Alliant Energy (LNT) 10-K 22 Feb 19 2018 FY Annual report Financial data

Document And Entity Information

Document And Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2018	Jan. 31, 2019	Jun. 30, 2018
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
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		236,074,475
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 9.9
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, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues:
Electric utility	$ 3,000.3	$ 2,894.7	$ 2,875.5
Gas utility	446.6	400.9	355.4
Other utility	48	47.5	48.6
Non-utility	39.6	39.1	40.5
Total revenues	3,534.5	3,382.2	3,320
Operating expenses:
Electric production fuel and purchased power	855	818.1	854
Electric transmission service	495.7	480.9	527.9
Cost of gas sold	232.3	211.4	194.3
Asset valuation charges for Franklin County wind farm	0	0	86.4
Other operation and maintenance	645.8	633.2	589.4
Depreciation and amortization	506.9	461.8	411.6
Taxes other than income taxes	104.4	105.6	102.3
Total operating expenses	2,840.1	2,711	2,765.9
Operating income (loss)	694.4	671.2	554.1
Other (income) and deductions:
Interest expense	247	215.6	196.2
Equity (income) loss from unconsolidated investments, net	(54.6)	(44.8)	(39.6)
Allowance for funds used during construction	(75.6)	(49.7)	(62.5)
Other	7.6	17.3	16.6
Total other (income) and deductions	124.4	138.4	110.7
Income from continuing operations before income taxes	570	532.8	443.4
Income tax expense (benefit)	47.7	66.7	59.4
Income from continuing operations, net of tax	522.3	466.1	384
Income (loss) from discontinued operations, net of tax	0	1.4	(2.3)
Net income	522.3	467.5	381.7
Preferred dividend requirements of Interstate Power and Light Company	10.2	10.2	10.2
Net income attributable to common shareowners	$ 512.1	$ 457.3	$ 371.5
Weighted average number of common shares outstanding (basic and diluted) (in shares)	233.6	229.7	227.1
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)	$ 2.19	$ 1.99	$ 1.65
Loss from discontinued operations, net of tax (in dollars per share)	0	0	(0.01)
Net income (in dollars per share)	$ 2.19	$ 1.99	$ 1.64
Amounts attributable to common shareowners:
Income from continuing operations, net of tax	$ 512.1	$ 455.9	$ 373.8
Income (loss) from discontinued operations, net of tax	0	1.4	(2.3)
Net income	512.1	457.3	371.5
IPL [Member]
Revenues:
Electric utility	1,731.1	1,598.9	1,569.7
Gas utility	266.2	226	204
Other utility	45	45.4	46.7
Total revenues	2,042.3	1,870.3	1,820.4
Operating expenses:
Electric production fuel and purchased power	469	443.6	430.5
Electric transmission service	352.9	310.4	359.7
Cost of gas sold	129.6	115.6	111
Other operation and maintenance	402.6	396.6	376.9
Depreciation and amortization	283.5	245	210.8
Taxes other than income taxes	53.9	55	53.9
Total operating expenses	1,691.5	1,566.2	1,542.8
Operating income (loss)	350.8	304.1	277.6
Other (income) and deductions:
Interest expense	119.4	112.4	103.2
Allowance for funds used during construction	(42.2)	(31.4)	(52)
Other	2.6	7	6.5
Total other (income) and deductions	79.8	88	57.7
Income from continuing operations before income taxes	271	216.1	219.9
Income tax expense (benefit)	(3.2)	(10.9)	(5.9)
Net income	274.2	227	225.8
Preferred dividend requirements of Interstate Power and Light Company	10.2	10.2	10.2
Net income attributable to common shareowners	264	216.8	215.6
Amounts attributable to common shareowners:
Net income	264	216.8	215.6
WPL [Member]
Revenues:
Electric utility	1,269.2	1,295.8	1,305.8
Gas utility	180.4	174.9	151.4
Other utility	3	2.1	1.9
Total revenues	1,452.6	1,472.8	1,459.1
Operating expenses:
Electric production fuel and purchased power	386	374.5	423.5
Electric transmission service	142.8	170.5	168.2
Cost of gas sold	102.7	95.8	83.3
Other operation and maintenance	241.6	238.5	209.6
Depreciation and amortization	219.4	212.9	192.5
Taxes other than income taxes	47.2	46.9	44.8
Total operating expenses	1,139.7	1,139.1	1,121.9
Operating income (loss)	312.9	333.7	337.2
Other (income) and deductions:
Interest expense	97.8	93.8	91.4
Equity (income) loss from unconsolidated investments, net	(0.9)	(0.7)	(39.8)
Allowance for funds used during construction	(33.4)	(18.3)	(10.5)
Other	5.1	10.4	10
Total other (income) and deductions	68.6	85.2	51.1
Income from continuing operations before income taxes	244.3	248.5	286.1
Income tax expense (benefit)	36.2	61.9	93.3
Net income	208.1	186.6	192.8
Net income attributable to noncontrolling interest	0	0	2.4
Net income attributable to common shareowners	208.1	186.6	190.4
Amounts attributable to common shareowners:
Net income	$ 208.1	$ 186.6	$ 190.4

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents		$ 20.9	$ 27.9
Accounts receivable, less allowance for doubtful accounts		350.4	482.8
Production fuel, at weighted average cost		61.4	72.3
Gas stored underground, at weighted average cost		49	44.5
Materials and supplies, at weighted average cost		101.4	105.6
Regulatory assets		79.8	84.3
Prepaid gross receipts tax		42.2	41.3
Other		80	46.4
Total current assets		785.1	905.1
Property, plant and equipment, net		12,462.4	11,234.5
Investments:
ATC Holdings		293.6	274.2
Other		137.7	121.9
Total investments		431.3	396.1
Other assets:
Regulatory assets		1,657.5	1,582.4
Deferred charges and other		89.7	69.7
Total other assets		1,747.2	1,652.1
Total assets		15,426	14,187.8
Current liabilities:
Current maturities of long-term debt		256.5	855.7
Commercial paper		441.2	320.2
Other short-term borrowings		0	95
Accounts payable		543.3	477.3
Regulatory liabilities		142.7	140
Other		260.4	260.8
Total current liabilities		1,644.1	2,149
Long-term debt, net (excluding current portion)	[1]	5,246.3	4,010.6
Other liabilities:
Deferred tax liabilities		1,603.1	1,478.4
Regulatory liabilities		1,350.5	1,357.2
Pension and other benefit obligations		509.1	504
Other		287.2	306.4
Total other liabilities		3,749.9	3,646
Commitments and contingencies (Note 17)
Common equity:
Common stock		2.4	2.3
Additional paid-in capital		2,045.5	1,845.5
Retained earnings		2,545.9	2,346
Accumulated other comprehensive income (loss)		1.7	(0.5)
Shares in deferred compensation trust - 384,580 and 463,365 shares at a weighted average cost of $25.60 and $23.91 per share		(9.8)	(11.1)
Total common equity		4,585.7	4,182.2
Cumulative preferred stock of Interstate Power and Light Company		200	200
Total equity		4,785.7	4,382.2
Total liabilities and equity		15,426	14,187.8
IPL [Member]
Current assets:
Cash and cash equivalents		9.7	3.6
Accounts receivable, less allowance for doubtful accounts		153.5	264.9
Production fuel, at weighted average cost		44.8	52.4
Gas stored underground, at weighted average cost		26.1	20.3
Materials and supplies, at weighted average cost		55.4	60.6
Regulatory assets		39.2	41.9
Other		43.1	32.3
Total current assets		371.8	476
Property, plant and equipment, net		6,781.5	5,926.2
Other assets:
Regulatory assets		1,239.8	1,189.7
Deferred charges and other		18.3	14.1
Total other assets		1,258.1	1,203.8
Total assets		8,411.4	7,606
Current liabilities:
Current maturities of long-term debt		0	350
Commercial paper		50.4	0
Accounts payable		304.9	220.3
Accounts payable to associated companies		28.8	50.1
Regulatory liabilities		90	69.7
Accrued taxes		45.8	47.1
Accrued interest		31.2	32.1
Other		56	58.4
Total current liabilities		607.1	827.7
Long-term debt, net (excluding current portion)	[1]	2,552.3	2,056
Other liabilities:
Deferred tax liabilities		957.3	910.7
Regulatory liabilities		664.9	685.7
Pension and other benefit obligations		178.4	173.8
Other		220.7	242.4
Total other liabilities		2,021.3	2,012.6
Commitments and contingencies (Note 17)
Common equity:
Common stock		33.4	33.4
Additional paid-in capital		2,222.8	1,797.8
Retained earnings		774.5	678.5
Total common equity		3,030.7	2,509.7
Cumulative preferred stock of Interstate Power and Light Company		200	200
Total equity		3,230.7	2,709.7
Total liabilities and equity		8,411.4	7,606
WPL [Member]
Current assets:
Cash and cash equivalents		8.7	23.1
Accounts receivable, less allowance for doubtful accounts		190.1	212.2
Production fuel, at weighted average cost		16.6	19.9
Gas stored underground, at weighted average cost		22.9	24.2
Materials and supplies, at weighted average cost		42.9	42.1
Regulatory assets		40.6	42.4
Prepaid gross receipts tax		42.2	41.3
Other		20.6	13.4
Total current assets		384.6	418.6
Property, plant and equipment, net		5,287.3	4,917.9
Other assets:
Regulatory assets		417.7	392.7
Deferred charges and other		62.9	27.3
Total other assets		480.6	420
Total assets		6,152.5	5,756.5
Current liabilities:
Current maturities of long-term debt		250	0
Commercial paper		105.5	25
Accounts payable		180.9	201.7
Accounts payable to associated companies		31.8	22.2
Regulatory liabilities		52.7	70.3
Accrued interest		25.5	25.6
Other		48.2	51.4
Total current liabilities		694.6	396.2
Long-term debt, net (excluding current portion)	[1]	1,584.9	1,833.4
Other liabilities:
Deferred tax liabilities		582	522.4
Regulatory liabilities		685.6	671.5
Capital lease obligations - Sheboygan Falls Energy Facility		60	70.2
Pension and other benefit obligations		217.7	213.7
Other		178.2	167.6
Total other liabilities		1,723.5	1,645.4
Commitments and contingencies (Note 17)
Common equity:
Common stock		66.2	66.2
Additional paid-in capital		1,309	1,109
Retained earnings		774.3	706.3
Total common equity		2,149.5	1,881.5
Total equity		2,149.5	1,881.5
Total liabilities and equity		$ 6,152.5	$ 5,756.5
[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, 2018	Dec. 31, 2017
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)	236,063,279	231,348,646
Shares in deferred compensation trust (in shares)	384,580	463,365
Shares in deferred compensation trust, weighted average cost per share (in dollars per share)	$ 25.60	$ 23.91
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, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from (used for) operating activities:
Net income	$ 522.3	$ 467.5	$ 381.7
Adjustments to reconcile net income to net cash flows from (used for) operating activities:
Depreciation and amortization	506.9	461.8	411.6
Other amortizations	10.6	21.7	(4.8)
Deferred tax expense and tax credits	67	139.6	84.6
Equity (income) loss from unconsolidated investments, net	(54.6)	(44.8)	(39.6)
Distributions from equity method investments	43.9	38.1	28.3
Equity component of allowance for funds used during construction	(51.4)	(33.6)	(42.3)
Asset valuation charges for Franklin County wind farm	0	0	86.4
Other	7.8	6.7	0.8
Other changes in assets and liabilities:
Accounts receivable	(475.4)	(441.2)	(572.2)
Regulatory assets	(16.2)	(130.8)	(3.6)
Regulatory liabilities	1.3	(83.8)	(63)
Deferred income taxes	55.9	81.7	102.4
Other	(90.4)	38.7	22.5
Net cash flows from (used for) operating activities	527.7	521.6	392.8
Cash flows from (used for) investing activities:
Utility construction and acquisition expenditures	(1,568.3)	(1,281.8)	(1,131.2)
Other construction and acquisition expenditures	(65.6)	(185.1)	(65.6)
Cash receipts on sold receivables	605.3	461.8	466.8
Other	(38.2)	(28.3)	9.8
Net cash flows from (used for) investing activities	(1,066.8)	(1,033.4)	(720.2)
Cash flows from (used for) financing activities:
Common stock dividends	(312.2)	(288.3)	(266.5)
Proceeds from issuance of common stock, net	196.6	149.6	26.6
Proceeds from issuance of long-term debt	1,500	550	800
Payments to retire long-term debt	(855.7)	(4.6)	(313.4)
Net change in commercial paper and other short-term borrowings	26	171.1	84.3
Other	(24)	(45.2)	(1.7)
Net cash flows from (used for) financing activities	530.7	532.6	329.3
Net increase (decrease) in cash, cash equivalents and restricted cash	(8.4)	20.8	1.9
Cash, cash equivalents and restricted cash at beginning of period	33.9	13.1	11.2
Cash, cash equivalents and restricted cash at end of period	25.5	33.9	13.1
Supplemental cash flows information:
Interest, net of capitalized interest	(247.5)	(212.6)	(192.4)
Income taxes, net	(5)	(11.3)	(9.8)
Significant non-cash investing and financing activities:
Accrued capital expenditures	299.5	196.5	154.4
Beneficial interest obtained in exchange for securitized accounts receivable	119.4	222.1	211.1
IPL [Member]
Cash flows from (used for) operating activities:
Net income	274.2	227	225.8
Adjustments to reconcile net income to net cash flows from (used for) operating activities:
Depreciation and amortization	283.5	245	210.8
Deferred tax expense and tax credits	2.2	55.8	35.6
Equity component of allowance for funds used during construction	(28.6)	(21.1)	(35.2)
Other	3.6	1.5	2.9
Other changes in assets and liabilities:
Accounts receivable	(494)	(478.7)	(510.5)
Regulatory assets	(20.2)	(126.2)	(54.7)
Accounts payable	(24.9)	24	8
Regulatory liabilities	0.6	(71.2)	(67.3)
Deferred income taxes	43.8	103.7	97.7
Other	(45.2)	18.4	(18)
Net cash flows from (used for) operating activities	(5)	(21.8)	(104.9)
Cash flows from (used for) investing activities:
Utility construction and acquisition expenditures	(990.7)	(676)	(689.7)
Cash receipts on sold receivables	605.3	461.8	466.8
Other	(44)	(27.7)	(5.7)
Net cash flows from (used for) investing activities	(429.4)	(241.9)	(228.6)
Cash flows from (used for) financing activities:
Common stock dividends	(168)	(156.1)	(151.9)
Capital contributions from parent	425	200	190
Proceeds from issuance of long-term debt	500	250	300
Payments to retire long-term debt	(350)	0	0
Net change in commercial paper and other short-term borrowings	50.4	0	0
Other	(17.8)	(27.2)	(7.6)
Net cash flows from (used for) financing activities	439.6	266.7	330.5
Net increase (decrease) in cash, cash equivalents and restricted cash	5.2	3	(3)
Cash, cash equivalents and restricted cash at beginning of period	7.2	4.2	7.2
Cash, cash equivalents and restricted cash at end of period	12.4	7.2	4.2
Supplemental cash flows information:
Interest, net of capitalized interest	(120.3)	(111.8)	(99.7)
Income taxes, net	(23.8)	8.6	(11.1)
Significant non-cash investing and financing activities:
Accrued capital expenditures	186.6	76.4	53.8
Beneficial interest obtained in exchange for securitized accounts receivable	119.4	222.1	211.1
WPL [Member]
Cash flows from (used for) operating activities:
Net income	208.1	186.6	192.8
Adjustments to reconcile net income to net cash flows from (used for) operating activities:
Depreciation and amortization	219.4	212.9	192.5
Other amortizations	5.8	17.7	(8.7)
Deferred tax expense and tax credits	49.8	53.9	114.5
Equity (income) loss from unconsolidated investments, net	(0.9)	(0.7)	(39.8)
Other	(23.3)	(13.3)	(17.8)
Other changes in assets and liabilities:
Accounts receivable	19.7	17.7	(47.6)
Regulatory assets	4	(4.7)	51.1
Other	(26.5)	(5.1)	44.6
Net cash flows from (used for) operating activities	457	465.7	521.4
Cash flows from (used for) investing activities:
Utility construction and acquisition expenditures	(577.6)	(637.4)	(453)
Other	(29.9)	(29.9)	(25.9)
Net cash flows from (used for) investing activities	(607.5)	(667.3)	(478.9)
Cash flows from (used for) financing activities:
Common stock dividends	(140.1)	(125.9)	(135)
Capital contributions from parent	200	90	60
Proceeds from issuance of long-term debt	0	300	0
Net change in commercial paper and other short-term borrowings	80.5	(27.3)	32.4
Other	(4.9)	(17.9)	3.9
Net cash flows from (used for) financing activities	135.5	218.9	(38.7)
Net increase (decrease) in cash, cash equivalents and restricted cash	(15)	17.3	3.8
Cash, cash equivalents and restricted cash at beginning of period	24.2	6.9	3.1
Cash, cash equivalents and restricted cash at end of period	9.2	24.2	6.9
Supplemental cash flows information:
Interest, net of capitalized interest	(98.1)	(91.7)	(91.5)
Income taxes, net	14	(8.4)	27.8
Significant non-cash investing and financing activities:
Accrued capital expenditures	102.5	114.5	93.1
Transfer of investment in ATC and tax liability to ATI	$ 0	$ 0	$ (163.6)

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]	Preferred Stock [Member]	Preferred Stock [Member]IPL [Member]	Noncontrolling Interest [Member]WPL [Member]
Beginning balance at Dec. 31, 2015				$ 2.3	$ 33.4	$ 66.2	$ 1,661.8	$ 1,407.8	$ 959	$ 2,068.9	$ 554.1	$ 731.1	$ (0.4)	$ (8.5)	$ 200	$ 200	$ 11.3
Beginning balance at Dec. 31, 2015	$ 3,924.1	$ 2,195.3	$ 1,767.6
Increase (Decrease) in Shareowners' 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	4,062			2.3	33.4	66.2	1,693.1	1,597.8	1,019	2,177	617.8	645.6	(0.4)	(10)	200	200	0
Ending balance at Dec. 31, 2016		2,449	1,730.8
Increase (Decrease) in Shareowners' 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)	200	200	0
Ending balance at Dec. 31, 2017	4,382.2	2,709.7	1,881.5
Increase (Decrease) in Shareowners' Equity [Roll Forward]
Net income (loss) attributable to common shareowners	512.1	264	208.1							512.1	264	208.1
Net income	522.3	274.2	208.1
Common stock dividends	(312.2)	(168)	(140.1)							(312.2)	(168)	(140.1)
Common stock issued, net	196.6			0.1			196.5
Capital contributions from parent		425	200					425	200
Transfer of investment in ATC to ATI			0
Other	4.8						3.5							1.3
Other comprehensive income (loss), net of tax	2.2	0	0										2.2
Ending balance at Dec. 31, 2018	4,585.7	3,030.7	2,149.5	$ 2.4	$ 33.4	$ 66.2	$ 2,045.5	$ 2,222.8	$ 1,309	$ 2,545.9	$ 774.5	$ 774.3	$ 1.7	$ (9.8)	$ 200	$ 200	$ 0
Ending balance at Dec. 31, 2018	$ 4,785.7	$ 3,230.7	$ 2,149.5

Consolidated Statements of Co_2

Consolidated Statements of Common Equity (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Stockholders' Equity [Abstract]
Common stock dividends (in dollars per share)	$ 1.34	$ 1.26	$ 1.175

Summary Of Significant Accounti

Summary Of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
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 previously held Alliant Energy’s interest in ATC. 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 farm, the Sheboygan Falls Energy Facility and other non-utility holdings. 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 interest in ATC Holdings. The non-utility wind farm includes a 50% cash equity ownership interest in a 225 MW 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. 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 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 modifications to the presentation of the components of net periodic benefit costs for defined benefit pension and other postretirement plans in the income statements, and restricted cash and cash receipts on sold receivables in the cash flows statements, as discussed in Note 1(n) . Discontinued operations reported in Alliant Energy’s income statements are related to various warranty claims associated with the sale of RMT, Inc. in 2013, which has 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. (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 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. 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. Federal Tax Reform repealed corporate federal AMT and allows unutilized AMT credits to be refunded over 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 2018 , 2017 and 2016 . (d) Cash, Cash Equivalents and Restricted Cash - Cash and cash equivalents include short-term liquid investments that have original maturities of less than 90 days. At December 31, 2018 and 2017 , restricted cash primarily related to deposits with trustees and requirements in Sheboygan Power, LLC’s debt agreement. (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. Property, plant and equipment that is probable of being retired early is classified as plant anticipated to be retired early. 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. 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 collected from customers. The average rates of depreciation for electric, gas and other properties, consistent with current rate-making practices, were as follows: IPL WPL 2018 2017 2016 2018 2017 2016 Electric - generation 3.6% 3.5% 3.5% 3.6% 3.5% 3.1% Electric - distribution 2.8% 2.4% 2.4% 2.6% 2.6% 2.6% Electric - other 4.7% 4.5% 4.2% 5.7% 6.9% 4.7% Gas 3.2% 3.4% 3.3% 2.5% 2.5% 2.5% Other 5.2% 4.0% 3.9% 5.8% 6.0% 5.9% 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: 2018 2017 2016 IPL (Marshalltown CWIP) N/A 7.8% 7.9% IPL (Wind generation CWIP) 7.5% 7.6% N/A IPL (other CWIP) 7.5% 7.6% 7.7% WPL (retail jurisdiction) 7.7% 7.6% 8.2% WPL (wholesale jurisdiction) 7.2% 6.0% 6.7% In accordance with their respective regulatory commission decisions, 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 and Other 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 remaining book value is recorded as a loss in the income statements. (f) Revenue Recognition - Utility - Revenues from Alliant Energy’s utility business are primarily from electric and gas sales to customers. Utility revenues are recognized over time as services are rendered or commodities are delivered to customers, and include billed and unbilled components. The billed component is based on the reading of customers’ meters, which occurs on a systematic basis throughout each reporting period and represents the fair value of the services provided or commodities delivered. The unbilled component is estimated and recorded at the end of each reporting period based on estimated amounts of energy delivered to customers since the end of each customer’s last billing period. The unbilled revenue is based on estimates of daily system demand volumes, 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. As of December 31, 2018 , the related amounts accrued for IPL and WPL were not material. IPL and WPL participate in bid/offer-based wholesale energy and ancillary services markets operated by MISO. 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 as bulk power sales in “Electric utility 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 over time as services are rendered 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 revenues. Other - Alliant Energy, IPL and WPL do not disclose the value of unsatisfied performance obligations for: (i) contracts with an original expected length of one year or less; and (ii) contracts for which revenue is recognized at the amount to which they have the right to invoice for services performed. (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 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. 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 incurred to fund energy efficiency programs and initiatives that help customers reduce their energy usage 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. (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 derivative instruments. Refer to Notes 15 , 16 and 17(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. 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. (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 and 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. (k) Debt Issuance and Retirement Costs - Debt issuance costs and debt premiums or discounts are presented on the balance sheet as a direct adjustment to 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 record to interest 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. (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, the entity is structured with disproportionate voting rights and substantially all of the entity’s activities are conducted on behalf of the investor with disproportionately fewer voting rights, 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 consolidation of VIEs. (n) New Accounting Standards - Revenue Recognition - In May 2014, the FASB 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. Alliant Energy, IPL and WPL adopted this standard on January 1, 2018 using the modified retrospective method of adoption, which was applied to contracts with customers that were completed subsequent to January 1, 2018. Alliant Energy, IPL and WPL utilized a portfolio approach upon adoption, which involved evaluating portfolios of contracts with similar characteristics, where the effects of applying the standard were not expected to be materially different than evaluating on an individual contract basis. Upon adoption, there were no cumulative effect adjustments made to the January 1, 2018 retained earnings balances. In addition, prior period amounts have not been restated to reflect the adoption of this standard and continue to be reported under the accounting standards in effect for those periods. Alliant Energy, IPL and WPL did not have a material change in revenue recognition, including the timing and pattern of revenue recognition, as a result of the adoption of this standard. Leases - In February 2016, the FASB 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 adopted this standard on January 1, 2019 using an optional transition approach and there was no cumulative effect adjustment to their balance sheets as of January 1, 2019. Alliant Energy, IPL and WPL expect to recognize increases in assets and liabilities for certain operating leases of approximately $18 million , $11 million and $7 million , respectively, on January 1, 2019. After adoption, prior period amounts will continue to be reported under the accounting standards in effect for those periods. Alliant Energy, IPL and WPL do not expect a material change to their results of operations or cash flows statements. Upon transition to the new standard, Alliant Energy, IPL and WPL elected the land easement transition practical expedient, for which existing land easements that were not previously accounted for as leases under the original accounting standards did not need to be evaluated under the new accounting standard. In addition, Alliant Energy, IPL and WPL evaluated land easements that were previously accounted for as leases and determined that the majority of these land easements relate to joint-use land sites, and do not meet the criteria for leases under the new accounting standard. Therefore, these land easement arrangements are no longer reflected as operating leases effective
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 previously held Alliant Energy’s interest in ATC. 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 farm, the Sheboygan Falls Energy Facility and other non-utility holdings. 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 interest in ATC Holdings. The non-utility wind farm includes a 50% cash equity ownership interest in a 225 MW 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. 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 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 modifications to the presentation of the components of net periodic benefit costs for defined benefit pension and other postretirement plans in the income statements, and restricted cash and cash receipts on sold receivables in the cash flows statements, as discussed in Note 1(n) . Discontinued operations reported in Alliant Energy’s income statements are related to various warranty claims associated with the sale of RMT, Inc. in 2013, which has 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. (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 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. 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. Federal Tax Reform repealed corporate federal AMT and allows unutilized AMT credits to be refunded over 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 2018 , 2017 and 2016 . (d) Cash, Cash Equivalents and Restricted Cash - Cash and cash equivalents include short-term liquid investments that have original maturities of less than 90 days. At December 31, 2018 and 2017 , restricted cash primarily related to deposits with trustees and requirements in Sheboygan Power, LLC’s debt agreement. (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. Property, plant and equipment that is probable of being retired early is classified as plant anticipated to be retired early. 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. 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 collected from customers. The average rates of depreciation for electric, gas and other properties, consistent with current rate-making practices, were as follows: IPL WPL 2018 2017 2016 2018 2017 2016 Electric - generation 3.6% 3.5% 3.5% 3.6% 3.5% 3.1% Electric - distribution 2.8% 2.4% 2.4% 2.6% 2.6% 2.6% Electric - other 4.7% 4.5% 4.2% 5.7% 6.9% 4.7% Gas 3.2% 3.4% 3.3% 2.5% 2.5% 2.5% Other 5.2% 4.0% 3.9% 5.8% 6.0% 5.9% 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: 2018 2017 2016 IPL (Marshalltown CWIP) N/A 7.8% 7.9% IPL (Wind generation CWIP) 7.5% 7.6% N/A IPL (other CWIP) 7.5% 7.6% 7.7% WPL (retail jurisdiction) 7.7% 7.6% 8.2% WPL (wholesale jurisdiction) 7.2% 6.0% 6.7% In accordance with their respective regulatory commission decisions, 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 and Other 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 remaining book value is recorded as a loss in the income statements. (f) Revenue Recognition - Utility - Revenues from Alliant Energy’s utility business are primarily from electric and gas sales to customers. Utility revenues are recognized over time as services are rendered or commodities are delivered to customers, and include billed and unbilled components. The billed component is based on the reading of customers’ meters, which occurs on a systematic basis throughout each reporting period and represents the fair value of the services provided or commodities delivered. The unbilled component is estimated and recorded at the end of each reporting period based on estimated amounts of energy delivered to customers since the end of each customer’s last billing period. The unbilled revenue is based on estimates of daily system demand volumes, 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. As of December 31, 2018 , the related amounts accrued for IPL and WPL were not material. IPL and WPL participate in bid/offer-based wholesale energy and ancillary services markets operated by MISO. 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 as bulk power sales in “Electric utility 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 over time as services are rendered 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 revenues. Other - Alliant Energy, IPL and WPL do not disclose the value of unsatisfied performance obligations for: (i) contracts with an original expected length of one year or less; and (ii) contracts for which revenue is recognized at the amount to which they have the right to invoice for services performed. (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 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. 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 incurred to fund energy efficiency programs and initiatives that help customers reduce their energy usage 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. (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 derivative instruments. Refer to Notes 15 , 16 and 17(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. 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. (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 and 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. (k) Debt Issuance and Retirement Costs - Debt issuance costs and debt premiums or discounts are presented on the balance sheet as a direct adjustment to 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 record to interest 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. (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, the entity is structured with disproportionate voting rights and substantially all of the entity’s activities are conducted on behalf of the investor with disproportionately fewer voting rights, 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 consolidation of VIEs. (n) New Accounting Standards - Revenue Recognition - In May 2014, the FASB 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. Alliant Energy, IPL and WPL adopted this standard on January 1, 2018 using the modified retrospective method of adoption, which was applied to contracts with customers that were completed subsequent to January 1, 2018. Alliant Energy, IPL and WPL utilized a portfolio approach upon adoption, which involved evaluating portfolios of contracts with similar characteristics, where the effects of applying the standard were not expected to be materially different than evaluating on an individual contract basis. Upon adoption, there were no cumulative effect adjustments made to the January 1, 2018 retained earnings balances. In addition, prior period amounts have not been restated to reflect the adoption of this standard and continue to be reported under the accounting standards in effect for those periods. Alliant Energy, IPL and WPL did not have a material change in revenue recognition, including the timing and pattern of revenue recognition, as a result of the adoption of this standard. Leases - In February 2016, the FASB 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 adopted this standard on January 1, 2019 using an optional transition approach and there was no cumulative effect adjustment to their balance sheets as of January 1, 2019. Alliant Energy, IPL and WPL expect to recognize increases in assets and liabilities for certain operating leases of approximately $18 million , $11 million and $7 million , respectively, on January 1, 2019. After adoption, prior period amounts will continue to be reported under the accounting standards in effect for those periods. Alliant Energy, IPL and WPL do not expect a material change to their results of operations or cash flows statements. Upon transition to the new standard, Alliant Energy, IPL and WPL elected the land easement transition practical expedient, for which existing land easements that were not previously accounted for as leases under the original accounting standards did not need to be evaluated under the new accounting standard. In addition, Alliant Energy, IPL and WPL evaluated land easements that were previously accounted for as leases and determined that the majority of these land easements relate to joint-use land sites, and do not meet the criteria for leases under the new accounting standard. Therefore, these land easement arrangements are no longer reflected as operating leases effective
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 previously held Alliant Energy’s interest in ATC. 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 farm, the Sheboygan Falls Energy Facility and other non-utility holdings. 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 interest in ATC Holdings. The non-utility wind farm includes a 50% cash equity ownership interest in a 225 MW 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. 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 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 modifications to the presentation of the components of net periodic benefit costs for defined benefit pension and other postretirement plans in the income statements, and restricted cash and cash receipts on sold receivables in the cash flows statements, as discussed in Note 1(n) . Discontinued operations reported in Alliant Energy’s income statements are related to various warranty claims associated with the sale of RMT, Inc. in 2013, which has 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. (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 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. 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. Federal Tax Reform repealed corporate federal AMT and allows unutilized AMT credits to be refunded over 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 2018 , 2017 and 2016 . (d) Cash, Cash Equivalents and Restricted Cash - Cash and cash equivalents include short-term liquid investments that have original maturities of less than 90 days. At December 31, 2018 and 2017 , restricted cash primarily related to deposits with trustees and requirements in Sheboygan Power, LLC’s debt agreement. (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. Property, plant and equipment that is probable of being retired early is classified as plant anticipated to be retired early. 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. 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 collected from customers. The average rates of depreciation for electric, gas and other properties, consistent with current rate-making practices, were as follows: IPL WPL 2018 2017 2016 2018 2017 2016 Electric - generation 3.6% 3.5% 3.5% 3.6% 3.5% 3.1% Electric - distribution 2.8% 2.4% 2.4% 2.6% 2.6% 2.6% Electric - other 4.7% 4.5% 4.2% 5.7% 6.9% 4.7% Gas 3.2% 3.4% 3.3% 2.5% 2.5% 2.5% Other 5.2% 4.0% 3.9% 5.8% 6.0% 5.9% 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: 2018 2017 2016 IPL (Marshalltown CWIP) N/A 7.8% 7.9% IPL (Wind generation CWIP) 7.5% 7.6% N/A IPL (other CWIP) 7.5% 7.6% 7.7% WPL (retail jurisdiction) 7.7% 7.6% 8.2% WPL (wholesale jurisdiction) 7.2% 6.0% 6.7% In accordance with their respective regulatory commission decisions, 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 and Other 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 remaining book value is recorded as a loss in the income statements. (f) Revenue Recognition - Utility - Revenues from Alliant Energy’s utility business are primarily from electric and gas sales to customers. Utility revenues are recognized over time as services are rendered or commodities are delivered to customers, and include billed and unbilled components. The billed component is based on the reading of customers’ meters, which occurs on a systematic basis throughout each reporting period and represents the fair value of the services provided or commodities delivered. The unbilled component is estimated and recorded at the end of each reporting period based on estimated amounts of energy delivered to customers since the end of each customer’s last billing period. The unbilled revenue is based on estimates of daily system demand volumes, 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. As of December 31, 2018 , the related amounts accrued for IPL and WPL were not material. IPL and WPL participate in bid/offer-based wholesale energy and ancillary services markets operated by MISO. 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 as bulk power sales in “Electric utility 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 over time as services are rendered 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 revenues. Other - Alliant Energy, IPL and WPL do not disclose the value of unsatisfied performance obligations for: (i) contracts with an original expected length of one year or less; and (ii) contracts for which revenue is recognized at the amount to which they have the right to invoice for services performed. (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 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. 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 incurred to fund energy efficiency programs and initiatives that help customers reduce their energy usage 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. (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 derivative instruments. Refer to Notes 15 , 16 and 17(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. 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. (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 and 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. (k) Debt Issuance and Retirement Costs - Debt issuance costs and debt premiums or discounts are presented on the balance sheet as a direct adjustment to 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 record to interest 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. (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, the entity is structured with disproportionate voting rights and substantially all of the entity’s activities are conducted on behalf of the investor with disproportionately fewer voting rights, 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 consolidation of VIEs. (n) New Accounting Standards - Revenue Recognition - In May 2014, the FASB 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. Alliant Energy, IPL and WPL adopted this standard on January 1, 2018 using the modified retrospective method of adoption, which was applied to contracts with customers that were completed subsequent to January 1, 2018. Alliant Energy, IPL and WPL utilized a portfolio approach upon adoption, which involved evaluating portfolios of contracts with similar characteristics, where the effects of applying the standard were not expected to be materially different than evaluating on an individual contract basis. Upon adoption, there were no cumulative effect adjustments made to the January 1, 2018 retained earnings balances. In addition, prior period amounts have not been restated to reflect the adoption of this standard and continue to be reported under the accounting standards in effect for those periods. Alliant Energy, IPL and WPL did not have a material change in revenue recognition, including the timing and pattern of revenue recognition, as a result of the adoption of this standard. Leases - In February 2016, the FASB 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 adopted this standard on January 1, 2019 using an optional transition approach and there was no cumulative effect adjustment to their balance sheets as of January 1, 2019. Alliant Energy, IPL and WPL expect to recognize increases in assets and liabilities for certain operating leases of approximately $18 million , $11 million and $7 million , respectively, on January 1, 2019. After adoption, prior period amounts will continue to be reported under the accounting standards in effect for those periods. Alliant Energy, IPL and WPL do not expect a material change to their results of operations or cash flows statements. Upon transition to the new standard, Alliant Energy, IPL and WPL elected the land easement transition practical expedient, for which existing land easements that were not previously accounted for as leases under the original accounting standards did not need to be evaluated under the new accounting standard. In addition, Alliant Energy, IPL and WPL evaluated land easements that were previously accounted for as leases and determined that the majority of these land easements relate to joint-use land sites, and do not meet the criteria for leases under the new accounting standard. Therefore, these land easement arrangements are no longer reflected as operating leases effective

