investment [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value	[1]	274.2	317.6
Equity (income) / loss	[1]	$ (42.4)	(39.1)	(34.2)
Wind Investment in Oklahoma [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest		50.00%
Carrying value		$ 98.3	0
Equity (income) / loss		(1.8)	0	0
Other [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value		8.9	8.4
Equity (income) / loss		$ (0.6)	(0.5)	0.4
ATC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest		16.00%
ATC [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest		0.00%
Carrying value		$ 0	0
Equity (income) / loss		$ 0	(39.1)	(34.2)
Wisconsin River Power Company [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest		50.00%
Carrying value		$ 8.3	7.7
Equity (income) / loss		(0.7)	(0.7)	(0.9)
Totals [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value		381.4	326
Equity (income) / loss		(44.8)	(39.6)	(33.8)
Totals [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value		8.3	7.7
Equity (income) / loss		$ (0.7)	$ (39.8)	$ (35.1)
ATC Holdco LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest		20.00%
[1]	As of December 31, 2017, Alliant Energy’s ATC Investment is comprised of a 16% ownership interest in ATC and a 20% ownership interest in ATC Holdco LLC, which are described below. In 2017, Alliant Energy’s investment in ATC decreased due to the impacts of Tax Reform. Refer to Note 11 for further discussion. Alliant Energy currently has the ability to exercise significant influence over ATC’s financial and operating policies through its participation on ATC’s Board of Directors. Refer to Note 18 for information regarding related party transactions with ATC.

Investments (Summary Financial

Investments (Summary Financial Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating revenues	$ 741	$ 658	$ 624
Operating income	374	331	299
Net income	267	232	186
Current assets	104	82
Non-current assets	5,041	4,340
Current liabilities	770	498
Non-current liabilities	2,038	2,144
Minority interest	255	0
WPL [Member]
Operating revenues	8	658	624
Operating income	4	331	299
Net income	2	234	$ 202
Current assets	7	6
Non-current assets	20	19
Current liabilities	2	3
Non-current liabilities	8	7
Minority interest	$ 0	$ 0

Investments (Cash Surrender Val

Investments (Cash Surrender Value Of Life Insurance Policies) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Cash surrender value	$ 10.4	$ 10.6
WPL [Member]
Cash surrender value	$ 5.6	$ 5.8

Common Equity (Narrative) (Deta

Common Equity (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Common Equity [Line Items]
Shares available for issuance under the Amended and Restated OIP, Shareowner Direct Plan and 401(k) Savings Plan (in shares)		12,400,000
Proceeds from issuance of common stock, net		$ 149.6	$ 26.6	$ 151.2
Ownership percentage		15.00%
Other comprehensive income (loss)		$ (0.1)	0	0.2
IPL [Member]
Schedule of Common Equity [Line Items]
Retained earnings free of dividend restrictions		$ 679
Common equity ratio, computation of ratio average, number of months		13 months
Common equity ratio		42.00%
Restricted net assets of subsidiaries		$ 1,800
Other comprehensive income (loss)		0	0	0
WPL [Member]
Schedule of Common Equity [Line Items]
Retained earnings free of dividend restrictions		140
Restricted net assets of subsidiaries		1,700
Other comprehensive income (loss)		0	$ 0	0
At The Market Offering Program [Member]
Schedule of Common Equity [Line Items]
Maximum aggregate gross sales price of common stock that can be offered and sold		$ 125		$ 150
Common stock issued during the period, At-the-market offering programs (in shares)		3,074,931	0	4,373,234
Proceeds from issuance of common stock, net		$ 124		$ 133
Fees and commissions from issuance of common stock		$ 1		$ 2
Scenario, Forecast [Member] | WPL [Member]
Schedule of Common Equity [Line Items]
Retained earnings free of dividend restrictions	$ 140
Common equity ratio, computation of ratio average, number of months	13 months
Common equity ratio	51.00%

Common Equity (Common Share Act

Common Equity (Common Share Activity) (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Common Stock Oustanding [Roll Forward]
Shares outstanding, January 1 (in shares)	227,673,654	226,918,432	221,871,360
Common stock issued during the period, Shareowner Direct Plan (in shares)	640,723	732,814	606,010
Equity-based compensation plans (in shares)	5,185	22,408	112,756
Other (in shares)	(45,847)	0	(44,928)
Shares outstanding, December 31 (in shares)	231,348,646	227,673,654	226,918,432
At The Market Offering Program [Member]
Common Stock Oustanding [Roll Forward]
Common stock issued during the period, At-the-market offering programs (in shares)	3,074,931	0	4,373,234

Redeemable Preferred Stock (Nar

Redeemable Preferred Stock (Narrative) (Details) - IPL [Member]	12 Months Ended
	Dec. 31, 2017Qboard_member$ / sharesshares	Dec. 31, 2016shares
Temporary Equity [Line Items]
Number of board members to be elected (in members) | board_member	2
Number of quarterly dividend requirements (in quarters) | Q	6
Redeemable Preferred Stock [Member]
Temporary Equity [Line Items]
Shares Authorized (in shares)	16,000,000
Redeemable Preferred Stock [Member] | 5.1% [Member]
Temporary Equity [Line Items]
Shares Authorized (in shares)	8,000,000	8,000,000
Cumulative preferred stock rate	5.10%	5.10%
Preferred stock redemption price per share (in dollars per share) | $ / shares	$ 25

Redeemable Preferred Stock (Car

Redeemable Preferred Stock (Carrying Value Of Cumulative Preferred Stock) (Details) - IPL [Member] - Redeemable Preferred Stock [Member] - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Temporary Equity [Line Items]
Shares Authorized (in shares)	16,000,000
5.1% [Member]
Temporary Equity [Line Items]
Series	5.10%	5.10%
Liquidation Preference/Stated Value (in dollars per share)	$ 25	$ 25
Shares Authorized (in shares)	8,000,000	8,000,000
Shares Outstanding (in shares)	8,000,000	8,000,000
Cumulative preferred stock	$ 200	$ 200

Debt (Narrative) (Details)

Debt (Narrative) (Details) - USD ($)	1 Months Ended
	Jul. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity			$ 1,000,000,000
Non-recourse debt and hybrid securities consolidated capital maximum limit			15.00%
Parent Company [Member]
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity			$ 400,000,000
IPL [Member]
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity			250,000,000
WPL [Member]
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity			$ 350,000,000
Scenario, Forecast [Member] | Parent Company [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		$ 500,000,000
Scenario, Forecast [Member] | IPL [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		400,000,000
Scenario, Forecast [Member] | WPL [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		$ 500,000,000
Term Loan Credit Agreement [Member] | Alliant Energy Finance, LLC [Member]
Debt Instrument [Line Items]
Proceeds from the issuance of debt	$ 95,000,000
Variable-rate term loan credit agreement, period	364 days
Interest rate			2.20%

Debt (Credit Facilities) (Detai

Debt (Credit Facilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Commercial paper:
Amount outstanding	$ 320.2	$ 244.1
Weighted average interest rates	2.00%	0.90%
Available credit facility capacity	$ 679.8	$ 755.9
IPL [Member]
Commercial paper:
Amount outstanding	0	0
Available credit facility capacity	250	300
WPL [Member]
Commercial paper:
Amount outstanding	$ 25	$ 52.3
Weighted average interest rates	1.50%	0.70%
Available credit facility capacity	$ 325	$ 347.7

Debt (Other Short-Term Borrowin

Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 424.4	$ 251.8
Average amount outstanding (based on daily outstanding balances)	$ 294.3	$ 179
Weighted average interest rates	1.20%	0.60%
IPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 20	$ 3.1
Average amount outstanding (based on daily outstanding balances)	$ 0.5	$ 0
Weighted average interest rates	1.30%	0.70%
WPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 271.2	$ 118.3
Average amount outstanding (based on daily outstanding balances)	$ 118.2	$ 38.1
Weighted average interest rates	1.00%	0.40%

Debt (Schedule Of Debt-To-Capit

Debt (Schedule Of Debt-To-Capital Ratios) (Details)	Dec. 31, 2017
Actual debt-to-capital ratio	54.00%
IPL [Member]
Actual debt-to-capital ratio	47.00%
WPL [Member]
Actual debt-to-capital ratio	51.00%
Maximum [Member]
Requirement, not to exceed	65.00%
Maximum [Member] | IPL [Member]
Requirement, not to exceed	65.00%
Maximum [Member] | WPL [Member]
Requirement, not to exceed	65.00%

Debt (Long-term Debt) (Details)

Debt (Long-term Debt) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term debt		$ 4,902.5	$ 4,357.1
Current maturities		(855.7)	(4.6)
Unamortized debt issuance costs		(25.4)	(23.4)
Unamortized debt (discount) and premium, net		(10.8)	(13.5)
Long-term debt, net	[1]	4,010.6	4,315.6
Senior Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	2,425	2,175
Senior Debentures [Member] | 5.875% senior debenture, due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 100	$ 100
Interest rate		5.875%	5.875%
Senior Debentures [Member] | 7.25% senior debenture, due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250	$ 250
Interest rate		7.25%	7.25%
Senior Debentures [Member] | 3.65% senior debenture, due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 200	$ 200
Interest rate		3.65%	3.65%
Senior Debentures [Member] | 3.25% senior debenture, due 2024 [Member]
Debt Instrument [Line Items]
Long-term debt	[2],[3]	$ 500	$ 250
Interest rate		3.25%	3.25%
Senior Debentures [Member] | 3.4% senior debenture, due 2025 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250	$ 250
Interest rate		3.40%	3.40%
Senior Debentures [Member] | 5.5% senior debenture, due 2025 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 50	$ 50
Interest rate		5.50%	5.50%
Senior Debentures [Member] | 6.45% senior debenture, due 2033 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 100	$ 100
Interest rate		6.45%	6.45%
Senior Debentures [Member] | 6.3% senior debenture, due 2034 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 125	$ 125
Interest rate		6.30%	6.30%
Senior Debentures [Member] | 6.25% senior debenture, due 2039 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 300	$ 300
Interest rate		6.25%	6.25%
Senior Debentures [Member] | 4.7% senior debenture, due 2043 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250	$ 250
Interest rate		4.70%	4.70%
Senior Debentures [Member] | 3.7% senior debenture, due 2046 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 300	$ 300
Interest rate		3.70%	3.70%
Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 1,850	$ 1,550
Debentures [Member] | 5% debenture, due 2019 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250	$ 250
Interest rate		5.00%	5.00%
Debentures [Member] | 4.6% debenture, due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 150	$ 150
Interest rate		4.60%	4.60%
Debentures [Member] | 2.25% debenture, due 2022 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250	$ 250
Interest rate		2.25%	2.25%
Debentures [Member] | 3.05% debenture, due 2027 [Member]
Debt Instrument [Line Items]
Long-term debt	[2],[4]	$ 300
Interest rate		3.05%
Debentures [Member] | 6.25% debenture, due 2034 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 100	$ 100
Interest rate		6.25%	6.25%
Debentures [Member] | 6.375% debenture, due 2037 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 300	$ 300
Interest rate		6.375%	6.375%
Debentures [Member] | 7.6% debenture, due 2038 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250	$ 250
Interest rate		7.60%	7.60%
Debentures [Member] | 4.1% debenture, due 2044 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250	$ 250
Interest rate		4.10%	4.10%
Other Long Term Debt [Member]
Debt Instrument [Line Items]
Long-term debt		$ 627.5	$ 632.1
Other [Member] | Other, 1% at December 31, 2017, due 2018 to 2025 [Member]
Debt Instrument [Line Items]
Long-term debt		$ 2.9	3.3
Interest rate		1.00%
IPL [Member]
Debt Instrument [Line Items]
Long-term debt		$ 2,425	2,175
Current maturities		(350)	0
Unamortized debt issuance costs		(14.3)	(13.7)
Unamortized debt (discount) and premium, net		(4.7)	(7.8)
Long-term debt, net	[1]	2,056	2,153.5
IPL [Member] | Senior Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	2,425	2,175
IPL [Member] | Senior Debentures [Member] | 5.875% senior debenture, due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 100	$ 100
Interest rate		5.875%	5.875%
IPL [Member] | Senior Debentures [Member] | 7.25% senior debenture, due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250	$ 250
Interest rate		7.25%	7.25%
IPL [Member] | Senior Debentures [Member] | 3.65% senior debenture, due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 200	$ 200
Interest rate		3.65%	3.65%
IPL [Member] | Senior Debentures [Member] | 3.25% senior debenture, due 2024 [Member]
Debt Instrument [Line Items]
Long-term debt	[2],[3]	$ 500	$ 250
Proceeds from the issuance of debt		$ 250
Interest rate		3.25%	3.25%
IPL [Member] | Senior Debentures [Member] | 3.4% senior debenture, due 2025 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250	$ 250
Interest rate		3.40%	3.40%
IPL [Member] | Senior Debentures [Member] | 5.5% senior debenture, due 2025 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 50	$ 50
Interest rate		5.50%	5.50%
IPL [Member] | Senior Debentures [Member] | 6.45% senior debenture, due 2033 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 100	$ 100
Interest rate		6.45%	6.45%
IPL [Member] | Senior Debentures [Member] | 6.3% senior debenture, due 2034 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 125	$ 125
Interest rate		6.30%	6.30%
IPL [Member] | Senior Debentures [Member] | 6.25% senior debenture, due 2039 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 300	$ 300
Interest rate		6.25%	6.25%
IPL [Member] | Senior Debentures [Member] | 4.7% senior debenture, due 2043 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250	$ 250
Interest rate		4.70%	4.70%
IPL [Member] | Senior Debentures [Member] | 3.7% senior debenture, due 2046 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 300	$ 300
Interest rate		3.70%	3.70%
WPL [Member]
Debt Instrument [Line Items]
Long-term debt		$ 1,850	$ 1,550
Current maturities		0	0
Unamortized debt issuance costs		(10.5)	(9.1)
Unamortized debt (discount) and premium, net		(6.1)	(5.7)
Long-term debt, net	[1]	1,833.4	1,535.2
WPL [Member] | Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	1,850	1,550
WPL [Member] | Debentures [Member] | 5% debenture, due 2019 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250	$ 250
Interest rate		5.00%	5.00%
WPL [Member] | Debentures [Member] | 4.6% debenture, due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 150	$ 150
Interest rate		4.60%	4.60%
WPL [Member] | Debentures [Member] | 2.25% debenture, due 2022 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250	$ 250
Interest rate		2.25%	2.25%
WPL [Member] | Debentures [Member] | 3.05% debenture, due 2027 [Member]
Debt Instrument [Line Items]
Long-term debt	[2],[4]	$ 300
Proceeds from the issuance of debt	[2]	$ 300
Interest rate		3.05%
WPL [Member] | Debentures [Member] | 6.25% debenture, due 2034 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 100	$ 100
Interest rate		6.25%	6.25%
WPL [Member] | Debentures [Member] | 6.375% debenture, due 2037 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 300	$ 300
Interest rate		6.375%	6.375%
WPL [Member] | Debentures [Member] | 7.6% debenture, due 2038 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250	$ 250
Interest rate		7.60%	7.60%
WPL [Member] | Debentures [Member] | 4.1% debenture, due 2044 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250	$ 250
Interest rate		4.10%	4.10%
Alliant Energy Finance, LLC [Member] | Term Loan Credit Agreement [Member] | Term loan credit agreement through 2018, 2% at December 31, 2017 [Member]
Debt Instrument [Line Items]
Long-term debt	[5]	$ 500	$ 500
Interest rate		2.00%
Corporate Services [Member] | Senior Notes [Member] | 3.45% senior notes, due 2022 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 75	$ 75
Interest rate		3.45%	3.45%
Sheboygan Power, LLC [Member] | Senior Secured Notes [Member] | 5.06% senior secured notes, due 2018 to 2024 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 49.6	$ 53.8
Interest rate		5.06%	5.06%
[1]	There were no significant sinking fund requirements related to the outstanding long-term debt.
[2]	Contains optional redemption provisions which, if elected by the issuer at its sole discretion, could require material redemption premium payments by the issuer. The redemption premium payments under these optional redemption provisions are variable and dependent on applicable U.S. Treasury rates at the time of redemption.
[3]	In 2017, IPL issued $250 million of 3.25% senior debentures due 2024. The proceeds from the issuance were used by IPL to reduce commercial paper classified as long-term debt, reduce cash amounts received from its sales of accounts receivable program and for general corporate purposes.
[4]	In 2017, WPL issued $300 million of 3.05% debentures due 2027. The proceeds from the issuance were used by WPL to reduce commercial paper and for general corporate purposes.
[5]	Refer to Note 9(a) for discussion of a financial covenant contained in AEF’s term loan credit agreement.