Regulatory Matters

Regulatory Matters	12 Months Ended
	Dec. 31, 2018
Regulatory Matters [Line Items]
Regulatory Matters	REGULATORY MATTERS Regulatory Assets - 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, 2018 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. At December 31, regulatory assets were comprised of the following items (in millions): Alliant Energy IPL WPL 2018 2017 2018 2017 2018 2017 Tax-related $820.6 $778.2 $783.1 $750.5 $37.5 $27.7 Pension and OPEB costs 542.3 548.0 274.0 274.4 268.3 273.6 EGUs retired early 111.6 63.8 55.4 31.6 56.2 32.2 AROs 110.8 109.3 76.3 72.5 34.5 36.8 Derivatives 28.0 45.3 15.1 21.8 12.9 23.5 Emission allowances 23.6 25.5 23.6 25.5 — — Other 100.4 96.6 51.5 55.3 48.9 41.3 $1,737.3 $1,666.7 $1,279.0 $1,231.6 $458.3 $435.1 At December 31, 2018 , IPL and WPL had $77 million and $6 million , respectively, of regulatory assets that were not earning a return. IPL’s regulatory assets that were not earning a return consisted primarily of certain EGUs retired early, 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. 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. During 2018 , Alliant Energy’s and IPL’s tax-related regulatory assets increased primarily due to property-related differences for qualifying repairs expenditures. Partially offsetting this increase was a decrease due to the impacts of Iowa tax reform as discussed in Note 12 . Pension and other postretirement benefits costs - The IUB, PSCW and FERC have authorized IPL and WPL to record the previously unrecognized net actuarial gains and losses, and prior service costs and credits, as regulatory assets in lieu of accumulated other comprehensive loss on the balance sheets, as these amounts are expected to be recovered in future rates. 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 retail and wholesale customers, which are based upon pension and OPEB costs determined in accordance with GAAP and are calculated in accordance with IPL’s and WPL’s respective regulatory jurisdictions. Electric generating units retired early - IPL and WPL have retired various natural gas-fired and coal-fired EGUs and reclassified the remaining net book value of these EGUs from property, plant and equipment to regulatory assets on their respective balance sheets. Details regarding the recovery of the remaining net book value of these EGUs from IPL’s and WPL’s customers are as follows: Entity EGU Retirement Date Regulatory Asset Balance as of Dec. 31, 2018 Recovery Regulatory Approval IPL Sutherland Units 1 and 3 June 2017 $27.6 Return of remaining net book value over 10 years IUB and FERC IPL M.L. Kapp Unit 2 June 2018 27.8 Return of and return on remaining net book value over 10 years Pending with FERC; to be addressed with IUB WPL Nelson Dewey Units 1 and 2 and Edgewater Unit 3 December 2015 27.4 Return of and return on remaining net book value over 10 years PSCW and FERC WPL Edgewater Unit 4 September 2018 28.8 Return of and return on remaining net book value over 10 years PSCW and pending with FERC AROs - Alliant Energy, IPL and WPL believe it is probable that certain 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. 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. 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 and are authorized to recover these amounts from its retail customers over a 10 -year period. Regulatory Liabilities - At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2018 2017 2018 2017 2018 2017 Tax-related $890.6 $899.4 $390.1 $399.5 $500.5 $499.9 Cost of removal obligations 401.2 410.0 273.3 274.5 127.9 135.5 Electric transmission cost recovery 104.0 90.4 47.7 26.4 56.3 64.0 WPL earnings sharing mechanism 25.4 8.0 — — 25.4 8.0 Commodity cost recovery 16.8 21.0 11.9 14.6 4.9 6.4 IPL’s tax benefit riders 6.4 25.0 6.4 25.0 — — Other 48.8 43.4 25.5 15.4 23.3 28.0 $1,493.2 $1,497.2 $754.9 $755.4 $738.3 $741.8 Regulatory liabilities related to cost of removal obligations, to the extent expensed through depreciation rates, reduce rate base. Tax-related regulatory liabilities related to excess deferred taxes also reduce revenue requirement calculations utilized in IPL’s and WPL’s respective rate proceedings. A significant portion of the remaining regulatory liabilities are not used to adjust revenue requirement calculations. Tax-related - Alliant Energy’s, IPL’s and WPL’s tax-related regulatory liabilities are primarily related to excess deferred tax benefits resulting from the remeasurement of accumulated deferred income taxes caused by Federal Tax Reform. A portion of these benefits related to accelerated depreciation are subject to tax normalization rules. These rules limit the rate at which these tax benefits are allowed to be passed on to customers. 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 amounts collected in rates for these future removal costs and reduce the regulatory liability for amounts spent on removal activities. Electric transmission cost recovery - Refer to Note 1(g) for details of IPL’s and WPL’s electric transmission cost recovery mechanisms. Starting in March 2018, amounts billed by transmission providers decreased due to the impacts from Federal Tax Reform. During 2018, Alliant Energy, IPL and WPL recorded the benefits associated with lower transmission expense as regulatory liabilities. In May 2018, IPL starting providing these benefits back to its retail electric customers utilizing the transmission recovery mechanism. WPL is deferring these benefits until a future rate proceeding. Offsetting WPL’s increase in regulatory liabilities was transmission expense amortizations as authorized in the WPL retail electric rate review (2017/2018 Test Period). WPL earnings sharing mechanism - Pursuant to PSCW orders, WPL must defer a portion of its earnings if its annual regulatory return on common equity exceeds certain levels during the related test periods. As of December 31, 2018 , Alliant Energy and WPL deferred $20 million of WPL’s 2018 earnings related to this provision. The timing of the refund of these regulatory liabilities to customers will be determined in a future WPL regulatory proceeding. Commodity cost recovery - Refer to Note 1(g) for details of IPL’s and WPL’s commodity 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 2018 , Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $19 million as follows (in millions): Electric tax benefit rider credits $17 Gas tax benefit rider credits 2 $19 Other - In January 2018, the IUB issued an order requiring IPL and other investor-owned utilities in Iowa to track all calculated differences since January 1, 2018 resulting from Federal Tax Reform, such that any over-collections can be refunded to its customers at a future date. Pursuant to IUB approval, the retail electric portion of IPL’s Federal Tax Reform benefits is currently being refunded to customers, beginning May 2018. In January 2018, the PSCW issued an order directing WPL and other investor-owned utilities in Wisconsin to defer the revenue requirement impacts resulting from Federal Tax Reform since its inception. Pursuant to PSCW approval, the retail electric and gas portions of WPL’s Federal Tax Reform benefits are currently being refunded to customers, beginning June 2018. In 2018, Alliant Energy, IPL, and WPL refunded benefits of $66 million , $25 million and $41 million , respectively, from the 2018 impact of Federal Tax Reform, which were recorded as a reduction in revenues. As of December 31, 2018 , Alliant Energy’s, IPL’s and WPL’s remaining deferrals related to Federal Tax Reform were $20 million , $15 million and $5 million , respectively, which are included in “Other” in the regulatory liabilities table above. 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 and 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 September 2017, IPL reached a partial, non-unanimous settlement agreement with intervener groups 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 were effective May 1, 2018. 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 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 with implementation of final rates, 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 (2019/2020 Test Period) - In December 2018, the PSCW issued an order approving WPL’s proposed settlement for its retail electric and gas rate review covering the 2019/2020 Test Period, which was based on a stipulated agreement between WPL and intervener groups. Under the settlement, WPL retail electric and gas base rates will not change from current levels through the end of 2020. 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 order included utilization of amounts that WPL previously over-recovered from its customers for energy efficiency cost recovery and electric transmission cost recovery to reduce the requested base rate increase. The order also included provisions that required WPL to defer a portion of its earnings if its annual regulatory return on common equity exceeded certain levels in 2017 or 2018.
IPL [Member]
Regulatory Matters [Line Items]
Regulatory Matters	REGULATORY MATTERS Regulatory Assets - 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, 2018 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. At December 31, regulatory assets were comprised of the following items (in millions): Alliant Energy IPL WPL 2018 2017 2018 2017 2018 2017 Tax-related $820.6 $778.2 $783.1 $750.5 $37.5 $27.7 Pension and OPEB costs 542.3 548.0 274.0 274.4 268.3 273.6 EGUs retired early 111.6 63.8 55.4 31.6 56.2 32.2 AROs 110.8 109.3 76.3 72.5 34.5 36.8 Derivatives 28.0 45.3 15.1 21.8 12.9 23.5 Emission allowances 23.6 25.5 23.6 25.5 — — Other 100.4 96.6 51.5 55.3 48.9 41.3 $1,737.3 $1,666.7 $1,279.0 $1,231.6 $458.3 $435.1 At December 31, 2018 , IPL and WPL had $77 million and $6 million , respectively, of regulatory assets that were not earning a return. IPL’s regulatory assets that were not earning a return consisted primarily of certain EGUs retired early, 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. 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. During 2018 , Alliant Energy’s and IPL’s tax-related regulatory assets increased primarily due to property-related differences for qualifying repairs expenditures. Partially offsetting this increase was a decrease due to the impacts of Iowa tax reform as discussed in Note 12 . Pension and other postretirement benefits costs - The IUB, PSCW and FERC have authorized IPL and WPL to record the previously unrecognized net actuarial gains and losses, and prior service costs and credits, as regulatory assets in lieu of accumulated other comprehensive loss on the balance sheets, as these amounts are expected to be recovered in future rates. 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 retail and wholesale customers, which are based upon pension and OPEB costs determined in accordance with GAAP and are calculated in accordance with IPL’s and WPL’s respective regulatory jurisdictions. Electric generating units retired early - IPL and WPL have retired various natural gas-fired and coal-fired EGUs and reclassified the remaining net book value of these EGUs from property, plant and equipment to regulatory assets on their respective balance sheets. Details regarding the recovery of the remaining net book value of these EGUs from IPL’s and WPL’s customers are as follows: Entity EGU Retirement Date Regulatory Asset Balance as of Dec. 31, 2018 Recovery Regulatory Approval IPL Sutherland Units 1 and 3 June 2017 $27.6 Return of remaining net book value over 10 years IUB and FERC IPL M.L. Kapp Unit 2 June 2018 27.8 Return of and return on remaining net book value over 10 years Pending with FERC; to be addressed with IUB WPL Nelson Dewey Units 1 and 2 and Edgewater Unit 3 December 2015 27.4 Return of and return on remaining net book value over 10 years PSCW and FERC WPL Edgewater Unit 4 September 2018 28.8 Return of and return on remaining net book value over 10 years PSCW and pending with FERC AROs - Alliant Energy, IPL and WPL believe it is probable that certain 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. 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. 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 and are authorized to recover these amounts from its retail customers over a 10 -year period. Regulatory Liabilities - At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2018 2017 2018 2017 2018 2017 Tax-related $890.6 $899.4 $390.1 $399.5 $500.5 $499.9 Cost of removal obligations 401.2 410.0 273.3 274.5 127.9 135.5 Electric transmission cost recovery 104.0 90.4 47.7 26.4 56.3 64.0 WPL earnings sharing mechanism 25.4 8.0 — — 25.4 8.0 Commodity cost recovery 16.8 21.0 11.9 14.6 4.9 6.4 IPL’s tax benefit riders 6.4 25.0 6.4 25.0 — — Other 48.8 43.4 25.5 15.4 23.3 28.0 $1,493.2 $1,497.2 $754.9 $755.4 $738.3 $741.8 Regulatory liabilities related to cost of removal obligations, to the extent expensed through depreciation rates, reduce rate base. Tax-related regulatory liabilities related to excess deferred taxes also reduce revenue requirement calculations utilized in IPL’s and WPL’s respective rate proceedings. A significant portion of the remaining regulatory liabilities are not used to adjust revenue requirement calculations. Tax-related - Alliant Energy’s, IPL’s and WPL’s tax-related regulatory liabilities are primarily related to excess deferred tax benefits resulting from the remeasurement of accumulated deferred income taxes caused by Federal Tax Reform. A portion of these benefits related to accelerated depreciation are subject to tax normalization rules. These rules limit the rate at which these tax benefits are allowed to be passed on to customers. 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 amounts collected in rates for these future removal costs and reduce the regulatory liability for amounts spent on removal activities. Electric transmission cost recovery - Refer to Note 1(g) for details of IPL’s and WPL’s electric transmission cost recovery mechanisms. Starting in March 2018, amounts billed by transmission providers decreased due to the impacts from Federal Tax Reform. During 2018, Alliant Energy, IPL and WPL recorded the benefits associated with lower transmission expense as regulatory liabilities. In May 2018, IPL starting providing these benefits back to its retail electric customers utilizing the transmission recovery mechanism. WPL is deferring these benefits until a future rate proceeding. Offsetting WPL’s increase in regulatory liabilities was transmission expense amortizations as authorized in the WPL retail electric rate review (2017/2018 Test Period). WPL earnings sharing mechanism - Pursuant to PSCW orders, WPL must defer a portion of its earnings if its annual regulatory return on common equity exceeds certain levels during the related test periods. As of December 31, 2018 , Alliant Energy and WPL deferred $20 million of WPL’s 2018 earnings related to this provision. The timing of the refund of these regulatory liabilities to customers will be determined in a future WPL regulatory proceeding. Commodity cost recovery - Refer to Note 1(g) for details of IPL’s and WPL’s commodity 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 2018 , Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $19 million as follows (in millions): Electric tax benefit rider credits $17 Gas tax benefit rider credits 2 $19 Other - In January 2018, the IUB issued an order requiring IPL and other investor-owned utilities in Iowa to track all calculated differences since January 1, 2018 resulting from Federal Tax Reform, such that any over-collections can be refunded to its customers at a future date. Pursuant to IUB approval, the retail electric portion of IPL’s Federal Tax Reform benefits is currently being refunded to customers, beginning May 2018. In January 2018, the PSCW issued an order directing WPL and other investor-owned utilities in Wisconsin to defer the revenue requirement impacts resulting from Federal Tax Reform since its inception. Pursuant to PSCW approval, the retail electric and gas portions of WPL’s Federal Tax Reform benefits are currently being refunded to customers, beginning June 2018. In 2018, Alliant Energy, IPL, and WPL refunded benefits of $66 million , $25 million and $41 million , respectively, from the 2018 impact of Federal Tax Reform, which were recorded as a reduction in revenues. As of December 31, 2018 , Alliant Energy’s, IPL’s and WPL’s remaining deferrals related to Federal Tax Reform were $20 million , $15 million and $5 million , respectively, which are included in “Other” in the regulatory liabilities table above. 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 and 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 September 2017, IPL reached a partial, non-unanimous settlement agreement with intervener groups 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 were effective May 1, 2018. 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 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 with implementation of final rates, 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 (2019/2020 Test Period) - In December 2018, the PSCW issued an order approving WPL’s proposed settlement for its retail electric and gas rate review covering the 2019/2020 Test Period, which was based on a stipulated agreement between WPL and intervener groups. Under the settlement, WPL retail electric and gas base rates will not change from current levels through the end of 2020. 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 order included utilization of amounts that WPL previously over-recovered from its customers for energy efficiency cost recovery and electric transmission cost recovery to reduce the requested base rate increase. The order also included provisions that required WPL to defer a portion of its earnings if its annual regulatory return on common equity exceeded certain levels in 2017 or 2018.
WPL [Member]
Regulatory Matters [Line Items]
Regulatory Matters	REGULATORY MATTERS Regulatory Assets - 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, 2018 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. At December 31, regulatory assets were comprised of the following items (in millions): Alliant Energy IPL WPL 2018 2017 2018 2017 2018 2017 Tax-related $820.6 $778.2 $783.1 $750.5 $37.5 $27.7 Pension and OPEB costs 542.3 548.0 274.0 274.4 268.3 273.6 EGUs retired early 111.6 63.8 55.4 31.6 56.2 32.2 AROs 110.8 109.3 76.3 72.5 34.5 36.8 Derivatives 28.0 45.3 15.1 21.8 12.9 23.5 Emission allowances 23.6 25.5 23.6 25.5 — — Other 100.4 96.6 51.5 55.3 48.9 41.3 $1,737.3 $1,666.7 $1,279.0 $1,231.6 $458.3 $435.1 At December 31, 2018 , IPL and WPL had $77 million and $6 million , respectively, of regulatory assets that were not earning a return. IPL’s regulatory assets that were not earning a return consisted primarily of certain EGUs retired early, 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. 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. During 2018 , Alliant Energy’s and IPL’s tax-related regulatory assets increased primarily due to property-related differences for qualifying repairs expenditures. Partially offsetting this increase was a decrease due to the impacts of Iowa tax reform as discussed in Note 12 . Pension and other postretirement benefits costs - The IUB, PSCW and FERC have authorized IPL and WPL to record the previously unrecognized net actuarial gains and losses, and prior service costs and credits, as regulatory assets in lieu of accumulated other comprehensive loss on the balance sheets, as these amounts are expected to be recovered in future rates. 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 retail and wholesale customers, which are based upon pension and OPEB costs determined in accordance with GAAP and are calculated in accordance with IPL’s and WPL’s respective regulatory jurisdictions. Electric generating units retired early - IPL and WPL have retired various natural gas-fired and coal-fired EGUs and reclassified the remaining net book value of these EGUs from property, plant and equipment to regulatory assets on their respective balance sheets. Details regarding the recovery of the remaining net book value of these EGUs from IPL’s and WPL’s customers are as follows: Entity EGU Retirement Date Regulatory Asset Balance as of Dec. 31, 2018 Recovery Regulatory Approval IPL Sutherland Units 1 and 3 June 2017 $27.6 Return of remaining net book value over 10 years IUB and FERC IPL M.L. Kapp Unit 2 June 2018 27.8 Return of and return on remaining net book value over 10 years Pending with FERC; to be addressed with IUB WPL Nelson Dewey Units 1 and 2 and Edgewater Unit 3 December 2015 27.4 Return of and return on remaining net book value over 10 years PSCW and FERC WPL Edgewater Unit 4 September 2018 28.8 Return of and return on remaining net book value over 10 years PSCW and pending with FERC AROs - Alliant Energy, IPL and WPL believe it is probable that certain 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. 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. 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 and are authorized to recover these amounts from its retail customers over a 10 -year period. Regulatory Liabilities - At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2018 2017 2018 2017 2018 2017 Tax-related $890.6 $899.4 $390.1 $399.5 $500.5 $499.9 Cost of removal obligations 401.2 410.0 273.3 274.5 127.9 135.5 Electric transmission cost recovery 104.0 90.4 47.7 26.4 56.3 64.0 WPL earnings sharing mechanism 25.4 8.0 — — 25.4 8.0 Commodity cost recovery 16.8 21.0 11.9 14.6 4.9 6.4 IPL’s tax benefit riders 6.4 25.0 6.4 25.0 — — Other 48.8 43.4 25.5 15.4 23.3 28.0 $1,493.2 $1,497.2 $754.9 $755.4 $738.3 $741.8 Regulatory liabilities related to cost of removal obligations, to the extent expensed through depreciation rates, reduce rate base. Tax-related regulatory liabilities related to excess deferred taxes also reduce revenue requirement calculations utilized in IPL’s and WPL’s respective rate proceedings. A significant portion of the remaining regulatory liabilities are not used to adjust revenue requirement calculations. Tax-related - Alliant Energy’s, IPL’s and WPL’s tax-related regulatory liabilities are primarily related to excess deferred tax benefits resulting from the remeasurement of accumulated deferred income taxes caused by Federal Tax Reform. A portion of these benefits related to accelerated depreciation are subject to tax normalization rules. These rules limit the rate at which these tax benefits are allowed to be passed on to customers. 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 amounts collected in rates for these future removal costs and reduce the regulatory liability for amounts spent on removal activities. Electric transmission cost recovery - Refer to Note 1(g) for details of IPL’s and WPL’s electric transmission cost recovery mechanisms. Starting in March 2018, amounts billed by transmission providers decreased due to the impacts from Federal Tax Reform. During 2018, Alliant Energy, IPL and WPL recorded the benefits associated with lower transmission expense as regulatory liabilities. In May 2018, IPL starting providing these benefits back to its retail electric customers utilizing the transmission recovery mechanism. WPL is deferring these benefits until a future rate proceeding. Offsetting WPL’s increase in regulatory liabilities was transmission expense amortizations as authorized in the WPL retail electric rate review (2017/2018 Test Period). WPL earnings sharing mechanism - Pursuant to PSCW orders, WPL must defer a portion of its earnings if its annual regulatory return on common equity exceeds certain levels during the related test periods. As of December 31, 2018 , Alliant Energy and WPL deferred $20 million of WPL’s 2018 earnings related to this provision. The timing of the refund of these regulatory liabilities to customers will be determined in a future WPL regulatory proceeding. Commodity cost recovery - Refer to Note 1(g) for details of IPL’s and WPL’s commodity 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 2018 , Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $19 million as follows (in millions): Electric tax benefit rider credits $17 Gas tax benefit rider credits 2 $19 Other - In January 2018, the IUB issued an order requiring IPL and other investor-owned utilities in Iowa to track all calculated differences since January 1, 2018 resulting from Federal Tax Reform, such that any over-collections can be refunded to its customers at a future date. Pursuant to IUB approval, the retail electric portion of IPL’s Federal Tax Reform benefits is currently being refunded to customers, beginning May 2018. In January 2018, the PSCW issued an order directing WPL and other investor-owned utilities in Wisconsin to defer the revenue requirement impacts resulting from Federal Tax Reform since its inception. Pursuant to PSCW approval, the retail electric and gas portions of WPL’s Federal Tax Reform benefits are currently being refunded to customers, beginning June 2018. In 2018, Alliant Energy, IPL, and WPL refunded benefits of $66 million , $25 million and $41 million , respectively, from the 2018 impact of Federal Tax Reform, which were recorded as a reduction in revenues. As of December 31, 2018 , Alliant Energy’s, IPL’s and WPL’s remaining deferrals related to Federal Tax Reform were $20 million , $15 million and $5 million , respectively, which are included in “Other” in the regulatory liabilities table above. 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 and 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 September 2017, IPL reached a partial, non-unanimous settlement agreement with intervener groups 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 were effective May 1, 2018. 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 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 with implementation of final rates, 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 (2019/2020 Test Period) - In December 2018, the PSCW issued an order approving WPL’s proposed settlement for its retail electric and gas rate review covering the 2019/2020 Test Period, which was based on a stipulated agreement between WPL and intervener groups. Under the settlement, WPL retail electric and gas base rates will not change from current levels through the end of 2020. 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 order included utilization of amounts that WPL previously over-recovered from its customers for energy efficiency cost recovery and electric transmission cost recovery to reduce the requested base rate increase. The order also included provisions that required WPL to defer a portion of its earnings if its annual regulatory return on common equity exceeded certain levels in 2017 or 2018.

Property, Plant and Equipment

Property, Plant and Equipment	12 Months Ended
	Dec. 31, 2018
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 2018 2017 2018 2017 2018 2017 Utility: Electric plant: Generation in service $6,800.6 $6,655.3 $3,610.4 $3,715.9 $3,190.2 $2,939.4 Distribution in service 5,452.2 5,123.5 3,023.7 2,820.9 2,428.5 2,302.6 Other in service 410.8 425.1 260.4 282.3 150.4 142.8 Anticipated to be retired early (a) — 93.0 — — — 93.0 Total electric plant 12,663.6 12,296.9 6,894.5 6,819.1 5,769.1 5,477.8 Gas plant in service 1,387.6 1,244.0 763.1 654.8 624.5 589.2 Other plant in service 513.2 571.9 322.4 333.4 190.8 238.5 Accumulated depreciation (4,314.6 ) (4,283.1 ) (2,294.7 ) (2,311.0 ) (2,019.9 ) (1,972.1 ) Net plant 10,249.8 9,829.7 5,685.3 5,496.3 4,564.5 4,333.4 Leased Sheboygan Falls Energy Facility, net (b) — — — — 38.1 46.2 Construction work in progress 1,774.8 962.2 1,091.2 424.4 683.6 537.8 Other, net 6.1 6.0 5.0 5.5 1.1 0.5 Total utility 12,030.7 10,797.9 6,781.5 5,926.2 5,287.3 4,917.9 Non-utility and other: Non-utility Generation, net (c) 86.9 90.9 — — — — Corporate Services and other, net (d) 344.8 345.7 — — — — Total non-utility and other 431.7 436.6 — — — — Total property, plant and equipment $12,462.4 $11,234.5 $6,781.5 $5,926.2 $5,287.3 $4,917.9 (a) In 2018, WPL retired Edgewater Unit 4 and reclassified the remaining net book value of this EGU from property, plant and equipment to a regulatory asset on Alliant Energy’s and WPL’s balance sheets. (b) Less accumulated amortization of $82.8 million and $77.6 million for WPL as of December 31, 2018 and 2017 , 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. (c) Less accumulated depreciation of $54.5 million and $50.5 million for Alliant Energy as of December 31, 2018 and 2017 , respectively. (d) Less accumulated depreciation of $167.5 million and $285.6 million for Alliant Energy as of December 31, 2018 and 2017 , respectively. Generation in Service - WPL’s Acquisition of Forward Wind Energy Center - In 2018, WPL, along with WPSC and MGE, received approval from the PSCW and FERC to acquire a partial ownership interest in the assets of FWEC, which is a 129 MW wind farm located in Wisconsin. In April 2018, WPL acquired 55 MW of the FWEC wind farm for approximately $74 million . WPL, WPSC and MGE had been receiving electricity from the FWEC wind farm under PPAs since FWEC began commercial operations in 2008. Upon completion of the acquisitions, such PPAs terminated. As of the closing date, the estimated fair value of the assets purchased and the liabilities assumed by WPL were as follows (in millions): Property, plant and equipment, net $81 Liabilities 7 Net assets acquired $74 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 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 2018 2017 2016 2018 2017 2016 2018 2017 2016 Equity $51.4 $33.6 $42.3 $28.6 $21.1 $35.2 $22.8 $12.5 $7.1 Debt 24.2 16.1 20.2 13.6 10.3 16.8 10.6 5.8 3.4 $75.6 $49.7 $62.5 $42.2 $31.4 $52.0 $33.4 $18.3 $10.5 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, 2018 , Alliant Energy recorded $87 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 2018 2017 2018 2017 2018 2017 Utility: Electric plant: Generation in service $6,800.6 $6,655.3 $3,610.4 $3,715.9 $3,190.2 $2,939.4 Distribution in service 5,452.2 5,123.5 3,023.7 2,820.9 2,428.5 2,302.6 Other in service 410.8 425.1 260.4 282.3 150.4 142.8 Anticipated to be retired early (a) — 93.0 — — — 93.0 Total electric plant 12,663.6 12,296.9 6,894.5 6,819.1 5,769.1 5,477.8 Gas plant in service 1,387.6 1,244.0 763.1 654.8 624.5 589.2 Other plant in service 513.2 571.9 322.4 333.4 190.8 238.5 Accumulated depreciation (4,314.6 ) (4,283.1 ) (2,294.7 ) (2,311.0 ) (2,019.9 ) (1,972.1 ) Net plant 10,249.8 9,829.7 5,685.3 5,496.3 4,564.5 4,333.4 Leased Sheboygan Falls Energy Facility, net (b) — — — — 38.1 46.2 Construction work in progress 1,774.8 962.2 1,091.2 424.4 683.6 537.8 Other, net 6.1 6.0 5.0 5.5 1.1 0.5 Total utility 12,030.7 10,797.9 6,781.5 5,926.2 5,287.3 4,917.9 Non-utility and other: Non-utility Generation, net (c) 86.9 90.9 — — — — Corporate Services and other, net (d) 344.8 345.7 — — — — Total non-utility and other 431.7 436.6 — — — — Total property, plant and equipment $12,462.4 $11,234.5 $6,781.5 $5,926.2 $5,287.3 $4,917.9 (a) In 2018, WPL retired Edgewater Unit 4 and reclassified the remaining net book value of this EGU from property, plant and equipment to a regulatory asset on Alliant Energy’s and WPL’s balance sheets. (b) Less accumulated amortization of $82.8 million and $77.6 million for WPL as of December 31, 2018 and 2017 , 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. (c) Less accumulated depreciation of $54.5 million and $50.5 million for Alliant Energy as of December 31, 2018 and 2017 , respectively. (d) Less accumulated depreciation of $167.5 million and $285.6 million for Alliant Energy as of December 31, 2018 and 2017 , respectively. Generation in Service - WPL’s Acquisition of Forward Wind Energy Center - In 2018, WPL, along with WPSC and MGE, received approval from the PSCW and FERC to acquire a partial ownership interest in the assets of FWEC, which is a 129 MW wind farm located in Wisconsin. In April 2018, WPL acquired 55 MW of the FWEC wind farm for approximately $74 million . WPL, WPSC and MGE had been receiving electricity from the FWEC wind farm under PPAs since FWEC began commercial operations in 2008. Upon completion of the acquisitions, such PPAs terminated. As of the closing date, the estimated fair value of the assets purchased and the liabilities assumed by WPL were as follows (in millions): Property, plant and equipment, net $81 Liabilities 7 Net assets acquired $74 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 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 2018 2017 2016 2018 2017 2016 2018 2017 2016 Equity $51.4 $33.6 $42.3 $28.6 $21.1 $35.2 $22.8 $12.5 $7.1 Debt 24.2 16.1 20.2 13.6 10.3 16.8 10.6 5.8 3.4 $75.6 $49.7 $62.5 $42.2 $31.4 $52.0 $33.4 $18.3 $10.5
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 2018 2017 2018 2017 2018 2017 Utility: Electric plant: Generation in service $6,800.6 $6,655.3 $3,610.4 $3,715.9 $3,190.2 $2,939.4 Distribution in service 5,452.2 5,123.5 3,023.7 2,820.9 2,428.5 2,302.6 Other in service 410.8 425.1 260.4 282.3 150.4 142.8 Anticipated to be retired early (a) — 93.0 — — — 93.0 Total electric plant 12,663.6 12,296.9 6,894.5 6,819.1 5,769.1 5,477.8 Gas plant in service 1,387.6 1,244.0 763.1 654.8 624.5 589.2 Other plant in service 513.2 571.9 322.4 333.4 190.8 238.5 Accumulated depreciation (4,314.6 ) (4,283.1 ) (2,294.7 ) (2,311.0 ) (2,019.9 ) (1,972.1 ) Net plant 10,249.8 9,829.7 5,685.3 5,496.3 4,564.5 4,333.4 Leased Sheboygan Falls Energy Facility, net (b) — — — — 38.1 46.2 Construction work in progress 1,774.8 962.2 1,091.2 424.4 683.6 537.8 Other, net 6.1 6.0 5.0 5.5 1.1 0.5 Total utility 12,030.7 10,797.9 6,781.5 5,926.2 5,287.3 4,917.9 Non-utility and other: Non-utility Generation, net (c) 86.9 90.9 — — — — Corporate Services and other, net (d) 344.8 345.7 — — — — Total non-utility and other 431.7 436.6 — — — — Total property, plant and equipment $12,462.4 $11,234.5 $6,781.5 $5,926.2 $5,287.3 $4,917.9 (a) In 2018, WPL retired Edgewater Unit 4 and reclassified the remaining net book value of this EGU from property, plant and equipment to a regulatory asset on Alliant Energy’s and WPL’s balance sheets. (b) Less accumulated amortization of $82.8 million and $77.6 million for WPL as of December 31, 2018 and 2017 , 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. (c) Less accumulated depreciation of $54.5 million and $50.5 million for Alliant Energy as of December 31, 2018 and 2017 , respectively. (d) Less accumulated depreciation of $167.5 million and $285.6 million for Alliant Energy as of December 31, 2018 and 2017 , respectively. Generation in Service - WPL’s Acquisition of Forward Wind Energy Center - In 2018, WPL, along with WPSC and MGE, received approval from the PSCW and FERC to acquire a partial ownership interest in the assets of FWEC, which is a 129 MW wind farm located in Wisconsin. In April 2018, WPL acquired 55 MW of the FWEC wind farm for approximately $74 million . WPL, WPSC and MGE had been receiving electricity from the FWEC wind farm under PPAs since FWEC began commercial operations in 2008. Upon completion of the acquisitions, such PPAs terminated. As of the closing date, the estimated fair value of the assets purchased and the liabilities assumed by WPL were as follows (in millions): Property, plant and equipment, net $81 Liabilities 7 Net assets acquired $74 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 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 2018 2017 2016 2018 2017 2016 2018 2017 2016 Equity $51.4 $33.6 $42.3 $28.6 $21.1 $35.2 $22.8 $12.5 $7.1 Debt 24.2 16.1 20.2 13.6 10.3 16.8 10.6 5.8 3.4 $75.6 $49.7 $62.5 $42.2 $31.4 $52.0 $33.4 $18.3 $10.5

Jointly-Owned Electric Utility

Jointly-Owned Electric Utility Plant	12 Months Ended
	Dec. 31, 2018
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 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 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 EGUs at December 31, 2018 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0 % $507.1 $161.7 $65.3 George Neal Unit 4 25.7 % 187.3 89.8 1.2 George Neal Unit 3 28.0 % 156.7 57.8 1.4 Louisa Unit 1 4.0 % 39.2 23.9 0.8 890.3 333.2 68.7 WPL Columbia Units 1-2 52.5 % 779.4 247.9 9.8 FWEC 42.6 % 119.7 41.2 0.1 West Riverside (a) 91.8 % — — 538.4 899.1 289.1 548.3 Alliant Energy $1,789.4 $622.3 $617.0 (a) In January 2018, certain electric cooperatives, which currently have wholesale power supply agreements with WPL, acquired a partial ownership interest in West Riverside.
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 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 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 EGUs at December 31, 2018 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0 % $507.1 $161.7 $65.3 George Neal Unit 4 25.7 % 187.3 89.8 1.2 George Neal Unit 3 28.0 % 156.7 57.8 1.4 Louisa Unit 1 4.0 % 39.2 23.9 0.8 890.3 333.2 68.7 WPL Columbia Units 1-2 52.5 % 779.4 247.9 9.8 FWEC 42.6 % 119.7 41.2 0.1 West Riverside (a) 91.8 % — — 538.4 899.1 289.1 548.3 Alliant Energy $1,789.4 $622.3 $617.0 (a) In January 2018, certain electric cooperatives, which currently have wholesale power supply agreements with WPL, acquired a partial ownership interest in West Riverside.
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 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 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 EGUs at December 31, 2018 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0 % $507.1 $161.7 $65.3 George Neal Unit 4 25.7 % 187.3 89.8 1.2 George Neal Unit 3 28.0 % 156.7 57.8 1.4 Louisa Unit 1 4.0 % 39.2 23.9 0.8 890.3 333.2 68.7 WPL Columbia Units 1-2 52.5 % 779.4 247.9 9.8 FWEC 42.6 % 119.7 41.2 0.1 West Riverside (a) 91.8 % — — 538.4 899.1 289.1 548.3 Alliant Energy $1,789.4 $622.3 $617.0 (a) In January 2018, certain electric cooperatives, which currently have wholesale power supply agreements with WPL, acquired a partial ownership interest in West Riverside.