Debt (Schedule Of Debt Maturiti

Debt (Schedule Of Debt Maturities) (Details) $ in Millions	Dec. 31, 2017USD ($)
2,018	$ 856
2,019	256
2,020	357
2,021	8
2,022	333
IPL [Member]
2,018	350
2,019	0
2,020	200
2,021	0
2,022	0
WPL [Member]
2,018	0
2,019	250
2,020	150
2,021	0
2,022	250
Corporate Services [Member]
2,018	0
2,019	0
2,020	0
2,021	0
2,022	75
Alliant Energy Finance, LLC [Member]
2,018	506
2,019	6
2,020	7
2,021	8
2,022	$ 8

Leases (Narrative) (Details)

Leases (Narrative) (Details) - Sheboygan Falls Energy Facility [Member] - WPL [Member]	12 Months Ended
	Dec. 31, 2005lease_renewal_period_option
Leases [Line Items]
Capital lease, agreement term	20 years
Capital leases, renewal options (in number of renewal periods)	2
Capital lease asset amortization term	20 years

Leases (Future Minimum Operatin

Leases (Future Minimum Operating Lease Payments, Excluding Contingent Rentals) (Details) $ in Millions	Dec. 31, 2017USD ($)
Operating Leased Assets [Line Items]
2,018	$ 6
2,019	5
2,020	2
2,021	2
2,022	1
Thereafter	13
Total future minimum operating lease payments, excluding contingent rentals	29
IPL [Member]
Operating Leased Assets [Line Items]
2,018	3
2,019	2
2,020	1
2,021	1
2,022	1
Thereafter	13
Total future minimum operating lease payments, excluding contingent rentals	21
WPL [Member]
Operating Leased Assets [Line Items]
2,018	3
2,019	3
2,020	1
2,021	0
2,022	0
Thereafter	0
Total future minimum operating lease payments, excluding contingent rentals	$ 7

Leases (Schedule Of Lease Expen

Leases (Schedule Of Lease Expenses Included In Consolidated Statements Of Income) (Details) - WPL [Member] - Sheboygan Falls Energy Facility [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Interest expense	$ 8.7	$ 9.3	$ 9.9
Depreciation and amortization	6.2	6.2	6.2
Total lease expenses included in income statements	$ 14.9	$ 15.5	$ 16.1

Leases (Future Minimum Capital

Leases (Future Minimum Capital Lease Payments) (Details) - WPL [Member] - Sheboygan Falls Energy Facility [Member] $ in Millions	Dec. 31, 2017USD ($)
Schedule of Future Lease Payments for Capital Leases [Line Items]
2,018	$ 15
2,019	15
2,020	15
2,021	15
2,022	15
Thereafter	35
Total estimated future minimum capital lease payments	110
Less: amount representing interest	33
Present value of minimum capital lease payments	$ 77

Income Taxes (Narrative) (Detai

Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax [Line Items]
Statutory federal income tax rate		35.00%	35.00%	35.00%
Other assets, deferred charges and other		$ 41
Scenario, Forecast [Member]
Income Tax [Line Items]
Deductions in income tax return	$ 670
IPL [Member]
Income Tax [Line Items]
Statutory federal income tax rate		35.00%	35.00%	35.00%
Other assets, deferred charges and other		$ 0
IPL [Member] | Scenario, Forecast [Member]
Income Tax [Line Items]
Deductions in income tax return	500
WPL [Member]
Income Tax [Line Items]
Statutory federal income tax rate		35.00%	35.00%	35.00%
Other assets, deferred charges and other		$ 0
WPL [Member] | Scenario, Forecast [Member]
Income Tax [Line Items]
Deductions in income tax return	$ 140
Subsequent Event [Member]
Income Tax [Line Items]
Statutory federal income tax rate	21.00%
Subsequent Event [Member] | IPL [Member]
Income Tax [Line Items]
Statutory federal income tax rate	21.00%
Subsequent Event [Member] | WPL [Member]
Income Tax [Line Items]
Statutory federal income tax rate	21.00%

Income Taxes (Schedule Of Impac

Income Taxes (Schedule Of Impacts Of the Tax Cuts and Jobs Act of 2017) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)
Income Tax [Line Items]
Other current assets	$ 0
Investments, ATC Investment	(75)
Other assets, regulatory assets	(387.6)
Other assets, deferred charges and other	41
Total Tax Reform impact on assets	(421.6)
Other liabilities, deferred tax liabilities	(1,331.9)
Other liabilities, regulatory liabilities	885.9
Other liabilities, other	6.3
Total Tax Reform impact on liabilities	(439.7)
Income tax expense (benefit)	(18.1)
IPL [Member]
Income Tax [Line Items]
Other current assets	4.7
Investments, ATC Investment	0
Other assets, regulatory assets	(375)
Other assets, deferred charges and other	0
Total Tax Reform impact on assets	(370.3)
Other liabilities, deferred tax liabilities	(757.2)
Other liabilities, regulatory liabilities	390.7
Other liabilities, other	0
Total Tax Reform impact on liabilities	(366.5)
Income tax expense (benefit)	3.8
WPL [Member]
Income Tax [Line Items]
Other current assets	(1.4)
Investments, ATC Investment	0
Other assets, regulatory assets	(12.7)
Other assets, deferred charges and other	0
Total Tax Reform impact on assets	(14.1)
Other liabilities, deferred tax liabilities	(523.8)
Other liabilities, regulatory liabilities	495.2
Other liabilities, other	0
Total Tax Reform impact on liabilities	(28.6)
Income tax expense (benefit)	$ (14.5)

Income Taxes (Schedule Of Compo

Income Taxes (Schedule Of Components Of Income Tax) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current tax expense (benefit):
Federal	$ (41)	$ 1.8	$ 2
State	8.5	17.2	3.2
Deferred tax expense (benefit):
Federal	159.5	112.8	120.8
State	12.3	4.9	27.9
Production tax credits	(31.1)	(31.8)	(33.1)
Investment tax credits	(1.1)	(1.3)	(1.4)
Income tax expense (benefit)	66.7	59.4	70.4
IPL's tax benefit riders [Member]
Current tax expense (benefit):
IPL's tax benefit riders	(40.4)	(44.2)	(49)
IPL [Member]
Current tax expense (benefit):
Federal	(27.9)	(12.8)	(14.1)
State	1.6	15.5	11.5
Deferred tax expense (benefit):
Federal	72.5	59.1	40.7
State	(2.2)	(9)	3.3
Production tax credits	(14.1)	(14)	(14.5)
Investment tax credits	(0.4)	(0.5)	(0.6)
Income tax expense (benefit)	(10.9)	(5.9)	(22.7)
IPL [Member] | IPL's tax benefit riders [Member]
Current tax expense (benefit):
IPL's tax benefit riders	(40.4)	(44.2)	(49)
WPL [Member]
Current tax expense (benefit):
Federal	5.5	(22.3)	4.7
State	2.5	1.1	0.6
Deferred tax expense (benefit):
Federal	55	112.3	76.8
State	16.6	20.8	20.2
Production tax credits	(17)	(17.8)	(18.6)
Investment tax credits	(0.7)	(0.8)	(0.8)
Income tax expense (benefit)	$ 61.9	$ 93.3	$ 82.9

Income Taxes (Schedule Of Effec

Income Taxes (Schedule Of Effective Income Tax Rates) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Tax Rate [Line Items]
Statutory federal income tax rate	35.00%	35.00%	35.00%
State income taxes, net of federal benefits	5.50%	5.40%	5.20%
Effect of rate-making on property-related differences	(8.50%)	(8.50%)	(6.80%)
Production tax credits	(6.10%)	(7.20%)	(7.20%)
Impact of Tax Reform	(3.40%)	(0.00%)	(0.00%)
Other items, net	(2.40%)	(1.30%)	(0.30%)
Overall income tax rate	12.50%	13.40%	15.30%
IPL [Member]
Effective Tax Rate [Line Items]
Statutory federal income tax rate	35.00%	35.00%	35.00%
State income taxes, net of federal benefits	6.50%	6.40%	6.20%
Effect of rate-making on property-related differences	(19.10%)	(16.20%)	(17.20%)
Production tax credits	(6.70%)	(6.30%)	(8.30%)
Impact of Tax Reform	1.70%	(0.00%)	(0.00%)
Other items, net	(3.70%)	(1.50%)	(0.50%)
Overall income tax rate	(5.00%)	(2.70%)	(13.10%)
WPL [Member]
Effective Tax Rate [Line Items]
Statutory federal income tax rate	35.00%	35.00%	35.00%
State income taxes, net of federal benefits	5.10%	5.10%	5.10%
Effect of rate-making on property-related differences	(1.70%)	(0.70%)	(0.50%)
Production tax credits	(7.10%)	(6.20%)	(7.10%)
Impact of Tax Reform	(5.80%)	(0.00%)	(0.00%)
Other items, net	(0.60%)	(0.60%)	(0.70%)
Overall income tax rate	24.90%	32.60%	31.80%
IPL's tax benefit riders [Member]
Effective Tax Rate [Line Items]
IPL’s tax benefit riders	(7.60%)	(10.00%)	(10.60%)
IPL's tax benefit riders [Member] | IPL [Member]
Effective Tax Rate [Line Items]
IPL’s tax benefit riders	(18.70%)	(20.10%)	(28.30%)