Receivables

Receivables	12 Months Ended
	Dec. 31, 2018
Accounts 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 2018 2017 2018 2017 2018 2017 Customer $91.0 $103.3 $— $— $84.8 $97.7 Unbilled utility revenues 74.2 85.1 — — 74.2 85.1 Deferred proceeds 119.4 222.1 119.4 222.1 — — Other 76.3 84.3 37.2 44.1 38.5 40.1 Allowance for doubtful accounts (10.5 ) (12.0 ) (3.1 ) (1.3 ) (7.4 ) (10.7 ) $350.4 $482.8 $153.5 $264.9 $190.1 $212.2 (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. In March 2018, IPL amended and extended through March 2021 the purchase commitment from the third party to which it sells its receivables. 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. 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. Refer to Note 16 for discussion of the fair value of deferred proceeds. 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 April 2018, the limit on cash proceeds fluctuates between $90 million and $110 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, 2018 , IPL had no available capacity under its sales of accounts receivable program. 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 2018 2017 2016 2018 2017 2016 Outstanding aggregate cash proceeds $116.0 $112.0 $172.0 $53.4 $62.2 $73.2 As of December 31, the attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2018 2017 Customer accounts receivable $140.1 $133.8 Unbilled utility revenues 97.1 112.7 Other receivables 0.1 0.3 Receivables sold to third party 237.3 246.8 Less: cash proceeds 108.0 12.0 Deferred proceeds 129.3 234.8 Less: allowance for doubtful accounts 9.9 12.7 Fair value of deferred proceeds $119.4 $222.1 Outstanding receivables past due $35.5 $44.7 Additional attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2018 2017 2016 Collections $2,076.7 $1,647.1 $1,818.1 Write-offs, net of recoveries 21.3 17.7 4.8 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 compared to 2017 and 2018.
IPL [Member]
Accounts 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 2018 2017 2018 2017 2018 2017 Customer $91.0 $103.3 $— $— $84.8 $97.7 Unbilled utility revenues 74.2 85.1 — — 74.2 85.1 Deferred proceeds 119.4 222.1 119.4 222.1 — — Other 76.3 84.3 37.2 44.1 38.5 40.1 Allowance for doubtful accounts (10.5 ) (12.0 ) (3.1 ) (1.3 ) (7.4 ) (10.7 ) $350.4 $482.8 $153.5 $264.9 $190.1 $212.2 (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. In March 2018, IPL amended and extended through March 2021 the purchase commitment from the third party to which it sells its receivables. 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. 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. Refer to Note 16 for discussion of the fair value of deferred proceeds. 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 April 2018, the limit on cash proceeds fluctuates between $90 million and $110 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, 2018 , IPL had no available capacity under its sales of accounts receivable program. 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 2018 2017 2016 2018 2017 2016 Outstanding aggregate cash proceeds $116.0 $112.0 $172.0 $53.4 $62.2 $73.2 As of December 31, the attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2018 2017 Customer accounts receivable $140.1 $133.8 Unbilled utility revenues 97.1 112.7 Other receivables 0.1 0.3 Receivables sold to third party 237.3 246.8 Less: cash proceeds 108.0 12.0 Deferred proceeds 129.3 234.8 Less: allowance for doubtful accounts 9.9 12.7 Fair value of deferred proceeds $119.4 $222.1 Outstanding receivables past due $35.5 $44.7 Additional attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2018 2017 2016 Collections $2,076.7 $1,647.1 $1,818.1 Write-offs, net of recoveries 21.3 17.7 4.8 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 compared to 2017 and 2018.
WPL [Member]
Accounts 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 2018 2017 2018 2017 2018 2017 Customer $91.0 $103.3 $— $— $84.8 $97.7 Unbilled utility revenues 74.2 85.1 — — 74.2 85.1 Deferred proceeds 119.4 222.1 119.4 222.1 — — Other 76.3 84.3 37.2 44.1 38.5 40.1 Allowance for doubtful accounts (10.5 ) (12.0 ) (3.1 ) (1.3 ) (7.4 ) (10.7 ) $350.4 $482.8 $153.5 $264.9 $190.1 $212.2

Investments

Investments	12 Months Ended
	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Investments	INVESTMENTS 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, 2018 2018 2017 2018 2017 2016 Alliant Energy ATC Holdings (a) 16%-20% $293.6 $274.2 ($38.1 ) ($42.4 ) ($39.1 ) Non-utility wind farm in Oklahoma 50% 105.1 98.3 (15.6 ) (1.8 ) — Other Various 22.6 8.9 (0.9 ) (0.6 ) (0.5 ) $421.3 $381.4 ($54.6 ) ($44.8 ) ($39.6 ) WPL ATC —% $— $— $— $— ($39.1 ) Wisconsin River Power Company 50% 8.1 8.3 (0.9 ) (0.7 ) (0.7 ) $8.1 $8.3 ($0.9 ) ($0.7 ) ($39.8 ) (a) As of December 31, 2018 , Alliant Energy’s interest in ATC Holdings is comprised of a 16% ownership interest in ATC and a 20% ownership interest in ATC Holdco LLC, which are described below. Alliant Energy currently has the ability to exercise significant influence over ATC’s and ATC Holdco LLC’s financial and operating policies through its participation on ATC’s Board of Directors. Summary aggregate financial information from the financial statements of these holdings is as follows (in millions): Alliant Energy WPL 2018 2017 2016 2018 2017 2016 Revenues $724 $741 $658 $8 $8 $658 Operating income 325 374 331 2 4 331 Net income 217 267 232 1 2 234 As of December 31: Current assets 144 104 4 7 Non-current assets 5,498 5,041 19 20 Current liabilities 644 770 1 2 Non-current liabilities 2,315 2,038 6 8 Noncontrolling interest 223 255 — — Interest in ATC and WPL’s Noncontrolling Interest - Prior to 2014, WPL owned 100% of WPL Transco, which held Alliant Energy’s interest 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. Prior to the transfer of the interest 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 PSCW order, WPL Transco was liquidated and WPL transferred its interest in ATC to ATI. As a result, WPL no longer records equity income from its prior interest in ATC. In conjunction with the transfer of the interest in ATC, a deferred intercompany tax gain recognized by WPL was assumed by ATI. The impact of WPL’s transfer of the interest in ATC Holdings, 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 interest in ATC Holdings. Interest in ATC Holdco LLC - In 2011, Duke Energy Corporation and ATC announced the creation of Duke-American Transmission Company, LLC, a joint venture that is expected to acquire, build, own and operate new electric transmission infrastructure in North America. In 2017, ATC transferred a portion of its interest in Duke-American Transmission Company, LLC 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 the proceeds from the transfer was distributed to all ATC Holdco LLC’s owners based on their ATC ownership percentage. Non-utility Wind Farm 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. The wind farm provides electricity to a third-party under a long-term PPA, and has both cash and tax equity ownership. The equity income recognized in 2018 was primarily related to the impacts of Federal Tax Reform. The liquidation method utilized to recognize Alliant Energy’s share of the wind farm’s earnings includes utilizing the federal income tax rate in effect as of the end of the measurement period. The lower federal income tax rate effective as of January 1, 2018 resulted in an acceleration of earnings attributable to Alliant Energy’s interest in the Oklahoma wind farm. This increase in earnings is expected to reverse over time. Alliant Energy does 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 17(d) for discussion of the guarantee.
WPL [Member]
Schedule of Equity Method Investments [Line Items]
Investments	INVESTMENTS 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, 2018 2018 2017 2018 2017 2016 Alliant Energy ATC Holdings (a) 16%-20% $293.6 $274.2 ($38.1 ) ($42.4 ) ($39.1 ) Non-utility wind farm in Oklahoma 50% 105.1 98.3 (15.6 ) (1.8 ) — Other Various 22.6 8.9 (0.9 ) (0.6 ) (0.5 ) $421.3 $381.4 ($54.6 ) ($44.8 ) ($39.6 ) WPL ATC —% $— $— $— $— ($39.1 ) Wisconsin River Power Company 50% 8.1 8.3 (0.9 ) (0.7 ) (0.7 ) $8.1 $8.3 ($0.9 ) ($0.7 ) ($39.8 ) (a) As of December 31, 2018 , Alliant Energy’s interest in ATC Holdings is comprised of a 16% ownership interest in ATC and a 20% ownership interest in ATC Holdco LLC, which are described below. Alliant Energy currently has the ability to exercise significant influence over ATC’s and ATC Holdco LLC’s financial and operating policies through its participation on ATC’s Board of Directors. Summary aggregate financial information from the financial statements of these holdings is as follows (in millions): Alliant Energy WPL 2018 2017 2016 2018 2017 2016 Revenues $724 $741 $658 $8 $8 $658 Operating income 325 374 331 2 4 331 Net income 217 267 232 1 2 234 As of December 31: Current assets 144 104 4 7 Non-current assets 5,498 5,041 19 20 Current liabilities 644 770 1 2 Non-current liabilities 2,315 2,038 6 8 Noncontrolling interest 223 255 — — Interest in ATC and WPL’s Noncontrolling Interest - Prior to 2014, WPL owned 100% of WPL Transco, which held Alliant Energy’s interest 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. Prior to the transfer of the interest 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 PSCW order, WPL Transco was liquidated and WPL transferred its interest in ATC to ATI. As a result, WPL no longer records equity income from its prior interest in ATC. In conjunction with the transfer of the interest in ATC, a deferred intercompany tax gain recognized by WPL was assumed by ATI. The impact of WPL’s transfer of the interest in ATC Holdings, 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 interest in ATC Holdings. Interest in ATC Holdco LLC - In 2011, Duke Energy Corporation and ATC announced the creation of Duke-American Transmission Company, LLC, a joint venture that is expected to acquire, build, own and operate new electric transmission infrastructure in North America. In 2017, ATC transferred a portion of its interest in Duke-American Transmission Company, LLC 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 the proceeds from the transfer was distributed to all ATC Holdco LLC’s owners based on their ATC ownership percentage.

Common Equity

Common Equity	12 Months Ended
	Dec. 31, 2018
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: 2018 2017 2016 Shares outstanding, January 1 231,348,646 227,673,654 226,918,432 At-the-market offering programs 4,171,013 3,074,931 — Shareowner Direct Plan issuances 576,965 640,723 732,814 Equity-based compensation plans ( Note 13(b) ) 5,078 5,185 22,408 Other (38,423 ) (45,847 ) — Shares outstanding, December 31 236,063,279 231,348,646 227,673,654 At December 31, 2018 , Alliant Energy had a total of 11.2 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 2018 and 2017, Alliant Energy filed prospectus supplements under which it could sell up to $175 million and $125 million of its common stock, respectively, through at-the-market offering programs. In 2018, Alliant Energy issued 4,171,013 shares of common stock through this program and received cash proceeds of $173 million , net of $2 million in commissions and fees. 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. The 2017 at-the-market offering program expired in 2018, and Alliant Energy currently has no plans to issue any additional common stock through the 2018 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. Equity Forward Agreements - In December 2018, Alliant Energy entered into forward sale agreements with various counterparties in connection with a public offering of 8,358,973 shares of Alliant Energy common stock. In the forward sale transaction, the counterparties, or their affiliates, borrowed an aggregate of 8,358,973 shares of Alliant Energy common stock from third parties and sold such shares to the related underwriters. Alliant Energy has concluded that the forward sale agreements meet the derivative scope exception for certain contracts involving an entity’s own equity. Alliant Energy has not yet received any proceeds from the offering and no amounts have been or will be recorded in equity on Alliant Energy’s balance sheets until the forward sale agreements settle. Alliant Energy expects to settle the forward sale agreements prior to December 31, 2019 through physical delivery of shares of common stock in exchange for cash proceeds at the then-applicable forward sale price; however, Alliant Energy may elect cash settlement or net share settlement for all or a portion of the obligations under the forward sale agreements. The initial forward sale price of $44.33 per share, is subject to daily adjustment based on a floating interest rate factor, and will decrease by other fixed amounts specified in the forward sale agreement. Until settlement of the forward sale agreements, Alliant Energy’s EPS dilution resulting from the agreements, if any, is determined using the treasury stock method. Share dilution occurs when the average market price of Alliant Energy stock during the reporting period is higher than the forward sale price as of the end of the reporting period. As of December 31, 2018, the securities under the forward sale agreements were excluded from the calculation of diluted EPS as they were antidilutive. 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. Comprehensive Income (Loss) - In 2018 , 2017 and 2016 , Alliant Energy’s other comprehensive income (loss) was $2.2 million , ($0.1) million and $0 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 2018 , 2017 and 2016 , 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: 2018 2017 2016 Shares outstanding, January 1 231,348,646 227,673,654 226,918,432 At-the-market offering programs 4,171,013 3,074,931 — Shareowner Direct Plan issuances 576,965 640,723 732,814 Equity-based compensation plans ( Note 13(b) ) 5,078 5,185 22,408 Other (38,423 ) (45,847 ) — Shares outstanding, December 31 236,063,279 231,348,646 227,673,654 At December 31, 2018 , Alliant Energy had a total of 11.2 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 2018 and 2017, Alliant Energy filed prospectus supplements under which it could sell up to $175 million and $125 million of its common stock, respectively, through at-the-market offering programs. In 2018, Alliant Energy issued 4,171,013 shares of common stock through this program and received cash proceeds of $173 million , net of $2 million in commissions and fees. 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. The 2017 at-the-market offering program expired in 2018, and Alliant Energy currently has no plans to issue any additional common stock through the 2018 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. Equity Forward Agreements - In December 2018, Alliant Energy entered into forward sale agreements with various counterparties in connection with a public offering of 8,358,973 shares of Alliant Energy common stock. In the forward sale transaction, the counterparties, or their affiliates, borrowed an aggregate of 8,358,973 shares of Alliant Energy common stock from third parties and sold such shares to the related underwriters. Alliant Energy has concluded that the forward sale agreements meet the derivative scope exception for certain contracts involving an entity’s own equity. Alliant Energy has not yet received any proceeds from the offering and no amounts have been or will be recorded in equity on Alliant Energy’s balance sheets until the forward sale agreements settle. Alliant Energy expects to settle the forward sale agreements prior to December 31, 2019 through physical delivery of shares of common stock in exchange for cash proceeds at the then-applicable forward sale price; however, Alliant Energy may elect cash settlement or net share settlement for all or a portion of the obligations under the forward sale agreements. The initial forward sale price of $44.33 per share, is subject to daily adjustment based on a floating interest rate factor, and will decrease by other fixed amounts specified in the forward sale agreement. Until settlement of the forward sale agreements, Alliant Energy’s EPS dilution resulting from the agreements, if any, is determined using the treasury stock method. Share dilution occurs when the average market price of Alliant Energy stock during the reporting period is higher than the forward sale price as of the end of the reporting period. As of December 31, 2018, the securities under the forward sale agreements were excluded from the calculation of diluted EPS as they were antidilutive. 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. Comprehensive Income (Loss) - In 2018 , 2017 and 2016 , Alliant Energy’s other comprehensive income (loss) was $2.2 million , ($0.1) million and $0 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 2018 , 2017 and 2016 , 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: 2018 2017 2016 Shares outstanding, January 1 231,348,646 227,673,654 226,918,432 At-the-market offering programs 4,171,013 3,074,931 — Shareowner Direct Plan issuances 576,965 640,723 732,814 Equity-based compensation plans ( Note 13(b) ) 5,078 5,185 22,408 Other (38,423 ) (45,847 ) — Shares outstanding, December 31 236,063,279 231,348,646 227,673,654 At December 31, 2018 , Alliant Energy had a total of 11.2 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 2018 and 2017, Alliant Energy filed prospectus supplements under which it could sell up to $175 million and $125 million of its common stock, respectively, through at-the-market offering programs. In 2018, Alliant Energy issued 4,171,013 shares of common stock through this program and received cash proceeds of $173 million , net of $2 million in commissions and fees. 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. The 2017 at-the-market offering program expired in 2018, and Alliant Energy currently has no plans to issue any additional common stock through the 2018 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. Equity Forward Agreements - In December 2018, Alliant Energy entered into forward sale agreements with various counterparties in connection with a public offering of 8,358,973 shares of Alliant Energy common stock. In the forward sale transaction, the counterparties, or their affiliates, borrowed an aggregate of 8,358,973 shares of Alliant Energy common stock from third parties and sold such shares to the related underwriters. Alliant Energy has concluded that the forward sale agreements meet the derivative scope exception for certain contracts involving an entity’s own equity. Alliant Energy has not yet received any proceeds from the offering and no amounts have been or will be recorded in equity on Alliant Energy’s balance sheets until the forward sale agreements settle. Alliant Energy expects to settle the forward sale agreements prior to December 31, 2019 through physical delivery of shares of common stock in exchange for cash proceeds at the then-applicable forward sale price; however, Alliant Energy may elect cash settlement or net share settlement for all or a portion of the obligations under the forward sale agreements. The initial forward sale price of $44.33 per share, is subject to daily adjustment based on a floating interest rate factor, and will decrease by other fixed amounts specified in the forward sale agreement. Until settlement of the forward sale agreements, Alliant Energy’s EPS dilution resulting from the agreements, if any, is determined using the treasury stock method. Share dilution occurs when the average market price of Alliant Energy stock during the reporting period is higher than the forward sale price as of the end of the reporting period. As of December 31, 2018, the securities under the forward sale agreements were excluded from the calculation of diluted EPS as they were antidilutive. 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. Comprehensive Income (Loss) - In 2018 , 2017 and 2016 , Alliant Energy’s other comprehensive income (loss) was $2.2 million , ($0.1) million and $0 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 2018 , 2017 and 2016 , 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.

Preferred Stock

Preferred Stock	12 Months Ended
	Dec. 31, 2018
Redeemable Preferred Stock	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 2018 2017 (in millions) 5.1% $25 8,000,000 8,000,000 $200.0 $200.0 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 Board of Directors and could not force IPL to redeem its preferred stock.
IPL [Member]
Redeemable Preferred Stock	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 2018 2017 (in millions) 5.1% $25 8,000,000 8,000,000 $200.0 $200.0 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 Board of Directors and could not force IPL to redeem its preferred stock.

Debt

Debt	12 Months Ended
	Dec. 31, 2018
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. At December 31, 2018 , the short-term borrowing capacity under a single credit facility agreement, which expires in August 2022, 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 2018 2017 2018 2017 2018 2017 Commercial paper outstanding $441.2 $320.2 $50.4 $— $105.5 $25.0 Commercial paper weighted average interest rates 2.8% 2.0% 2.8% N/A 2.5% 1.5% Available credit facility capacity $558.8 $679.8 $199.6 $250.0 $244.5 $325.0 Alliant Energy IPL WPL For the year ended 2018 2017 2018 2017 2018 2017 Maximum amount outstanding (based on daily outstanding balances) $446.5 $424.4 $50.4 $20.0 $126.0 $271.2 Average amount outstanding (based on daily outstanding balances) $221.4 $294.3 $1.5 $0.5 $36.6 $118.2 Weighted average interest rates 2.2% 1.2% 2.3% 1.3% 2.1% 1.0% In June 2018, AEF retired its $95 million , 364 -day variable-rate ( 2.2% at December 31, 2017) term loan credit agreement. Financial Covenants - The single 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, 2018 were as follows: Alliant Energy IPL WPL Requirement, not to exceed 65% 65% 65% Actual 55% 45% 48% 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): 2018 2017 Alliant Energy IPL WPL Alliant Energy IPL WPL Senior Debentures (a): 3.65%, due 2020 $200.0 $200.0 $— $200.0 $200.0 $— 3.25%, due 2024 500.0 500.0 — 500.0 500.0 — 3.4%, due 2025 250.0 250.0 — 250.0 250.0 — 5.5%, due 2025 50.0 50.0 — 50.0 50.0 — 4.1%, due 2028 (b) 500.0 500.0 — — — — 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 — 5.875% (Retired in 2018) — — — 100.0 100.0 — 7.25% (Retired in 2018) — — — 250.0 250.0 — 2,575.0 2,575.0 — 2,425.0 2,425.0 — 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 300.0 — 300.0 300.0 — 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 1,850.0 — 1,850.0 Other: AEF term loan credit agreement through April 2020, 3% at December 31, 2018 (with Alliant Energy as guarantor) (c) 300.0 — — — — — Corporate Services 3.45% senior notes, due 2022 (a) 75.0 — — 75.0 — — AEF 3.75% senior notes, due 2023 (with Alliant Energy as guarantor) (a)(d) 400.0 — — — — — AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a)(d) 300.0 — — — — — Sheboygan Power, LLC 5.06% senior secured notes, due 2019 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 44.3 — — 49.6 — — AEF term loan credit agreement, 2% at December 31, 2017 (with Alliant Energy as guarantor) (Retired in 2018) (d) — — — 500.0 — — Other, 1% at December 31, 2018, due 2019 to 2025 2.4 — — 2.9 — — 1,121.7 — — 627.5 — — Subtotal 5,546.7 2,575.0 1,850.0 4,902.5 2,425.0 1,850.0 Current maturities (256.5 ) — (250.0 ) (855.7 ) (350.0 ) — Unamortized debt issuance costs (32.1 ) (17.2 ) (9.6 ) (25.4 ) (14.3 ) (10.5 ) Unamortized debt (discount) and premium, net (11.8 ) (5.5 ) (5.5 ) (10.8 ) (4.7 ) (6.1 ) Long-term debt, net (e) $5,246.3 $2,552.3 $1,584.9 $4,010.6 $2,056.0 $1,833.4 (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 September 2018, IPL issued $500 million of 4.1% senior debentures due 2028. The senior debentures were issued as green bonds, and all of the net proceeds were allocated for the construction and development of wind and solar projects. (c) In April 2018, AEF entered into a $300 million variable-rate term loan credit agreement and used the proceeds from borrowings under this agreement for general corporate purposes. Refer to Note 9(a) for discussion of a financial covenant contained in AEF’s term loan credit agreement. (d) In June 2018, AEF issued $400 million of 3.75% senior notes due 2023 and $300 million of 4.25% senior notes due 2028. The proceeds from the issuances were used by AEF to retire its $500 million and $95 million variable-rate term loan credit agreements expiring in 2018, to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes. (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, 2018 , long-term debt maturities for 2019 through 2023 were as follows (in millions): 2019 2020 2021 2022 2023 IPL $— $200 $— $— $— WPL 250 150 — 250 — Corporate Services — — — 75 — AEF 6 307 8 8 408 Alliant Energy $256 $657 $8 $333 $408 Fair Value of Long-term Debt - Refer to Note 16 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. At December 31, 2018 , the short-term borrowing capacity under a single credit facility agreement, which expires in August 2022, 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 2018 2017 2018 2017 2018 2017 Commercial paper outstanding $441.2 $320.2 $50.4 $— $105.5 $25.0 Commercial paper weighted average interest rates 2.8% 2.0% 2.8% N/A 2.5% 1.5% Available credit facility capacity $558.8 $679.8 $199.6 $250.0 $244.5 $325.0 Alliant Energy IPL WPL For the year ended 2018 2017 2018 2017 2018 2017 Maximum amount outstanding (based on daily outstanding balances) $446.5 $424.4 $50.4 $20.0 $126.0 $271.2 Average amount outstanding (based on daily outstanding balances) $221.4 $294.3 $1.5 $0.5 $36.6 $118.2 Weighted average interest rates 2.2% 1.2% 2.3% 1.3% 2.1% 1.0% In June 2018, AEF retired its $95 million , 364 -day variable-rate ( 2.2% at December 31, 2017) term loan credit agreement. Financial Covenants - The single 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, 2018 were as follows: Alliant Energy IPL WPL Requirement, not to exceed 65% 65% 65% Actual 55% 45% 48% 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): 2018 2017 Alliant Energy IPL WPL Alliant Energy IPL WPL Senior Debentures (a): 3.65%, due 2020 $200.0 $200.0 $— $200.0 $200.0 $— 3.25%, due 2024 500.0 500.0 — 500.0 500.0 — 3.4%, due 2025 250.0 250.0 — 250.0 250.0 — 5.5%, due 2025 50.0 50.0 — 50.0 50.0 — 4.1%, due 2028 (b) 500.0 500.0 — — — — 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 — 5.875% (Retired in 2018) — — — 100.0 100.0 — 7.25% (Retired in 2018) — — — 250.0 250.0 — 2,575.0 2,575.0 — 2,425.0 2,425.0 — 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 300.0 — 300.0 300.0 — 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 1,850.0 — 1,850.0 Other: AEF term loan credit agreement through April 2020, 3% at December 31, 2018 (with Alliant Energy as guarantor) (c) 300.0 — — — — — Corporate Services 3.45% senior notes, due 2022 (a) 75.0 — — 75.0 — — AEF 3.75% senior notes, due 2023 (with Alliant Energy as guarantor) (a)(d) 400.0 — — — — — AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a)(d) 300.0 — — — — — Sheboygan Power, LLC 5.06% senior secured notes, due 2019 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 44.3 — — 49.6 — — AEF term loan credit agreement, 2% at December 31, 2017 (with Alliant Energy as guarantor) (Retired in 2018) (d) — — — 500.0 — — Other, 1% at December 31, 2018, due 2019 to 2025 2.4 — — 2.9 — — 1,121.7 — — 627.5 — — Subtotal 5,546.7 2,575.0 1,850.0 4,902.5 2,425.0 1,850.0 Current maturities (256.5 ) — (250.0 ) (855.7 ) (350.0 ) — Unamortized debt issuance costs (32.1 ) (17.2 ) (9.6 ) (25.4 ) (14.3 ) (10.5 ) Unamortized debt (discount) and premium, net (11.8 ) (5.5 ) (5.5 ) (10.8 ) (4.7 ) (6.1 ) Long-term debt, net (e) $5,246.3 $2,552.3 $1,584.9 $4,010.6 $2,056.0 $1,833.4 (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 September 2018, IPL issued $500 million of 4.1% senior debentures due 2028. The senior debentures were issued as green bonds, and all of the net proceeds were allocated for the construction and development of wind and solar projects. (c) In April 2018, AEF entered into a $300 million variable-rate term loan credit agreement and used the proceeds from borrowings under this agreement for general corporate purposes. Refer to Note 9(a) for discussion of a financial covenant contained in AEF’s term loan credit agreement. (d) In June 2018, AEF issued $400 million of 3.75% senior notes due 2023 and $300 million of 4.25% senior notes due 2028. The proceeds from the issuances were used by AEF to retire its $500 million and $95 million variable-rate term loan credit agreements expiring in 2018, to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes. (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, 2018 , long-term debt maturities for 2019 through 2023 were as follows (in millions): 2019 2020 2021 2022 2023 IPL $— $200 $— $— $— WPL 250 150 — 250 — Corporate Services — — — 75 — AEF 6 307 8 8 408 Alliant Energy $256 $657 $8 $333 $408 Fair Value of Long-term Debt - Refer to Note 16 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. At December 31, 2018 , the short-term borrowing capacity under a single credit facility agreement, which expires in August 2022, 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 2018 2017 2018 2017 2018 2017 Commercial paper outstanding $441.2 $320.2 $50.4 $— $105.5 $25.0 Commercial paper weighted average interest rates 2.8% 2.0% 2.8% N/A 2.5% 1.5% Available credit facility capacity $558.8 $679.8 $199.6 $250.0 $244.5 $325.0 Alliant Energy IPL WPL For the year ended 2018 2017 2018 2017 2018 2017 Maximum amount outstanding (based on daily outstanding balances) $446.5 $424.4 $50.4 $20.0 $126.0 $271.2 Average amount outstanding (based on daily outstanding balances) $221.4 $294.3 $1.5 $0.5 $36.6 $118.2 Weighted average interest rates 2.2% 1.2% 2.3% 1.3% 2.1% 1.0% In June 2018, AEF retired its $95 million , 364 -day variable-rate ( 2.2% at December 31, 2017) term loan credit agreement. Financial Covenants - The single 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, 2018 were as follows: Alliant Energy IPL WPL Requirement, not to exceed 65% 65% 65% Actual 55% 45% 48% 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): 2018 2017 Alliant Energy IPL WPL Alliant Energy IPL WPL Senior Debentures (a): 3.65%, due 2020 $200.0 $200.0 $— $200.0 $200.0 $— 3.25%, due 2024 500.0 500.0 — 500.0 500.0 — 3.4%, due 2025 250.0 250.0 — 250.0 250.0 — 5.5%, due 2025 50.0 50.0 — 50.0 50.0 — 4.1%, due 2028 (b) 500.0 500.0 — — — — 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 — 5.875% (Retired in 2018) — — — 100.0 100.0 — 7.25% (Retired in 2018) — — — 250.0 250.0 — 2,575.0 2,575.0 — 2,425.0 2,425.0 — 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 300.0 — 300.0 300.0 — 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 1,850.0 — 1,850.0 Other: AEF term loan credit agreement through April 2020, 3% at December 31, 2018 (with Alliant Energy as guarantor) (c) 300.0 — — — — — Corporate Services 3.45% senior notes, due 2022 (a) 75.0 — — 75.0 — — AEF 3.75% senior notes, due 2023 (with Alliant Energy as guarantor) (a)(d) 400.0 — — — — — AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a)(d) 300.0 — — — — — Sheboygan Power, LLC 5.06% senior secured notes, due 2019 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 44.3 — — 49.6 — — AEF term loan credit agreement, 2% at December 31, 2017 (with Alliant Energy as guarantor) (Retired in 2018) (d) — — — 500.0 — — Other, 1% at December 31, 2018, due 2019 to 2025 2.4 — — 2.9 — — 1,121.7 — — 627.5 — — Subtotal 5,546.7 2,575.0 1,850.0 4,902.5 2,425.0 1,850.0 Current maturities (256.5 ) — (250.0 ) (855.7 ) (350.0 ) — Unamortized debt issuance costs (32.1 ) (17.2 ) (9.6 ) (25.4 ) (14.3 ) (10.5 ) Unamortized debt (discount) and premium, net (11.8 ) (5.5 ) (5.5 ) (10.8 ) (4.7 ) (6.1 ) Long-term debt, net (e) $5,246.3 $2,552.3 $1,584.9 $4,010.6 $2,056.0 $1,833.4 (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 September 2018, IPL issued $500 million of 4.1% senior debentures due 2028. The senior debentures were issued as green bonds, and all of the net proceeds were allocated for the construction and development of wind and solar projects. (c) In April 2018, AEF entered into a $300 million variable-rate term loan credit agreement and used the proceeds from borrowings under this agreement for general corporate purposes. Refer to Note 9(a) for discussion of a financial covenant contained in AEF’s term loan credit agreement. (d) In June 2018, AEF issued $400 million of 3.75% senior notes due 2023 and $300 million of 4.25% senior notes due 2028. The proceeds from the issuances were used by AEF to retire its $500 million and $95 million variable-rate term loan credit agreements expiring in 2018, to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes. (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, 2018 , long-term debt maturities for 2019 through 2023 were as follows (in millions): 2019 2020 2021 2022 2023 IPL $— $200 $— $— $— WPL 250 150 — 250 — Corporate Services — — — 75 — AEF 6 307 8 8 408 Alliant Energy $256 $657 $8 $333 $408 Fair Value of Long-term Debt - Refer to Note 16 for information on the fair value of long-term debt outstanding.

Leases

Leases	12 Months Ended
	Dec. 31, 2018
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 2018 , 2017 and 2016 , rental expenses associated with operating leases were not material. At December 31, 2018 , future minimum operating lease payments, excluding contingent rentals, were as follows (in millions): 2019 2020 2021 2022 2023 Thereafter Total Alliant Energy $5 $5 $3 $3 $2 $12 $30 IPL 3 2 2 2 2 12 23 WPL 2 3 1 — — — 6
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 2018 , 2017 and 2016 , rental expenses associated with operating leases were not material. At December 31, 2018 , future minimum operating lease payments, excluding contingent rentals, were as follows (in millions): 2019 2020 2021 2022 2023 Thereafter Total Alliant Energy $5 $5 $3 $3 $2 $12 $30 IPL 3 2 2 2 2 12 23 WPL 2 3 1 — — — 6
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 2018 , 2017 and 2016 , rental expenses associated with operating leases were not material. At December 31, 2018 , future minimum operating lease payments, excluding contingent rentals, were as follows (in millions): 2019 2020 2021 2022 2023 Thereafter Total Alliant Energy $5 $5 $3 $3 $2 $12 $30 IPL 3 2 2 2 2 12 23 WPL 2 3 1 — — — 6 (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): 2018 2017 2016 Interest expense $8 $9 $9 Depreciation and amortization 6 6 6 $14 $15 $15 At December 31, 2018 , WPL’s estimated future minimum capital lease payments for the Sheboygan Falls Energy Facility were as follows (in millions): 2019 2020 2021 2022 2023 Thereafter Total Less: amount representing interest Present value of minimum capital lease payments Sheboygan Falls Energy Facility $15 $15 $15 $15 $15 $19 $94 $26 $68

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax [Line Items]
Income Taxes	INCOME TAXES Federal Tax Reform Adjustments - In December 2017, Federal Tax Reform was enacted, which had a material impact on Alliant Energy’s, IPL’s and WPL’s 2017 financial statements since changes in tax laws must be recognized in the period in which the law is enacted. The most significant provision of Federal Tax Reform that impact Alliant Energy, IPL and WPL was the reduction in the federal corporate tax rate from 35% to 21% . As a result of Federal Tax Reform, at December 31, 2017, Alliant Energy’s, IPL’s and WPL’s regulated utility operations recorded the net impacts from re-measuring deferred tax assets and liabilities as a change in regulatory liabilities or regulatory assets. 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 Holdings” on Alliant Energy’s balance sheet. As a result of Federal Tax Reform, Alliant Energy, IPL and WPL recorded tax expense (benefits) in their 2017 income statements of ($18.1) million , $3.8 million , and ($14.5) million , respectively. In 2018, additional rules were issued related to Federal Tax Reform, including clarifications of the treatment of bonus depreciation deductions, which impacted the federal income tax return for the calendar year 2017. As result of these clarifying rules, Alliant Energy, IPL and WPL recorded tax benefits of $5.6 million , $1.1 million and $5.5 million , respectively, in 2018. Iowa Tax Reform - In May 2018, Iowa tax reform was enacted, resulting in a reduction in the Iowa income tax rate from 12% to 9.8% , effective January 1, 2021 and the elimination of the deduction for federal income taxes, effective January 1, 2022. Alliant Energy’s and IPL’s deferred tax assets and liabilities as of June 30, 2018 were remeasured based upon the new tax rate, and as a result, Alliant Energy’s and IPL’s tax-related regulatory assets decreased $33.7 million and tax-related regulatory liabilities increased $7.3 million in the second quarter of 2018. Income Tax Expense (Benefit) - The components of “Income tax expense (benefit)” in the income statements were as follows (in millions): Alliant Energy IPL WPL 2018 2017 2016 2018 2017 2016 2018 2017 2016 Current tax expense (benefit): Federal ($1.0 ) ($41.0 ) $1.8 $14.9 ($27.9 ) ($12.8 ) ($9.2 ) $5.5 ($22.3 ) State (5.1 ) 8.5 17.2 (7.1 ) 1.6 15.5 (4.4 ) 2.5 1.1 IPL’s tax benefit riders (13.2 ) (40.4 ) (44.2 ) (13.2 ) (40.4 ) (44.2 ) — — — Deferred tax expense (benefit): Federal 67.9 159.5 112.8 9.5 72.5 59.1 43.8 55.0 112.3 State 29.8 12.3 4.9 7.3 (2.2 ) (9.0 ) 22.1 16.6 20.8 Production tax credits (29.5 ) (31.1 ) (31.8 ) (14.0 ) (14.1 ) (14.0 ) (15.5 ) (17.0 ) (17.8 ) Investment tax credits (1.2 ) (1.1 ) (1.3 ) (0.6 ) (0.4 ) (0.5 ) (0.6 ) (0.7 ) (0.8 ) $47.7 $66.7 $59.4 ($3.2 ) ($10.9 ) ($5.9 ) $36.2 $61.9 $93.3 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 2018 2017 2016 2018 2017 2016 2018 2017 2016 Statutory federal income tax rate 21.0 % 35.0 % 35.0 % 21.0 % 35.0 % 35.0 % 21.0 % 35.0 % 35.0 % State income taxes, net of federal benefits 7.0 5.5 5.4 7.7 6.5 6.4 6.2 5.1 5.1 Effect of rate-making on property-related differences (7.6 ) (8.5 ) (8.5 ) (14.0 ) (19.1 ) (16.2 ) (2.3 ) (1.7 ) (0.7 ) Production tax credits (5.2 ) (6.1 ) (7.2 ) (5.2 ) (6.7 ) (6.3 ) (6.4 ) (7.1 ) (6.2 ) IPL’s tax benefit riders (2.3 ) (7.6 ) (10.0 ) (4.9 ) (18.7 ) (20.1 ) — — — Adjustment for prior period taxes (2.3 ) (1.5 ) (0.8 ) (4.8 ) (3.4 ) (1.2 ) (0.2 ) — (0.1 ) Federal Tax Reform adjustments (1.0 ) (3.4 ) — (0.4 ) 1.7 — (2.3 ) (5.8 ) — Other items, net (1.2 ) (0.9 ) (0.5 ) (0.6 ) (0.3 ) (0.3 ) (1.2 ) (0.6 ) (0.5 ) Overall income tax rate 8.4 % 12.5 % 13.4 % (1.2 %) (5.0 %) (2.7 %) 14.8 % 24.9 % 32.6 % 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 2018 2017 2018 2017 2018 2017 Deferred tax liabilities: Property $1,975.5 $1,852.7 $1,158.4 $1,102.6 $735.2 $674.2 ATC Holdings 102.4 86.4 — — — — Other 80.5 75.9 69.4 63.4 34.4 36.5 Total deferred tax liabilities 2,158.4 2,015.0 1,227.8 1,166.0 769.6 710.7 Deferred tax assets: Federal credit carryforwards 299.1 260.7 133.2 113.1 147.4 131.0 Net operating losses carryforwards - federal 158.6 174.4 114.1 107.4 26.4 43.7 Net operating losses carryforwards - state 47.0 41.3 0.9 0.9 0.5 0.2 Other 59.8 69.2 22.8 34.5 14.1 14.7 Subtotal deferred tax assets 564.5 545.6 271.0 255.9 188.4 189.6 Valuation allowances (9.2 ) (9.0 ) (0.5 ) (0.6 ) (0.8 ) (1.3 ) Total deferred tax assets 555.3 536.6 270.5 255.3 187.6 188.3 Total deferred tax liabilities, net $1,603.1 $1,478.4 $957.3 $910.7 $582.0 $522.4 Carryforwards - At December 31, 2018 , carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2031-2037 $766 $551 $126 State net operating losses 2018-2038 792 13 8 Federal tax credits 2022-2038 299 133 147 Valuation Allowances - Due to the anticipated future reductions in revenues from utility customers due to Federal Tax Reform, Alliant Energy expects its Federal net operating losses carryforwards will not be fully utilized until 2024. Because taxable income must be reduced by net operating losses carryforwards prior to utilizing federal tax credit carryforwards, Alliant Energy currently does not expect to utilize 2002 and 2003 vintage federal credit carryforwards prior to their expiration in 2022 and 2023, respectively. As a result, Alliant Energy established valuation allowances for the 2002 and 2003 vintage federal credit carryforwards in 2017 that remain as of December 31, 2018. Uncertain Tax Positions - At December 31, 2018 , 2017 and 2016 , there were no uncertain tax positions or penalties accrued related to uncertain tax positions. As of December 31, 2018 , 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) 2015 - 2017 Consolidated Iowa income tax returns (b) 2015 - 2017 Wisconsin combined tax returns (c) 2014 - 2017 (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 Federal Tax Reform Adjustments - In December 2017, Federal Tax Reform was enacted, which had a material impact on Alliant Energy’s, IPL’s and WPL’s 2017 financial statements since changes in tax laws must be recognized in the period in which the law is enacted. The most significant provision of Federal Tax Reform that impact Alliant Energy, IPL and WPL was the reduction in the federal corporate tax rate from 35% to 21% . As a result of Federal Tax Reform, at December 31, 2017, Alliant Energy’s, IPL’s and WPL’s regulated utility operations recorded the net impacts from re-measuring deferred tax assets and liabilities as a change in regulatory liabilities or regulatory assets. 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 Holdings” on Alliant Energy’s balance sheet. As a result of Federal Tax Reform, Alliant Energy, IPL and WPL recorded tax expense (benefits) in their 2017 income statements of ($18.1) million , $3.8 million , and ($14.5) million , respectively. In 2018, additional rules were issued related to Federal Tax Reform, including clarifications of the treatment of bonus depreciation deductions, which impacted the federal income tax return for the calendar year 2017. As result of these clarifying rules, Alliant Energy, IPL and WPL recorded tax benefits of $5.6 million , $1.1 million and $5.5 million , respectively, in 2018. Iowa Tax Reform - In May 2018, Iowa tax reform was enacted, resulting in a reduction in the Iowa income tax rate from 12% to 9.8% , effective January 1, 2021 and the elimination of the deduction for federal income taxes, effective January 1, 2022. Alliant Energy’s and IPL’s deferred tax assets and liabilities as of June 30, 2018 were remeasured based upon the new tax rate, and as a result, Alliant Energy’s and IPL’s tax-related regulatory assets decreased $33.7 million and tax-related regulatory liabilities increased $7.3 million in the second quarter of 2018. Income Tax Expense (Benefit) - The components of “Income tax expense (benefit)” in the income statements were as follows (in millions): Alliant Energy IPL WPL 2018 2017 2016 2018 2017 2016 2018 2017 2016 Current tax expense (benefit): Federal ($1.0 ) ($41.0 ) $1.8 $14.9 ($27.9 ) ($12.8 ) ($9.2 ) $5.5 ($22.3 ) State (5.1 ) 8.5 17.2 (7.1 ) 1.6 15.5 (4.4 ) 2.5 1.1 IPL’s tax benefit riders (13.2 ) (40.4 ) (44.2 ) (13.2 ) (40.4 ) (44.2 ) — — — Deferred tax expense (benefit): Federal 67.9 159.5 112.8 9.5 72.5 59.1 43.8 55.0 112.3 State 29.8 12.3 4.9 7.3 (2.2 ) (9.0 ) 22.1 16.6 20.8 Production tax credits (29.5 ) (31.1 ) (31.8 ) (14.0 ) (14.1 ) (14.0 ) (15.5 ) (17.0 ) (17.8 ) Investment tax credits (1.2 ) (1.1 ) (1.3 ) (0.6 ) (0.4 ) (0.5 ) (0.6 ) (0.7 ) (0.8 ) $47.7 $66.7 $59.4 ($3.2 ) ($10.9 ) ($5.9 ) $36.2 $61.9 $93.3 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 2018 2017 2016 2018 2017 2016 2018 2017 2016 Statutory federal income tax rate 21.0 % 35.0 % 35.0 % 21.0 % 35.0 % 35.0 % 21.0 % 35.0 % 35.0 % State income taxes, net of federal benefits 7.0 5.5 5.4 7.7 6.5 6.4 6.2 5.1 5.1 Effect of rate-making on property-related differences (7.6 ) (8.5 ) (8.5 ) (14.0 ) (19.1 ) (16.2 ) (2.3 ) (1.7 ) (0.7 ) Production tax credits (5.2 ) (6.1 ) (7.2 ) (5.2 ) (6.7 ) (6.3 ) (6.4 ) (7.1 ) (6.2 ) IPL’s tax benefit riders (2.3 ) (7.6 ) (10.0 ) (4.9 ) (18.7 ) (20.1 ) — — — Adjustment for prior period taxes (2.3 ) (1.5 ) (0.8 ) (4.8 ) (3.4 ) (1.2 ) (0.2 ) — (0.1 ) Federal Tax Reform adjustments (1.0 ) (3.4 ) — (0.4 ) 1.7 — (2.3 ) (5.8 ) — Other items, net (1.2 ) (0.9 ) (0.5 ) (0.6 ) (0.3 ) (0.3 ) (1.2 ) (0.6 ) (0.5 ) Overall income tax rate 8.4 % 12.5 % 13.4 % (1.2 %) (5.0 %) (2.7 %) 14.8 % 24.9 % 32.6 % 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 2018 2017 2018 2017 2018 2017 Deferred tax liabilities: Property $1,975.5 $1,852.7 $1,158.4 $1,102.6 $735.2 $674.2 ATC Holdings 102.4 86.4 — — — — Other 80.5 75.9 69.4 63.4 34.4 36.5 Total deferred tax liabilities 2,158.4 2,015.0 1,227.8 1,166.0 769.6 710.7 Deferred tax assets: Federal credit carryforwards 299.1 260.7 133.2 113.1 147.4 131.0 Net operating losses carryforwards - federal 158.6 174.4 114.1 107.4 26.4 43.7 Net operating losses carryforwards - state 47.0 41.3 0.9 0.9 0.5 0.2 Other 59.8 69.2 22.8 34.5 14.1 14.7 Subtotal deferred tax assets 564.5 545.6 271.0 255.9 188.4 189.6 Valuation allowances (9.2 ) (9.0 ) (0.5 ) (0.6 ) (0.8 ) (1.3 ) Total deferred tax assets 555.3 536.6 270.5 255.3 187.6 188.3 Total deferred tax liabilities, net $1,603.1 $1,478.4 $957.3 $910.7 $582.0 $522.4 Carryforwards - At December 31, 2018 , carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2031-2037 $766 $551 $126 State net operating losses 2018-2038 792 13 8 Federal tax credits 2022-2038 299 133 147 Valuation Allowances - Due to the anticipated future reductions in revenues from utility customers due to Federal Tax Reform, Alliant Energy expects its Federal net operating losses carryforwards will not be fully utilized until 2024. Because taxable income must be reduced by net operating losses carryforwards prior to utilizing federal tax credit carryforwards, Alliant Energy currently does not expect to utilize 2002 and 2003 vintage federal credit carryforwards prior to their expiration in 2022 and 2023, respectively. As a result, Alliant Energy established valuation allowances for the 2002 and 2003 vintage federal credit carryforwards in 2017 that remain as of December 31, 2018. Uncertain Tax Positions - At December 31, 2018 , 2017 and 2016 , there were no uncertain tax positions or penalties accrued related to uncertain tax positions. As of December 31, 2018 , 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) 2015 - 2017 Consolidated Iowa income tax returns (b) 2015 - 2017 Wisconsin combined tax returns (c) 2014 - 2017 (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 Federal Tax Reform Adjustments - In December 2017, Federal Tax Reform was enacted, which had a material impact on Alliant Energy’s, IPL’s and WPL’s 2017 financial statements since changes in tax laws must be recognized in the period in which the law is enacted. The most significant provision of Federal Tax Reform that impact Alliant Energy, IPL and WPL was the reduction in the federal corporate tax rate from 35% to 21% . As a result of Federal Tax Reform, at December 31, 2017, Alliant Energy’s, IPL’s and WPL’s regulated utility operations recorded the net impacts from re-measuring deferred tax assets and liabilities as a change in regulatory liabilities or regulatory assets. 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 Holdings” on Alliant Energy’s balance sheet. As a result of Federal Tax Reform, Alliant Energy, IPL and WPL recorded tax expense (benefits) in their 2017 income statements of ($18.1) million , $3.8 million , and ($14.5) million , respectively. In 2018, additional rules were issued related to Federal Tax Reform, including clarifications of the treatment of bonus depreciation deductions, which impacted the federal income tax return for the calendar year 2017. As result of these clarifying rules, Alliant Energy, IPL and WPL recorded tax benefits of $5.6 million , $1.1 million and $5.5 million , respectively, in 2018. Iowa Tax Reform - In May 2018, Iowa tax reform was enacted, resulting in a reduction in the Iowa income tax rate from 12% to 9.8% , effective January 1, 2021 and the elimination of the deduction for federal income taxes, effective January 1, 2022. Alliant Energy’s and IPL’s deferred tax assets and liabilities as of June 30, 2018 were remeasured based upon the new tax rate, and as a result, Alliant Energy’s and IPL’s tax-related regulatory assets decreased $33.7 million and tax-related regulatory liabilities increased $7.3 million in the second quarter of 2018. Income Tax Expense (Benefit) - The components of “Income tax expense (benefit)” in the income statements were as follows (in millions): Alliant Energy IPL WPL 2018 2017 2016 2018 2017 2016 2018 2017 2016 Current tax expense (benefit): Federal ($1.0 ) ($41.0 ) $1.8 $14.9 ($27.9 ) ($12.8 ) ($9.2 ) $5.5 ($22.3 ) State (5.1 ) 8.5 17.2 (7.1 ) 1.6 15.5 (4.4 ) 2.5 1.1 IPL’s tax benefit riders (13.2 ) (40.4 ) (44.2 ) (13.2 ) (40.4 ) (44.2 ) — — — Deferred tax expense (benefit): Federal 67.9 159.5 112.8 9.5 72.5 59.1 43.8 55.0 112.3 State 29.8 12.3 4.9 7.3 (2.2 ) (9.0 ) 22.1 16.6 20.8 Production tax credits (29.5 ) (31.1 ) (31.8 ) (14.0 ) (14.1 ) (14.0 ) (15.5 ) (17.0 ) (17.8 ) Investment tax credits (1.2 ) (1.1 ) (1.3 ) (0.6 ) (0.4 ) (0.5 ) (0.6 ) (0.7 ) (0.8 ) $47.7 $66.7 $59.4 ($3.2 ) ($10.9 ) ($5.9 ) $36.2 $61.9 $93.3 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 2018 2017 2016 2018 2017 2016 2018 2017 2016 Statutory federal income tax rate 21.0 % 35.0 % 35.0 % 21.0 % 35.0 % 35.0 % 21.0 % 35.0 % 35.0 % State income taxes, net of federal benefits 7.0 5.5 5.4 7.7 6.5 6.4 6.2 5.1 5.1 Effect of rate-making on property-related differences (7.6 ) (8.5 ) (8.5 ) (14.0 ) (19.1 ) (16.2 ) (2.3 ) (1.7 ) (0.7 ) Production tax credits (5.2 ) (6.1 ) (7.2 ) (5.2 ) (6.7 ) (6.3 ) (6.4 ) (7.1 ) (6.2 ) IPL’s tax benefit riders (2.3 ) (7.6 ) (10.0 ) (4.9 ) (18.7 ) (20.1 ) — — — Adjustment for prior period taxes (2.3 ) (1.5 ) (0.8 ) (4.8 ) (3.4 ) (1.2 ) (0.2 ) — (0.1 ) Federal Tax Reform adjustments (1.0 ) (3.4 ) — (0.4 ) 1.7 — (2.3 ) (5.8 ) — Other items, net (1.2 ) (0.9 ) (0.5 ) (0.6 ) (0.3 ) (0.3 ) (1.2 ) (0.6 ) (0.5 ) Overall income tax rate 8.4 % 12.5 % 13.4 % (1.2 %) (5.0 %) (2.7 %) 14.8 % 24.9 % 32.6 % 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 2018 2017 2018 2017 2018 2017 Deferred tax liabilities: Property $1,975.5 $1,852.7 $1,158.4 $1,102.6 $735.2 $674.2 ATC Holdings 102.4 86.4 — — — — Other 80.5 75.9 69.4 63.4 34.4 36.5 Total deferred tax liabilities 2,158.4 2,015.0 1,227.8 1,166.0 769.6 710.7 Deferred tax assets: Federal credit carryforwards 299.1 260.7 133.2 113.1 147.4 131.0 Net operating losses carryforwards - federal 158.6 174.4 114.1 107.4 26.4 43.7 Net operating losses carryforwards - state 47.0 41.3 0.9 0.9 0.5 0.2 Other 59.8 69.2 22.8 34.5 14.1 14.7 Subtotal deferred tax assets 564.5 545.6 271.0 255.9 188.4 189.6 Valuation allowances (9.2 ) (9.0 ) (0.5 ) (0.6 ) (0.8 ) (1.3 ) Total deferred tax assets 555.3 536.6 270.5 255.3 187.6 188.3 Total deferred tax liabilities, net $1,603.1 $1,478.4 $957.3 $910.7 $582.0 $522.4 Carryforwards - At December 31, 2018 , carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2031-2037 $766 $551 $126 State net operating losses 2018-2038 792 13 8 Federal tax credits 2022-2038 299 133 147 Valuation Allowances - Due to the anticipated future reductions in revenues from utility customers due to Federal Tax Reform, Alliant Energy expects its Federal net operating losses carryforwards will not be fully utilized until 2024. Because taxable income must be reduced by net operating losses carryforwards prior to utilizing federal tax credit carryforwards, Alliant Energy currently does not expect to utilize 2002 and 2003 vintage federal credit carryforwards prior to their expiration in 2022 and 2023, respectively. As a result, Alliant Energy established valuation allowances for the 2002 and 2003 vintage federal credit carryforwards in 2017 that remain as of December 31, 2018. Uncertain Tax Positions - At December 31, 2018 , 2017 and 2016 , there were no uncertain tax positions or penalties accrued related to uncertain tax positions. As of December 31, 2018 , 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) 2015 - 2017 Consolidated Iowa income tax returns (b) 2015 - 2017 Wisconsin combined tax returns (c) 2014 - 2017 (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.

Revenues

Revenues	12 Months Ended
	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	REVENUES Revenues from Alliant Energy’s, IPL’s and WPL’s utility businesses are primarily from electric and gas sales provided to customers based on approved tariffs or specific contracts with customers. IPL’s and WPL’s primary performance obligations under such arrangements are to deliver electricity and gas, and their customers simultaneously receive and consume the electricity and gas. For such arrangements, revenues are recognized equivalent to the value of the electricity or gas supplied during each period, including amounts billed during each period and changes in amounts estimated to be billed at the end of each period. IPL and WPL apply the right to invoice method to measure progress towards completing performance obligations to transfer electricity and gas to their customers. IPL provides retail electric and gas service to customers in Iowa, and WPL provides retail and wholesale electric and retail gas service to customers in Wisconsin. IPL also sells electricity to wholesale customers in Minnesota, Illinois and Iowa, as well as steam from its Prairie Creek Generating Station to high-pressure steam customers in Iowa. IPL’s and WPL’s retail electric and gas revenues include sales to residential, commercial and industrial customers. IPL’s and WPL’s retail electric and gas customer prices are based on IPL’s and WPL’s cost of service and are determined through general rate review proceedings and various tariff filings with the IUB and PSCW, respectively. Such tariff-based services provide electricity or gas to customers without a defined contractual term. IPL and WPL have wholesale electric market-based rate authority from FERC allowing them to participate in wholesale energy markets (e.g. MISO) and transact directly with third parties. This authority from FERC allows sales of electricity referred to as bulk power sales based on current market values. FERC also allows IPL and WPL to enter into power supply agreements with municipalities and rural electric cooperatives with defined contractual terms, which include standard pricing mechanisms that are detailed in current tariffs accepted by FERC through wholesale rate review proceedings. Revenues from Alliant Energy’s non-utility business customers are primarily from its Transportation business, which includes a short-line railway that provides freight service, hauling services on the Mississippi River, and other transfer and storage services. As of December 31, 2018 , revenue expected to be recognized in any future year related to remaining performance obligations is not material, as the majority of revenues are recognized as services are rendered or commodities are delivered, and are from contracts with durations of less than one year. Alliant Energy, IPL and WPL do not have any material contract assets or contract liabilities, or contract acquisition fulfillment costs. Disaggregation of revenues from contracts with customers, which correlates to revenues for each reportable segment, was as follows (in millions): Alliant Energy IPL WPL 2018 2017 2016 2018 2017 2016 2018 2017 2016 Electric Utility: Retail - residential $1,063.4 $1,006.2 $1,001.1 $590.6 $535.6 $536.7 $472.8 $470.6 $464.4 Retail - commercial 735.6 710.3 712.6 486.8 452.7 445.4 248.8 257.6 267.2 Retail - industrial 888.8 853.1 851.1 500.8 459.7 460.4 388.0 393.4 390.7 Wholesale 188.4 238.4 256.6 71.2 95.5 94.2 117.2 142.9 162.4 Bulk power and other 124.1 86.7 54.1 81.7 55.4 33.0 42.4 31.3 21.1 Total Electric Utility 3,000.3 2,894.7 2,875.5 1,731.1 1,598.9 1,569.7 1,269.2 1,295.8 1,305.8 Gas Utility: Retail - residential 254.4 224.7 197.6 152.3 123.2 110.6 102.1 101.5 87.0 Retail - commercial 133.0 123.2 109.6 75.9 67.9 61.9 57.1 55.3 47.7 Retail - industrial 14.9 16.7 15.2 10.2 11.1 10.6 4.7 5.6 4.6 Transportation/other 44.3 36.3 33.0 27.8 23.8 20.9 16.5 12.5 12.1 Total Gas Utility 446.6 400.9 355.4 266.2 226.0 204.0 180.4 174.9 151.4 Other Utility: Steam 35.2 34.6 33.9 35.2 34.6 33.9 — — — Other utility 12.8 12.9 14.7 9.8 10.8 12.8 3.0 2.1 1.9 Total Other Utility 48.0 47.5 48.6 45.0 45.4 46.7 3.0 2.1 1.9 Non-Utility and Other: Transportation and other 39.6 39.1 40.5 — — — — — — Total Non-Utility and Other 39.6 39.1 40.5 — — — — — — Total revenues $3,534.5 $3,382.2 $3,320.0 $2,042.3 $1,870.3 $1,820.4 $1,452.6 $1,472.8 $1,459.1
IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	REVENUES Revenues from Alliant Energy’s, IPL’s and WPL’s utility businesses are primarily from electric and gas sales provided to customers based on approved tariffs or specific contracts with customers. IPL’s and WPL’s primary performance obligations under such arrangements are to deliver electricity and gas, and their customers simultaneously receive and consume the electricity and gas. For such arrangements, revenues are recognized equivalent to the value of the electricity or gas supplied during each period, including amounts billed during each period and changes in amounts estimated to be billed at the end of each period. IPL and WPL apply the right to invoice method to measure progress towards completing performance obligations to transfer electricity and gas to their customers. IPL provides retail electric and gas service to customers in Iowa, and WPL provides retail and wholesale electric and retail gas service to customers in Wisconsin. IPL also sells electricity to wholesale customers in Minnesota, Illinois and Iowa, as well as steam from its Prairie Creek Generating Station to high-pressure steam customers in Iowa. IPL’s and WPL’s retail electric and gas revenues include sales to residential, commercial and industrial customers. IPL’s and WPL’s retail electric and gas customer prices are based on IPL’s and WPL’s cost of service and are determined through general rate review proceedings and various tariff filings with the IUB and PSCW, respectively. Such tariff-based services provide electricity or gas to customers without a defined contractual term. IPL and WPL have wholesale electric market-based rate authority from FERC allowing them to participate in wholesale energy markets (e.g. MISO) and transact directly with third parties. This authority from FERC allows sales of electricity referred to as bulk power sales based on current market values. FERC also allows IPL and WPL to enter into power supply agreements with municipalities and rural electric cooperatives with defined contractual terms, which include standard pricing mechanisms that are detailed in current tariffs accepted by FERC through wholesale rate review proceedings. Revenues from Alliant Energy’s non-utility business customers are primarily from its Transportation business, which includes a short-line railway that provides freight service, hauling services on the Mississippi River, and other transfer and storage services. As of December 31, 2018 , revenue expected to be recognized in any future year related to remaining performance obligations is not material, as the majority of revenues are recognized as services are rendered or commodities are delivered, and are from contracts with durations of less than one year. Alliant Energy, IPL and WPL do not have any material contract assets or contract liabilities, or contract acquisition fulfillment costs. Disaggregation of revenues from contracts with customers, which correlates to revenues for each reportable segment, was as follows (in millions): Alliant Energy IPL WPL 2018 2017 2016 2018 2017 2016 2018 2017 2016 Electric Utility: Retail - residential $1,063.4 $1,006.2 $1,001.1 $590.6 $535.6 $536.7 $472.8 $470.6 $464.4 Retail - commercial 735.6 710.3 712.6 486.8 452.7 445.4 248.8 257.6 267.2 Retail - industrial 888.8 853.1 851.1 500.8 459.7 460.4 388.0 393.4 390.7 Wholesale 188.4 238.4 256.6 71.2 95.5 94.2 117.2 142.9 162.4 Bulk power and other 124.1 86.7 54.1 81.7 55.4 33.0 42.4 31.3 21.1 Total Electric Utility 3,000.3 2,894.7 2,875.5 1,731.1 1,598.9 1,569.7 1,269.2 1,295.8 1,305.8 Gas Utility: Retail - residential 254.4 224.7 197.6 152.3 123.2 110.6 102.1 101.5 87.0 Retail - commercial 133.0 123.2 109.6 75.9 67.9 61.9 57.1 55.3 47.7 Retail - industrial 14.9 16.7 15.2 10.2 11.1 10.6 4.7 5.6 4.6 Transportation/other 44.3 36.3 33.0 27.8 23.8 20.9 16.5 12.5 12.1 Total Gas Utility 446.6 400.9 355.4 266.2 226.0 204.0 180.4 174.9 151.4 Other Utility: Steam 35.2 34.6 33.9 35.2 34.6 33.9 — — — Other utility 12.8 12.9 14.7 9.8 10.8 12.8 3.0 2.1 1.9 Total Other Utility 48.0 47.5 48.6 45.0 45.4 46.7 3.0 2.1 1.9 Non-Utility and Other: Transportation and other 39.6 39.1 40.5 — — — — — — Total Non-Utility and Other 39.6 39.1 40.5 — — — — — — Total revenues $3,534.5 $3,382.2 $3,320.0 $2,042.3 $1,870.3 $1,820.4 $1,452.6 $1,472.8 $1,459.1
WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	REVENUES Revenues from Alliant Energy’s, IPL’s and WPL’s utility businesses are primarily from electric and gas sales provided to customers based on approved tariffs or specific contracts with customers. IPL’s and WPL’s primary performance obligations under such arrangements are to deliver electricity and gas, and their customers simultaneously receive and consume the electricity and gas. For such arrangements, revenues are recognized equivalent to the value of the electricity or gas supplied during each period, including amounts billed during each period and changes in amounts estimated to be billed at the end of each period. IPL and WPL apply the right to invoice method to measure progress towards completing performance obligations to transfer electricity and gas to their customers. IPL provides retail electric and gas service to customers in Iowa, and WPL provides retail and wholesale electric and retail gas service to customers in Wisconsin. IPL also sells electricity to wholesale customers in Minnesota, Illinois and Iowa, as well as steam from its Prairie Creek Generating Station to high-pressure steam customers in Iowa. IPL’s and WPL’s retail electric and gas revenues include sales to residential, commercial and industrial customers. IPL’s and WPL’s retail electric and gas customer prices are based on IPL’s and WPL’s cost of service and are determined through general rate review proceedings and various tariff filings with the IUB and PSCW, respectively. Such tariff-based services provide electricity or gas to customers without a defined contractual term. IPL and WPL have wholesale electric market-based rate authority from FERC allowing them to participate in wholesale energy markets (e.g. MISO) and transact directly with third parties. This authority from FERC allows sales of electricity referred to as bulk power sales based on current market values. FERC also allows IPL and WPL to enter into power supply agreements with municipalities and rural electric cooperatives with defined contractual terms, which include standard pricing mechanisms that are detailed in current tariffs accepted by FERC through wholesale rate review proceedings. Revenues from Alliant Energy’s non-utility business customers are primarily from its Transportation business, which includes a short-line railway that provides freight service, hauling services on the Mississippi River, and other transfer and storage services. As of December 31, 2018 , revenue expected to be recognized in any future year related to remaining performance obligations is not material, as the majority of revenues are recognized as services are rendered or commodities are delivered, and are from contracts with durations of less than one year. Alliant Energy, IPL and WPL do not have any material contract assets or contract liabilities, or contract acquisition fulfillment costs. Disaggregation of revenues from contracts with customers, which correlates to revenues for each reportable segment, was as follows (in millions): Alliant Energy IPL WPL 2018 2017 2016 2018 2017 2016 2018 2017 2016 Electric Utility: Retail - residential $1,063.4 $1,006.2 $1,001.1 $590.6 $535.6 $536.7 $472.8 $470.6 $464.4 Retail - commercial 735.6 710.3 712.6 486.8 452.7 445.4 248.8 257.6 267.2 Retail - industrial 888.8 853.1 851.1 500.8 459.7 460.4 388.0 393.4 390.7 Wholesale 188.4 238.4 256.6 71.2 95.5 94.2 117.2 142.9 162.4 Bulk power and other 124.1 86.7 54.1 81.7 55.4 33.0 42.4 31.3 21.1 Total Electric Utility 3,000.3 2,894.7 2,875.5 1,731.1 1,598.9 1,569.7 1,269.2 1,295.8 1,305.8 Gas Utility: Retail - residential 254.4 224.7 197.6 152.3 123.2 110.6 102.1 101.5 87.0 Retail - commercial 133.0 123.2 109.6 75.9 67.9 61.9 57.1 55.3 47.7 Retail - industrial 14.9 16.7 15.2 10.2 11.1 10.6 4.7 5.6 4.6 Transportation/other 44.3 36.3 33.0 27.8 23.8 20.9 16.5 12.5 12.1 Total Gas Utility 446.6 400.9 355.4 266.2 226.0 204.0 180.4 174.9 151.4 Other Utility: Steam 35.2 34.6 33.9 35.2 34.6 33.9 — — — Other utility 12.8 12.9 14.7 9.8 10.8 12.8 3.0 2.1 1.9 Total Other Utility 48.0 47.5 48.6 45.0 45.4 46.7 3.0 2.1 1.9 Non-Utility and Other: Transportation and other 39.6 39.1 40.5 — — — — — — Total Non-Utility and Other 39.6 39.1 40.5 — — — — — — Total revenues $3,534.5 $3,382.2 $3,320.0 $2,042.3 $1,870.3 $1,820.4 $1,452.6 $1,472.8 $1,459.1