Income Taxes (Schedule Of Defer

Income Taxes (Schedule Of Deferred Tax Assets And Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred tax liabilities, property	$ 1,852.7	$ 2,919
Deferred tax liabilities, other	75.9	95.1
Total deferred tax liabilities	2,015	3,167.2
Deferred tax assets, federal credit carryforwards	260.7	268.4
Deferred tax assets, net operating losses carryforwards - federal	174.4	173.3
Deferred tax assets, net operating losses carryforwards - state	41.3	32.9
Deferred tax assets, other	61.8	87.9
Subtotal deferred tax assets	545.6	597.2
Deferred tax assets, valuation allowance	(9)	(0.2)
Total deferred tax assets	536.6	597
Total deferred tax liabilities, net	1,478.4	2,570.2
IPL [Member]
Deferred tax liabilities, property	1,102.6	1,677
Deferred tax liabilities, other	63.4	71.4
Total deferred tax liabilities	1,166	1,748.4
Deferred tax assets, federal credit carryforwards	113.1	95.9
Deferred tax assets, net operating losses carryforwards - federal	107.4	69.6
Deferred tax assets, net operating losses carryforwards - state	0.9	0.6
Deferred tax assets, other	27.1	35.8
Subtotal deferred tax assets	255.9	236.6
Deferred tax assets, valuation allowance	(0.6)	0
Total deferred tax assets	255.3	236.6
Total deferred tax liabilities, net	910.7	1,511.8
WPL [Member]
Deferred tax liabilities, property	674.2	1,124.5
Deferred tax liabilities, other	36.5	58.8
Total deferred tax liabilities	710.7	1,183.3
Deferred tax assets, federal credit carryforwards	131	112.9
Deferred tax assets, net operating losses carryforwards - federal	43.7	75.4
Deferred tax assets, net operating losses carryforwards - state	0.2	0.1
Deferred tax assets, other	14.7	23.6
Subtotal deferred tax assets	189.6	212
Deferred tax assets, valuation allowance	(1.3)	(0.3)
Total deferred tax assets	188.3	211.7
Total deferred tax liabilities, net	522.4	971.6
ATC Investment [Member]
Deferred tax liabilities, ATC Investment	86.4	153.1
IPL's tax benefit riders [Member]
Deferred tax assets, regulatory liability - IPL's tax benefit riders	7.4	34.7
IPL's tax benefit riders [Member] | IPL [Member]
Deferred tax assets, regulatory liability - IPL's tax benefit riders	$ 7.4	$ 34.7

Income Taxes (Summary Of Tax Cr

Income Taxes (Summary Of Tax Credit Carryforwards) (Details) $ in Millions	Dec. 31, 2017USD ($)
Federal [Member]
Tax carryforwards, net operating losses	$ 852
Tax carryforwards, tax credits	267
Federal [Member] | IPL [Member]
Tax carryforwards, net operating losses	537
Tax carryforwards, tax credits	119
Federal [Member] | WPL [Member]
Tax carryforwards, net operating losses	208
Tax carryforwards, tax credits	131
State [Member]
Tax carryforwards, net operating losses	700
State [Member] | IPL [Member]
Tax carryforwards, net operating losses	13
State [Member] | WPL [Member]
Tax carryforwards, net operating losses	$ 2

Income Taxes (Schedule Of Uncer

Income Taxes (Schedule Of Uncertain Tax Positions) (Details)	12 Months Ended
	Dec. 31, 2017
Statute of limitations, expiration period from extended due date of federal tax return	3 years
Statute of limitations, expiration period from extended due date of Iowa tax return	3 years
Statute of limitations, expiration period from extended due date of Wisconsin tax return	4 years

Benefit Plans (Narrative) (Deta

Benefit Plans (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Benefit Plans [Line Items]
Total plan assets, percentage of common stock (less than 1%)	1.00%	1.00%
Common stock percentage in assets held in 401(k) saving plans	11.50%	12.60%
Unrecognized compensation cost	$ 5.6
Percentage of base salary and performance-based compensation	100.00%
Carrying value of deferred compensation obligations	$ 21.8	$ 19.4
Performance Shares and Performance Units [Member]
Benefit Plans [Line Items]
Performance period	3 years
Actual number of shares paid out upon vesting, maximum percentage of target shares	200.00%
Actual number of shares paid out upon vesting, minimum percentage of target shares	0.00%
Performance Shares [Member]
Benefit Plans [Line Items]
Instrument valuation based on shares of common stock, number of shares	1
Performance Units [Member]
Benefit Plans [Line Items]
Instrument valuation based on shares of common stock, number of shares	1
Performance Restricted Stock Unit And Performance Restricted Unit [Member]
Benefit Plans [Line Items]
Performance period	3 years
Actual number of shares paid out upon vesting, maximum percentage of target shares	200.00%
Actual number of shares paid out upon vesting, minimum percentage of target shares	0.00%
Performance Restricted Stock Unit [Member]
Benefit Plans [Line Items]
Instrument valuation based on shares of common stock, number of shares	1
Restricted Stock Unit And Restricted Unit [Member]
Benefit Plans [Line Items]
Performance period	3 years
Instrument valuation based on shares of common stock, number of shares	1
Minimum [Member]
Benefit Plans [Line Items]
Unrecognized compensation cost recognized over a weighted average period	1 year
Maximum [Member]
Benefit Plans [Line Items]
Unrecognized compensation cost recognized over a weighted average period	2 years
IPL [Member]
Benefit Plans [Line Items]
Unrecognized compensation cost	$ 3.1
WPL [Member]
Benefit Plans [Line Items]
Unrecognized compensation cost	2.2
Other Postretirement Benefits Plans [Member]
Benefit Plans [Line Items]
Plan asset threshold for long-term allocation targets	5
Other Postretirement Benefits Plans [Member] | IPL [Member]
Benefit Plans [Line Items]
Plan asset threshold for long-term allocation targets	5
Other Postretirement Benefits Plans [Member] | WPL [Member]
Benefit Plans [Line Items]
Plan asset threshold for long-term allocation targets	$ 5
Omnibus Incentive Plan [Member]
Benefit Plans [Line Items]
Shares available for issuance under the Amended and Restated OIP	7,100,000
2015 Grant [Member] | Performance Units [Member]
Benefit Plans [Line Items]
Instrument valuation based on shares of common stock, number of shares	1
2016 Grant [Member] | Performance Units [Member]
Benefit Plans [Line Items]
Instrument valuation based on shares of common stock, number of shares	1

Benefit Plans (Assumptions Used

Benefit Plans (Assumptions Used To Measure Benefit Plans) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate for benefit obligations	3.66%	4.19%	4.47%
Discount rate for net periodic cost	4.19%	4.47%	4.18%
Expected rate of return on plan assets	7.60%	7.60%	7.60%
Pension Plans, Defined Benefit [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate for benefit obligations	3.68%	4.22%	4.50%
Discount rate for net periodic cost	4.22%	4.50%	4.20%
Expected rate of return on plan assets	7.60%	7.60%	7.60%
Rate of compensation increase	3.65%	3.65%	3.65%
Pension Plans, Defined Benefit [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate for benefit obligations	3.69%	4.23%	4.51%
Discount rate for net periodic cost	4.23%	4.51%	4.20%
Expected rate of return on plan assets	7.60%	7.60%	7.60%
Rate of compensation increase	3.65%	3.65%	3.65%
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate for benefit obligations	3.53%	3.98%	4.30%
Discount rate for net periodic cost	3.98%	4.30%	3.97%
Expected rate of return on plan assets	5.80%	6.30%	6.20%
Medical cost trend on covered charges, initial trend rate (end of year)	6.75%	7.00%	7.25%
Medical cost trend on covered charges, ultimate trend rate	5.00%	5.00%	5.00%
Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate for benefit obligations	3.51%	3.95%	4.28%
Discount rate for net periodic cost	3.95%	4.28%	3.94%
Expected rate of return on plan assets	6.20%	6.60%	6.60%
Medical cost trend on covered charges, initial trend rate (end of year)	6.75%	7.00%	7.25%
Medical cost trend on covered charges, ultimate trend rate	5.00%	5.00%	5.00%
Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate for benefit obligations	3.51%	3.96%	4.28%
Discount rate for net periodic cost	3.96%	4.28%	3.96%
Expected rate of return on plan assets	3.50%	4.70%	4.60%
Medical cost trend on covered charges, initial trend rate (end of year)	6.75%	7.00%	7.25%
Medical cost trend on covered charges, ultimate trend rate	5.00%	5.00%	5.00%
Minimum [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Rate of compensation increase	3.65%	3.65%	3.65%
Maximum [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Rate of compensation increase	4.50%	4.50%	4.50%

Benefit Plans (Defined Benefit

Benefit Plans (Defined Benefit Pension And Other Postretirement Benefits Plans) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 12.5	$ 12.6	$ 15.9
Interest cost		51	53	53.6
Expected return on plan assets	[1]	(65.5)	(65.5)	(75)
Amortization of prior service cost (credit)		(0.4)	(0.3)	(0.2)
Amortization of actuarial loss	[2]	37.6	37.4	35.4
Additional benefit costs		0	0	0.5
Settlement losses	[3]	0.9	0	0
Total		36.1	37.2	30.2
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		5	5.3	5.5
Interest cost		8.6	9.4	9.1
Expected return on plan assets	[1]	(6.1)	(6.1)	(8.4)
Amortization of prior service cost (credit)	[4]	(0.2)	(4.1)	(11.3)
Amortization of actuarial loss	[2]	3.8	4.7	4.8
Additional benefit costs		0	0	0
Settlement losses		0	0	0
Total		11.1	9.2	(0.3)
IPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		7.3	7.5	8.8
Interest cost		23.5	24.5	25
Expected return on plan assets	[1]	(30.8)	(30.9)	(35.8)
Amortization of prior service cost (credit)		(0.2)	(0.2)	(0.1)
Amortization of actuarial loss	[2]	16.1	16.5	15.3
Total		15.9	17.4	13.2
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		2.1	2.3	2.4
Interest cost		3.5	3.8	3.8
Expected return on plan assets	[1]	(4.3)	(4.3)	(5.7)
Amortization of prior service cost (credit)	[4]	0	(2.6)	(6.1)
Amortization of actuarial loss	[2]	2	2.6	2.3
Total		3.3	1.8	(3.3)
WPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		4.9	4.9	5.8
Interest cost		21.8	22.3	22.6
Expected return on plan assets	[1]	(28.5)	(28.3)	(32.4)
Amortization of prior service cost (credit)		0.1	0.2	0.2
Amortization of actuarial loss	[2]	18.5	17.6	16.8
Additional benefit costs		0	0	0.5
Total		16.8	16.7	13.5
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		1.9	2	2.1
Interest cost		3.4	3.8	3.7
Expected return on plan assets	[1]	(0.8)	(0.8)	(1.5)
Amortization of prior service cost (credit)	[4]	(0.2)	(0.9)	(3.5)
Amortization of actuarial loss	[2]	1.6	1.8	2.2
Additional benefit costs		0	0	0
Total		$ 5.9	$ 5.9	$ 3
[1]	The expected return on plan assets is based on the expected rate of return on plan assets and the fair value approach to the market-related value of plan assets.
[2]	Unrecognized net actuarial gains or losses in excess of 10% of the greater of the plans’ benefit obligations or assets are amortized over the average future service lives of plan participants, except for the Alliant Energy Cash Balance Pension Plan where gains or losses outside the 10% threshold are amortized over the time period the participants are expected to receive benefits.
[3]	Settlement losses related to payments made to retired executives of Alliant Energy.
[4]	Unrecognized prior service costs (credits) for the OPEB plans are amortized over the average future service period to full eligibility of the participants of each plan.