Benefit Plans

Benefit Plans	12 Months Ended
	Dec. 31, 2018
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 2018 2017 2016 2018 2017 2016 Discount rate for benefit obligations 4.34% 3.66% 4.19% 4.24% 3.53% 3.98% Discount rate for net periodic cost 3.66% 4.19% 4.47% 3.53% 3.98% 4.30% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.44% 5.80% 6.30% Interest crediting rate for Alliant Energy Cash Balance Pension Plan 5.04% 4.64% 3.17% N/A N/A N/A 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 6.50% 6.75% 7.00% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans IPL 2018 2017 2016 2018 2017 2016 Discount rate for benefit obligations 4.35% 3.68% 4.22% 4.23% 3.51% 3.95% Discount rate for net periodic cost 3.68% 4.22% 4.50% 3.51% 3.95% 4.28% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.60% 6.20% 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 6.50% 6.75% 7.00% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans WPL 2018 2017 2016 2018 2017 2016 Discount rate for benefit obligations 4.35% 3.69% 4.23% 4.23% 3.51% 3.96% Discount rate for net periodic cost 3.69% 4.23% 4.51% 3.51% 3.96% 4.28% Expected rate of return on plan assets 7.60% 7.60% 7.60% 3.84% 3.50% 4.70% 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 6.50% 6.75% 7.00% 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 utilizes base mortality tables that were released in 2014 by the Society of Actuaries, and was updated in 2016 and 2018. Net Periodic Benefit Costs - The components of net periodic benefit costs for sponsored defined benefit pension and OPEB plans are included in the tables below (in millions). The service cost component of net periodic benefit costs is included in “Other operation and maintenance” expenses in the income statements and all other components of net periodic benefit costs are included in “Other (income) and deductions” in the income statements. Alliant Energy Defined Benefit Pension Plans OPEB Plans 2018 2017 2016 2018 2017 2016 Service cost $12.1 $12.5 $12.6 $4.2 $5.0 $5.3 Interest cost 46.8 51.0 53.0 7.7 8.6 9.4 Expected return on plan assets (a) (69.7 ) (65.5 ) (65.5 ) (6.0 ) (6.1 ) (6.1 ) Amortization of prior service credit (b) (0.7 ) (0.4 ) (0.3 ) (0.2 ) (0.2 ) (4.1 ) Amortization of actuarial loss (c) 35.2 37.6 37.4 3.4 3.8 4.7 Settlement losses (d) — 0.9 — — — — $23.7 $36.1 $37.2 $9.1 $11.1 $9.2 IPL Defined Benefit Pension Plans OPEB Plans 2018 2017 2016 2018 2017 2016 Service cost $7.4 $7.3 $7.5 $1.7 $2.1 $2.3 Interest cost 21.4 23.5 24.5 3.1 3.5 3.8 Expected return on plan assets (a) (32.6 ) (30.8 ) (30.9 ) (4.4 ) (4.3 ) (4.3 ) Amortization of prior service credit (b) (0.2 ) (0.2 ) (0.2 ) — — (2.6 ) Amortization of actuarial loss (c) 14.9 16.1 16.5 1.3 2.0 2.6 $10.9 $15.9 $17.4 $1.7 $3.3 $1.8 WPL Defined Benefit Pension Plans OPEB Plans 2018 2017 2016 2018 2017 2016 Service cost $4.4 $4.9 $4.9 $1.6 $1.9 $2.0 Interest cost 20.2 21.8 22.3 3.1 3.4 3.8 Expected return on plan assets (a) (30.4 ) (28.5 ) (28.3 ) (0.6 ) (0.8 ) (0.8 ) Amortization of prior service cost (credit) (b) (0.1 ) 0.1 0.2 (0.2 ) (0.2 ) (0.9 ) Amortization of actuarial loss (c) 17.2 18.5 17.6 2.0 1.6 1.8 $11.3 $16.8 $16.7 $5.9 $5.9 $5.9 (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. 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 2018 2017 2018 2017 Change in benefit obligation: Net benefit obligation at January 1 $1,303.1 $1,244.3 $222.3 $220.1 Service cost 12.1 12.5 4.2 5.0 Interest cost 46.8 51.0 7.7 8.6 Plan participants’ contributions — — 3.1 2.9 Actuarial (gain) loss (96.2 ) 83.6 (8.2 ) 5.4 Gross benefits paid (90.8 ) (88.3 ) (23.0 ) (19.7 ) Net benefit obligation at December 31 1,175.0 1,303.1 206.1 222.3 Change in plan assets: Fair value of plan assets at January 1 950.7 895.7 111.1 105.8 Actual return on plan assets (57.8 ) 136.7 (2.6 ) 12.9 Employer contributions 6.5 6.6 10.5 9.2 Plan participants’ contributions — — 3.1 2.9 Gross benefits paid (90.8 ) (88.3 ) (23.0 ) (19.7 ) Fair value of plan assets at December 31 808.6 950.7 99.1 111.1 Under funded status at December 31 ($366.4 ) ($352.4 ) ($107.0 ) ($111.2 ) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2018 2017 2018 2017 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $7.5 $8.8 Current liabilities (2.3 ) (2.2 ) (9.6 ) (9.1 ) Pension and other benefit obligations (364.1 ) (350.2 ) (104.9 ) (110.9 ) Net amounts recognized at December 31 ($366.4 ) ($352.4 ) ($107.0 ) ($111.2 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $505.2 $509.1 $44.5 $47.4 Prior service credit (5.8 ) (6.5 ) (1.1 ) (1.3 ) $499.4 $502.6 $43.4 $46.1 Defined Benefit Pension Plans OPEB Plans IPL 2018 2017 2018 2017 Change in benefit obligation: Net benefit obligation at January 1 $592.9 $570.4 $89.4 $90.1 Service cost 7.4 7.3 1.7 2.1 Interest cost 21.4 23.5 3.1 3.5 Plan participants’ contributions — — 1.0 1.0 Actuarial (gain) loss (44.4 ) 34.9 (4.3 ) (0.1 ) Gross benefits paid (43.4 ) (43.2 ) (8.4 ) (7.2 ) Net benefit obligation at December 31 533.9 592.9 82.5 89.4 Change in plan assets: Fair value of plan assets at January 1 443.7 422.0 72.9 68.2 Actual return on plan assets (26.8 ) 64.3 (1.4 ) 8.9 Employer contributions 4.3 0.6 2.6 2.0 Plan participants’ contributions — — 1.0 1.0 Gross benefits paid (43.4 ) (43.2 ) (8.4 ) (7.2 ) Fair value of plan assets at December 31 377.8 443.7 66.7 72.9 Under funded status at December 31 ($156.1 ) ($149.2 ) ($15.8 ) ($16.5 ) Defined Benefit Pension Plans OPEB Plans IPL 2018 2017 2018 2017 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $4.7 $5.9 Current liabilities (0.6 ) (0.5 ) (1.9 ) (2.0 ) Pension and other benefit obligations (155.5 ) (148.7 ) (18.6 ) (20.4 ) Net amounts recognized at December 31 ($156.1 ) ($149.2 ) ($15.8 ) ($16.5 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $218.8 $218.9 $18.8 $18.7 Prior service credit (1.8 ) (2.1 ) — — $217.0 $216.8 $18.8 $18.7 Defined Benefit Pension Plans OPEB Plans WPL 2018 2017 2018 2017 Change in benefit obligation: Net benefit obligation at January 1 $559.8 $529.2 $90.4 $88.9 Service cost 4.4 4.9 1.6 1.9 Interest cost 20.2 21.8 3.1 3.4 Plan participants’ contributions — — 1.4 1.4 Actuarial (gain) loss (40.0 ) 38.3 (2.5 ) 4.1 Gross benefits paid (37.8 ) (34.4 ) (10.8 ) (9.3 ) Net benefit obligation at December 31 506.6 559.8 83.2 90.4 Change in plan assets: Fair value of plan assets at January 1 415.0 389.7 18.7 18.6 Actual return on plan assets (25.1 ) 59.6 (0.1 ) 1.2 Employer contributions 0.1 0.1 7.5 6.8 Plan participants’ contributions — — 1.4 1.4 Gross benefits paid (37.8 ) (34.4 ) (10.8 ) (9.3 ) Fair value of plan assets at December 31 352.2 415.0 16.7 18.7 Under funded status at December 31 ($154.4 ) ($144.8 ) ($66.5 ) ($71.7 ) Defined Benefit Pension Plans OPEB Plans WPL 2018 2017 2018 2017 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $2.8 $2.9 Current liabilities (0.1 ) (0.1 ) (7.4 ) (6.8 ) Pension and other benefit obligations (154.3 ) (144.7 ) (61.9 ) (67.8 ) Net amounts recognized at December 31 ($154.4 ) ($144.8 ) ($66.5 ) ($71.7 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $223.0 $224.7 $19.9 $23.6 Prior service credit (1.3 ) (1.5 ) (1.1 ) (1.3 ) $221.7 $223.2 $18.8 $22.3 Actuarial gains related to benefit obligations in 2018 for defined benefit pension and OPEB plans were primarily due to increases in the discount rates and the impact of the updated mortality table, partially offset by experience losses. Actuarial losses related to benefit obligations in 2017 for defined benefit pension and OPEB plans were primarily due to decreases in the discount rates. 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 2018 2017 2018 2017 Accumulated benefit obligations $1,139.9 $1,269.0 $206.1 $222.3 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,139.9 1,269.0 206.1 222.3 Fair value of plan assets 808.6 950.7 99.1 111.1 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,175.0 1,303.1 N/A N/A Fair value of plan assets 808.6 950.7 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2018 2017 2018 2017 Accumulated benefit obligations $514.3 $573.1 $82.5 $89.4 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 514.3 573.1 82.5 89.4 Fair value of plan assets 377.8 443.7 66.7 72.9 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 533.9 592.9 N/A N/A Fair value of plan assets 377.8 443.7 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2018 2017 2018 2017 Accumulated benefit obligations $494.8 $548.1 $83.2 $90.4 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 494.8 548.1 83.2 90.4 Fair value of plan assets 352.2 415.0 16.7 18.7 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 506.6 559.8 N/A N/A Fair value of plan assets 352.2 415.0 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 2018 2017 2018 2017 Regulatory assets $38.2 $38.9 $27.7 $28.1 Estimated Future Employer Contributions and Benefit Payments - Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2019 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $33.8 $16.5 $15.6 OPEB plans 9.6 1.9 7.4 (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 2019 2020 2021 2022 2023 2024 - 2028 Defined benefit pension benefits $71.7 $86.1 $74.7 $76.3 $77.8 $388.3 OPEB 19.3 18.1 17.8 17.5 17.0 77.7 $91.0 $104.2 $92.5 $93.8 $94.8 $466.0 IPL 2019 2020 2021 2022 2023 2024 - 2028 Defined benefit pension benefits $33.0 $35.1 $35.1 $36.4 $37.5 $182.9 OPEB 7.3 7.3 7.2 7.1 6.9 31.3 $40.3 $42.4 $42.3 $43.5 $44.4 $214.2 WPL 2019 2020 2021 2022 2023 2024 - 2028 Defined benefit pension benefits $31.4 $31.8 $31.9 $32.0 $32.5 $165.3 OPEB 8.8 7.6 7.4 7.1 6.8 30.7 $40.2 $39.4 $39.3 $39.1 $39.3 $196.0 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 may 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 are not limited to, direct ownership of real estate, oil and gas limited partnerships, securities of the managers’ firms or affiliate firms, and Alliant Energy securities. At December 31, 2018 , 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% 4% Equity securities - U.S. 11 % - 41% 25% Equity securities - international 14 % - 34% 22% Global asset securities 5 % - 15% 9% 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. At December 31, 2018 , 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 % - 46% 24% Equity securities - international 0 % - 34% 2% Fixed income securities 20 % - 100% 73% 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 16 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, 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): 2018 2017 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $36.4 $3.0 $33.4 $— $28.2 $4.5 $23.7 $— Equity securities - U.S. 130.2 130.2 — — 158.3 158.3 — — Equity securities - international 116.0 116.0 — — 137.5 137.5 — — Global asset securities 42.1 42.1 — — 49.4 49.4 — — Fixed income securities 127.8 52.5 75.3 — 135.9 55.8 80.1 — Total assets in fair value hierarchy 452.5 $343.8 $108.7 $— 509.3 $405.5 $103.8 $— Assets measured at net asset value 355.4 441.1 Accrued investment income 1.2 1.0 Due to brokers, net (pending trades with brokers) (0.5 ) (0.7 ) Total pension plan assets $808.6 $950.7 2018 2017 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $17.0 $1.4 $15.6 $— $13.2 $2.2 $11.0 $— Equity securities - U.S. 60.8 60.8 — — 73.9 73.9 — — Equity securities - international 54.2 54.2 — — 64.2 64.2 — — Global asset securities 19.7 19.7 — — 23.0 23.0 — — Fixed income securities 59.7 24.5 35.2 — 63.4 26.0 37.4 — Total assets in fair value hierarchy 211.4 $160.6 $50.8 $— 237.7 $189.3 $48.4 $— Assets measured at net asset value 166.1 205.8 Accrued investment income 0.6 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.3 ) Total pension plan assets $377.8 $443.7 2018 2017 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $15.9 $1.3 $14.6 $— $12.3 $2.0 $10.3 $— Equity securities - U.S. 56.7 56.7 — — 69.1 69.1 — — Equity securities - international 50.5 50.5 — — 60.0 60.0 — — Global asset securities 18.4 18.4 — — 21.6 21.6 — — Fixed income securities 55.6 22.8 32.8 — 59.3 24.3 35.0 — Total assets in fair value hierarchy 197.1 $149.7 $47.4 $— 222.3 $177.0 $45.3 $— Assets measured at net asset value 154.8 192.5 Accrued investment income 0.5 0.5 Due to brokers, net (pending trades with brokers) (0.2 ) (0.3 ) Total pension plan assets $352.2 $415.0 At December 31, the fair values of OPEB plan assets were as follows (in millions): 2018 2017 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $1.4 $1.1 $0.3 $— $1.2 $0.7 $0.5 $— Equity securities - U.S. 3.9 3.9 — — 27.9 27.9 — — Equity securities - international 2.9 2.9 — — 11.4 11.4 — — Global asset securities 0.4 0.4 — — 0.4 0.4 — — Fixed income securities 68.2 67.5 0.7 — 66.6 66.0 0.6 — Total assets in fair value hierarchy 76.8 $75.8 $1.0 $— 107.5 $106.4 $1.1 $— Assets measured at net asset value 22.3 3.6 Total OPEB plan assets $99.1 $111.1 2018 2017 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.7 $0.7 $— $— $0.3 $0.3 $— $— Equity securities - U.S. — — — — 22.3 22.3 — — Equity securities - international — — — — 7.5 7.5 — — Fixed income securities 47.0 47.0 — — 42.8 42.8 — — Total assets in fair value hierarchy 47.7 $47.7 $— $— 72.9 $72.9 $— $— Assets measured at net asset value 19.0 — Total OPEB plan assets $66.7 $72.9 2018 2017 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.1 $0.1 $— $— $0.6 $0.3 $0.3 $— Fixed income securities 16.6 16.6 — — 18.1 18.1 — — Total OPEB plan assets $16.7 $16.7 $— $— $18.7 $18.4 $0.3 $— 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, 2018 and 2017 . 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 11.5% of total assets in the 401(k) savings plans at December 31, 2018 and 2017 , 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 2018 2017 2016 2018 2017 2016 2018 2017 2016 401(k) costs $25.1 $24.8 $23.6 $13.0 $12.8 $12.0 $11.2 $11.1 $10.7 (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, 2018 , performance shares and restricted stock units (performance- and time-vesting) were outstanding under the Amended and Restated OIP, and 6.8 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, 2018 , performance units and restricted units (performance- and time-vesting) were outstanding under the DLIP, and the the amount of nonvested restricted units was not material. 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 2018 2017 2016 2018 2017 2016 2018 2017 2016 Compensation expense $17.0 $15.1 $18.0 $9.4 $8.3 $9.5 $6.9 $6.4 $7.9 Income tax benefits 4.9 6.2 7.4 2.8 3.4 4.0 1.9 2.6 3.2 As of December 31, 2018 , Alliant Energy’s, IPL’s and WPL’s total unrecognized compensation cost related to share-based compensation awards was $4.9 million , $2.7 million and $2.1 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 2018 2017 2016 2018 2017 2016 Nonvested awards, January 1 223,511 257,599 288,430 71,737 93,320 116,412 Granted 74,163 65,350 68,585 19,840 21,558 23,918 Vested (90,806 ) (99,438 ) (98,186 ) (31,910 ) (37,395 ) (42,760 ) Forfeited (3,680 ) — (1,230 ) (1,906 ) (5,746 ) (4,250 ) Nonvested awards, December 31 203,188 223,511 257,599 57,761 71,737 93,320 Granted Awards - Each performance share’s and performance unit’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. 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 2018 2017 2016 2018 2017 2016 2015 Grant 2014 Grant 2013 Grant 2015 Grant 2014 Grant 2013 Grant Performance awards vested 90,806 99,438 98,186 31,910 37,395 42,760 Percentage of target number of performance awards 137.5% 147.5% 165.0% 137.5% 147.5% 165.0% Aggregate payout value (in millions) $5.3 $5.6 $5.1 $1.4 $1.5 $1.7 Payout - cash (in millions) $4.9 $5.1 $2.9 $1.4 $1.5 $1.7 Payout - common stock shares issued 5,078 5,185 22,408 N/A N/A N/A Fair Value of Awards - At December 31, 2018 , Alliant Energy’s common stock closing price was $42.25 . Additio nal i nformation related to fair values of nonvested performance shares and performance units at December 31, 2018 , by year of grant, was as follows: Performance Shares Performance Units 2018 Grant 2017 Grant 2016 Grant 2018 Grant 2017 Grant 2016 Grant Nonvested awards at target 70,483 65,350 67,355 19,196 18,062 20,503 Estimated payout percentage based on performance criteria 85 % 85 % 143 % 85 % 85 % 143 % Fair values of each nonvested award $35.91 $35.91 $60.42 $35.91 $35.91 $60.42 Performance Restricted Stock Units - Payouts of performance restricted stock units under the Amended and Restated OIP are based on the achievement of certain performance targets (currently specified growth of consolidated net 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. 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: 2018 2017 2016 Units Weighted Average Grant Date Fair Value Units Weighted Average Grant Date Fair Value Units Weighted Average Grant Date Fair Value Nonvested units, January 1 132,705 $36.50 67,355 $33.96 — $— Granted 74,163 38.60 65,350 39.12 68,585 33.96 Forfeited (3,680 ) 38.60 — — (1,230 ) 33.90 Nonvested units, December 31 203,188 37.23 132,705 36.50 67,355 33.96 Restricted Stock Units - Payouts of restricted stock units under the Amended and Restated OIP 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. A summary of the restricted stock units activity was as follows: 2018 2017 2016 Nonvested units, January 1 113,749 57,736 — Granted 63,568 56,013 58,790 Forfeited (3,154 ) — (1,054 ) Nonvested units, December 31 174,163 113,749 57,736 (c) Deferred Compensation Plan - Alliant Energy maintains a DCP under which key employees may defer up to 100% of base salary and short-term cash incentive compensation and directors may elect to defer all or part of their retainer and committee fees. Key employees who have made the maximum allowed contribution to the Alliant Energy 401(k) Savings Plan may receive an additional credit to the DCP. Key employees and directors may elect to have their deferrals credited to a company stock account, an interest account, equity accounts or mutual fund accounts based on certain benchmark funds. Company Stock Account - The DCP does not permit diversification of deferrals credited to the company stock account and all distributions from participants’ company stock accounts are made in the form of shares of Alliant Energy common stock. The deferred compensation obligations for participants’ company stock accounts are recorded in “Additional paid-in capital” and the shares of Alliant Energy common stock held in a rabbi trust to satisfy this obligation are recorded in “Shares in deferred compensation trust” on Alliant Energy’s balance sheets. At December 31, the carrying value of the deferred c
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 2018 2017 2016 2018 2017 2016 Discount rate for benefit obligations 4.34% 3.66% 4.19% 4.24% 3.53% 3.98% Discount rate for net periodic cost 3.66% 4.19% 4.47% 3.53% 3.98% 4.30% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.44% 5.80% 6.30% Interest crediting rate for Alliant Energy Cash Balance Pension Plan 5.04% 4.64% 3.17% N/A N/A N/A 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 6.50% 6.75% 7.00% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans IPL 2018 2017 2016 2018 2017 2016 Discount rate for benefit obligations 4.35% 3.68% 4.22% 4.23% 3.51% 3.95% Discount rate for net periodic cost 3.68% 4.22% 4.50% 3.51% 3.95% 4.28% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.60% 6.20% 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 6.50% 6.75% 7.00% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans WPL 2018 2017 2016 2018 2017 2016 Discount rate for benefit obligations 4.35% 3.69% 4.23% 4.23% 3.51% 3.96% Discount rate for net periodic cost 3.69% 4.23% 4.51% 3.51% 3.96% 4.28% Expected rate of return on plan assets 7.60% 7.60% 7.60% 3.84% 3.50% 4.70% 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 6.50% 6.75% 7.00% 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 utilizes base mortality tables that were released in 2014 by the Society of Actuaries, and was updated in 2016 and 2018. Net Periodic Benefit Costs - The components of net periodic benefit costs for sponsored defined benefit pension and OPEB plans are included in the tables below (in millions). The service cost component of net periodic benefit costs is included in “Other operation and maintenance” expenses in the income statements and all other components of net periodic benefit costs are included in “Other (income) and deductions” in the income statements. Alliant Energy Defined Benefit Pension Plans OPEB Plans 2018 2017 2016 2018 2017 2016 Service cost $12.1 $12.5 $12.6 $4.2 $5.0 $5.3 Interest cost 46.8 51.0 53.0 7.7 8.6 9.4 Expected return on plan assets (a) (69.7 ) (65.5 ) (65.5 ) (6.0 ) (6.1 ) (6.1 ) Amortization of prior service credit (b) (0.7 ) (0.4 ) (0.3 ) (0.2 ) (0.2 ) (4.1 ) Amortization of actuarial loss (c) 35.2 37.6 37.4 3.4 3.8 4.7 Settlement losses (d) — 0.9 — — — — $23.7 $36.1 $37.2 $9.1 $11.1 $9.2 IPL Defined Benefit Pension Plans OPEB Plans 2018 2017 2016 2018 2017 2016 Service cost $7.4 $7.3 $7.5 $1.7 $2.1 $2.3 Interest cost 21.4 23.5 24.5 3.1 3.5 3.8 Expected return on plan assets (a) (32.6 ) (30.8 ) (30.9 ) (4.4 ) (4.3 ) (4.3 ) Amortization of prior service credit (b) (0.2 ) (0.2 ) (0.2 ) — — (2.6 ) Amortization of actuarial loss (c) 14.9 16.1 16.5 1.3 2.0 2.6 $10.9 $15.9 $17.4 $1.7 $3.3 $1.8 WPL Defined Benefit Pension Plans OPEB Plans 2018 2017 2016 2018 2017 2016 Service cost $4.4 $4.9 $4.9 $1.6 $1.9 $2.0 Interest cost 20.2 21.8 22.3 3.1 3.4 3.8 Expected return on plan assets (a) (30.4 ) (28.5 ) (28.3 ) (0.6 ) (0.8 ) (0.8 ) Amortization of prior service cost (credit) (b) (0.1 ) 0.1 0.2 (0.2 ) (0.2 ) (0.9 ) Amortization of actuarial loss (c) 17.2 18.5 17.6 2.0 1.6 1.8 $11.3 $16.8 $16.7 $5.9 $5.9 $5.9 (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. 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 2018 2017 2018 2017 Change in benefit obligation: Net benefit obligation at January 1 $1,303.1 $1,244.3 $222.3 $220.1 Service cost 12.1 12.5 4.2 5.0 Interest cost 46.8 51.0 7.7 8.6 Plan participants’ contributions — — 3.1 2.9 Actuarial (gain) loss (96.2 ) 83.6 (8.2 ) 5.4 Gross benefits paid (90.8 ) (88.3 ) (23.0 ) (19.7 ) Net benefit obligation at December 31 1,175.0 1,303.1 206.1 222.3 Change in plan assets: Fair value of plan assets at January 1 950.7 895.7 111.1 105.8 Actual return on plan assets (57.8 ) 136.7 (2.6 ) 12.9 Employer contributions 6.5 6.6 10.5 9.2 Plan participants’ contributions — — 3.1 2.9 Gross benefits paid (90.8 ) (88.3 ) (23.0 ) (19.7 ) Fair value of plan assets at December 31 808.6 950.7 99.1 111.1 Under funded status at December 31 ($366.4 ) ($352.4 ) ($107.0 ) ($111.2 ) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2018 2017 2018 2017 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $7.5 $8.8 Current liabilities (2.3 ) (2.2 ) (9.6 ) (9.1 ) Pension and other benefit obligations (364.1 ) (350.2 ) (104.9 ) (110.9 ) Net amounts recognized at December 31 ($366.4 ) ($352.4 ) ($107.0 ) ($111.2 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $505.2 $509.1 $44.5 $47.4 Prior service credit (5.8 ) (6.5 ) (1.1 ) (1.3 ) $499.4 $502.6 $43.4 $46.1 Defined Benefit Pension Plans OPEB Plans IPL 2018 2017 2018 2017 Change in benefit obligation: Net benefit obligation at January 1 $592.9 $570.4 $89.4 $90.1 Service cost 7.4 7.3 1.7 2.1 Interest cost 21.4 23.5 3.1 3.5 Plan participants’ contributions — — 1.0 1.0 Actuarial (gain) loss (44.4 ) 34.9 (4.3 ) (0.1 ) Gross benefits paid (43.4 ) (43.2 ) (8.4 ) (7.2 ) Net benefit obligation at December 31 533.9 592.9 82.5 89.4 Change in plan assets: Fair value of plan assets at January 1 443.7 422.0 72.9 68.2 Actual return on plan assets (26.8 ) 64.3 (1.4 ) 8.9 Employer contributions 4.3 0.6 2.6 2.0 Plan participants’ contributions — — 1.0 1.0 Gross benefits paid (43.4 ) (43.2 ) (8.4 ) (7.2 ) Fair value of plan assets at December 31 377.8 443.7 66.7 72.9 Under funded status at December 31 ($156.1 ) ($149.2 ) ($15.8 ) ($16.5 ) Defined Benefit Pension Plans OPEB Plans IPL 2018 2017 2018 2017 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $4.7 $5.9 Current liabilities (0.6 ) (0.5 ) (1.9 ) (2.0 ) Pension and other benefit obligations (155.5 ) (148.7 ) (18.6 ) (20.4 ) Net amounts recognized at December 31 ($156.1 ) ($149.2 ) ($15.8 ) ($16.5 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $218.8 $218.9 $18.8 $18.7 Prior service credit (1.8 ) (2.1 ) — — $217.0 $216.8 $18.8 $18.7 Defined Benefit Pension Plans OPEB Plans WPL 2018 2017 2018 2017 Change in benefit obligation: Net benefit obligation at January 1 $559.8 $529.2 $90.4 $88.9 Service cost 4.4 4.9 1.6 1.9 Interest cost 20.2 21.8 3.1 3.4 Plan participants’ contributions — — 1.4 1.4 Actuarial (gain) loss (40.0 ) 38.3 (2.5 ) 4.1 Gross benefits paid (37.8 ) (34.4 ) (10.8 ) (9.3 ) Net benefit obligation at December 31 506.6 559.8 83.2 90.4 Change in plan assets: Fair value of plan assets at January 1 415.0 389.7 18.7 18.6 Actual return on plan assets (25.1 ) 59.6 (0.1 ) 1.2 Employer contributions 0.1 0.1 7.5 6.8 Plan participants’ contributions — — 1.4 1.4 Gross benefits paid (37.8 ) (34.4 ) (10.8 ) (9.3 ) Fair value of plan assets at December 31 352.2 415.0 16.7 18.7 Under funded status at December 31 ($154.4 ) ($144.8 ) ($66.5 ) ($71.7 ) Defined Benefit Pension Plans OPEB Plans WPL 2018 2017 2018 2017 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $2.8 $2.9 Current liabilities (0.1 ) (0.1 ) (7.4 ) (6.8 ) Pension and other benefit obligations (154.3 ) (144.7 ) (61.9 ) (67.8 ) Net amounts recognized at December 31 ($154.4 ) ($144.8 ) ($66.5 ) ($71.7 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $223.0 $224.7 $19.9 $23.6 Prior service credit (1.3 ) (1.5 ) (1.1 ) (1.3 ) $221.7 $223.2 $18.8 $22.3 Actuarial gains related to benefit obligations in 2018 for defined benefit pension and OPEB plans were primarily due to increases in the discount rates and the impact of the updated mortality table, partially offset by experience losses. Actuarial losses related to benefit obligations in 2017 for defined benefit pension and OPEB plans were primarily due to decreases in the discount rates. 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 2018 2017 2018 2017 Accumulated benefit obligations $1,139.9 $1,269.0 $206.1 $222.3 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,139.9 1,269.0 206.1 222.3 Fair value of plan assets 808.6 950.7 99.1 111.1 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,175.0 1,303.1 N/A N/A Fair value of plan assets 808.6 950.7 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2018 2017 2018 2017 Accumulated benefit obligations $514.3 $573.1 $82.5 $89.4 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 514.3 573.1 82.5 89.4 Fair value of plan assets 377.8 443.7 66.7 72.9 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 533.9 592.9 N/A N/A Fair value of plan assets 377.8 443.7 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2018 2017 2018 2017 Accumulated benefit obligations $494.8 $548.1 $83.2 $90.4 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 494.8 548.1 83.2 90.4 Fair value of plan assets 352.2 415.0 16.7 18.7 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 506.6 559.8 N/A N/A Fair value of plan assets 352.2 415.0 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 2018 2017 2018 2017 Regulatory assets $38.2 $38.9 $27.7 $28.1 Estimated Future Employer Contributions and Benefit Payments - Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2019 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $33.8 $16.5 $15.6 OPEB plans 9.6 1.9 7.4 (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 2019 2020 2021 2022 2023 2024 - 2028 Defined benefit pension benefits $71.7 $86.1 $74.7 $76.3 $77.8 $388.3 OPEB 19.3 18.1 17.8 17.5 17.0 77.7 $91.0 $104.2 $92.5 $93.8 $94.8 $466.0 IPL 2019 2020 2021 2022 2023 2024 - 2028 Defined benefit pension benefits $33.0 $35.1 $35.1 $36.4 $37.5 $182.9 OPEB 7.3 7.3 7.2 7.1 6.9 31.3 $40.3 $42.4 $42.3 $43.5 $44.4 $214.2 WPL 2019 2020 2021 2022 2023 2024 - 2028 Defined benefit pension benefits $31.4 $31.8 $31.9 $32.0 $32.5 $165.3 OPEB 8.8 7.6 7.4 7.1 6.8 30.7 $40.2 $39.4 $39.3 $39.1 $39.3 $196.0 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 may 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 are not limited to, direct ownership of real estate, oil and gas limited partnerships, securities of the managers’ firms or affiliate firms, and Alliant Energy securities. At December 31, 2018 , 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% 4% Equity securities - U.S. 11 % - 41% 25% Equity securities - international 14 % - 34% 22% Global asset securities 5 % - 15% 9% 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. At December 31, 2018 , 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 % - 46% 24% Equity securities - international 0 % - 34% 2% Fixed income securities 20 % - 100% 73% 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 16 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, 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): 2018 2017 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $36.4 $3.0 $33.4 $— $28.2 $4.5 $23.7 $— Equity securities - U.S. 130.2 130.2 — — 158.3 158.3 — — Equity securities - international 116.0 116.0 — — 137.5 137.5 — — Global asset securities 42.1 42.1 — — 49.4 49.4 — — Fixed income securities 127.8 52.5 75.3 — 135.9 55.8 80.1 — Total assets in fair value hierarchy 452.5 $343.8 $108.7 $— 509.3 $405.5 $103.8 $— Assets measured at net asset value 355.4 441.1 Accrued investment income 1.2 1.0 Due to brokers, net (pending trades with brokers) (0.5 ) (0.7 ) Total pension plan assets $808.6 $950.7 2018 2017 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $17.0 $1.4 $15.6 $— $13.2 $2.2 $11.0 $— Equity securities - U.S. 60.8 60.8 — — 73.9 73.9 — — Equity securities - international 54.2 54.2 — — 64.2 64.2 — — Global asset securities 19.7 19.7 — — 23.0 23.0 — — Fixed income securities 59.7 24.5 35.2 — 63.4 26.0 37.4 — Total assets in fair value hierarchy 211.4 $160.6 $50.8 $— 237.7 $189.3 $48.4 $— Assets measured at net asset value 166.1 205.8 Accrued investment income 0.6 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.3 ) Total pension plan assets $377.8 $443.7 2018 2017 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $15.9 $1.3 $14.6 $— $12.3 $2.0 $10.3 $— Equity securities - U.S. 56.7 56.7 — — 69.1 69.1 — — Equity securities - international 50.5 50.5 — — 60.0 60.0 — — Global asset securities 18.4 18.4 — — 21.6 21.6 — — Fixed income securities 55.6 22.8 32.8 — 59.3 24.3 35.0 — Total assets in fair value hierarchy 197.1 $149.7 $47.4 $— 222.3 $177.0 $45.3 $— Assets measured at net asset value 154.8 192.5 Accrued investment income 0.5 0.5 Due to brokers, net (pending trades with brokers) (0.2 ) (0.3 ) Total pension plan assets $352.2 $415.0 At December 31, the fair values of OPEB plan assets were as follows (in millions): 2018 2017 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $1.4 $1.1 $0.3 $— $1.2 $0.7 $0.5 $— Equity securities - U.S. 3.9 3.9 — — 27.9 27.9 — — Equity securities - international 2.9 2.9 — — 11.4 11.4 — — Global asset securities 0.4 0.4 — — 0.4 0.4 — — Fixed income securities 68.2 67.5 0.7 — 66.6 66.0 0.6 — Total assets in fair value hierarchy 76.8 $75.8 $1.0 $— 107.5 $106.4 $1.1 $— Assets measured at net asset value 22.3 3.6 Total OPEB plan assets $99.1 $111.1 2018 2017 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.7 $0.7 $— $— $0.3 $0.3 $— $— Equity securities - U.S. — — — — 22.3 22.3 — — Equity securities - international — — — — 7.5 7.5 — — Fixed income securities 47.0 47.0 — — 42.8 42.8 — — Total assets in fair value hierarchy 47.7 $47.7 $— $— 72.9 $72.9 $— $— Assets measured at net asset value 19.0 — Total OPEB plan assets $66.7 $72.9 2018 2017 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.1 $0.1 $— $— $0.6 $0.3 $0.3 $— Fixed income securities 16.6 16.6 — — 18.1 18.1 — — Total OPEB plan assets $16.7 $16.7 $— $— $18.7 $18.4 $0.3 $— 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, 2018 and 2017 . 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 11.5% of total assets in the 401(k) savings plans at December 31, 2018 and 2017 , 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 2018 2017 2016 2018 2017 2016 2018 2017 2016 401(k) costs $25.1 $24.8 $23.6 $13.0 $12.8 $12.0 $11.2 $11.1 $10.7 (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, 2018 , performance shares and restricted stock units (performance- and time-vesting) were outstanding under the Amended and Restated OIP, and 6.8 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, 2018 , performance units and restricted units (performance- and time-vesting) were outstanding under the DLIP, and the the amount of nonvested restricted units was not material. 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 2018 2017 2016 2018 2017 2016 2018 2017 2016 Compensation expense $17.0 $15.1 $18.0 $9.4 $8.3 $9.5 $6.9 $6.4 $7.9 Income tax benefits 4.9 6.2 7.4 2.8 3.4 4.0 1.9 2.6 3.2 As of December 31, 2018 , Alliant Energy’s, IPL’s and WPL’s total unrecognized compensation cost related to share-based compensation awards was $4.9 million , $2.7 million and $2.1 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 2018 2017 2016 2018 2017 2016 Nonvested awards, January 1 223,511 257,599 288,430 71,737 93,320 116,412 Granted 74,163 65,350 68,585 19,840 21,558 23,918 Vested (90,806 ) (99,438 ) (98,186 ) (31,910 ) (37,395 ) (42,760 ) Forfeited (3,680 ) — (1,230 ) (1,906 ) (5,746 ) (4,250 ) Nonvested awards, December 31 203,188 223,511 257,599 57,761 71,737 93,320 Granted Awards - Each performance share’s and performance unit’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. 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 2018 2017 2016 2018 2017 2016 2015 Grant 2014 Grant 2013 Grant 2015 Grant 2014 Grant 2013 Grant Performance awards vested 90,806 99,438 98,186 31,910 37,395 42,760 Percentage of target number of performance awards 137.5% 147.5% 165.0% 137.5% 147.5% 165.0% Aggregate payout value (in millions) $5.3 $5.6 $5.1 $1.4 $1.5 $1.7 Payout - cash (in millions) $4.9 $5.1 $2.9 $1.4 $1.5 $1.7 Payout - common stock shares issued 5,078 5,185 22,408 N/A N/A N/A Fair Value of Awards - At December 31, 2018 , Alliant Energy’s common stock closing price was $42.25 . Additio nal i nformation related to fair values of nonvested performance shares and performance units at December 31, 2018 , by year of grant, was as follows: Performance Shares Performance Units 2018 Grant 2017 Grant 2016 Grant 2018 Grant 2017 Grant 2016 Grant Nonvested awards at target 70,483 65,350 67,355 19,196 18,062 20,503 Estimated payout percentage based on performance criteria 85 % 85 % 143 % 85 % 85 % 143 % Fair values of each nonvested award $35.91 $35.91 $60.42 $35.91 $35.91 $60.42 Performance Restricted Stock Units - Payouts of performance restricted stock units under the Amended and Restated OIP are based on the achievement of certain performance targets (currently specified growth of consolidated net 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. 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: 2018 2017 2016 Units Weighted Average Grant Date Fair Value Units Weighted Average Grant Date Fair Value Units Weighted Average Grant Date Fair Value Nonvested units, January 1 132,705 $36.50 67,355 $33.96 — $— Granted 74,163 38.60 65,350 39.12 68,585 33.96 Forfeited (3,680 ) 38.60 — — (1,230 ) 33.90 Nonvested units, December 31 203,188 37.23 132,705 36.50 67,355 33.96 Restricted Stock Units - Payouts of restricted stock units under the Amended and Restated OIP 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. A summary of the restricted stock units activity was as follows: 2018 2017 2016 Nonvested units, January 1 113,749 57,736 — Granted 63,568 56,013 58,790 Forfeited (3,154 ) — (1,054 ) Nonvested units, December 31 174,163 113,749 57,736
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 2018 2017 2016 2018 2017 2016 Discount rate for benefit obligations 4.34% 3.66% 4.19% 4.24% 3.53% 3.98% Discount rate for net periodic cost 3.66% 4.19% 4.47% 3.53% 3.98% 4.30% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.44% 5.80% 6.30% Interest crediting rate for Alliant Energy Cash Balance Pension Plan 5.04% 4.64% 3.17% N/A N/A N/A 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 6.50% 6.75% 7.00% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans IPL 2018 2017 2016 2018 2017 2016 Discount rate for benefit obligations 4.35% 3.68% 4.22% 4.23% 3.51% 3.95% Discount rate for net periodic cost 3.68% 4.22% 4.50% 3.51% 3.95% 4.28% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.60% 6.20% 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 6.50% 6.75% 7.00% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans WPL 2018 2017 2016 2018 2017 2016 Discount rate for benefit obligations 4.35% 3.69% 4.23% 4.23% 3.51% 3.96% Discount rate for net periodic cost 3.69% 4.23% 4.51% 3.51% 3.96% 4.28% Expected rate of return on plan assets 7.60% 7.60% 7.60% 3.84% 3.50% 4.70% 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 6.50% 6.75% 7.00% 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 utilizes base mortality tables that were released in 2014 by the Society of Actuaries, and was updated in 2016 and 2018. Net Periodic Benefit Costs - The components of net periodic benefit costs for sponsored defined benefit pension and OPEB plans are included in the tables below (in millions). The service cost component of net periodic benefit costs is included in “Other operation and maintenance” expenses in the income statements and all other components of net periodic benefit costs are included in “Other (income) and deductions” in the income statements. Alliant Energy Defined Benefit Pension Plans OPEB Plans 2018 2017 2016 2018 2017 2016 Service cost $12.1 $12.5 $12.6 $4.2 $5.0 $5.3 Interest cost 46.8 51.0 53.0 7.7 8.6 9.4 Expected return on plan assets (a) (69.7 ) (65.5 ) (65.5 ) (6.0 ) (6.1 ) (6.1 ) Amortization of prior service credit (b) (0.7 ) (0.4 ) (0.3 ) (0.2 ) (0.2 ) (4.1 ) Amortization of actuarial loss (c) 35.2 37.6 37.4 3.4 3.8 4.7 Settlement losses (d) — 0.9 — — — — $23.7 $36.1 $37.2 $9.1 $11.1 $9.2 IPL Defined Benefit Pension Plans OPEB Plans 2018 2017 2016 2018 2017 2016 Service cost $7.4 $7.3 $7.5 $1.7 $2.1 $2.3 Interest cost 21.4 23.5 24.5 3.1 3.5 3.8 Expected return on plan assets (a) (32.6 ) (30.8 ) (30.9 ) (4.4 ) (4.3 ) (4.3 ) Amortization of prior service credit (b) (0.2 ) (0.2 ) (0.2 ) — — (2.6 ) Amortization of actuarial loss (c) 14.9 16.1 16.5 1.3 2.0 2.6 $10.9 $15.9 $17.4 $1.7 $3.3 $1.8 WPL Defined Benefit Pension Plans OPEB Plans 2018 2017 2016 2018 2017 2016 Service cost $4.4 $4.9 $4.9 $1.6 $1.9 $2.0 Interest cost 20.2 21.8 22.3 3.1 3.4 3.8 Expected return on plan assets (a) (30.4 ) (28.5 ) (28.3 ) (0.6 ) (0.8 ) (0.8 ) Amortization of prior service cost (credit) (b) (0.1 ) 0.1 0.2 (0.2 ) (0.2 ) (0.9 ) Amortization of actuarial loss (c) 17.2 18.5 17.6 2.0 1.6 1.8 $11.3 $16.8 $16.7 $5.9 $5.9 $5.9 (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. 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 2018 2017 2018 2017 Change in benefit obligation: Net benefit obligation at January 1 $1,303.1 $1,244.3 $222.3 $220.1 Service cost 12.1 12.5 4.2 5.0 Interest cost 46.8 51.0 7.7 8.6 Plan participants’ contributions — — 3.1 2.9 Actuarial (gain) loss (96.2 ) 83.6 (8.2 ) 5.4 Gross benefits paid (90.8 ) (88.3 ) (23.0 ) (19.7 ) Net benefit obligation at December 31 1,175.0 1,303.1 206.1 222.3 Change in plan assets: Fair value of plan assets at January 1 950.7 895.7 111.1 105.8 Actual return on plan assets (57.8 ) 136.7 (2.6 ) 12.9 Employer contributions 6.5 6.6 10.5 9.2 Plan participants’ contributions — — 3.1 2.9 Gross benefits paid (90.8 ) (88.3 ) (23.0 ) (19.7 ) Fair value of plan assets at December 31 808.6 950.7 99.1 111.1 Under funded status at December 31 ($366.4 ) ($352.4 ) ($107.0 ) ($111.2 ) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2018 2017 2018 2017 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $7.5 $8.8 Current liabilities (2.3 ) (2.2 ) (9.6 ) (9.1 ) Pension and other benefit obligations (364.1 ) (350.2 ) (104.9 ) (110.9 ) Net amounts recognized at December 31 ($366.4 ) ($352.4 ) ($107.0 ) ($111.2 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $505.2 $509.1 $44.5 $47.4 Prior service credit (5.8 ) (6.5 ) (1.1 ) (1.3 ) $499.4 $502.6 $43.4 $46.1 Defined Benefit Pension Plans OPEB Plans IPL 2018 2017 2018 2017 Change in benefit obligation: Net benefit obligation at January 1 $592.9 $570.4 $89.4 $90.1 Service cost 7.4 7.3 1.7 2.1 Interest cost 21.4 23.5 3.1 3.5 Plan participants’ contributions — — 1.0 1.0 Actuarial (gain) loss (44.4 ) 34.9 (4.3 ) (0.1 ) Gross benefits paid (43.4 ) (43.2 ) (8.4 ) (7.2 ) Net benefit obligation at December 31 533.9 592.9 82.5 89.4 Change in plan assets: Fair value of plan assets at January 1 443.7 422.0 72.9 68.2 Actual return on plan assets (26.8 ) 64.3 (1.4 ) 8.9 Employer contributions 4.3 0.6 2.6 2.0 Plan participants’ contributions — — 1.0 1.0 Gross benefits paid (43.4 ) (43.2 ) (8.4 ) (7.2 ) Fair value of plan assets at December 31 377.8 443.7 66.7 72.9 Under funded status at December 31 ($156.1 ) ($149.2 ) ($15.8 ) ($16.5 ) Defined Benefit Pension Plans OPEB Plans IPL 2018 2017 2018 2017 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $4.7 $5.9 Current liabilities (0.6 ) (0.5 ) (1.9 ) (2.0 ) Pension and other benefit obligations (155.5 ) (148.7 ) (18.6 ) (20.4 ) Net amounts recognized at December 31 ($156.1 ) ($149.2 ) ($15.8 ) ($16.5 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $218.8 $218.9 $18.8 $18.7 Prior service credit (1.8 ) (2.1 ) — — $217.0 $216.8 $18.8 $18.7 Defined Benefit Pension Plans OPEB Plans WPL 2018 2017 2018 2017 Change in benefit obligation: Net benefit obligation at January 1 $559.8 $529.2 $90.4 $88.9 Service cost 4.4 4.9 1.6 1.9 Interest cost 20.2 21.8 3.1 3.4 Plan participants’ contributions — — 1.4 1.4 Actuarial (gain) loss (40.0 ) 38.3 (2.5 ) 4.1 Gross benefits paid (37.8 ) (34.4 ) (10.8 ) (9.3 ) Net benefit obligation at December 31 506.6 559.8 83.2 90.4 Change in plan assets: Fair value of plan assets at January 1 415.0 389.7 18.7 18.6 Actual return on plan assets (25.1 ) 59.6 (0.1 ) 1.2 Employer contributions 0.1 0.1 7.5 6.8 Plan participants’ contributions — — 1.4 1.4 Gross benefits paid (37.8 ) (34.4 ) (10.8 ) (9.3 ) Fair value of plan assets at December 31 352.2 415.0 16.7 18.7 Under funded status at December 31 ($154.4 ) ($144.8 ) ($66.5 ) ($71.7 ) Defined Benefit Pension Plans OPEB Plans WPL 2018 2017 2018 2017 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $2.8 $2.9 Current liabilities (0.1 ) (0.1 ) (7.4 ) (6.8 ) Pension and other benefit obligations (154.3 ) (144.7 ) (61.9 ) (67.8 ) Net amounts recognized at December 31 ($154.4 ) ($144.8 ) ($66.5 ) ($71.7 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $223.0 $224.7 $19.9 $23.6 Prior service credit (1.3 ) (1.5 ) (1.1 ) (1.3 ) $221.7 $223.2 $18.8 $22.3 Actuarial gains related to benefit obligations in 2018 for defined benefit pension and OPEB plans were primarily due to increases in the discount rates and the impact of the updated mortality table, partially offset by experience losses. Actuarial losses related to benefit obligations in 2017 for defined benefit pension and OPEB plans were primarily due to decreases in the discount rates. 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 2018 2017 2018 2017 Accumulated benefit obligations $1,139.9 $1,269.0 $206.1 $222.3 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,139.9 1,269.0 206.1 222.3 Fair value of plan assets 808.6 950.7 99.1 111.1 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,175.0 1,303.1 N/A N/A Fair value of plan assets 808.6 950.7 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2018 2017 2018 2017 Accumulated benefit obligations $514.3 $573.1 $82.5 $89.4 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 514.3 573.1 82.5 89.4 Fair value of plan assets 377.8 443.7 66.7 72.9 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 533.9 592.9 N/A N/A Fair value of plan assets 377.8 443.7 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2018 2017 2018 2017 Accumulated benefit obligations $494.8 $548.1 $83.2 $90.4 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 494.8 548.1 83.2 90.4 Fair value of plan assets 352.2 415.0 16.7 18.7 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 506.6 559.8 N/A N/A Fair value of plan assets 352.2 415.0 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 2018 2017 2018 2017 Regulatory assets $38.2 $38.9 $27.7 $28.1 Estimated Future Employer Contributions and Benefit Payments - Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2019 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $33.8 $16.5 $15.6 OPEB plans 9.6 1.9 7.4 (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 2019 2020 2021 2022 2023 2024 - 2028 Defined benefit pension benefits $71.7 $86.1 $74.7 $76.3 $77.8 $388.3 OPEB 19.3 18.1 17.8 17.5 17.0 77.7 $91.0 $104.2 $92.5 $93.8 $94.8 $466.0 IPL 2019 2020 2021 2022 2023 2024 - 2028 Defined benefit pension benefits $33.0 $35.1 $35.1 $36.4 $37.5 $182.9 OPEB 7.3 7.3 7.2 7.1 6.9 31.3 $40.3 $42.4 $42.3 $43.5 $44.4 $214.2 WPL 2019 2020 2021 2022 2023 2024 - 2028 Defined benefit pension benefits $31.4 $31.8 $31.9 $32.0 $32.5 $165.3 OPEB 8.8 7.6 7.4 7.1 6.8 30.7 $40.2 $39.4 $39.3 $39.1 $39.3 $196.0 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 may 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 are not limited to, direct ownership of real estate, oil and gas limited partnerships, securities of the managers’ firms or affiliate firms, and Alliant Energy securities. At December 31, 2018 , 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% 4% Equity securities - U.S. 11 % - 41% 25% Equity securities - international 14 % - 34% 22% Global asset securities 5 % - 15% 9% 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. At December 31, 2018 , 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 % - 46% 24% Equity securities - international 0 % - 34% 2% Fixed income securities 20 % - 100% 73% 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 16 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, 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): 2018 2017 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $36.4 $3.0 $33.4 $— $28.2 $4.5 $23.7 $— Equity securities - U.S. 130.2 130.2 — — 158.3 158.3 — — Equity securities - international 116.0 116.0 — — 137.5 137.5 — — Global asset securities 42.1 42.1 — — 49.4 49.4 — — Fixed income securities 127.8 52.5 75.3 — 135.9 55.8 80.1 — Total assets in fair value hierarchy 452.5 $343.8 $108.7 $— 509.3 $405.5 $103.8 $— Assets measured at net asset value 355.4 441.1 Accrued investment income 1.2 1.0 Due to brokers, net (pending trades with brokers) (0.5 ) (0.7 ) Total pension plan assets $808.6 $950.7 2018 2017 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $17.0 $1.4 $15.6 $— $13.2 $2.2 $11.0 $— Equity securities - U.S. 60.8 60.8 — — 73.9 73.9 — — Equity securities - international 54.2 54.2 — — 64.2 64.2 — — Global asset securities 19.7 19.7 — — 23.0 23.0 — — Fixed income securities 59.7 24.5 35.2 — 63.4 26.0 37.4 — Total assets in fair value hierarchy 211.4 $160.6 $50.8 $— 237.7 $189.3 $48.4 $— Assets measured at net asset value 166.1 205.8 Accrued investment income 0.6 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.3 ) Total pension plan assets $377.8 $443.7 2018 2017 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $15.9 $1.3 $14.6 $— $12.3 $2.0 $10.3 $— Equity securities - U.S. 56.7 56.7 — — 69.1 69.1 — — Equity securities - international 50.5 50.5 — — 60.0 60.0 — — Global asset securities 18.4 18.4 — — 21.6 21.6 — — Fixed income securities 55.6 22.8 32.8 — 59.3 24.3 35.0 — Total assets in fair value hierarchy 197.1 $149.7 $47.4 $— 222.3 $177.0 $45.3 $— Assets measured at net asset value 154.8 192.5 Accrued investment income 0.5 0.5 Due to brokers, net (pending trades with brokers) (0.2 ) (0.3 ) Total pension plan assets $352.2 $415.0 At December 31, the fair values of OPEB plan assets were as follows (in millions): 2018 2017 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $1.4 $1.1 $0.3 $— $1.2 $0.7 $0.5 $— Equity securities - U.S. 3.9 3.9 — — 27.9 27.9 — — Equity securities - international 2.9 2.9 — — 11.4 11.4 — — Global asset securities 0.4 0.4 — — 0.4 0.4 — — Fixed income securities 68.2 67.5 0.7 — 66.6 66.0 0.6 — Total assets in fair value hierarchy 76.8 $75.8 $1.0 $— 107.5 $106.4 $1.1 $— Assets measured at net asset value 22.3 3.6 Total OPEB plan assets $99.1 $111.1 2018 2017 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.7 $0.7 $— $— $0.3 $0.3 $— $— Equity securities - U.S. — — — — 22.3 22.3 — — Equity securities - international — — — — 7.5 7.5 — — Fixed income securities 47.0 47.0 — — 42.8 42.8 — — Total assets in fair value hierarchy 47.7 $47.7 $— $— 72.9 $72.9 $— $— Assets measured at net asset value 19.0 — Total OPEB plan assets $66.7 $72.9 2018 2017 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.1 $0.1 $— $— $0.6 $0.3 $0.3 $— Fixed income securities 16.6 16.6 — — 18.1 18.1 — — Total OPEB plan assets $16.7 $16.7 $— $— $18.7 $18.4 $0.3 $— 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, 2018 and 2017 . 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 11.5% of total assets in the 401(k) savings plans at December 31, 2018 and 2017 , 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 2018 2017 2016 2018 2017 2016 2018 2017 2016 401(k) costs $25.1 $24.8 $23.6 $13.0 $12.8 $12.0 $11.2 $11.1 $10.7 (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, 2018 , performance shares and restricted stock units (performance- and time-vesting) were outstanding under the Amended and Restated OIP, and 6.8 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, 2018 , performance units and restricted units (performance- and time-vesting) were outstanding under the DLIP, and the the amount of nonvested restricted units was not material. 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 2018 2017 2016 2018 2017 2016 2018 2017 2016 Compensation expense $17.0 $15.1 $18.0 $9.4 $8.3 $9.5 $6.9 $6.4 $7.9 Income tax benefits 4.9 6.2 7.4 2.8 3.4 4.0 1.9 2.6 3.2 As of December 31, 2018 , Alliant Energy’s, IPL’s and WPL’s total unrecognized compensation cost related to share-based compensation awards was $4.9 million , $2.7 million and $2.1 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 2018 2017 2016 2018 2017 2016 Nonvested awards, January 1 223,511 257,599 288,430 71,737 93,320 116,412 Granted 74,163 65,350 68,585 19,840 21,558 23,918 Vested (90,806 ) (99,438 ) (98,186 ) (31,910 ) (37,395 ) (42,760 ) Forfeited (3,680 ) — (1,230 ) (1,906 ) (5,746 ) (4,250 ) Nonvested awards, December 31 203,188 223,511 257,599 57,761 71,737 93,320 Granted Awards - Each performance share’s and performance unit’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. 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 2018 2017 2016 2018 2017 2016 2015 Grant 2014 Grant 2013 Grant 2015 Grant 2014 Grant 2013 Grant Performance awards vested 90,806 99,438 98,186 31,910 37,395 42,760 Percentage of target number of performance awards 137.5% 147.5% 165.0% 137.5% 147.5% 165.0% Aggregate payout value (in millions) $5.3 $5.6 $5.1 $1.4 $1.5 $1.7 Payout - cash (in millions) $4.9 $5.1 $2.9 $1.4 $1.5 $1.7 Payout - common stock shares issued 5,078 5,185 22,408 N/A N/A N/A Fair Value of Awards - At December 31, 2018 , Alliant Energy’s common stock closing price was $42.25 . Additio nal i nformation related to fair values of nonvested performance shares and performance units at December 31, 2018 , by year of grant, was as follows: Performance Shares Performance Units 2018 Grant 2017 Grant 2016 Grant 2018 Grant 2017 Grant 2016 Grant Nonvested awards at target 70,483 65,350 67,355 19,196 18,062 20,503 Estimated payout percentage based on performance criteria 85 % 85 % 143 % 85 % 85 % 143 % Fair values of each nonvested award $35.91 $35.91 $60.42 $35.91 $35.91 $60.42 Performance Restricted Stock Units - Payouts of performance restricted stock units under the Amended and Restated OIP are based on the achievement of certain performance targets (currently specified growth of consolidated net 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. 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: 2018 2017 2016 Units Weighted Average Grant Date Fair Value Units Weighted Average Grant Date Fair Value Units Weighted Average Grant Date Fair Value Nonvested units, January 1 132,705 $36.50 67,355 $33.96 — $— Granted 74,163 38.60 65,350 39.12 68,585 33.96 Forfeited (3,680 ) 38.60 — — (1,230 ) 33.90 Nonvested units, December 31 203,188 37.23 132,705 36.50 67,355 33.96 Restricted Stock Units - Payouts of restricted stock units under the Amended and Restated OIP 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. A summary of the restricted stock units activity was as follows: 2018 2017 2016 Nonvested units, January 1 113,749 57,736 — Granted 63,568 56,013 58,790 Forfeited (3,154 ) — (1,054 ) Nonvested units, December 31 174,163 113,749 57,736