Benefit Plans (Estimated Amorit

Benefit Plans (Estimated Amoritization From Regulatory Assets, Regulatory Liabilities And Accumulated Other Comprehensive Loss) (Details) - Scenario, Forecast [Member] $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	$ 35.2
Prior service credit	(0.7)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	34.5
Defined Benefit Pension Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	15
Prior service credit	(0.2)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	14.8
Defined Benefit Pension Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	17.2
Prior service credit	(0.2)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	17
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	3.4
Prior service credit	(0.2)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	3.2
Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	1.2
Prior service credit	0
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	1.2
Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	2
Prior service credit	(0.2)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	$ 1.8

Benefit Plans (Funded Status of

Benefit Plans (Funded Status of Benefits Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plans, Defined Benefit [Member]
Change in benefit obligation:
Net benefit obligation at January 1	$ 1,244.3	$ 1,206.3
Service cost	12.5	12.6	$ 15.9
Interest cost	51	53	53.6
Plan participants' contributions	0	0
Actuarial (gain) loss	83.6	48.3
Gross benefits paid	(88.3)	(75.9)
Net benefit obligation at December 31	1,303.1	1,244.3	1,206.3
Change in plan assets:
Fair value of plan assets at January 1	895.7	895
Actual return on plan assets	136.7	74.3
Employer contributions	6.6	2.3
Plan participants' contributions	0	0
Gross benefits paid	(88.3)	(75.9)
Fair value of plan assets at December 31	950.7	895.7	895
Under funded status at December 31	(352.4)	(348.6)
Pension Plans, Defined Benefit [Member] | IPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	570.4	556.1
Service cost	7.3	7.5	8.8
Interest cost	23.5	24.5	25
Plan participants' contributions	0	0
Actuarial (gain) loss	34.9	19.1
Gross benefits paid	(43.2)	(36.8)
Net benefit obligation at December 31	592.9	570.4	556.1
Change in plan assets:
Fair value of plan assets at January 1	422	422.7
Actual return on plan assets	64.3	35.3
Employer contributions	0.6	0.8
Plan participants' contributions	0	0
Gross benefits paid	(43.2)	(36.8)
Fair value of plan assets at December 31	443.7	422	422.7
Under funded status at December 31	(149.2)	(148.4)
Pension Plans, Defined Benefit [Member] | WPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	529.2	505.9
Service cost	4.9	4.9	5.8
Interest cost	21.8	22.3	22.6
Plan participants' contributions	0	0
Actuarial (gain) loss	38.3	25.7
Gross benefits paid	(34.4)	(29.6)
Net benefit obligation at December 31	559.8	529.2	505.9
Change in plan assets:
Fair value of plan assets at January 1	389.7	386.8
Actual return on plan assets	59.6	32.4
Employer contributions	0.1	0.1
Plan participants' contributions	0	0
Gross benefits paid	(34.4)	(29.6)
Fair value of plan assets at December 31	415	389.7	386.8
Under funded status at December 31	(144.8)	(139.5)
Other Postretirement Benefits Plans [Member]
Change in benefit obligation:
Net benefit obligation at January 1	220.1	221.4
Service cost	5	5.3	5.5
Interest cost	8.6	9.4	9.1
Plan participants' contributions	2.9	2.4
Actuarial (gain) loss	5.4	(0.3)
Gross benefits paid	(19.7)	(18.1)
Net benefit obligation at December 31	222.3	220.1	221.4
Change in plan assets:
Fair value of plan assets at January 1	105.8	106.9
Actual return on plan assets	12.9	8.2
Employer contributions	9.2	6.4
Plan participants' contributions	2.9	2.4
Gross benefits paid	(19.7)	(18.1)
Fair value of plan assets at December 31	111.1	105.8	106.9
Under funded status at December 31	(111.2)	(114.3)
Other Postretirement Benefits Plans [Member] | IPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	90.1	91.3
Service cost	2.1	2.3	2.4
Interest cost	3.5	3.8	3.8
Plan participants' contributions	1	0.9
Actuarial (gain) loss	(0.1)	(0.7)
Gross benefits paid	(7.2)	(7.5)
Net benefit obligation at December 31	89.4	90.1	91.3
Change in plan assets:
Fair value of plan assets at January 1	68.2	69.2
Actual return on plan assets	8.9	5.3
Employer contributions	2	0.3
Plan participants' contributions	1	0.9
Gross benefits paid	(7.2)	(7.5)
Fair value of plan assets at December 31	72.9	68.2	69.2
Under funded status at December 31	(16.5)	(21.9)
Other Postretirement Benefits Plans [Member] | WPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	88.9	89.7
Service cost	1.9	2	2.1
Interest cost	3.4	3.8	3.7
Plan participants' contributions	1.4	1.2
Actuarial (gain) loss	4.1	0.5
Gross benefits paid	(9.3)	(8.3)
Net benefit obligation at December 31	90.4	88.9	89.7
Change in plan assets:
Fair value of plan assets at January 1	18.6	18.7
Actual return on plan assets	1.2	1.2
Employer contributions	6.8	5.8
Plan participants' contributions	1.4	1.2
Gross benefits paid	(9.3)	(8.3)
Fair value of plan assets at December 31	18.7	18.6	$ 18.7
Under funded status at December 31	$ (71.7)	$ (70.3)

Benefit Plans (Amounts Recogniz

Benefit Plans (Amounts Recognized On The Consolidated Balance Sheets) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Amounts recognized on the balance sheets consist of:
Pension and other benefit obligations	$ (504)	$ (489.9)
Pension Plans, Defined Benefit [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	0	0
Current liabilities	(2.2)	(6.5)
Pension and other benefit obligations	(350.2)	(342.1)
Net amounts recognized at December 31	(352.4)	(348.6)
Amounts recognized in Regulatory Assets and AOCL consist of:
Net actuarial loss	509.1	535.1
Prior service credit	(6.5)	(6.9)
Net amount recognized at December 31	502.6	528.2
Other Postretirement Benefits Plans [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	8.8	3.2
Current liabilities	(9.1)	(8.6)
Pension and other benefit obligations	(110.9)	(108.9)
Net amounts recognized at December 31	(111.2)	(114.3)
Amounts recognized in Regulatory Assets and AOCL consist of:
Net actuarial loss	47.4	52.6
Prior service credit	(1.3)	(1.5)
Net amount recognized at December 31	46.1	51.1
IPL [Member]
Amounts recognized on the balance sheets consist of:
Pension and other benefit obligations	(173.8)	(173.2)
IPL [Member] | Pension Plans, Defined Benefit [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	0	0
Current liabilities	(0.5)	(0.7)
Pension and other benefit obligations	(148.7)	(147.7)
Net amounts recognized at December 31	(149.2)	(148.4)
Amounts recognized in Regulatory Assets and AOCL consist of:
Net actuarial loss	218.9	233.6
Prior service credit	(2.1)	(2.3)
Net amount recognized at December 31	216.8	231.3
IPL [Member] | Other Postretirement Benefits Plans [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	5.9	0.4
Current liabilities	(2)	(1.9)
Pension and other benefit obligations	(20.4)	(20.4)
Net amounts recognized at December 31	(16.5)	(21.9)
Amounts recognized in Regulatory Assets and AOCL consist of:
Net actuarial loss	18.7	25.4
Prior service credit	0	0
Net amount recognized at December 31	18.7	25.4
WPL [Member]
Amounts recognized on the balance sheets consist of:
Pension and other benefit obligations	(213.7)	(207.8)
WPL [Member] | Pension Plans, Defined Benefit [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	0	0
Current liabilities	(0.1)	(0.1)
Pension and other benefit obligations	(144.7)	(139.4)
Net amounts recognized at December 31	(144.8)	(139.5)
Amounts recognized in Regulatory Assets and AOCL consist of:
Net actuarial loss	224.7	236.1
Prior service credit	(1.5)	(1.4)
Net amount recognized at December 31	223.2	234.7
WPL [Member] | Other Postretirement Benefits Plans [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	2.9	2.7
Current liabilities	(6.8)	(6.4)
Pension and other benefit obligations	(67.8)	(66.6)
Net amounts recognized at December 31	(71.7)	(70.3)
Amounts recognized in Regulatory Assets and AOCL consist of:
Net actuarial loss	23.6	21.5
Prior service credit	(1.3)	(1.5)
Net amount recognized at December 31	$ 22.3	$ 20

Benefit Plans (Accumulated Bene

Benefit Plans (Accumulated Benefit Obligations) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	$ 1,269	$ 1,201.5
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	1,269	1,201.5
Fair value of plan assets	950.7	895.7
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	1,303.1	1,244.3
Fair value of plan assets	950.7	895.7
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	222.3	220.1
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	222.3	220.1
Fair value of plan assets	111.1	105.8
Plans with projected benefit obligations in excess of plan assets:
Fair value of plan assets	107.5	102.8
IPL [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	573.1	546.7
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	573.1	546.7
Fair value of plan assets	443.7	422
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	592.9	570.4
Fair value of plan assets	443.7	422
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	89.4	90.1
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	89.4	90.1
Fair value of plan assets	72.9	68.2
Plans with projected benefit obligations in excess of plan assets:
Fair value of plan assets		68.2
WPL [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	548.1	513.2
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	548.1	513.2
Fair value of plan assets	415	389.7
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	559.8	529.2
Fair value of plan assets	415	389.7
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	90.4	88.9
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	90.4	88.9
Fair value of plan assets	$ 18.7	$ 18.6

Benefit Plans (Regulatory Asset

Benefit Plans (Regulatory Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	$ 38.9	$ 37.3
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	$ 28.1	$ 30

Benefit Plans (Estimated Future

Benefit Plans (Estimated Future Funding) (Details) - Scenario, Forecast [Member] $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2018	$ 6.3	[1]
Defined Benefit Pension Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2018	4.4	[1]
Defined Benefit Pension Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2018	0.3	[1]
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2018	9
Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2018	2
Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2018	$ 6.8
[1]	Alliant Energy sponsors several non-qualified defined benefit pension plans that cover certain current and former key employees of IPL and WPL. Alliant Energy allocates pension costs to IPL and WPL for these plans. In addition, IPL and WPL amounts reflect funding for their non-bargaining employees who are participants in the Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans.

Benefit Plans (Expected Benefit

Benefit Plans (Expected Benefit Payments) (Details) $ in Millions	Dec. 31, 2017USD ($)
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2018	$ 90.4
Expected benefit payments, 2019	92.3
Expected benefit payments, 2020	94.1
Expected benefit payments, 2021	94.6
Expected benefit payments, 2022	110.1
Expected benefit payments, 2023 - 2027	478.3
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2018	72.2
Expected benefit payments, 2019	73.9
Expected benefit payments, 2020	76.4
Expected benefit payments, 2021	77.1
Expected benefit payments, 2022	92.9
Expected benefit payments, 2023 - 2027	398.2
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2018	18.2
Expected benefit payments, 2019	18.4
Expected benefit payments, 2020	17.7
Expected benefit payments, 2021	17.5
Expected benefit payments, 2022	17.2
Expected benefit payments, 2023 - 2027	80.1
IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2018	41
Expected benefit payments, 2019	40.9
Expected benefit payments, 2020	43.7
Expected benefit payments, 2021	43.7
Expected benefit payments, 2022	44.6
Expected benefit payments, 2023 - 2027	221
IPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2018	33.9
Expected benefit payments, 2019	33.8
Expected benefit payments, 2020	36.5
Expected benefit payments, 2021	36.6
Expected benefit payments, 2022	37.6
Expected benefit payments, 2023 - 2027	188.4
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2018	7.1
Expected benefit payments, 2019	7.1
Expected benefit payments, 2020	7.2
Expected benefit payments, 2021	7.1
Expected benefit payments, 2022	7
Expected benefit payments, 2023 - 2027	32.6
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2018	39.7
Expected benefit payments, 2019	40.3
Expected benefit payments, 2020	39.9
Expected benefit payments, 2021	39.7
Expected benefit payments, 2022	39.7
Expected benefit payments, 2023 - 2027	199.6
WPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2018	31.4
Expected benefit payments, 2019	32
Expected benefit payments, 2020	32.4
Expected benefit payments, 2021	32.4
Expected benefit payments, 2022	32.7
Expected benefit payments, 2023 - 2027	167.8
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2018	8.3
Expected benefit payments, 2019	8.3
Expected benefit payments, 2020	7.5
Expected benefit payments, 2021	7.3
Expected benefit payments, 2022	7
Expected benefit payments, 2023 - 2027	$ 31.8

Benefit Plans (Allocation Of Pl

Benefit Plans (Allocation Of Plan Assets) (Details)	12 Months Ended
	Dec. 31, 2017
Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, minimum	0.00%
Target range allocation, maximum	5.00%
Actual allocation	3.00%
Pension Plans, Defined Benefit [Member] | Equity securites - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, minimum	11.00%
Target range allocation, maximum	41.00%
Actual allocation	24.00%
Pension Plans, Defined Benefit [Member] | Equity securities - international [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, minimum	14.00%
Target range allocation, maximum	34.00%
Actual allocation	23.00%
Pension Plans, Defined Benefit [Member] | Global asset securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, minimum	5.00%
Target range allocation, maximum	15.00%
Actual allocation	10.00%
Pension Plans, Defined Benefit [Member] | Risk parity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, minimum	5.00%
Target range allocation, maximum	15.00%
Actual allocation	10.00%
Pension Plans, Defined Benefit [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, minimum	20.00%
Target range allocation, maximum	40.00%
Actual allocation	30.00%
Other Postretirement Benefits Plans [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, minimum	0.00%
Target range allocation, maximum	5.00%
Actual allocation	1.00%
Other Postretirement Benefits Plans [Member] | Equity securites - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, minimum	0.00%
Target range allocation, maximum	50.00%
Actual allocation	26.00%
Other Postretirement Benefits Plans [Member] | Equity securities - international [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, minimum	0.00%
Target range allocation, maximum	34.00%
Actual allocation	10.00%
Other Postretirement Benefits Plans [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, minimum	20.00%
Target range allocation, maximum	100.00%
Actual allocation	63.00%

Benefit Plans (Fair Value Of Pl

Benefit Plans (Fair Value Of Plan Assets By Asset Category And Fair Value Hierarchy Level) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 950.7	$ 895.7	$ 895
Assets measured at net asset value	441.1	405.9
Total assets in fair value hierarchy	950.7	895.7
Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	28.2	30.4
Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4.5	5
Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	23.7	25.4
Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | Equity securites - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	158.3	183.6
Pension Plans, Defined Benefit [Member] | Equity securites - U.S. [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	158.3	183.6
Pension Plans, Defined Benefit [Member] | Equity securites - U.S. [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | Equity securites - U.S. [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | Equity securities - international [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	137.5	97.4
Pension Plans, Defined Benefit [Member] | Equity securities - international [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	137.5	97.4
Pension Plans, Defined Benefit [Member] | Equity securities - international [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | Equity securities - international [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | Global asset securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	49.4	53
Pension Plans, Defined Benefit [Member] | Global asset securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	49.4	53
Pension Plans, Defined Benefit [Member] | Global asset securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | Global asset securities [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	135.9	125.4
Pension Plans, Defined Benefit [Member] | Fixed income securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	55.8	53.6
Pension Plans, Defined Benefit [Member] | Fixed income securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	80.1	71.8
Pension Plans, Defined Benefit [Member] | Fixed income securities [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | Subtotal [Member]
Defined Benefit Plan Disclosure [Line Items]
Total assets in fair value hierarchy	509.3	489.8
Pension Plans, Defined Benefit [Member] | Subtotal [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	405.5	392.6
Pension Plans, Defined Benefit [Member] | Subtotal [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	103.8	97.2
Pension Plans, Defined Benefit [Member] | Subtotal [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | Accrued investment income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	1.1
Pension Plans, Defined Benefit [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	443.7	422	422.7
Assets measured at net asset value	205.8	191.2
Total assets in fair value hierarchy	443.7	422
Pension Plans, Defined Benefit [Member] | IPL [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13.2	14.4
Pension Plans, Defined Benefit [Member] | IPL [Member] | Cash and equivalents [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2.2	2.4
Pension Plans, Defined Benefit [Member] | IPL [Member] | Cash and equivalents [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11	12
Pension Plans, Defined Benefit [Member] | IPL [Member] | Cash and equivalents [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | IPL [Member] | Equity securites - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	73.9	86.5
Pension Plans, Defined Benefit [Member] | IPL [Member] | Equity securites - U.S. [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	73.9	86.5
Pension Plans, Defined Benefit [Member] | IPL [Member] | Equity securites - U.S. [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | IPL [Member] | Equity securites - U.S. [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | IPL [Member] | Equity securities - international [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	64.2	45.9
Pension Plans, Defined Benefit [Member] | IPL [Member] | Equity securities - international [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	64.2	45.9
Pension Plans, Defined Benefit [Member] | IPL [Member] | Equity securities - international [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | IPL [Member] | Equity securities - international [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | IPL [Member] | Global asset securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	23	24.9
Pension Plans, Defined Benefit [Member] | IPL [Member] | Global asset securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	23	24.9
Pension Plans, Defined Benefit [Member] | IPL [Member] | Global asset securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | IPL [Member] | Global asset securities [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | IPL [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	63.4	59.1
Pension Plans, Defined Benefit [Member] | IPL [Member] | Fixed income securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	26	25.3
Pension Plans, Defined Benefit [Member] | IPL [Member] | Fixed income securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	37.4	33.8
Pension Plans, Defined Benefit [Member] | IPL [Member] | Fixed income securities [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | IPL [Member] | Subtotal [Member]
Defined Benefit Plan Disclosure [Line Items]
Total assets in fair value hierarchy	237.7	230.8
Pension Plans, Defined Benefit [Member] | IPL [Member] | Subtotal [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	189.3	185
Pension Plans, Defined Benefit [Member] | IPL [Member] | Subtotal [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	48.4	45.8
Pension Plans, Defined Benefit [Member] | IPL [Member] | Subtotal [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | IPL [Member] | Accrued investment income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.5	0.5
Pension Plans, Defined Benefit [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	415	389.7	386.8
Assets measured at net asset value	192.5	176.6
Total assets in fair value hierarchy	415	389.7
Pension Plans, Defined Benefit [Member] | WPL [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12.3	13.3
Pension Plans, Defined Benefit [Member] | WPL [Member] | Cash and equivalents [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2	2.2
Pension Plans, Defined Benefit [Member] | WPL [Member] | Cash and equivalents [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10.3	11.1
Pension Plans, Defined Benefit [Member] | WPL [Member] | Cash and equivalents [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | WPL [Member] | Equity securites - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	69.1	79.9
Pension Plans, Defined Benefit [Member] | WPL [Member] | Equity securites - U.S. [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	69.1	79.9
Pension Plans, Defined Benefit [Member] | WPL [Member] | Equity securites - U.S. [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | WPL [Member] | Equity securites - U.S. [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | WPL [Member] | Equity securities - international [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	60	42.4
Pension Plans, Defined Benefit [Member] | WPL [Member] | Equity securities - international [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	60	42.4
Pension Plans, Defined Benefit [Member] | WPL [Member] | Equity securities - international [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | WPL [Member] | Equity securities - international [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | WPL [Member] | Global asset securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	21.6	23
Pension Plans, Defined Benefit [Member] | WPL [Member] | Global asset securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	21.6	23
Pension Plans, Defined Benefit [Member] | WPL [Member] | Global asset securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | WPL [Member] | Global asset securities [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | WPL [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	59.3	54.5
Pension Plans, Defined Benefit [Member] | WPL [Member] | Fixed income securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	24.3	23.3
Pension Plans, Defined Benefit [Member] | WPL [Member] | Fixed income securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	35	31.2
Pension Plans, Defined Benefit [Member] | WPL [Member] | Fixed income securities [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | WPL [Member] | Subtotal [Member]
Defined Benefit Plan Disclosure [Line Items]
Total assets in fair value hierarchy	222.3	213.1
Pension Plans, Defined Benefit [Member] | WPL [Member] | Subtotal [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	177	170.8
Pension Plans, Defined Benefit [Member] | WPL [Member] | Subtotal [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	45.3	42.3
Pension Plans, Defined Benefit [Member] | WPL [Member] | Subtotal [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | WPL [Member] | Accrued investment income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.5	0.5
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	111.1	105.8	106.9
Assets measured at net asset value	3.6	3
Total assets in fair value hierarchy	107.5	102.8
Other Postretirement Benefit Plans, Defined Benefit [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	106.4	100.7
Other Postretirement Benefit Plans, Defined Benefit [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1.1	2.1
Other Postretirement Benefit Plans, Defined Benefit [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1.2	3.5
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash and equivalents [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.7	2
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash and equivalents [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.5	1.5
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash and equivalents [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Equity securites - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	27.9	22.5
Other Postretirement Benefit Plans, Defined Benefit [Member] | Equity securites - U.S. [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	27.9	22.5
Other Postretirement Benefit Plans, Defined Benefit [Member] | Equity securites - U.S. [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Equity securites - U.S. [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Equity securities - international [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11.4	13.5
Other Postretirement Benefit Plans, Defined Benefit [Member] | Equity securities - international [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11.4	13.5
Other Postretirement Benefit Plans, Defined Benefit [Member] | Equity securities - international [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Equity securities - international [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Global asset securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.4	16.5
Other Postretirement Benefit Plans, Defined Benefit [Member] | Global asset securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.4	16.5
Other Postretirement Benefit Plans, Defined Benefit [Member] | Global asset securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Global asset securities [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	66.6	46.8
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fixed income securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	66	46.2
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fixed income securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.6	0.6
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fixed income securities [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	72.9	68.2	69.2
Total assets in fair value hierarchy		68.2
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	72.9	68.2
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.3	0.8
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Cash and equivalents [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.3	0.8
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Cash and equivalents [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Cash and equivalents [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Equity securites - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	22.3	17
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Equity securites - U.S. [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	22.3	17
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Equity securites - U.S. [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Equity securites - U.S. [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Equity securities - international [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7.5	11
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Equity securities - international [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7.5	11
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Equity securities - international [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Equity securities - international [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Global asset securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	7
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Global asset securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	7
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Global asset securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Global asset securities [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	42.8	32.4
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Fixed income securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	42.8	32.4
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Fixed income securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Fixed income securities [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18.7	18.6	$ 18.7
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18.4	17.3
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.3	1.3
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.6	2
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Cash and equivalents [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.3	0.7
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Cash and equivalents [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.3	1.3
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Cash and equivalents [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Global asset securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	5.5
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Global asset securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	5.5
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Global asset securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Global asset securities [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18.1	11.1
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Fixed income securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18.1	11.1
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Fixed income securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Fixed income securities [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Defined Benefit Plan Due To Brokers Net [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(0.7)	(1.1)
Defined Benefit Plan Due To Brokers Net [Member] | Pension Plans, Defined Benefit [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(0.3)	(0.5)
Defined Benefit Plan Due To Brokers Net [Member] | Pension Plans, Defined Benefit [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ (0.3)	$ (0.5)

Benefit Plans (Employees Partic

Benefit Plans (Employees Participate In Defined Contribution Retirement Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
401(k) costs	$ 24.8	$ 23.6	$ 24.9
IPL [Member]
401(k) costs	12.8	12	12.7
WPL [Member]
401(k) costs	$ 11.1	$ 10.7	$ 11.2

Benefit Plans (Recognized Compe

Benefit Plans (Recognized Compensation Expense And Income Tax Benefits) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Compensation expense	$ 15.1	$ 18	$ 10.7
Income tax benefits	6.2	7.4	4.4
IPL [Member]
Compensation expense	8.3	9.5	5.7
Income tax benefits	3.4	4	2.4
WPL [Member]
Compensation expense	6.4	7.9	4.7
Income tax benefits	$ 2.6	$ 3.2	$ 1.9

Benefit Plans (Summary Of Perfo

Benefit Plans (Summary Of Performance Shares and Performance Units Activity) (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Nonvested, January 1 (in shares/awards)	257,599	288,430	288,848
Granted (in shares/awards)	65,350	68,585	90,806
Vested (in shares/awards)	(99,438)	(98,186)	(91,224)
Forfeited (in shares/awards)	0	(1,230)	0
Nonvested, December 31 (in shares/awards)	223,511	257,599	288,430
Performance Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Nonvested, January 1 (in shares/awards)	93,320	116,412	127,330
Granted (in shares/awards)	21,558	23,918	35,674
Vested (in shares/awards)	(37,395)	(42,760)	(45,690)
Forfeited (in shares/awards)	(5,746)	(4,250)	(902)
Nonvested, December 31 (in shares/awards)	71,737	93,320	116,412

Benefit Plans (Summary Of Pe111

Benefit Plans (Summary Of Performance Shares and Performance Units Vested Awards) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares/awards)	99,438	98,186	91,224
Performance Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares/awards)	37,395	42,760	45,690
2014 Grant [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares/awards)	99,438
Vested percentage of the target	147.50%
Cash and stock payout value	$ 5.6
Cash payout value	$ 5.1
Common stock shares from vested performance shares (in shares)	5,185
2014 Grant [Member] | Performance Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares/awards)	37,395
Vested percentage of the target	147.50%
Cash and stock payout value	$ 1.5
Cash payout value	$ 1.5
2013 Grant [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares/awards)		98,186
Vested percentage of the target		165.00%
Cash and stock payout value		$ 5.1
Cash payout value		$ 2.9
Common stock shares from vested performance shares (in shares)		22,408
2013 Grant [Member] | Performance Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares/awards)		42,760
Vested percentage of the target		165.00%
Cash and stock payout value		$ 1.7
Cash payout value		$ 1.7
2012 Grant [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares/awards)			91,224
Vested percentage of the target			167.50%
Cash and stock payout value			$ 5.1
Cash payout value			$ 3.2
Common stock shares from vested performance shares (in shares)			21,950
2012 Grant [Member] | Performance Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares/awards)			45,690
Vested percentage of the target			167.50%
Cash and stock payout value			$ 1.6
Cash payout value			$ 1.6

Benefit Plans (Fair Values Of N

Benefit Plans (Fair Values Of Nonvested Performance Shares And Performance Units) (Details)	12 Months Ended
	Dec. 31, 2017$ / sharesshares
Performance Shares [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards) | shares	223,511
Performance Units [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards) | shares	71,737
2017 Grant [Member] | Performance Shares [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards) | shares	65,350
Alliant Energy common stock closing price on December 29, 2017 (in dollars per share)	$ 42.61
Estimated payout percentage based on performance criteria	105.00%
Fair values of each nonvested award (in dollars per share)	$ 44.74
2017 Grant [Member] | Performance Units [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards) | shares	18,600
Alliant Energy common stock closing price on December 29, 2017 (in dollars per share)	$ 42.61
Estimated payout percentage based on performance criteria	105.00%
Fair values of each nonvested award (in dollars per share)	$ 44.74
2016 Grant [Member] | Performance Shares [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards) | shares	67,355
Alliant Energy common stock closing price on December 29, 2017 (in dollars per share)	$ 42.61
Estimated payout percentage based on performance criteria	150.00%
Fair values of each nonvested award (in dollars per share)	$ 63.92
2016 Grant [Member] | Performance Units [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards) | shares	21,227
Alliant Energy common stock closing price on December 29, 2017 (in dollars per share)	$ 42.61
Estimated payout percentage based on performance criteria	150.00%
Fair values of each nonvested award (in dollars per share)	$ 63.92
2015 Grant [Member] | Performance Shares [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards) | shares	90,806
Alliant Energy common stock closing price on December 29, 2017 (in dollars per share)	$ 42.61
Estimated payout percentage based on performance criteria	138.00%
Fair values of each nonvested award (in dollars per share)	$ 58.80
2015 Grant [Member] | Performance Units [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards) | shares	31,910
Alliant Energy common stock closing price on grant date (in dollars per share)	$ 32.55
Estimated payout percentage based on performance criteria	138.00%
Fair values of each nonvested award (in dollars per share)	$ 44.92