Asset Retirement Obligations

Asset Retirement Obligations	12 Months Ended
	Dec. 31, 2018
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, wind farms, ash ponds, active ash landfills, solar generation and above ground storage tanks. Recognized AROs also include legal obligations for the management and final disposition of asbestos and polychlorinated biphenyls. 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 2018 2017 2018 2017 2018 2017 Balance, January 1 $184.5 $195.7 $134.1 $124.7 $50.4 $61.4 Revisions in estimated cash flows (10.1 ) 4.3 (10.1 ) 7.0 — (2.7 ) Liabilities settled (10.4 ) (23.5 ) (9.7 ) (13.1 ) (0.7 ) (10.4 ) Liabilities incurred 7.3 2.0 — 11.7 7.3 — Accretion expense 6.2 6.0 4.0 3.8 2.2 2.1 Balance, December 31 $177.5 $184.5 $118.3 $134.1 $59.2 $50.4
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, wind farms, ash ponds, active ash landfills, solar generation and above ground storage tanks. Recognized AROs also include legal obligations for the management and final disposition of asbestos and polychlorinated biphenyls. 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 2018 2017 2018 2017 2018 2017 Balance, January 1 $184.5 $195.7 $134.1 $124.7 $50.4 $61.4 Revisions in estimated cash flows (10.1 ) 4.3 (10.1 ) 7.0 — (2.7 ) Liabilities settled (10.4 ) (23.5 ) (9.7 ) (13.1 ) (0.7 ) (10.4 ) Liabilities incurred 7.3 2.0 — 11.7 7.3 — Accretion expense 6.2 6.0 4.0 3.8 2.2 2.1 Balance, December 31 $177.5 $184.5 $118.3 $134.1 $59.2 $50.4
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, wind farms, ash ponds, active ash landfills, solar generation and above ground storage tanks. Recognized AROs also include legal obligations for the management and final disposition of asbestos and polychlorinated biphenyls. 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 2018 2017 2018 2017 2018 2017 Balance, January 1 $184.5 $195.7 $134.1 $124.7 $50.4 $61.4 Revisions in estimated cash flows (10.1 ) 4.3 (10.1 ) 7.0 — (2.7 ) Liabilities settled (10.4 ) (23.5 ) (9.7 ) (13.1 ) (0.7 ) (10.4 ) Liabilities incurred 7.3 2.0 — 11.7 7.3 — Accretion expense 6.2 6.0 4.0 3.8 2.2 2.1 Balance, December 31 $177.5 $184.5 $118.3 $134.1 $59.2 $50.4

Derivative Instruments

Derivative Instruments	12 Months Ended
	Dec. 31, 2018
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. 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: 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 swap, options and physical forward contracts (IPL and WPL) 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) Notional Amounts - As of December 31, 2018 , 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): FTRs Natural Gas Coal Diesel Fuel MWhs Years Dths Years Tons Years Gallons Years Alliant Energy 10,399 2019 181,694 2019-2026 10,467 2019-2021 3,024 2019 IPL 5,954 2019 80,150 2019-2026 4,410 2019-2021 — WPL 4,445 2019 101,544 2019-2026 6,057 2019-2021 3,024 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 2018 2017 2018 2017 2018 2017 Current derivative assets $24.6 $21.1 $16.1 $15.8 $8.5 $5.3 Non-current derivative assets 3.7 4.0 1.6 1.3 2.1 2.7 Current derivative liabilities 5.6 18.7 3.1 5.0 2.5 13.7 Non-current derivative liabilities 17.7 23.0 8.1 14.4 9.6 8.6 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, 2018 and 2017 , 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, 2018 and 2017 . 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. 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: 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 swap, options and physical forward contracts (IPL and WPL) 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) Notional Amounts - As of December 31, 2018 , 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): FTRs Natural Gas Coal Diesel Fuel MWhs Years Dths Years Tons Years Gallons Years Alliant Energy 10,399 2019 181,694 2019-2026 10,467 2019-2021 3,024 2019 IPL 5,954 2019 80,150 2019-2026 4,410 2019-2021 — WPL 4,445 2019 101,544 2019-2026 6,057 2019-2021 3,024 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 2018 2017 2018 2017 2018 2017 Current derivative assets $24.6 $21.1 $16.1 $15.8 $8.5 $5.3 Non-current derivative assets 3.7 4.0 1.6 1.3 2.1 2.7 Current derivative liabilities 5.6 18.7 3.1 5.0 2.5 13.7 Non-current derivative liabilities 17.7 23.0 8.1 14.4 9.6 8.6 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, 2018 and 2017 , 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, 2018 and 2017 . 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. 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: 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 swap, options and physical forward contracts (IPL and WPL) 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) Notional Amounts - As of December 31, 2018 , 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): FTRs Natural Gas Coal Diesel Fuel MWhs Years Dths Years Tons Years Gallons Years Alliant Energy 10,399 2019 181,694 2019-2026 10,467 2019-2021 3,024 2019 IPL 5,954 2019 80,150 2019-2026 4,410 2019-2021 — WPL 4,445 2019 101,544 2019-2026 6,057 2019-2021 3,024 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 2018 2017 2018 2017 2018 2017 Current derivative assets $24.6 $21.1 $16.1 $15.8 $8.5 $5.3 Non-current derivative assets 3.7 4.0 1.6 1.3 2.1 2.7 Current derivative liabilities 5.6 18.7 3.1 5.0 2.5 13.7 Non-current derivative liabilities 17.7 23.0 8.1 14.4 9.6 8.6 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, 2018 and 2017 , 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, 2018 and 2017 . 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.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2018
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 - 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 a discounted cash flow methodology using observable data from comparably traded securities with similar credit profiles, and was substantially classified as Level 2. Refer to Note 9(b) for additional information regarding long-term debt. 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 2018 2017 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $28.3 $— $8.9 $19.4 $28.3 $25.1 $— $4.1 $21.0 $25.1 Deferred proceeds 119.4 — — 119.4 119.4 222.1 — — 222.1 222.1 Liabilities and equity: Derivatives 23.3 — 16.1 7.2 23.3 41.7 — 8.5 33.2 41.7 Long-term debt (incl. current maturities) 5,502.8 — 5,858.4 2.4 5,860.8 4,866.3 — 5,444.6 2.9 5,447.5 IPL 2018 2017 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $17.7 $— $4.0 $13.7 $17.7 $17.1 $— $2.0 $15.1 $17.1 Deferred proceeds 119.4 — — 119.4 119.4 222.1 — — 222.1 222.1 Liabilities and equity: Derivatives 11.2 — 6.5 4.7 11.2 19.4 — 2.9 16.5 19.4 Long-term debt (incl. current maturities) 2,552.3 — 2,691.2 — 2,691.2 2,406.0 — 2,665.7 — 2,665.7 WPL 2018 2017 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $10.6 $— $4.9 $5.7 $10.6 $8.0 $— $2.1 $5.9 $8.0 Liabilities and equity: Derivatives 12.1 — 9.6 2.5 12.1 22.3 — 5.6 16.7 22.3 Long-term debt (incl. current maturities) 1,834.9 — 2,043.7 — 2,043.7 1,833.4 — 2,147.9 — 2,147.9 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 2018 2017 2018 2017 Beginning balance, January 1 ($12.2 ) $8.7 $222.1 $211.1 Total net gains (losses) included in changes in net assets (realized/unrealized) 9.1 (32.9 ) — — Transfers out of Level 3 (a) 16.1 12.2 — — Purchases 26.7 28.3 — — Sales (0.5 ) (0.3 ) — — Settlements (b) (27.0 ) (28.2 ) (102.7 ) 11.0 Ending balance, December 31 $12.2 ($12.2 ) $119.4 $222.1 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 $10.7 ($31.0 ) $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2018 2017 2018 2017 Beginning balance, January 1 ($1.4 ) $10.1 $222.1 $211.1 Total net losses included in changes in net assets (realized/unrealized) (0.5 ) (14.8 ) — — Transfers out of Level 3 (a) 11.0 3.1 — — Purchases 22.5 24.6 — — Sales (0.4 ) (0.2 ) — — Settlements (b) (22.2 ) (24.2 ) (102.7 ) 11.0 Ending balance, December 31 $9.0 ($1.4 ) $119.4 $222.1 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 $0.2 ($13.5 ) $— $— WPL Commodity Contract Derivative Assets and (Liabilities), net 2018 2017 Beginning balance, January 1 ($10.8 ) ($1.4 ) Total net gains (losses) included in changes in net assets (realized/unrealized) 9.6 (18.1 ) Transfers out of Level 3 (a) 5.1 9.1 Purchases 4.2 3.7 Sales (0.1 ) (0.1 ) Settlements (4.8 ) (4.0 ) Ending balance, December 31 $3.2 ($10.8 ) 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 $10.5 ($17.5 ) (a) Observable market inputs became available for certain commodity contracts previously classified as Level 3 for transfers out of Level 3. (b) 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, FTR and natural gas 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 2018 $3.2 $9.0 $1.8 $7.2 $1.4 $1.8 2017 (23.5 ) 11.3 (11.5 ) 10.1 (12.0 ) 1.2
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 - 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 a discounted cash flow methodology using observable data from comparably traded securities with similar credit profiles, and was substantially classified as Level 2. Refer to Note 9(b) for additional information regarding long-term debt. 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 2018 2017 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $28.3 $— $8.9 $19.4 $28.3 $25.1 $— $4.1 $21.0 $25.1 Deferred proceeds 119.4 — — 119.4 119.4 222.1 — — 222.1 222.1 Liabilities and equity: Derivatives 23.3 — 16.1 7.2 23.3 41.7 — 8.5 33.2 41.7 Long-term debt (incl. current maturities) 5,502.8 — 5,858.4 2.4 5,860.8 4,866.3 — 5,444.6 2.9 5,447.5 IPL 2018 2017 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $17.7 $— $4.0 $13.7 $17.7 $17.1 $— $2.0 $15.1 $17.1 Deferred proceeds 119.4 — — 119.4 119.4 222.1 — — 222.1 222.1 Liabilities and equity: Derivatives 11.2 — 6.5 4.7 11.2 19.4 — 2.9 16.5 19.4 Long-term debt (incl. current maturities) 2,552.3 — 2,691.2 — 2,691.2 2,406.0 — 2,665.7 — 2,665.7 WPL 2018 2017 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $10.6 $— $4.9 $5.7 $10.6 $8.0 $— $2.1 $5.9 $8.0 Liabilities and equity: Derivatives 12.1 — 9.6 2.5 12.1 22.3 — 5.6 16.7 22.3 Long-term debt (incl. current maturities) 1,834.9 — 2,043.7 — 2,043.7 1,833.4 — 2,147.9 — 2,147.9 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 2018 2017 2018 2017 Beginning balance, January 1 ($12.2 ) $8.7 $222.1 $211.1 Total net gains (losses) included in changes in net assets (realized/unrealized) 9.1 (32.9 ) — — Transfers out of Level 3 (a) 16.1 12.2 — — Purchases 26.7 28.3 — — Sales (0.5 ) (0.3 ) — — Settlements (b) (27.0 ) (28.2 ) (102.7 ) 11.0 Ending balance, December 31 $12.2 ($12.2 ) $119.4 $222.1 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 $10.7 ($31.0 ) $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2018 2017 2018 2017 Beginning balance, January 1 ($1.4 ) $10.1 $222.1 $211.1 Total net losses included in changes in net assets (realized/unrealized) (0.5 ) (14.8 ) — — Transfers out of Level 3 (a) 11.0 3.1 — — Purchases 22.5 24.6 — — Sales (0.4 ) (0.2 ) — — Settlements (b) (22.2 ) (24.2 ) (102.7 ) 11.0 Ending balance, December 31 $9.0 ($1.4 ) $119.4 $222.1 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 $0.2 ($13.5 ) $— $— WPL Commodity Contract Derivative Assets and (Liabilities), net 2018 2017 Beginning balance, January 1 ($10.8 ) ($1.4 ) Total net gains (losses) included in changes in net assets (realized/unrealized) 9.6 (18.1 ) Transfers out of Level 3 (a) 5.1 9.1 Purchases 4.2 3.7 Sales (0.1 ) (0.1 ) Settlements (4.8 ) (4.0 ) Ending balance, December 31 $3.2 ($10.8 ) 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 $10.5 ($17.5 ) (a) Observable market inputs became available for certain commodity contracts previously classified as Level 3 for transfers out of Level 3. (b) 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, FTR and natural gas 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 2018 $3.2 $9.0 $1.8 $7.2 $1.4 $1.8 2017 (23.5 ) 11.3 (11.5 ) 10.1 (12.0 ) 1.2
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 - 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 a discounted cash flow methodology using observable data from comparably traded securities with similar credit profiles, and was substantially classified as Level 2. Refer to Note 9(b) for additional information regarding long-term debt. 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 2018 2017 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $28.3 $— $8.9 $19.4 $28.3 $25.1 $— $4.1 $21.0 $25.1 Deferred proceeds 119.4 — — 119.4 119.4 222.1 — — 222.1 222.1 Liabilities and equity: Derivatives 23.3 — 16.1 7.2 23.3 41.7 — 8.5 33.2 41.7 Long-term debt (incl. current maturities) 5,502.8 — 5,858.4 2.4 5,860.8 4,866.3 — 5,444.6 2.9 5,447.5 IPL 2018 2017 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $17.7 $— $4.0 $13.7 $17.7 $17.1 $— $2.0 $15.1 $17.1 Deferred proceeds 119.4 — — 119.4 119.4 222.1 — — 222.1 222.1 Liabilities and equity: Derivatives 11.2 — 6.5 4.7 11.2 19.4 — 2.9 16.5 19.4 Long-term debt (incl. current maturities) 2,552.3 — 2,691.2 — 2,691.2 2,406.0 — 2,665.7 — 2,665.7 WPL 2018 2017 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $10.6 $— $4.9 $5.7 $10.6 $8.0 $— $2.1 $5.9 $8.0 Liabilities and equity: Derivatives 12.1 — 9.6 2.5 12.1 22.3 — 5.6 16.7 22.3 Long-term debt (incl. current maturities) 1,834.9 — 2,043.7 — 2,043.7 1,833.4 — 2,147.9 — 2,147.9 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 2018 2017 2018 2017 Beginning balance, January 1 ($12.2 ) $8.7 $222.1 $211.1 Total net gains (losses) included in changes in net assets (realized/unrealized) 9.1 (32.9 ) — — Transfers out of Level 3 (a) 16.1 12.2 — — Purchases 26.7 28.3 — — Sales (0.5 ) (0.3 ) — — Settlements (b) (27.0 ) (28.2 ) (102.7 ) 11.0 Ending balance, December 31 $12.2 ($12.2 ) $119.4 $222.1 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 $10.7 ($31.0 ) $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2018 2017 2018 2017 Beginning balance, January 1 ($1.4 ) $10.1 $222.1 $211.1 Total net losses included in changes in net assets (realized/unrealized) (0.5 ) (14.8 ) — — Transfers out of Level 3 (a) 11.0 3.1 — — Purchases 22.5 24.6 — — Sales (0.4 ) (0.2 ) — — Settlements (b) (22.2 ) (24.2 ) (102.7 ) 11.0 Ending balance, December 31 $9.0 ($1.4 ) $119.4 $222.1 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 $0.2 ($13.5 ) $— $— WPL Commodity Contract Derivative Assets and (Liabilities), net 2018 2017 Beginning balance, January 1 ($10.8 ) ($1.4 ) Total net gains (losses) included in changes in net assets (realized/unrealized) 9.6 (18.1 ) Transfers out of Level 3 (a) 5.1 9.1 Purchases 4.2 3.7 Sales (0.1 ) (0.1 ) Settlements (4.8 ) (4.0 ) Ending balance, December 31 $3.2 ($10.8 ) 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 $10.5 ($17.5 ) (a) Observable market inputs became available for certain commodity contracts previously classified as Level 3 for transfers out of Level 3. (b) 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, FTR and natural gas 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 2018 $3.2 $9.0 $1.8 $7.2 $1.4 $1.8 2017 (23.5 ) 11.3 (11.5 ) 10.1 (12.0 ) 1.2