Benefit Plans (Summary of Pe113

Benefit Plans (Summary of Performance Restricted Stock Units) (Details) - Performance Restricted Stock Unit [Member] - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Nonvested, January 1 (in shares/awards)	67,355	0
Nonvested shares, January 1, weighted average grant date fair value (in dollars per share)	$ 33.96	$ 0
Granted (in shares/awards)	65,350	68,585
Granted, weighted average grant date fair value (in dollars per share)	$ 39.12	$ 33.96
Forfeited (in shares/awards)	0	(1,230)
Forfeited, weighted average grant date fair value (in dollars per share)	$ 0	$ 33.90
Nonvested, December 31 (in shares/awards)	132,705	67,355
Nonvested shares, December 31, weighted average grant date fair value (in dollars per share)	$ 36.50	$ 33.96

Benefit Plans (Summary of Restr

Benefit Plans (Summary of Restricted Stock Units) (Details) - Restricted Stock Units [Member] - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Nonvested, January 1 (in shares/awards)	57,736	0
Granted (in shares/awards)	56,013	58,790
Forfeited (in shares/awards)	0	(1,054)
Nonvested, December 31 (in shares/awards)	113,749	57,736

Benefit Plans (Carrying Value A

Benefit Plans (Carrying Value And Fair Market Value Of The Deferred Compensation Obligation for Company Stock Account) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Carrying value of deferred compensation obligation	$ 11.1	$ 10
Fair market value of deferred compensation obligation	$ 19.7	$ 16.7

Asset Retirement Obligations (R

Asset Retirement Obligations (Reconciliation Of Changes In Asset Retirement Obligations) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1	$ 195.7	$ 214
Revisions in estimated cash flows	4.3	(13.3)
Liabilities settled	(23.5)	(14)
Liabilities incurred	2	2.6
Accretion expense	6	6.4
Balance, December 31	184.5	195.7
IPL [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1	124.7	132.9
Revisions in estimated cash flows	7	(5.8)
Liabilities settled	(13.1)	(6.8)
Liabilities incurred	11.7	0.7
Accretion expense	3.8	3.7
Balance, December 31	134.1	124.7
WPL [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1	61.4	71.9
Revisions in estimated cash flows	(2.7)	(7.5)
Liabilities settled	(10.4)	(7.2)
Liabilities incurred	0	1.9
Accretion expense	2.1	2.3
Balance, December 31	$ 50.4	$ 61.4

Fair Value Measurements (Narrat

Fair Value Measurements (Narrative) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
5.1% [Member] | Redeemable Preferred Stock [Member] | IPL [Member]
Cumulative preferred stock rate	5.10%	5.10%

Fair Value Measurements (Recurr

Fair Value Measurements (Recurring Fair Value Measurements) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets:
Deferred proceeds	$ 222.1	$ 211.1
Liabilities and equity:
Long-term debt (including current maturities)	5,447.5	4,799
IPL's cumulative preferred stock	203.8	194.8
Commodity Contracts [Member]
Assets:
Derivatives	25.1	41.4
Liabilities and equity:
Derivatives	41.7	28.6
Level 1 [Member]
Assets:
Deferred proceeds	0	0
Liabilities and equity:
Long-term debt (including current maturities)	0	0
IPL's cumulative preferred stock	203.8	194.8
Level 1 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	0	0
Liabilities and equity:
Derivatives	0	0
Level 2 [Member]
Assets:
Deferred proceeds	0	0
Liabilities and equity:
Long-term debt (including current maturities)	5,444.6	4,795.7
IPL's cumulative preferred stock	0	0
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	4.1	4.6
Liabilities and equity:
Derivatives	8.5	0.5
Level 3 [Member]
Assets:
Deferred proceeds	222.1	211.1
Liabilities and equity:
Long-term debt (including current maturities)	2.9	3.3
IPL's cumulative preferred stock	0	0
Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	21	36.8
Liabilities and equity:
Derivatives	33.2	28.1
Carrying Amount [Member]
Assets:
Deferred proceeds	222.1	211.1
Liabilities and equity:
Long-term debt (including current maturities)	4,866.3	4,320.2
IPL's cumulative preferred stock	200	200
Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	25.1	41.4
Liabilities and equity:
Derivatives	41.7	28.6
IPL [Member]
Assets:
Deferred proceeds	222.1	211.1
Liabilities and equity:
Long-term debt (including current maturities)	2,665.7	2,352.3
IPL's cumulative preferred stock	203.8	194.8
IPL [Member] | Commodity Contracts [Member]
Assets:
Derivatives	17.1	20.8
Liabilities and equity:
Derivatives	19.4	8.3
IPL [Member] | Level 1 [Member]
Assets:
Deferred proceeds	0	0
Liabilities and equity:
Long-term debt (including current maturities)	0	0
IPL's cumulative preferred stock	203.8	194.8
IPL [Member] | Level 1 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	0	0
Liabilities and equity:
Derivatives	0	0
IPL [Member] | Level 2 [Member]
Assets:
Deferred proceeds	0	0
Liabilities and equity:
Long-term debt (including current maturities)	2,665.7	2,352.3
IPL's cumulative preferred stock	0	0
IPL [Member] | Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	2	2.8
Liabilities and equity:
Derivatives	2.9	0.4
IPL [Member] | Level 3 [Member]
Assets:
Deferred proceeds	222.1	211.1
Liabilities and equity:
Long-term debt (including current maturities)	0	0
IPL's cumulative preferred stock	0	0
IPL [Member] | Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	15.1	18
Liabilities and equity:
Derivatives	16.5	7.9
IPL [Member] | Carrying Amount [Member]
Assets:
Deferred proceeds	222.1	211.1
Liabilities and equity:
Long-term debt (including current maturities)	2,406	2,153.5
IPL's cumulative preferred stock	200	200
IPL [Member] | Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	17.1	20.8
Liabilities and equity:
Derivatives	19.4	8.3
WPL [Member]
Liabilities and equity:
Long-term debt (including current maturities)	2,147.9	1,807.4
WPL [Member] | Commodity Contracts [Member]
Assets:
Derivatives	8	20.6
Liabilities and equity:
Derivatives	22.3	20.3
WPL [Member] | Level 1 [Member]
Liabilities and equity:
Long-term debt (including current maturities)	0	0
WPL [Member] | Level 1 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	0	0
Liabilities and equity:
Derivatives	0	0
WPL [Member] | Level 2 [Member]
Liabilities and equity:
Long-term debt (including current maturities)	2,147.9	1,807.4
WPL [Member] | Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	2.1	1.8
Liabilities and equity:
Derivatives	5.6	0.1
WPL [Member] | Level 3 [Member]
Liabilities and equity:
Long-term debt (including current maturities)	0	0
WPL [Member] | Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	5.9	18.8
Liabilities and equity:
Derivatives	16.7	20.2
WPL [Member] | Carrying Amount [Member]
Liabilities and equity:
Long-term debt (including current maturities)	1,833.4	1,535.2
WPL [Member] | Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	8	20.6
Liabilities and equity:
Derivatives	$ 22.3	$ 20.3

Fair Value Measurements (Fair V

Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Commodity Contracts Derivative Assets and (Liabilities), net [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance, January 1		$ 8.7	$ (32.7)
Total net gains (losses) included in changes in net assets (realized/unrealized)		(32.9)	30.7
Transfers into Level 3		0	0.9
Transfers out of Level 3		12.2	1.2
Purchases		28.3	22
Sales		(0.3)	(1)
Settlements		(28.2)	(12.4)
Ending balance, December 31		(12.2)	8.7
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31		(31)	32.7
Commodity Contracts Derivative Assets and (Liabilities), net [Member] | IPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance, January 1		10.1	(1.9)
Total net gains (losses) included in changes in net assets (realized/unrealized)		(14.8)	7.3
Transfers into Level 3		0	0.5
Transfers out of Level 3		3.1	0.2
Purchases		24.6	20.6
Sales		(0.2)	(1)
Settlements		(24.2)	(15.6)
Ending balance, December 31		(1.4)	10.1
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31		(13.5)	8.5
Commodity Contracts Derivative Assets and (Liabilities), net [Member] | WPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance, January 1		(1.4)	(30.8)
Total net gains (losses) included in changes in net assets (realized/unrealized)		(18.1)	23.4
Transfers into Level 3		0	0.4
Transfers out of Level 3		9.1	1
Purchases		3.7	1.4
Sales		(0.1)	0
Settlements		(4)	3.2
Ending balance, December 31		(10.8)	(1.4)
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31		(17.5)	24.2
Deferred Proceeds [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance, January 1		211.1	172
Total net gains (losses) included in changes in net assets (realized/unrealized)		0	0
Transfers into Level 3		0	0
Transfers out of Level 3		0	0
Purchases		0	0
Sales		0	0
Settlements	[1]	11	39.1
Ending balance, December 31		222.1	211.1
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31		0	0
Deferred Proceeds [Member] | IPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance, January 1		211.1	172
Total net gains (losses) included in changes in net assets (realized/unrealized)		0	0
Transfers into Level 3		0	0
Transfers out of Level 3		0	0
Purchases		0	0
Sales		0	0
Settlements	[1]	11	39.1
Ending balance, December 31		222.1	211.1
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at December 31		$ 0	$ 0
[1]	Settlements related to deferred proceeds are due to the change in the carrying amount of receivables sold less the allowance for doubtful accounts associated with the receivables sold and cash amounts received from the receivables sold.

Fair Value Measurements (Fai120

Fair Value Measurements (Fair Value Of Net Derivative Assets (Liabilities)) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	$ (12.2)	$ 8.7	$ (32.7)
Excluding FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative liabilities	23.5	2.3
FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	11.3	11
IPL [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	(1.4)	10.1	(1.9)
IPL [Member] | Excluding FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative liabilities	11.5
Fair value, net derivative assets		0.1
IPL [Member] | FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	10.1	10
WPL [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	(10.8)	(1.4)	$ (30.8)
WPL [Member] | Excluding FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative liabilities	12	2.4
WPL [Member] | FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	$ 1.2	$ 1

Derivative Instruments (Notiona

Derivative Instruments (Notional Amounts Of Derivative Instruments) (Details) - Commodity [Member] gal in Thousands, T in Thousands, MWh in Thousands, Dekatherms in Thousands	12 Months Ended
	Dec. 31, 2017DekathermsTMWhgal
Electricity (MWhs) [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs/Dths/tons)	1,314
Electricity (MWhs) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs/Dths/tons)	0
Electricity (MWhs) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs/Dths/tons)	1,314
FTRs (MWhs) [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs/Dths/tons)	8,970
FTRs (MWhs) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs/Dths/tons)	5,886
FTRs (MWhs) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs/Dths/tons)	3,084
Natural Gas (Dths) [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs/Dths/tons) | Dekatherms	170,463
Natural Gas (Dths) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs/Dths/tons) | Dekatherms	72,662
Natural Gas (Dths) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs/Dths/tons) | Dekatherms	97,801
Coal (tons) [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs/Dths/tons) | T	8,177
Coal (tons) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs/Dths/tons) | T	3,339
Coal (tons) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs/Dths/tons) | T	4,838
Diesel Fuel (gallons) [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in gallons) | gal	6,552
Diesel Fuel (gallons) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in gallons) | gal	0
Diesel Fuel (gallons) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in gallons) | gal	6,552

Derivative Instruments (Fair Va

Derivative Instruments (Fair Value Of Financial Instruments) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Current derivative assets	$ 21.1	$ 29.4
Non-current derivative assets	4	12
Current derivative liabilities	18.7	13.3
Non-current derivative liabilities	23	15.3
IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	15.8	19.1
Non-current derivative assets	1.3	1.7
Current derivative liabilities	5	2.7
Non-current derivative liabilities	14.4	5.6
WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	5.3	10.3
Non-current derivative assets	2.7	10.3
Current derivative liabilities	13.7	10.6
Non-current derivative liabilities	$ 8.6	$ 9.7

Commitments And Contingencie123

Commitments And Contingencies (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)
Long-term Purchase Commitment [Line Items]
Minimum future commitments	$ 2,410
Present value abandonment obligation	33
Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	1,232	[1]
Capital Purchase Obligation [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	82
IPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	1,669
Maximum indemnification limit	17
IPL [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	1,159	[1]
IPL [Member] | Capital Purchase Obligation [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	15
WPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	735
WPL [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	73	[1]
WPL [Member] | Capital Purchase Obligation [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	67
Indemnification Agreement [Member]
Long-term Purchase Commitment [Line Items]
Maximum indemnification limit	98
Indemnification Agreement [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Maximum indemnification limit	$ 17
[1]	Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased.

Commitments And Contingencie124

Commitments And Contingencies (Other Purchase Obligations) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)
Long-term Purchase Commitment [Line Items]
Minimum future commitments	$ 2,410
Individual commitments incurred	1
Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	1,232	[1]
Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	946
Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	192	[2]
Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	40	[3]
2018 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	545
2018 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	188	[1]
2018 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	229
2018 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	107	[2]
2018 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	21	[3]
2019 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	364
2019 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	159	[1]
2019 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	140
2019 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	59	[2]
2019 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	6	[3]
2020 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	292
2020 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	135	[1]
2020 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	130
2020 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	21	[2]
2020 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	6	[3]
2021 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	267
2021 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	149	[1]
2021 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	110
2021 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	5	[2]
2021 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	3	[3]
2022 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	226
2022 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	140	[1]
2022 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	84
2022 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	0	[2]
2022 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	2	[3]
Thereafter [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	716
Thereafter [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	461	[1]
Thereafter [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	253
Thereafter [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	0	[2]
Thereafter [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	2	[3]
IPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	1,669
IPL [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	1,159	[1]
IPL [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	384
IPL [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	98	[2]
IPL [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	28	[3]
IPL [Member] | 2018 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	322
IPL [Member] | 2018 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	130	[1]
IPL [Member] | 2018 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	126
IPL [Member] | 2018 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	51	[2]
IPL [Member] | 2018 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	15	[3]
IPL [Member] | 2019 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	236
IPL [Member] | 2019 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	144	[1]
IPL [Member] | 2019 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	58
IPL [Member] | 2019 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	31	[2]
IPL [Member] | 2019 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	3	[3]
IPL [Member] | 2020 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	193
IPL [Member] | 2020 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	135	[1]
IPL [Member] | 2020 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	44
IPL [Member] | 2020 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	11	[2]
IPL [Member] | 2020 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	3	[3]
IPL [Member] | 2021 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	189
IPL [Member] | 2021 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	149	[1]
IPL [Member] | 2021 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	32
IPL [Member] | 2021 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	5	[2]
IPL [Member] | 2021 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	3	[3]
IPL [Member] | 2022 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	164
IPL [Member] | 2022 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	140	[1]
IPL [Member] | 2022 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	22
IPL [Member] | 2022 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	0	[2]
IPL [Member] | 2022 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	2	[3]
IPL [Member] | Thereafter [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	565
IPL [Member] | Thereafter [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	461	[1]
IPL [Member] | Thereafter [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	102
IPL [Member] | Thereafter [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	0	[2]
IPL [Member] | Thereafter [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	2	[3]
WPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	735
WPL [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	73	[1]
WPL [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	562
WPL [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	94	[2]
WPL [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	6	[3]
WPL [Member] | 2018 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	222
WPL [Member] | 2018 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	58	[1]
WPL [Member] | 2018 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	103
WPL [Member] | 2018 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	56	[2]
WPL [Member] | 2018 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	5	[3]
WPL [Member] | 2019 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	126
WPL [Member] | 2019 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	15	[1]
WPL [Member] | 2019 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	82
WPL [Member] | 2019 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	28	[2]
WPL [Member] | 2019 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	1	[3]
WPL [Member] | 2020 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	96
WPL [Member] | 2020 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	0	[1]
WPL [Member] | 2020 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	86
WPL [Member] | 2020 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	10	[2]
WPL [Member] | 2020 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	0	[3]
WPL [Member] | 2021 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	78
WPL [Member] | 2021 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	0	[1]
WPL [Member] | 2021 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	78
WPL [Member] | 2021 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	0	[2]
WPL [Member] | 2021 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	0	[3]
WPL [Member] | 2022 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	62
WPL [Member] | 2022 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	0	[1]
WPL [Member] | 2022 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	62
WPL [Member] | 2022 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	0	[2]
WPL [Member] | 2022 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	0	[3]
WPL [Member] | Thereafter [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	151
WPL [Member] | Thereafter [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	0	[1]
WPL [Member] | Thereafter [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	151
WPL [Member] | Thereafter [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	0	[2]
WPL [Member] | Thereafter [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	$ 0	[3]
[1]	Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased.
[2]	Corporate Services entered into system-wide coal contracts on behalf of IPL and WPL that include minimum future commitments. These commitments were assigned to IPL and WPL based on information available as of December 31, 2017 regarding expected future usage, which is subject to change.
[3]	Includes individual commitments incurred during the normal course of business that exceeded $1 million at December 31, 2017.

Commitments And Contingencie125

Commitments And Contingencies (Schedule Of Environmental Liabilities) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)
Minimum range of estimated future cost incurred for investigation, remediation and monitoring	$ 11
Maximum remaining estimated cost incurred for investigation, remediation and monitoring	30
IPL [Member]
Minimum range of estimated future cost incurred for investigation, remediation and monitoring	9
Maximum remaining estimated cost incurred for investigation, remediation and monitoring	27
Manufactured Gas Plant [Member]
Total environmental liabilities	15
Manufactured Gas Plant [Member] | IPL [Member]
Total environmental liabilities	$ 13

Segments Of Business (Narrative

Segments Of Business (Narrative) (Details)	12 Months Ended
	Dec. 31, 2017segment
Utility Business [Member]
Segment Reporting Information [Line Items]
Number of reportable segments (in segments)	3
IPL [Member]
Segment Reporting Information [Line Items]
Number of reportable segments (in segments)	3
WPL [Member]
Segment Reporting Information [Line Items]
Number of reportable segments (in segments)	3

Segments Of Business (Schedule

Segments Of Business (Schedule Of Segment Of Business) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Operating revenues	$ 856.1	$ 906.9	$ 765.3	$ 853.9	$ 797	$ 924.6	$ 754.6	$ 843.8	$ 3,382.2	$ 3,320	$ 3,253.6
Depreciation and amortization									461.8	411.6	401.3
Operating income (loss)	129.7	231.5	149.3	142.9	99.9	162.6	128.6	145.9	653.4	537	577
Interest expense									215.6	196.2	187.1
Equity (income) loss from unconsolidated investments, net									(44.8)	(39.6)	(33.8)
Income tax expense (benefit)									66.7	59.4	70.4
Net income (loss) attributable to common shareowners	93.8	168.8	94.3	100.4	62.7	128.4	83.9	96.5	457.3	371.5	378.2
Total assets	14,187.8				13,373.8				14,187.8	13,373.8	12,495.2
Investments in equity method subsidiaries	381.4				326				381.4	326	302.9
Construction and acquisition expenditures									1,466.9	1,196.8	1,034.3
IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	472.2	527.4	420.2	450.5	434.5	516.2	411	458.7	1,870.3	1,820.4	1,774.5
Depreciation and amortization									245	210.8	207.2
Operating income (loss)	49.2	131.8	66.3	49.6	34.9	125.9	48	62	296.9	270.8	241.9
Interest expense									112.4	103.2	96.8
Income tax expense (benefit)									(10.9)	(5.9)	(22.7)
Net income (loss) attributable to common shareowners	16.4	120.4	42.8	37.2	24	114.1	31.9	45.6	216.8	215.6	186
Total assets	7,606				7,304.7				7,606	7,304.7
WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	374.7	370.2	334.8	393.1	352.2	397	334.3	375.6	1,472.8	1,459.1	1,435.1
Depreciation and amortization									212.9	192.5	184.3
Operating income (loss)	72.8	90.7	73.7	86	58.2	115	75	78.8	323.2	327	308.7
Interest expense									93.8	91.4	92.4
Equity (income) loss from unconsolidated investments, net									(0.7)	(39.8)	(35.1)
Income tax expense (benefit)									61.9	93.3	82.9
Net income (loss) attributable to common shareowners	53.2	$ 49.8	$ 38.1	$ 45.5	31.7	$ 69	$ 43.2	$ 46.5	186.6	190.4	176.3
Total assets	5,756.5				5,290.3				5,756.5	5,290.3
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues									2,894.7	2,875.5	2,770.5
Depreciation and amortization									412	367	358.6
Operating income (loss)									586.5	571.9	514.1
Equity (income) loss from unconsolidated investments, net									(0.7)	(0.7)	(0.9)
Total assets	11,396.2				10,722.9				11,396.2	10,722.9	9,918
Investments in equity method subsidiaries	8.3				7.7				8.3	7.7	8.7
Construction and acquisition expenditures									1,154.9	994	852.5
Electric [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues									1,598.9	1,569.7	1,503.8
Depreciation and amortization									215.1	189.4	187.9
Operating income (loss)									281.1	252	218.8
Total assets	6,524.4				6,278.2				6,524.4	6,278.2	5,754.1
Construction and acquisition expenditures									594.1	598.1	561.2
Electric [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues									1,295.8	1,305.8	1,266.7
Depreciation and amortization									196.9	177.6	170.7
Operating income (loss)									305.4	319.9	295.3
Equity (income) loss from unconsolidated investments, net									(0.7)	(0.7)	(0.9)
Total assets	4,871.8				4,444.7				4,871.8	4,444.7	4,163.9
Investments in equity method subsidiaries	8.3				7.7				8.3	7.7	8.7
Construction and acquisition expenditures									592.4	395.9	291.3
Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues									400.9	355.4	381.2
Depreciation and amortization									38.2	34.2	31.1
Operating income (loss)									45.3	30.7	34.6
Equity (income) loss from unconsolidated investments, net									0	0	0
Total assets	1,199.8				1,091.1				1,199.8	1,091.1	939.3
Investments in equity method subsidiaries	0				0				0	0	0
Construction and acquisition expenditures									125.2	137.1	106.4
Gas [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues									226	204	217.3
Depreciation and amortization									22.2	19.3	17.5
Operating income (loss)									20.8	15.5	17.7
Total assets	727.9				653.3				727.9	653.3	548.2
Construction and acquisition expenditures									80.7	91.5	56.7
Gas [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues									174.9	151.4	163.9
Depreciation and amortization									16	14.9	13.6
Operating income (loss)									24.5	15.2	16.9
Equity (income) loss from unconsolidated investments, net									0	0	0
Total assets	471.9				437.8				471.9	437.8	391.1
Investments in equity method subsidiaries	0				0				0	0	0
Construction and acquisition expenditures									44.5	45.6	49.7
Other Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues									47.5	48.6	57.9
Depreciation and amortization									7.7	2.1	1.8
Operating income (loss)									(11.7)	(4.8)	1.9
Equity (income) loss from unconsolidated investments, net									0	0	0
Total assets	766.5				781				766.5	781	828.9
Investments in equity method subsidiaries	0				0				0	0	0
Construction and acquisition expenditures									1.7	0.1	1.4
Other Utility [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues									45.4	46.7	53.4
Depreciation and amortization									7.7	2.1	1.8
Operating income (loss)									(5)	3.3	5.4
Total assets	353.7				373.2				353.7	373.2	406.8
Construction and acquisition expenditures									1.2	0.1	1.4
Other Utility [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues									2.1	1.9	4.5
Depreciation and amortization									0	0	0
Operating income (loss)									(6.7)	(8.1)	(3.5)
Equity (income) loss from unconsolidated investments, net									0	(39.1)	(34.2)
Total assets	412.8				407.8				412.8	407.8	715.4
Investments in equity method subsidiaries	0				0				0	0	293.3
Construction and acquisition expenditures									0.5	11.5	3.3
Utility Business [Member]
Segment Reporting Information [Line Items]
Operating revenues									3,343.1	3,279.5	3,209.6
Depreciation and amortization									457.9	403.3	391.5
Operating income (loss)									620.1	597.8	550.6
Interest expense									206.2	194.6	189.2
Equity (income) loss from unconsolidated investments, net									(0.7)	(0.7)	(0.9)
Income tax expense (benefit)									51	71.4	46.2
Net income (loss) attributable to common shareowners									403.4	385.2	343.4
Total assets	13,362.5				12,595				13,362.5	12,595	11,686.2
Investments in equity method subsidiaries	8.3				7.7				8.3	7.7	8.7
Construction and acquisition expenditures									1,281.8	1,131.2	960.3
Utility Business [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues									1,870.3	1,820.4	1,774.5
Depreciation and amortization									245	210.8	207.2
Operating income (loss)									296.9	270.8	241.9
Interest expense									112.4	103.2	96.8
Income tax expense (benefit)									(10.9)	(5.9)	(22.7)
Net income (loss) attributable to common shareowners									216.8	215.6	186
Total assets	7,606				7,304.7				7,606	7,304.7	6,709.1
Construction and acquisition expenditures									676	689.7	619.3
Utility Business [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues									1,472.8	1,459.1	1,435.1
Depreciation and amortization									212.9	192.5	184.3
Operating income (loss)									323.2	327	308.7
Interest expense									93.8	91.4	92.4
Equity (income) loss from unconsolidated investments, net									(0.7)	(39.8)	(35.1)
Income tax expense (benefit)									61.9	93.3	82.9
Net income (loss) attributable to common shareowners									186.6	190.4	176.3
Total assets	5,756.5				5,290.3				5,756.5	5,290.3	5,270.4
Investments in equity method subsidiaries	8.3				7.7				8.3	7.7	302
Construction and acquisition expenditures									637.4	453	344.3
Non-Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues									39.1	40.5	44
Depreciation and amortization									3.9	8.3	9.8
Operating income (loss)									33.3	(60.8)	26.4
Interest expense									9.4	1.6
Interest income											(2.1)
Equity (income) loss from unconsolidated investments, net									(44.1)	(38.9)	(32.9)
Income tax expense (benefit)									15.7	(12)	24.2
Net income (loss) attributable to common shareowners									53.9	(13.7)	34.8
Total assets	825.3				778.8				825.3	778.8	809
Investments in equity method subsidiaries	$ 373.1				$ 318.3				373.1	318.3	294.2
Construction and acquisition expenditures									$ 185.1	$ 65.6	$ 74