Commitments And Contingencies

Commitments And Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies	COMMITMENTS AND CONTINGENCIES (a) Capital Purchase Commitments - Various contractual obligations contain minimum future commitments related to capital expenditures for certain construction projects. IPL’s projects include the expansion of wind generation. WPL’s projects include West Riverside. At December 31, 2018 , Alliant Energy’s, IPL’s and WPL’s minimum future commitments for these projects were $40 million , $14 million and $26 million , respectively. (b) Other Purchase Commitments - 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 commitments associated with other goods and services. At December 31, 2018 , the related minimum future commitments were as follows (in millions): Alliant Energy 2019 2020 2021 2022 2023 Thereafter Total Purchased power (a) $159 $135 $149 $140 $155 $307 $1,045 Natural gas 254 151 119 94 67 196 881 Coal (b) 102 49 26 9 9 — 195 Other (c) 60 4 3 2 2 — 71 $575 $339 $297 $245 $233 $503 $2,192 IPL 2019 2020 2021 2022 2023 Thereafter Total Purchased power (a) $144 $135 $149 $140 $155 $307 $1,030 Natural gas 124 55 42 33 26 83 363 Coal (b) 52 29 20 9 9 — 119 Other (c) 24 3 3 2 2 — 34 $344 $222 $214 $184 $192 $390 $1,546 WPL 2019 2020 2021 2022 2023 Thereafter Total Purchased power $15 $— $— $— $— $— $15 Natural gas 130 96 77 61 41 113 518 Coal (b) 50 20 6 — — — 76 Other (c) 23 — — — — — 23 $218 $116 $83 $61 $41 $113 $632 (a) Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. In July 2018, IPL entered into an amendment to shorten the term of the DAEC PPA by five years in exchange for a $110 million buyout payment by IPL in September 2020, subject to IUB approval. In December 2018, IPL received an order from the IUB, which was effective in January 2019, approving recovery of the buyout payment. As a result, Alliant Energy’s and IPL’s amounts in the above table do not include the September 2020 buyout payment, and the minimum future commitments reflect IPL’s purchase of capacity and the resulting energy from DAEC through December 2025, the original term of the PPA prior to the amendment. Amounts in the above table for 2021 and beyond relate to the DAEC PPA. (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, 2018 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, 2018 . (c) Legal Proceedings - Alliant Energy, IPL and WPL are involved in 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, 2018 , the present value of the abandonment obligations is estimated at $36 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, 2018 and 2017 . Non-utility Wind Farm 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 $90 million as of December 31, 2018 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, 2018 and 2017 . 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, 2018 and 2017 . The general terms of the indemnifications provided by IPL included a maximum limit of $17 million and expire in October 2020 . (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, 2018 , 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, 2018 , such amounts for WPL were not material. Alliant Energy IPL Range of estimated future costs $11 - $29 $8 - $24 Current and non-current environmental liabilities 15 12 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. 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: Effluent Limitation Guidelines, CCR Rule, and various legislation and EPA regulations to monitor and regulate the emission of GHG, including the CAA. (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. (g) Collective Bargaining Agreements - At December 31, 2018 , employees covered by collective bargaining agreements represented 55% , 63% and 82% of total employees of Alliant Energy, IPL and WPL, respectively. In May 2019, WPL’s collective bargaining agreement with International Brotherhood of Electrical Workers Local 965 expires, representing 26% and 82% of total employees of Alliant Energy and WPL, respectively.
IPL [Member]
Commitments And Contingencies	COMMITMENTS AND CONTINGENCIES (a) Capital Purchase Commitments - Various contractual obligations contain minimum future commitments related to capital expenditures for certain construction projects. IPL’s projects include the expansion of wind generation. WPL’s projects include West Riverside. At December 31, 2018 , Alliant Energy’s, IPL’s and WPL’s minimum future commitments for these projects were $40 million , $14 million and $26 million , respectively. (b) Other Purchase Commitments - 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 commitments associated with other goods and services. At December 31, 2018 , the related minimum future commitments were as follows (in millions): Alliant Energy 2019 2020 2021 2022 2023 Thereafter Total Purchased power (a) $159 $135 $149 $140 $155 $307 $1,045 Natural gas 254 151 119 94 67 196 881 Coal (b) 102 49 26 9 9 — 195 Other (c) 60 4 3 2 2 — 71 $575 $339 $297 $245 $233 $503 $2,192 IPL 2019 2020 2021 2022 2023 Thereafter Total Purchased power (a) $144 $135 $149 $140 $155 $307 $1,030 Natural gas 124 55 42 33 26 83 363 Coal (b) 52 29 20 9 9 — 119 Other (c) 24 3 3 2 2 — 34 $344 $222 $214 $184 $192 $390 $1,546 WPL 2019 2020 2021 2022 2023 Thereafter Total Purchased power $15 $— $— $— $— $— $15 Natural gas 130 96 77 61 41 113 518 Coal (b) 50 20 6 — — — 76 Other (c) 23 — — — — — 23 $218 $116 $83 $61 $41 $113 $632 (a) Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. In July 2018, IPL entered into an amendment to shorten the term of the DAEC PPA by five years in exchange for a $110 million buyout payment by IPL in September 2020, subject to IUB approval. In December 2018, IPL received an order from the IUB, which was effective in January 2019, approving recovery of the buyout payment. As a result, Alliant Energy’s and IPL’s amounts in the above table do not include the September 2020 buyout payment, and the minimum future commitments reflect IPL’s purchase of capacity and the resulting energy from DAEC through December 2025, the original term of the PPA prior to the amendment. Amounts in the above table for 2021 and beyond relate to the DAEC PPA. (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, 2018 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, 2018 . (c) Legal Proceedings - Alliant Energy, IPL and WPL are involved in 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, 2018 , the present value of the abandonment obligations is estimated at $36 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, 2018 and 2017 . Non-utility Wind Farm 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 $90 million as of December 31, 2018 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, 2018 and 2017 . 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, 2018 and 2017 . The general terms of the indemnifications provided by IPL included a maximum limit of $17 million and expire in October 2020 . (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, 2018 , 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, 2018 , such amounts for WPL were not material. Alliant Energy IPL Range of estimated future costs $11 - $29 $8 - $24 Current and non-current environmental liabilities 15 12 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. 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: Effluent Limitation Guidelines, CCR Rule, and various legislation and EPA regulations to monitor and regulate the emission of GHG, including the CAA. (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. (g) Collective Bargaining Agreements - At December 31, 2018 , employees covered by collective bargaining agreements represented 55% , 63% and 82% of total employees of Alliant Energy, IPL and WPL, respectively. In May 2019, WPL’s collective bargaining agreement with International Brotherhood of Electrical Workers Local 965 expires, representing 26% and 82% of total employees of Alliant Energy and WPL, respectively.
WPL [Member]
Commitments And Contingencies	COMMITMENTS AND CONTINGENCIES (a) Capital Purchase Commitments - Various contractual obligations contain minimum future commitments related to capital expenditures for certain construction projects. IPL’s projects include the expansion of wind generation. WPL’s projects include West Riverside. At December 31, 2018 , Alliant Energy’s, IPL’s and WPL’s minimum future commitments for these projects were $40 million , $14 million and $26 million , respectively. (b) Other Purchase Commitments - 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 commitments associated with other goods and services. At December 31, 2018 , the related minimum future commitments were as follows (in millions): Alliant Energy 2019 2020 2021 2022 2023 Thereafter Total Purchased power (a) $159 $135 $149 $140 $155 $307 $1,045 Natural gas 254 151 119 94 67 196 881 Coal (b) 102 49 26 9 9 — 195 Other (c) 60 4 3 2 2 — 71 $575 $339 $297 $245 $233 $503 $2,192 IPL 2019 2020 2021 2022 2023 Thereafter Total Purchased power (a) $144 $135 $149 $140 $155 $307 $1,030 Natural gas 124 55 42 33 26 83 363 Coal (b) 52 29 20 9 9 — 119 Other (c) 24 3 3 2 2 — 34 $344 $222 $214 $184 $192 $390 $1,546 WPL 2019 2020 2021 2022 2023 Thereafter Total Purchased power $15 $— $— $— $— $— $15 Natural gas 130 96 77 61 41 113 518 Coal (b) 50 20 6 — — — 76 Other (c) 23 — — — — — 23 $218 $116 $83 $61 $41 $113 $632 (a) Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. In July 2018, IPL entered into an amendment to shorten the term of the DAEC PPA by five years in exchange for a $110 million buyout payment by IPL in September 2020, subject to IUB approval. In December 2018, IPL received an order from the IUB, which was effective in January 2019, approving recovery of the buyout payment. As a result, Alliant Energy’s and IPL’s amounts in the above table do not include the September 2020 buyout payment, and the minimum future commitments reflect IPL’s purchase of capacity and the resulting energy from DAEC through December 2025, the original term of the PPA prior to the amendment. Amounts in the above table for 2021 and beyond relate to the DAEC PPA. (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, 2018 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, 2018 . (c) Legal Proceedings - Alliant Energy, IPL and WPL are involved in 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, 2018 , 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, 2018 , such amounts for WPL were not material. Alliant Energy IPL Range of estimated future costs $11 - $29 $8 - $24 Current and non-current environmental liabilities 15 12 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. 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: Effluent Limitation Guidelines, CCR Rule, and various legislation and EPA regulations to monitor and regulate the emission of GHG, including the CAA. (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. (g) Collective Bargaining Agreements - At December 31, 2018 , employees covered by collective bargaining agreements represented 55% , 63% and 82% of total employees of Alliant Energy, IPL and WPL, respectively. In May 2019, WPL’s collective bargaining agreement with International Brotherhood of Electrical Workers Local 965 expires, representing 26% and 82% of total employees of Alliant Energy and WPL, respectively.

Segments Of Business

Segments Of Business	12 Months Ended
	Dec. 31, 2018
Segment Reporting Information [Line Items]
Segments Of Business	SEGMENTS OF BUSINESS Alliant Energy - Alliant Energy’s principal businesses as of December 31, 2018 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 Holdings, 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 interest in ATC Holdings, Transportation, a non-utility wind farm, the Sheboygan Falls Energy Facility and other non-utility holdings. 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 Holdings, Utility Non-utility, Alliant Energy 2018 Electric Gas Other Total Parent and Other Consolidated Revenues $3,000.3 $446.6 $48.0 $3,494.9 $39.6 $3,534.5 Depreciation and amortization 457.3 42.0 3.6 502.9 4.0 506.9 Operating income 610.2 53.2 0.3 663.7 30.7 694.4 Interest expense 217.2 29.8 247.0 Equity income from unconsolidated investments, net (0.9 ) — — (0.9 ) (53.7 ) (54.6 ) Income taxes 33.0 14.7 47.7 Net income attributable to Alliant Energy common shareowners 472.1 40.0 512.1 Total assets 12,486.3 1,184.4 893.2 14,563.9 862.1 15,426.0 Investments in equity method subsidiaries 8.1 — — 8.1 413.2 421.3 Construction and acquisition expenditures 1,421.1 146.8 0.4 1,568.3 65.6 1,633.9 ATC Holdings, Utility Non-utility, Alliant Energy 2017 Electric Gas Other Total Parent and Other Consolidated 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) 601.7 47.7 (11.6 ) 637.8 33.4 671.2 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 Holdings, Utility Non-utility, Alliant Energy 2016 Electric Gas Other Total Parent and Other Consolidated 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) 586.5 33.0 (4.7 ) 614.8 (60.7 ) 554.1 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 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): 2018 Electric Gas Other Total Revenues $1,731.1 $266.2 $45.0 $2,042.3 Depreciation and amortization 254.7 25.2 3.6 283.5 Operating income 318.2 28.3 4.3 350.8 Interest expense 119.4 Income tax benefit (3.2 ) Earnings available for common stock 264.0 Total assets 7,219.9 687.5 504.0 8,411.4 Construction and acquisition expenditures 890.6 99.7 0.4 990.7 2017 Electric Gas Other Total 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) 287.3 21.7 (4.9 ) 304.1 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 Revenues $1,569.7 $204.0 $46.7 $1,820.4 Depreciation and amortization 189.4 19.3 2.1 210.8 Operating income 257.8 16.4 3.4 277.6 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 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 interest in ATC in 2016, 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 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): 2018 Electric Gas Other Total Revenues $1,269.2 $180.4 $3.0 $1,452.6 Depreciation and amortization 202.6 16.8 — 219.4 Operating income (loss) 292.0 24.9 (4.0 ) 312.9 Interest expense 97.8 Income taxes 36.2 Earnings available for common stock 208.1 Total assets 5,266.4 496.9 389.2 6,152.5 Construction and acquisition expenditures 530.5 47.1 — 577.6 2017 Electric Gas Other Total Revenues $1,295.8 $174.9 $2.1 $1,472.8 Depreciation and amortization 196.9 16.0 — 212.9 Operating income (loss) 314.4 26.0 (6.7 ) 333.7 Interest expense 93.8 Income taxes 61.9 Earnings available for common stock 186.6 Total assets 4,871.8 471.9 412.8 5,756.5 Construction and acquisition expenditures 592.4 44.5 0.5 637.4 2016 Electric Gas Other Total Revenues $1,305.8 $151.4 $1.9 $1,459.1 Depreciation and amortization 177.6 14.9 — 192.5 Operating income (loss) 328.7 16.6 (8.1 ) 337.2 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 Construction and acquisition expenditures 395.9 45.6 11.5 453.0
IPL [Member]
Segment Reporting Information [Line Items]
Segments Of Business	SEGMENTS OF BUSINESS Alliant Energy - Alliant Energy’s principal businesses as of December 31, 2018 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 Holdings, 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 interest in ATC Holdings, Transportation, a non-utility wind farm, the Sheboygan Falls Energy Facility and other non-utility holdings. 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 Holdings, Utility Non-utility, Alliant Energy 2018 Electric Gas Other Total Parent and Other Consolidated Revenues $3,000.3 $446.6 $48.0 $3,494.9 $39.6 $3,534.5 Depreciation and amortization 457.3 42.0 3.6 502.9 4.0 506.9 Operating income 610.2 53.2 0.3 663.7 30.7 694.4 Interest expense 217.2 29.8 247.0 Equity income from unconsolidated investments, net (0.9 ) — — (0.9 ) (53.7 ) (54.6 ) Income taxes 33.0 14.7 47.7 Net income attributable to Alliant Energy common shareowners 472.1 40.0 512.1 Total assets 12,486.3 1,184.4 893.2 14,563.9 862.1 15,426.0 Investments in equity method subsidiaries 8.1 — — 8.1 413.2 421.3 Construction and acquisition expenditures 1,421.1 146.8 0.4 1,568.3 65.6 1,633.9 ATC Holdings, Utility Non-utility, Alliant Energy 2017 Electric Gas Other Total Parent and Other Consolidated 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) 601.7 47.7 (11.6 ) 637.8 33.4 671.2 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 Holdings, Utility Non-utility, Alliant Energy 2016 Electric Gas Other Total Parent and Other Consolidated 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) 586.5 33.0 (4.7 ) 614.8 (60.7 ) 554.1 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 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): 2018 Electric Gas Other Total Revenues $1,731.1 $266.2 $45.0 $2,042.3 Depreciation and amortization 254.7 25.2 3.6 283.5 Operating income 318.2 28.3 4.3 350.8 Interest expense 119.4 Income tax benefit (3.2 ) Earnings available for common stock 264.0 Total assets 7,219.9 687.5 504.0 8,411.4 Construction and acquisition expenditures 890.6 99.7 0.4 990.7 2017 Electric Gas Other Total 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) 287.3 21.7 (4.9 ) 304.1 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 Revenues $1,569.7 $204.0 $46.7 $1,820.4 Depreciation and amortization 189.4 19.3 2.1 210.8 Operating income 257.8 16.4 3.4 277.6 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 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 interest in ATC in 2016, 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 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): 2018 Electric Gas Other Total Revenues $1,269.2 $180.4 $3.0 $1,452.6 Depreciation and amortization 202.6 16.8 — 219.4 Operating income (loss) 292.0 24.9 (4.0 ) 312.9 Interest expense 97.8 Income taxes 36.2 Earnings available for common stock 208.1 Total assets 5,266.4 496.9 389.2 6,152.5 Construction and acquisition expenditures 530.5 47.1 — 577.6 2017 Electric Gas Other Total Revenues $1,295.8 $174.9 $2.1 $1,472.8 Depreciation and amortization 196.9 16.0 — 212.9 Operating income (loss) 314.4 26.0 (6.7 ) 333.7 Interest expense 93.8 Income taxes 61.9 Earnings available for common stock 186.6 Total assets 4,871.8 471.9 412.8 5,756.5 Construction and acquisition expenditures 592.4 44.5 0.5 637.4 2016 Electric Gas Other Total Revenues $1,305.8 $151.4 $1.9 $1,459.1 Depreciation and amortization 177.6 14.9 — 192.5 Operating income (loss) 328.7 16.6 (8.1 ) 337.2 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 Construction and acquisition expenditures 395.9 45.6 11.5 453.0
WPL [Member]
Segment Reporting Information [Line Items]
Segments Of Business	SEGMENTS OF BUSINESS Alliant Energy - Alliant Energy’s principal businesses as of December 31, 2018 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 Holdings, 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 interest in ATC Holdings, Transportation, a non-utility wind farm, the Sheboygan Falls Energy Facility and other non-utility holdings. 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 Holdings, Utility Non-utility, Alliant Energy 2018 Electric Gas Other Total Parent and Other Consolidated Revenues $3,000.3 $446.6 $48.0 $3,494.9 $39.6 $3,534.5 Depreciation and amortization 457.3 42.0 3.6 502.9 4.0 506.9 Operating income 610.2 53.2 0.3 663.7 30.7 694.4 Interest expense 217.2 29.8 247.0 Equity income from unconsolidated investments, net (0.9 ) — — (0.9 ) (53.7 ) (54.6 ) Income taxes 33.0 14.7 47.7 Net income attributable to Alliant Energy common shareowners 472.1 40.0 512.1 Total assets 12,486.3 1,184.4 893.2 14,563.9 862.1 15,426.0 Investments in equity method subsidiaries 8.1 — — 8.1 413.2 421.3 Construction and acquisition expenditures 1,421.1 146.8 0.4 1,568.3 65.6 1,633.9 ATC Holdings, Utility Non-utility, Alliant Energy 2017 Electric Gas Other Total Parent and Other Consolidated 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) 601.7 47.7 (11.6 ) 637.8 33.4 671.2 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 Holdings, Utility Non-utility, Alliant Energy 2016 Electric Gas Other Total Parent and Other Consolidated 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) 586.5 33.0 (4.7 ) 614.8 (60.7 ) 554.1 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 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): 2018 Electric Gas Other Total Revenues $1,731.1 $266.2 $45.0 $2,042.3 Depreciation and amortization 254.7 25.2 3.6 283.5 Operating income 318.2 28.3 4.3 350.8 Interest expense 119.4 Income tax benefit (3.2 ) Earnings available for common stock 264.0 Total assets 7,219.9 687.5 504.0 8,411.4 Construction and acquisition expenditures 890.6 99.7 0.4 990.7 2017 Electric Gas Other Total 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) 287.3 21.7 (4.9 ) 304.1 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 Revenues $1,569.7 $204.0 $46.7 $1,820.4 Depreciation and amortization 189.4 19.3 2.1 210.8 Operating income 257.8 16.4 3.4 277.6 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 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 interest in ATC in 2016, 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 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): 2018 Electric Gas Other Total Revenues $1,269.2 $180.4 $3.0 $1,452.6 Depreciation and amortization 202.6 16.8 — 219.4 Operating income (loss) 292.0 24.9 (4.0 ) 312.9 Interest expense 97.8 Income taxes 36.2 Earnings available for common stock 208.1 Total assets 5,266.4 496.9 389.2 6,152.5 Construction and acquisition expenditures 530.5 47.1 — 577.6 2017 Electric Gas Other Total Revenues $1,295.8 $174.9 $2.1 $1,472.8 Depreciation and amortization 196.9 16.0 — 212.9 Operating income (loss) 314.4 26.0 (6.7 ) 333.7 Interest expense 93.8 Income taxes 61.9 Earnings available for common stock 186.6 Total assets 4,871.8 471.9 412.8 5,756.5 Construction and acquisition expenditures 592.4 44.5 0.5 637.4 2016 Electric Gas Other Total Revenues $1,305.8 $151.4 $1.9 $1,459.1 Depreciation and amortization 177.6 14.9 — 192.5 Operating income (loss) 328.7 16.6 (8.1 ) 337.2 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 Construction and acquisition expenditures 395.9 45.6 11.5 453.0

Related Parties

Related Parties	12 Months Ended
	Dec. 31, 2018
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 2018 2017 2016 2018 2017 2016 Corporate Services billings $170 $177 $161 $132 $135 $133 Sales credited 48 23 8 28 13 7 Purchases billed 358 364 433 81 115 102 As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2018 2017 IPL $95 $114 WPL 71 61 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): 2018 2017 2016 ATC billings to WPL $106 $105 $110 WPL billings to ATC 11 10 13 As of December 31, 2018 and 2017 , WPL owed ATC net amounts of $8 million and $9 million , respectively. Refer to Note 6 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 2018 2017 2016 2018 2017 2016 Corporate Services billings $170 $177 $161 $132 $135 $133 Sales credited 48 23 8 28 13 7 Purchases billed 358 364 433 81 115 102 As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2018 2017 IPL $95 $114 WPL 71 61 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): 2018 2017 2016 ATC billings to WPL $106 $105 $110 WPL billings to ATC 11 10 13 As of December 31, 2018 and 2017 , WPL owed ATC net amounts of $8 million and $9 million , respectively. Refer to Note 6 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 2018 2017 2016 2018 2017 2016 Corporate Services billings $170 $177 $161 $132 $135 $133 Sales credited 48 23 8 28 13 7 Purchases billed 358 364 433 81 115 102 As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2018 2017 IPL $95 $114 WPL 71 61 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): 2018 2017 2016 ATC billings to WPL $106 $105 $110 WPL billings to ATC 11 10 13 As of December 31, 2018 and 2017 , WPL owed ATC net amounts of $8 million and $9 million , respectively. Refer to Note 6 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, 2018
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. 2018 2017 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions, except per share data) Revenues $916.3 $816.1 $928.6 $873.5 $853.9 $765.3 $906.9 $856.1 Operating income 165.7 151.2 256.1 121.4 147.2 153.7 236.3 134.0 Amounts attributable to Alliant Energy common shareowners: Income from continuing operations, net of tax 120.9 100.4 205.5 85.3 99.0 94.3 168.8 93.8 Income from discontinued operations, net of tax — — — — 1.4 — — — Net income 120.9 100.4 205.5 85.3 100.4 94.3 168.8 93.8 Earnings per weighted average common share attributable to Alliant Energy common shareowners: Income from continuing operations, net of tax 0.52 0.43 0.87 0.36 0.43 0.41 0.73 0.41 Income from discontinued operations, net of tax — — — — 0.01 — — — Net income 0.52 0.43 0.87 0.36 0.44 0.41 0.73 0.41 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. 2018 2017 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Revenues $525.8 $474.8 $547.6 $494.1 $450.5 $420.2 $527.4 $472.2 Operating income 75.6 77.7 143.1 54.4 51.3 68.1 133.8 50.9 Net income 49.3 54.2 129.1 41.6 39.8 45.3 123.0 18.9 Earnings available for common stock 46.7 51.7 126.5 39.1 37.2 42.8 120.4 16.4 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. 2018 2017 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Revenues $381.7 $330.8 $370.7 $369.4 $393.1 $334.8 $370.2 $374.7 Operating income 84.0 63.4 104.2 61.3 88.6 76.2 93.5 75.4 Net income 54.0 39.8 76.3 38.0 45.5 38.1 49.8 53.2 Earnings available for common stock 54.0 39.8 76.3 38.0 45.5 38.1 49.8 53.2
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. 2018 2017 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions, except per share data) Revenues $916.3 $816.1 $928.6 $873.5 $853.9 $765.3 $906.9 $856.1 Operating income 165.7 151.2 256.1 121.4 147.2 153.7 236.3 134.0 Amounts attributable to Alliant Energy common shareowners: Income from continuing operations, net of tax 120.9 100.4 205.5 85.3 99.0 94.3 168.8 93.8 Income from discontinued operations, net of tax — — — — 1.4 — — — Net income 120.9 100.4 205.5 85.3 100.4 94.3 168.8 93.8 Earnings per weighted average common share attributable to Alliant Energy common shareowners: Income from continuing operations, net of tax 0.52 0.43 0.87 0.36 0.43 0.41 0.73 0.41 Income from discontinued operations, net of tax — — — — 0.01 — — — Net income 0.52 0.43 0.87 0.36 0.44 0.41 0.73 0.41 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. 2018 2017 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Revenues $525.8 $474.8 $547.6 $494.1 $450.5 $420.2 $527.4 $472.2 Operating income 75.6 77.7 143.1 54.4 51.3 68.1 133.8 50.9 Net income 49.3 54.2 129.1 41.6 39.8 45.3 123.0 18.9 Earnings available for common stock 46.7 51.7 126.5 39.1 37.2 42.8 120.4 16.4 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. 2018 2017 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Revenues $381.7 $330.8 $370.7 $369.4 $393.1 $334.8 $370.2 $374.7 Operating income 84.0 63.4 104.2 61.3 88.6 76.2 93.5 75.4 Net income 54.0 39.8 76.3 38.0 45.5 38.1 49.8 53.2 Earnings available for common stock 54.0 39.8 76.3 38.0 45.5 38.1 49.8 53.2
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. 2018 2017 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions, except per share data) Revenues $916.3 $816.1 $928.6 $873.5 $853.9 $765.3 $906.9 $856.1 Operating income 165.7 151.2 256.1 121.4 147.2 153.7 236.3 134.0 Amounts attributable to Alliant Energy common shareowners: Income from continuing operations, net of tax 120.9 100.4 205.5 85.3 99.0 94.3 168.8 93.8 Income from discontinued operations, net of tax — — — — 1.4 — — — Net income 120.9 100.4 205.5 85.3 100.4 94.3 168.8 93.8 Earnings per weighted average common share attributable to Alliant Energy common shareowners: Income from continuing operations, net of tax 0.52 0.43 0.87 0.36 0.43 0.41 0.73 0.41 Income from discontinued operations, net of tax — — — — 0.01 — — — Net income 0.52 0.43 0.87 0.36 0.44 0.41 0.73 0.41 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. 2018 2017 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Revenues $525.8 $474.8 $547.6 $494.1 $450.5 $420.2 $527.4 $472.2 Operating income 75.6 77.7 143.1 54.4 51.3 68.1 133.8 50.9 Net income 49.3 54.2 129.1 41.6 39.8 45.3 123.0 18.9 Earnings available for common stock 46.7 51.7 126.5 39.1 37.2 42.8 120.4 16.4 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. 2018 2017 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Revenues $381.7 $330.8 $370.7 $369.4 $393.1 $334.8 $370.2 $374.7 Operating income 84.0 63.4 104.2 61.3 88.6 76.2 93.5 75.4 Net income 54.0 39.8 76.3 38.0 45.5 38.1 49.8 53.2 Earnings available for common stock 54.0 39.8 76.3 38.0 45.5 38.1 49.8 53.2

Condensed Parent Company Financ

Condensed Parent Company Financial Statements	12 Months Ended
	Dec. 31, 2018
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) Year Ended December 31, CONDENSED STATEMENTS OF INCOME 2018 2017 2016 (in millions) Revenues $— $— $1 Operating expenses 5 2 3 Operating loss (5 ) (2 ) (2 ) Other (income) and deductions: Equity earnings from consolidated subsidiaries (523 ) (457 ) (374 ) Interest expense 4 3 3 Other 2 — (2 ) Total other (income) and deductions (517 ) (454 ) (373 ) Income before income taxes 512 452 371 Income tax benefit (1 ) (6 ) (1 ) Net income $513 $458 $372 Refer to accompanying Notes to Condensed Financial Statements. ALLIANT ENERGY CORPORATION (Parent Company Only) December 31, CONDENSED BALANCE SHEETS 2018 2017 (in millions) ASSETS Current assets: Notes receivable from affiliated companies $23 $50 Other 4 7 Total current assets 27 57 Investments: Investments in consolidated subsidiaries 5,518 4,676 Other 1 2 Total investments 5,519 4,678 Other assets 81 78 Total assets $5,627 $4,813 LIABILITIES AND EQUITY Current liabilities: Commercial paper $285 $295 Notes payable to affiliated companies 719 305 Other 21 12 Total current liabilities 1,025 612 Other liabilities 17 20 Common equity: Common stock and additional paid-in capital 2,048 1,848 Retained earnings 2,545 2,344 Accumulated other comprehensive income 2 — Shares in deferred compensation trust (10 ) (11 ) Total common equity 4,585 4,181 Total liabilities and equity $5,627 $4,813 Refer to accompanying Notes to Condensed Financial Statements. ALLIANT ENERGY CORPORATION (Parent Company Only) Year Ended December 31, CONDENSED STATEMENTS OF CASH FLOWS 2018 2017 2016 (in millions) Net cash flows from operating activities $311 $273 $254 Cash flows from (used for) investing activities: Capital contributions to consolidated subsidiaries (625 ) (290 ) (250 ) Capital repayments from consolidated subsidiaries — — 130 Net change in notes receivable from and payable to affiliates 441 54 294 Other — — 10 Net cash flows from (used for) investing activities (184 ) (236 ) 184 Cash flows used for financing activities: Common stock dividends (312 ) (288 ) (267 ) Proceeds from issuance of common stock, net 197 150 27 Payments to retire long-term debt — — (250 ) Net change in commercial paper (10 ) 103 52 Other (2 ) (2 ) — Net cash flows used for financing activities (127 ) (37 ) (438 ) Net increase (decrease) in cash, cash equivalents and restricted cash — — — Cash, cash equivalents and restricted cash at beginning of period — — — Cash, cash equivalents and restricted cash at end of period $— $— $— Supplemental cash flows information: Cash (paid) refunded during the period for: Interest, net of capitalized interest ($4 ) ($3 ) ($3 ) Income taxes, net 5 — (37 ) Refer to accompanying Notes to Condensed Financial 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 2018 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, 2018
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, 2018 $12.0 $21.2 $1.0 $23.7 $10.5 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 IPL (c) Year ended December 31, 2018 $1.3 $20.9 $— $19.1 $3.1 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 WPL Year ended December 31, 2018 $10.7 $0.3 $1.0 $4.6 $7.4 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 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, 2018 $23.0 $1.4 $— $9.5 $14.9 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 IPL Year ended December 31, 2018 $7.6 $0.9 $— $2.1 $6.4 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 WPL Year ended December 31, 2018 $6.4 $0.5 $— $2.3 $4.6 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 (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 Federal Tax Reform. (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 Federal 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, 2018 $12.0 $21.2 $1.0 $23.7 $10.5 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 IPL (c) Year ended December 31, 2018 $1.3 $20.9 $— $19.1 $3.1 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 WPL Year ended December 31, 2018 $10.7 $0.3 $1.0 $4.6 $7.4 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 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, 2018 $23.0 $1.4 $— $9.5 $14.9 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 IPL Year ended December 31, 2018 $7.6 $0.9 $— $2.1 $6.4 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 WPL Year ended December 31, 2018 $6.4 $0.5 $— $2.3 $4.6 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 (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 Federal Tax Reform. (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 Federal 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, 2018 $12.0 $21.2 $1.0 $23.7 $10.5 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 IPL (c) Year ended December 31, 2018 $1.3 $20.9 $— $19.1 $3.1 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 WPL Year ended December 31, 2018 $10.7 $0.3 $1.0 $4.6 $7.4 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 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, 2018 $23.0 $1.4 $— $9.5 $14.9 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 IPL Year ended December 31, 2018 $7.6 $0.9 $— $2.1 $6.4 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 WPL Year ended December 31, 2018 $6.4 $0.5 $— $2.3 $4.6 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 (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 Federal Tax Reform. (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 Federal Tax Reform.