Related Parties (Narrative) (De

Related Parties (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
WPL [Member] | WPL Owed ATC [Member]
Related Party Transaction [Line Items]
Net amounts owed	$ 9	$ 8

Related Parties (Service Agreem

Related Parties (Service Agreements) (Details) - Corporate Services [Member] - Subsidiary of Common Parent [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Administrative and General Services Billings [Member] | IPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	$ 177	$ 161	$ 150
Administrative and General Services Billings [Member] | WPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	135	133	121
Transmission Sales Credited [Member] | IPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	23	8	10
Transmission Sales Credited [Member] | WPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	13	7	24
Transmission Purchases Billed [Member] | IPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	364	433	366
Transmission Purchases Billed [Member] | WPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	$ 115	$ 102	$ 66

Related Parties (Net Intercompa

Related Parties (Net Intercompany Payables) (Details) - Corporate Services [Member] - Subsidiary of Common Parent [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
IPL [Member]
Related Party Transaction [Line Items]
Intercompany payables to Corporate Services	$ 114	$ 104
WPL [Member]
Related Party Transaction [Line Items]
Intercompany payables to Corporate Services	$ 61	$ 72

Related Parties (Related Amount

Related Parties (Related Amounts Billed Between Parties) (Details) - WPL [Member] - ATC [Member] - Equity Method Investment [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
ATC Billings To WPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	$ 105	$ 110	$ 101
WPL Billings To ATC [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	$ 10	$ 13	$ 13

Selected Consolidated Quarte132

Selected Consolidated Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating revenues	$ 856.1	$ 906.9	$ 765.3	$ 853.9	$ 797	$ 924.6	$ 754.6	$ 843.8	$ 3,382.2	$ 3,320	$ 3,253.6
Operating income	129.7	231.5	149.3	142.9	99.9	162.6	128.6	145.9	653.4	537	577
Income from continuing operations, net of tax	93.8	168.8	94.3	99	63	128.8	84.4	97.6	455.9	373.8	380.7
Income (loss) from discontinued operations, net of tax	0	0	0	1.4	(0.3)	(0.4)	(0.5)	(1.1)	1.4	(2.3)	(2.5)
Net income									467.5	381.7	388.4
Net income attributable to common shareowners	$ 93.8	$ 168.8	$ 94.3	$ 100.4	$ 62.7	$ 128.4	$ 83.9	$ 96.5	457.3	371.5	378.2
Income from continuing operations, net of tax (in dollars per share)	$ 0.41	$ 0.73	$ 0.41	$ 0.43	$ 0.28	$ 0.57	$ 0.37	$ 0.43
Income from discontinued operations, net of tax (in dollars per share)	0	0	0	0.01	0	0	0	0
Net income (in dollars per share)	$ 0.41	$ 0.73	$ 0.41	$ 0.44	$ 0.28	$ 0.57	$ 0.37	$ 0.43
IPL [Member]
Operating revenues	$ 472.2	$ 527.4	$ 420.2	$ 450.5	$ 434.5	$ 516.2	$ 411	$ 458.7	1,870.3	1,820.4	1,774.5
Operating income	49.2	131.8	66.3	49.6	34.9	125.9	48	62	296.9	270.8	241.9
Net income	18.9	123	45.3	39.8	26.5	116.7	34.4	48.2	227	225.8	196.2
Net income attributable to common shareowners	16.4	120.4	42.8	37.2	24	114.1	31.9	45.6	216.8	215.6	186
WPL [Member]
Operating revenues	374.7	370.2	334.8	393.1	352.2	397	334.3	375.6	1,472.8	1,459.1	1,435.1
Operating income	72.8	90.7	73.7	86	58.2	115	75	78.8	323.2	327	308.7
Net income	53.2	49.8	38.1	45.5	32.5	69.6	43.7	47	186.6	192.8	177.6
Net income attributable to common shareowners	$ 53.2	$ 49.8	$ 38.1	$ 45.5	$ 31.7	$ 69	$ 43.2	$ 46.5	$ 186.6	$ 190.4	$ 176.3

Condensed Parent Company Fin133

Condensed Parent Company Financial Statements (Condensed Balance Sheet) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Other	$ 87.7	$ 95.5
Total current assets	905.1	877.1
Investments:
Other	121.9	20
Total investments	396.1	337.6
Other assets	69.7	22.6
Total assets	14,187.8	13,373.8	$ 12,495.2
Current liabilities:
Commercial paper	320.2	244.1
Other	260.8	281.8
Total current liabilities	2,149	1,162
Other liabilities	306.4	279.3
Common Equity:
Retained earnings	2,346	2,177
Shares in deferred compensation trust	(11.1)	(10)
Total common equity	4,182.2	3,862	$ 3,724.1	$ 3,438.7
Total liabilities and equity	14,187.8	13,373.8
Parent Company [Member]
Current assets:
Notes receivable from affiliated companies	50	74
Other	7	5
Total current assets	57	79
Investments:
Investments in consolidated subsidiaries	4,676	4,211
Other	2	2
Total investments	4,678	4,213
Other assets	78	64
Total assets	4,813	4,356
Current liabilities:
Commercial paper	295	192
Notes payable to affiliated companies	305	275
Other	12	12
Total current liabilities	612	479
Other liabilities	20	18
Common Equity:
Common stock and additional paid-in capital	1,848	1,695
Retained earnings	2,344	2,174
Shares in deferred compensation trust	(11)	(10)
Total common equity	4,181	3,859
Total liabilities and equity	$ 4,813	$ 4,356

Condensed Parent Company Fin134

Condensed Parent Company Financial Statements (Condensed Statements Of Income) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating revenues	$ 856.1	$ 906.9	$ 765.3	$ 853.9	$ 797	$ 924.6	$ 754.6	$ 843.8	$ 3,382.2	$ 3,320	$ 3,253.6
Operating expenses									2,728.8	2,783	2,676.6
Operating income (loss)	$ 129.7	$ 231.5	$ 149.3	$ 142.9	$ 99.9	$ 162.6	$ 128.6	$ 145.9	653.4	537	577
Interest expense and other:
Equity earnings from consolidated subsidiaries									(44.8)	(39.6)	(33.8)
Interest expense									215.6	196.2	187.1
Total interest expense and other									120.6	93.6	115.7
Income from continuing operations before income taxes									532.8	443.4	461.3
Income tax benefit									66.7	59.4	70.4
Net income									467.5	381.7	388.4
Parent Company [Member]
Operating revenues									0	1	2
Operating expenses									2	3	3
Operating income (loss)									(2)	(2)	(1)
Interest expense and other:
Equity earnings from consolidated subsidiaries									(457)	(374)	(379)
Interest expense									3	3	3
Interest income									0	(2)	(3)
Total interest expense and other									(454)	(373)	(379)
Income from continuing operations before income taxes									452	371	378
Income tax benefit									(6)	(1)	(1)
Net income									$ 458	$ 372	$ 379

Condensed Parent Company Fin135

Condensed Parent Company Financial Statements (Condensed Statements of Cash Flow) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net cash flows from operating activities	$ 983.4	$ 859.6	$ 871.2
Cash flows from (used for) investing activities:
Other	(29.4)	10.3	(24.8)
Net cash flows from (used for) investing activities	(1,496.3)	(1,186.5)	(919.2)
Cash flows used for financing activities:
Common stock dividends	(288.3)	(266.5)	(247.3)
Proceeds from issuance of common stock, net	149.6	26.6	151.2
Payments to retire long-term debt	(4.6)	(313.4)	(183)
Net change in commercial paper	171.1	84.3	18.5
Other	(45.2)	(1.7)	6.8
Net cash flows from (used for) financing activities	532.6	329.3	(3.1)
Net increase (decrease) in cash and cash equivalents	19.7	2.4	(51.1)
Cash and cash equivalents at beginning of period	8.2	5.8	56.9
Cash and cash equivalents at end of period	27.9	8.2	5.8
Supplemental cash flows information:
Interest, net of capitalized interest	(212.6)	(192.4)	(184.8)
Income taxes, net	(11.3)	(9.8)	0
Parent Company [Member]
Net cash flows from operating activities	273	254	262
Cash flows from (used for) investing activities:
Capital contributions to consolidated subsidiaries	(290)	(250)	(165)
Captial repayments from consolidated subsidiaries	0	130	0
Net change in notes receivable from and payable to affiliates	54	294	2
Other	0	10	0
Net cash flows from (used for) investing activities	(236)	184	(163)
Cash flows used for financing activities:
Common stock dividends	(288)	(267)	(247)
Proceeds from issuance of common stock, net	150	27	151
Payments to retire long-term debt	0	(250)	0
Net change in commercial paper	103	52	(1)
Other	(2)	0	(2)
Net cash flows from (used for) financing activities	(37)	(438)	(99)
Net increase (decrease) in cash and cash equivalents	0	0	0
Cash and cash equivalents at beginning of period	0	0	0
Cash and cash equivalents at end of period	0	0	0
Supplemental cash flows information:
Interest, net of capitalized interest	(3)	(3)	(3)
Income taxes, net	$ 0	$ (37)	$ (9)

Valuation And Qualifying Acc136

Valuation And Qualifying Accounts and Reserves (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Provision for Uncollectible Accounts [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	[1]	$ 8.7	$ 4.8	$ 5.1
Charged to Expense	[1]	15.1	17.4	8.1
Charged to Other Accounts	[1],[2]	5.4	8.8	3
Deductions	[1],[3]	17.2	22.3	11.4
Ending Balance	[1]	12	8.7	4.8
Accumulated Provision for Uncollectible Accounts [Member] | IPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	[1]	1.1	0.6	0.4
Charged to Expense	[1]	14.9	17.2	8.1
Charged to Other Accounts	[1],[2]	0	0	0
Deductions	[1],[3]	14.7	16.7	7.9
Ending Balance	[1]	1.3	1.1	0.6
Accumulated Provision for Uncollectible Accounts [Member] | WPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance		7.1	3.7	4.2
Charged to Expense		0.2	0.1	0
Charged to Other Accounts	[2]	5.4	8.8	3
Deductions	[3]	2	5.5	3.5
Ending Balance		10.7	7.1	3.7
Accumulated Provision for Other Reserves [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	[4]	25.1	27.1	32.6
Charged to Expense	[4]	3.3	6.1	6.5
Charged to Other Accounts	[2],[4]	5.1	0	0
Deductions	[3],[4]	10.5	8.1	12
Ending Balance	[4]	23	25.1	27.1
Accumulated Provision for Other Reserves [Member] | IPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	[4]	8.7	9.4	10.6
Charged to Expense	[4]	0.3	1	2.1
Charged to Other Accounts	[2],[4]	0	0	0
Deductions	[3],[4]	1.4	1.7	3.3
Ending Balance	[4]	7.6	8.7	9.4
Accumulated Provision for Other Reserves [Member] | WPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	[4]	8.1	11.4	16.3
Charged to Expense	[4]	0.1	1.8	0.7
Charged to Other Accounts	[2],[4]	0	0	0
Deductions	[3],[4]	1.8	5.1	5.6
Ending Balance	[4]	$ 6.4	$ 8.1	$ 11.4
[1]	Refer to Note 5(b) for discussion of IPL’s sales of accounts receivable program.
[2]	Accumulated provision for uncollectible accounts: In accordance with its regulatory treatment, certain amounts provided by WPL are recorded in regulatory assets. WPL expenses these amounts when an uncollectible account is written-off.Accumulated provision for other reserves: In 2017, Alliant Energy recorded amounts to deferred tax liabilities related to the impacts of Tax Reform, which is discussed in Note 11.
[3]	Deductions are of the nature for which the reserves were created. In the case of the accumulated provision for uncollectible accounts, deductions from this reserve are reduced by recoveries of amounts previously written off.
[4]	Other reserves are largely related to injury and damage claims arising in the ordinary course of business, and the impacts of Tax Reform.