Summary Of Significant Accoun_2

Summary Of Significant Accounting Policies (Policy)	12 Months Ended
	Dec. 31, 2018
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. 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 are related to various warranty claims associated with the sale of RMT, Inc. in 2013, which has 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 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 modifications to the presentation of the components of net periodic benefit costs for defined benefit pension and other postretirement plans in the income statements, and restricted cash and cash receipts on sold receivables in the cash flows statements, as discussed in Note 1(n) .
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 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. 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. Federal Tax Reform repealed corporate federal AMT and allows unutilized AMT credits to be refunded over 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 2018 , 2017 and 2016 .
Cash, Cash Equivalents and Restricted Cash	Cash and cash equivalents include short-term liquid investments that have original maturities of less than 90 days. At December 31, 2018 and 2017 , restricted cash primarily related to deposits with trustees and requirements in Sheboygan Power, LLC’s debt agreement.
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. Property, plant and equipment that is probable of being retired early is classified as plant anticipated to be retired early. 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. Non-utility and Other 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 remaining book value 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 collected from customers. The average rates of depreciation for electric, gas and other properties, consistent with current rate-making practices, were as follows: IPL WPL 2018 2017 2016 2018 2017 2016 Electric - generation 3.6% 3.5% 3.5% 3.6% 3.5% 3.1% Electric - distribution 2.8% 2.4% 2.4% 2.6% 2.6% 2.6% Electric - other 4.7% 4.5% 4.2% 5.7% 6.9% 4.7% Gas 3.2% 3.4% 3.3% 2.5% 2.5% 2.5% Other 5.2% 4.0% 3.9% 5.8% 6.0% 5.9%
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: 2018 2017 2016 IPL (Marshalltown CWIP) N/A 7.8% 7.9% IPL (Wind generation CWIP) 7.5% 7.6% N/A IPL (other CWIP) 7.5% 7.6% 7.7% WPL (retail jurisdiction) 7.7% 7.6% 8.2% WPL (wholesale jurisdiction) 7.2% 6.0% 6.7% In accordance with their respective regulatory commission decisions, 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.
Revenue Recognition	Utility - Revenues from Alliant Energy’s utility business are primarily from electric and gas sales to customers. Utility revenues are recognized over time as services are rendered or commodities are delivered to customers, and include billed and unbilled components. The billed component is based on the reading of customers’ meters, which occurs on a systematic basis throughout each reporting period and represents the fair value of the services provided or commodities delivered. The unbilled component is estimated and recorded at the end of each reporting period based on estimated amounts of energy delivered to customers since the end of each customer’s last billing period. The unbilled revenue is based on estimates of daily system demand volumes, 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. As of December 31, 2018 , the related amounts accrued for IPL and WPL were not material. IPL and WPL participate in bid/offer-based wholesale energy and ancillary services markets operated by MISO. 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 as bulk power sales in “Electric utility 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 over time as services are rendered 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 revenues. Other - Alliant Energy, IPL and WPL do not disclose the value of unsatisfied performance obligations for: (i) contracts with an original expected length of one year or less; and (ii) contracts for which revenue is recognized at the amount to which they have the right to invoice for services performed.
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 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. 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 incurred to fund energy efficiency programs and initiatives that help customers reduce their energy usage 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. 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 and 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 adjustment to 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 record to interest 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, the entity is structured with disproportionate voting rights and substantially all of the entity’s activities are conducted on behalf of the investor with disproportionately fewer voting rights, 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 consolidation of VIEs.
New Accounting Standards	Revenue Recognition - In May 2014, the FASB 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. Alliant Energy, IPL and WPL adopted this standard on January 1, 2018 using the modified retrospective method of adoption, which was applied to contracts with customers that were completed subsequent to January 1, 2018. Alliant Energy, IPL and WPL utilized a portfolio approach upon adoption, which involved evaluating portfolios of contracts with similar characteristics, where the effects of applying the standard were not expected to be materially different than evaluating on an individual contract basis. Upon adoption, there were no cumulative effect adjustments made to the January 1, 2018 retained earnings balances. In addition, prior period amounts have not been restated to reflect the adoption of this standard and continue to be reported under the accounting standards in effect for those periods. Alliant Energy, IPL and WPL did not have a material change in revenue recognition, including the timing and pattern of revenue recognition, as a result of the adoption of this standard. Leases - In February 2016, the FASB 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 adopted this standard on January 1, 2019 using an optional transition approach and there was no cumulative effect adjustment to their balance sheets as of January 1, 2019. Alliant Energy, IPL and WPL expect to recognize increases in assets and liabilities for certain operating leases of approximately $18 million , $11 million and $7 million , respectively, on January 1, 2019. After adoption, prior period amounts will continue to be reported under the accounting standards in effect for those periods. Alliant Energy, IPL and WPL do not expect a material change to their results of operations or cash flows statements. Upon transition to the new standard, Alliant Energy, IPL and WPL elected the land easement transition practical expedient, for which existing land easements that were not previously accounted for as leases under the original accounting standards did not need to be evaluated under the new accounting standard. In addition, Alliant Energy, IPL and WPL evaluated land easements that were previously accounted for as leases and determined that the majority of these land easements relate to joint-use land sites, and do not meet the criteria for leases under the new accounting standard. Therefore, these land easement arrangements are no longer reflected as operating leases effective January 1, 2019. Alliant Energy’s, IPL’s and WPL’s operating leases under the new accounting standard are primarily comprised of leases of space on telecommunication towers and leases of property. Pension and Other Postretirement Benefits Plans - In March 2017, the FASB 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. Only the service cost component of net periodic benefit costs is eligible for capitalization into property, plant and equipment; however, IPL and WPL, as rate-regulated entities, 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. Alliant Energy, IPL and WPL used the actual net periodic benefit costs adjusted for approximately 40% of net periodic benefit costs allocated to capital projects for the retrospective method of adoption for the presentation requirements. The change in presentation resulted in a decrease in “Other operation and maintenance” expenses and an increase in “Other (income) and deductions” in Alliant Energy’s, IPL’s and WPL’s income statements of $17.8 million , $7.2 million and $10.5 million in 2017, and $17.1 million , $6.8 million and $10.2 million in 2016, respectively. Segment operating income (loss) was revised for these presentation requirements. In August 2018, the FASB issued an accounting standard modifying disclosure requirements for employers that sponsor defined benefit pension or OPEB plans. Alliant Energy, IPL and WPL early adopted this standard in the fourth quarter of 2018, which was applied retrospectively. Cash Flows Statements - In August 2016, the FASB issued an accounting standard providing specific guidance on several cash flow classification matters. The accounting standard requires classification of the consideration received for the beneficial interest obtained for transferring accounts receivable from IPL’s sales of accounts receivable program as an investing activity, instead of an operating activity. Alliant Energy, IPL and WPL retrospectively adopted this standard on January 1, 2018, and use a method of presentation that allocates cash flows between operating and investing activities based on daily transactional activity. Alliant Energy and IPL reclassified $461.8 million in 2017 and $466.8 million in 2016 of the related cash received from IPL’s sales of accounts receivable program from operating activities to investing activities based on daily transactional activity. In November 2016, the FASB issued an accounting standard requiring restricted cash to be included within beginning-of-period and end-of-period cash and cash equivalents in the cash flows statements. Alliant Energy, IPL and WPL adopted this standard on January 1, 2018, which was applied retrospectively.
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. 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 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 modifications to the presentation of the components of net periodic benefit costs for defined benefit pension and other postretirement plans in the income statements, and restricted cash and cash receipts on sold receivables in the cash flows statements, as discussed in Note 1(n) .
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 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. 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. Federal Tax Reform repealed corporate federal AMT and allows unutilized AMT credits to be refunded over 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 2018 , 2017 and 2016 .
Cash, Cash Equivalents and Restricted Cash	Cash and cash equivalents include short-term liquid investments that have original maturities of less than 90 days. At December 31, 2018 and 2017 , restricted cash primarily related to deposits with trustees and requirements in Sheboygan Power, LLC’s debt agreement.
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. Property, plant and equipment that is probable of being retired early is classified as plant anticipated to be retired early. 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.
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 collected from customers. The average rates of depreciation for electric, gas and other properties, consistent with current rate-making practices, were as follows: IPL WPL 2018 2017 2016 2018 2017 2016 Electric - generation 3.6% 3.5% 3.5% 3.6% 3.5% 3.1% Electric - distribution 2.8% 2.4% 2.4% 2.6% 2.6% 2.6% Electric - other 4.7% 4.5% 4.2% 5.7% 6.9% 4.7% Gas 3.2% 3.4% 3.3% 2.5% 2.5% 2.5% Other 5.2% 4.0% 3.9% 5.8% 6.0% 5.9%
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: 2018 2017 2016 IPL (Marshalltown CWIP) N/A 7.8% 7.9% IPL (Wind generation CWIP) 7.5% 7.6% N/A IPL (other CWIP) 7.5% 7.6% 7.7% WPL (retail jurisdiction) 7.7% 7.6% 8.2% WPL (wholesale jurisdiction) 7.2% 6.0% 6.7% In accordance with their respective regulatory commission decisions, 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.
Revenue Recognition	Utility - Revenues from Alliant Energy’s utility business are primarily from electric and gas sales to customers. Utility revenues are recognized over time as services are rendered or commodities are delivered to customers, and include billed and unbilled components. The billed component is based on the reading of customers’ meters, which occurs on a systematic basis throughout each reporting period and represents the fair value of the services provided or commodities delivered. The unbilled component is estimated and recorded at the end of each reporting period based on estimated amounts of energy delivered to customers since the end of each customer’s last billing period. The unbilled revenue is based on estimates of daily system demand volumes, 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. As of December 31, 2018 , the related amounts accrued for IPL and WPL were not material. IPL and WPL participate in bid/offer-based wholesale energy and ancillary services markets operated by MISO. 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 as bulk power sales in “Electric utility 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 over time as services are rendered 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 revenues. Other - Alliant Energy, IPL and WPL do not disclose the value of unsatisfied performance obligations for: (i) contracts with an original expected length of one year or less; and (ii) contracts for which revenue is recognized at the amount to which they have the right to invoice for services performed.
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 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. 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 incurred to fund energy efficiency programs and initiatives that help customers reduce their energy usage 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. 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 and 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 adjustment to 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 record to interest 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, the entity is structured with disproportionate voting rights and substantially all of the entity’s activities are conducted on behalf of the investor with disproportionately fewer voting rights, 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 consolidation of VIEs.
New Accounting Standards	Revenue Recognition - In May 2014, the FASB 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. Alliant Energy, IPL and WPL adopted this standard on January 1, 2018 using the modified retrospective method of adoption, which was applied to contracts with customers that were completed subsequent to January 1, 2018. Alliant Energy, IPL and WPL utilized a portfolio approach upon adoption, which involved evaluating portfolios of contracts with similar characteristics, where the effects of applying the standard were not expected to be materially different than evaluating on an individual contract basis. Upon adoption, there were no cumulative effect adjustments made to the January 1, 2018 retained earnings balances. In addition, prior period amounts have not been restated to reflect the adoption of this standard and continue to be reported under the accounting standards in effect for those periods. Alliant Energy, IPL and WPL did not have a material change in revenue recognition, including the timing and pattern of revenue recognition, as a result of the adoption of this standard. Leases - In February 2016, the FASB 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 adopted this standard on January 1, 2019 using an optional transition approach and there was no cumulative effect adjustment to their balance sheets as of January 1, 2019. Alliant Energy, IPL and WPL expect to recognize increases in assets and liabilities for certain operating leases of approximately $18 million , $11 million and $7 million , respectively, on January 1, 2019. After adoption, prior period amounts will continue to be reported under the accounting standards in effect for those periods. Alliant Energy, IPL and WPL do not expect a material change to their results of operations or cash flows statements. Upon transition to the new standard, Alliant Energy, IPL and WPL elected the land easement transition practical expedient, for which existing land easements that were not previously accounted for as leases under the original accounting standards did not need to be evaluated under the new accounting standard. In addition, Alliant Energy, IPL and WPL evaluated land easements that were previously accounted for as leases and determined that the majority of these land easements relate to joint-use land sites, and do not meet the criteria for leases under the new accounting standard. Therefore, these land easement arrangements are no longer reflected as operating leases effective January 1, 2019. Alliant Energy’s, IPL’s and WPL’s operating leases under the new accounting standard are primarily comprised of leases of space on telecommunication towers and leases of property. Pension and Other Postretirement Benefits Plans - In March 2017, the FASB 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. Only the service cost component of net periodic benefit costs is eligible for capitalization into property, plant and equipment; however, IPL and WPL, as rate-regulated entities, 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. Alliant Energy, IPL and WPL used the actual net periodic benefit costs adjusted for approximately 40% of net periodic benefit costs allocated to capital projects for the retrospective method of adoption for the presentation requirements. The change in presentation resulted in a decrease in “Other operation and maintenance” expenses and an increase in “Other (income) and deductions” in Alliant Energy’s, IPL’s and WPL’s income statements of $17.8 million , $7.2 million and $10.5 million in 2017, and $17.1 million , $6.8 million and $10.2 million in 2016, respectively. Segment operating income (loss) was revised for these presentation requirements. In August 2018, the FASB issued an accounting standard modifying disclosure requirements for employers that sponsor defined benefit pension or OPEB plans. Alliant Energy, IPL and WPL early adopted this standard in the fourth quarter of 2018, which was applied retrospectively. Cash Flows Statements - In August 2016, the FASB issued an accounting standard providing specific guidance on several cash flow classification matters. The accounting standard requires classification of the consideration received for the beneficial interest obtained for transferring accounts receivable from IPL’s sales of accounts receivable program as an investing activity, instead of an operating activity. Alliant Energy, IPL and WPL retrospectively adopted this standard on January 1, 2018, and use a method of presentation that allocates cash flows between operating and investing activities based on daily transactional activity. Alliant Energy and IPL reclassified $461.8 million in 2017 and $466.8 million in 2016 of the related cash received from IPL’s sales of accounts receivable program from operating activities to investing activities based on daily transactional activity. In November 2016, the FASB issued an accounting standard requiring restricted cash to be included within beginning-of-period and end-of-period cash and cash equivalents in the cash flows statements. Alliant Energy, IPL and WPL adopted this standard on January 1, 2018, which was applied retrospectively.
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. 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 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 modifications to the presentation of the components of net periodic benefit costs for defined benefit pension and other postretirement plans in the income statements, and restricted cash and cash receipts on sold receivables in the cash flows statements, as discussed in Note 1(n) .
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 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. 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. Federal Tax Reform repealed corporate federal AMT and allows unutilized AMT credits to be refunded over 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 2018 , 2017 and 2016 .
Cash, Cash Equivalents and Restricted Cash	Cash and cash equivalents include short-term liquid investments that have original maturities of less than 90 days. At December 31, 2018 and 2017 , restricted cash primarily related to deposits with trustees and requirements in Sheboygan Power, LLC’s debt agreement.
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. Property, plant and equipment that is probable of being retired early is classified as plant anticipated to be retired early. 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.
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 collected from customers. The average rates of depreciation for electric, gas and other properties, consistent with current rate-making practices, were as follows: IPL WPL 2018 2017 2016 2018 2017 2016 Electric - generation 3.6% 3.5% 3.5% 3.6% 3.5% 3.1% Electric - distribution 2.8% 2.4% 2.4% 2.6% 2.6% 2.6% Electric - other 4.7% 4.5% 4.2% 5.7% 6.9% 4.7% Gas 3.2% 3.4% 3.3% 2.5% 2.5% 2.5% Other 5.2% 4.0% 3.9% 5.8% 6.0% 5.9%
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: 2018 2017 2016 IPL (Marshalltown CWIP) N/A 7.8% 7.9% IPL (Wind generation CWIP) 7.5% 7.6% N/A IPL (other CWIP) 7.5% 7.6% 7.7% WPL (retail jurisdiction) 7.7% 7.6% 8.2% WPL (wholesale jurisdiction) 7.2% 6.0% 6.7% In accordance with their respective regulatory commission decisions, 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.
Revenue Recognition	Utility - Revenues from Alliant Energy’s utility business are primarily from electric and gas sales to customers. Utility revenues are recognized over time as services are rendered or commodities are delivered to customers, and include billed and unbilled components. The billed component is based on the reading of customers’ meters, which occurs on a systematic basis throughout each reporting period and represents the fair value of the services provided or commodities delivered. The unbilled component is estimated and recorded at the end of each reporting period based on estimated amounts of energy delivered to customers since the end of each customer’s last billing period. The unbilled revenue is based on estimates of daily system demand volumes, 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. As of December 31, 2018 , the related amounts accrued for IPL and WPL were not material. IPL and WPL participate in bid/offer-based wholesale energy and ancillary services markets operated by MISO. 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 as bulk power sales in “Electric utility 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 over time as services are rendered 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 revenues. Other - Alliant Energy, IPL and WPL do not disclose the value of unsatisfied performance obligations for: (i) contracts with an original expected length of one year or less; and (ii) contracts for which revenue is recognized at the amount to which they have the right to invoice for services performed.
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 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. 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 incurred to fund energy efficiency programs and initiatives that help customers reduce their energy usage 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. 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 and 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 adjustment to 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 record to interest 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, the entity is structured with disproportionate voting rights and substantially all of the entity’s activities are conducted on behalf of the investor with disproportionately fewer voting rights, 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 consolidation of VIEs.
New Accounting Standards	Revenue Recognition - In May 2014, the FASB 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. Alliant Energy, IPL and WPL adopted this standard on January 1, 2018 using the modified retrospective method of adoption, which was applied to contracts with customers that were completed subsequent to January 1, 2018. Alliant Energy, IPL and WPL utilized a portfolio approach upon adoption, which involved evaluating portfolios of contracts with similar characteristics, where the effects of applying the standard were not expected to be materially different than evaluating on an individual contract basis. Upon adoption, there were no cumulative effect adjustments made to the January 1, 2018 retained earnings balances. In addition, prior period amounts have not been restated to reflect the adoption of this standard and continue to be reported under the accounting standards in effect for those periods. Alliant Energy, IPL and WPL did not have a material change in revenue recognition, including the timing and pattern of revenue recognition, as a result of the adoption of this standard. Leases - In February 2016, the FASB 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 adopted this standard on January 1, 2019 using an optional transition approach and there was no cumulative effect adjustment to their balance sheets as of January 1, 2019. Alliant Energy, IPL and WPL expect to recognize increases in assets and liabilities for certain operating leases of approximately $18 million , $11 million and $7 million , respectively, on January 1, 2019. After adoption, prior period amounts will continue to be reported under the accounting standards in effect for those periods. Alliant Energy, IPL and WPL do not expect a material change to their results of operations or cash flows statements. Upon transition to the new standard, Alliant Energy, IPL and WPL elected the land easement transition practical expedient, for which existing land easements that were not previously accounted for as leases under the original accounting standards did not need to be evaluated under the new accounting standard. In addition, Alliant Energy, IPL and WPL evaluated land easements that were previously accounted for as leases and determined that the majority of these land easements relate to joint-use land sites, and do not meet the criteria for leases under the new accounting standard. Therefore, these land easement arrangements are no longer reflected as operating leases effective January 1, 2019. Alliant Energy’s, IPL’s and WPL’s operating leases under the new accounting standard are primarily comprised of leases of space on telecommunication towers and leases of property. Pension and Other Postretirement Benefits Plans - In March 2017, the FASB 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. Only the service cost component of net periodic benefit costs is eligible for capitalization into property, plant and equipment; however, IPL and WPL, as rate-regulated entities, 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. Alliant Energy, IPL and WPL used the actual net periodic benefit costs adjusted for approximately 40% of net periodic benefit costs allocated to capital projects for the retrospective method of adoption for the presentation requirements. The change in presentation resulted in a decrease in “Other operation and maintenance” expenses and an increase in “Other (income) and deductions” in Alliant Energy’s, IPL’s and WPL’s income statements of $17.8 million , $7.2 million and $10.5 million in 2017, and $17.1 million , $6.8 million and $10.2 million in 2016, respectively. Segment operating income (loss) was revised for these presentation requirements. In August 2018, the FASB issued an accounting standard modifying disclosure requirements for employers that sponsor defined benefit pension or OPEB plans. Alliant Energy, IPL and WPL early adopted this standard in the fourth quarter of 2018, which was applied retrospectively. Cash Flows Statements - In August 2016, the FASB issued an accounting standard providing specific guidance on several cash flow classification matters. The accounting standard requires classification of the consideration received for the beneficial interest obtained for transferring accounts receivable from IPL’s sales of accounts receivable program as an investing activity, instead of an operating activity. Alliant Energy, IPL and WPL retrospectively adopted this standard on January 1, 2018, and use a method of presentation that allocates cash flows between operating and investing activities based on daily transactional activity. Alliant Energy and IPL reclassified $461.8 million in 2017 and $466.8 million in 2016 of the related cash received from IPL’s sales of accounts receivable program from operating activities to investing activities based on daily transactional activity. In November 2016, the FASB issued an accounting standard requiring restricted cash to be included within beginning-of-period and end-of-period cash and cash equivalents in the cash flows statements. Alliant Energy, IPL and WPL adopted this standard on January 1, 2018, which was applied retrospectively.

Summary Of Significant Accoun_3

Summary Of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
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 2018 2017 2016 2018 2017 2016 Electric - generation 3.6% 3.5% 3.5% 3.6% 3.5% 3.1% Electric - distribution 2.8% 2.4% 2.4% 2.6% 2.6% 2.6% Electric - other 4.7% 4.5% 4.2% 5.7% 6.9% 4.7% Gas 3.2% 3.4% 3.3% 2.5% 2.5% 2.5% Other 5.2% 4.0% 3.9% 5.8% 6.0% 5.9%
Schedule of Allowance for Funds Used During Construction Rate	The AFUDC rates, computed in accordance with the prescribed regulatory formula, were as follows: 2018 2017 2016 IPL (Marshalltown CWIP) N/A 7.8% 7.9% IPL (Wind generation CWIP) 7.5% 7.6% N/A IPL (other CWIP) 7.5% 7.6% 7.7% WPL (retail jurisdiction) 7.7% 7.6% 8.2% WPL (wholesale jurisdiction) 7.2% 6.0% 6.7%
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 2018 2017 2016 2018 2017 2016 Electric - generation 3.6% 3.5% 3.5% 3.6% 3.5% 3.1% Electric - distribution 2.8% 2.4% 2.4% 2.6% 2.6% 2.6% Electric - other 4.7% 4.5% 4.2% 5.7% 6.9% 4.7% Gas 3.2% 3.4% 3.3% 2.5% 2.5% 2.5% Other 5.2% 4.0% 3.9% 5.8% 6.0% 5.9%
Schedule of Allowance for Funds Used During Construction Rate	The AFUDC rates, computed in accordance with the prescribed regulatory formula, were as follows: 2018 2017 2016 IPL (Marshalltown CWIP) N/A 7.8% 7.9% IPL (Wind generation CWIP) 7.5% 7.6% N/A IPL (other CWIP) 7.5% 7.6% 7.7% WPL (retail jurisdiction) 7.7% 7.6% 8.2% WPL (wholesale jurisdiction) 7.2% 6.0% 6.7%
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 2018 2017 2016 2018 2017 2016 Electric - generation 3.6% 3.5% 3.5% 3.6% 3.5% 3.1% Electric - distribution 2.8% 2.4% 2.4% 2.6% 2.6% 2.6% Electric - other 4.7% 4.5% 4.2% 5.7% 6.9% 4.7% Gas 3.2% 3.4% 3.3% 2.5% 2.5% 2.5% Other 5.2% 4.0% 3.9% 5.8% 6.0% 5.9%
Schedule of Allowance for Funds Used During Construction Rate	The AFUDC rates, computed in accordance with the prescribed regulatory formula, were as follows: 2018 2017 2016 IPL (Marshalltown CWIP) N/A 7.8% 7.9% IPL (Wind generation CWIP) 7.5% 7.6% N/A IPL (other CWIP) 7.5% 7.6% 7.7% WPL (retail jurisdiction) 7.7% 7.6% 8.2% WPL (wholesale jurisdiction) 7.2% 6.0% 6.7%

Regulatory Matters (Tables)

Regulatory Matters (Tables)	12 Months Ended
	Dec. 31, 2018
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 2018 2017 2018 2017 2018 2017 Tax-related $820.6 $778.2 $783.1 $750.5 $37.5 $27.7 Pension and OPEB costs 542.3 548.0 274.0 274.4 268.3 273.6 EGUs retired early 111.6 63.8 55.4 31.6 56.2 32.2 AROs 110.8 109.3 76.3 72.5 34.5 36.8 Derivatives 28.0 45.3 15.1 21.8 12.9 23.5 Emission allowances 23.6 25.5 23.6 25.5 — — Other 100.4 96.6 51.5 55.3 48.9 41.3 $1,737.3 $1,666.7 $1,279.0 $1,231.6 $458.3 $435.1
Electric Generating Units Retired Early	Details regarding the recovery of the remaining net book value of these EGUs from IPL’s and WPL’s customers are as follows: Entity EGU Retirement Date Regulatory Asset Balance as of Dec. 31, 2018 Recovery Regulatory Approval IPL Sutherland Units 1 and 3 June 2017 $27.6 Return of remaining net book value over 10 years IUB and FERC IPL M.L. Kapp Unit 2 June 2018 27.8 Return of and return on remaining net book value over 10 years Pending with FERC; to be addressed with IUB WPL Nelson Dewey Units 1 and 2 and Edgewater Unit 3 December 2015 27.4 Return of and return on remaining net book value over 10 years PSCW and FERC WPL Edgewater Unit 4 September 2018 28.8 Return of and return on remaining net book value over 10 years PSCW and pending with FERC
Schedule of Regulatory Liabilities	At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2018 2017 2018 2017 2018 2017 Tax-related $890.6 $899.4 $390.1 $399.5 $500.5 $499.9 Cost of removal obligations 401.2 410.0 273.3 274.5 127.9 135.5 Electric transmission cost recovery 104.0 90.4 47.7 26.4 56.3 64.0 WPL earnings sharing mechanism 25.4 8.0 — — 25.4 8.0 Commodity cost recovery 16.8 21.0 11.9 14.6 4.9 6.4 IPL’s tax benefit riders 6.4 25.0 6.4 25.0 — — Other 48.8 43.4 25.5 15.4 23.3 28.0 $1,493.2 $1,497.2 $754.9 $755.4 $738.3 $741.8
Tax Benefit Riders	In 2018 , Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $19 million as follows (in millions): Electric tax benefit rider credits $17 Gas tax benefit rider credits 2 $19
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 2018 2017 2018 2017 2018 2017 Tax-related $820.6 $778.2 $783.1 $750.5 $37.5 $27.7 Pension and OPEB costs 542.3 548.0 274.0 274.4 268.3 273.6 EGUs retired early 111.6 63.8 55.4 31.6 56.2 32.2 AROs 110.8 109.3 76.3 72.5 34.5 36.8 Derivatives 28.0 45.3 15.1 21.8 12.9 23.5 Emission allowances 23.6 25.5 23.6 25.5 — — Other 100.4 96.6 51.5 55.3 48.9 41.3 $1,737.3 $1,666.7 $1,279.0 $1,231.6 $458.3 $435.1
Electric Generating Units Retired Early	Details regarding the recovery of the remaining net book value of these EGUs from IPL’s and WPL’s customers are as follows: Entity EGU Retirement Date Regulatory Asset Balance as of Dec. 31, 2018 Recovery Regulatory Approval IPL Sutherland Units 1 and 3 June 2017 $27.6 Return of remaining net book value over 10 years IUB and FERC IPL M.L. Kapp Unit 2 June 2018 27.8 Return of and return on remaining net book value over 10 years Pending with FERC; to be addressed with IUB WPL Nelson Dewey Units 1 and 2 and Edgewater Unit 3 December 2015 27.4 Return of and return on remaining net book value over 10 years PSCW and FERC WPL Edgewater Unit 4 September 2018 28.8 Return of and return on remaining net book value over 10 years PSCW and pending with FERC
Schedule of Regulatory Liabilities	At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2018 2017 2018 2017 2018 2017 Tax-related $890.6 $899.4 $390.1 $399.5 $500.5 $499.9 Cost of removal obligations 401.2 410.0 273.3 274.5 127.9 135.5 Electric transmission cost recovery 104.0 90.4 47.7 26.4 56.3 64.0 WPL earnings sharing mechanism 25.4 8.0 — — 25.4 8.0 Commodity cost recovery 16.8 21.0 11.9 14.6 4.9 6.4 IPL’s tax benefit riders 6.4 25.0 6.4 25.0 — — Other 48.8 43.4 25.5 15.4 23.3 28.0 $1,493.2 $1,497.2 $754.9 $755.4 $738.3 $741.8
Tax Benefit Riders	In 2018 , Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $19 million as follows (in millions): Electric tax benefit rider credits $17 Gas tax benefit rider credits 2 $19
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 2018 2017 2018 2017 2018 2017 Tax-related $820.6 $778.2 $783.1 $750.5 $37.5 $27.7 Pension and OPEB costs 542.3 548.0 274.0 274.4 268.3 273.6 EGUs retired early 111.6 63.8 55.4 31.6 56.2 32.2 AROs 110.8 109.3 76.3 72.5 34.5 36.8 Derivatives 28.0 45.3 15.1 21.8 12.9 23.5 Emission allowances 23.6 25.5 23.6 25.5 — — Other 100.4 96.6 51.5 55.3 48.9 41.3 $1,737.3 $1,666.7 $1,279.0 $1,231.6 $458.3 $435.1
Electric Generating Units Retired Early	Details regarding the recovery of the remaining net book value of these EGUs from IPL’s and WPL’s customers are as follows: Entity EGU Retirement Date Regulatory Asset Balance as of Dec. 31, 2018 Recovery Regulatory Approval IPL Sutherland Units 1 and 3 June 2017 $27.6 Return of remaining net book value over 10 years IUB and FERC IPL M.L. Kapp Unit 2 June 2018 27.8 Return of and return on remaining net book value over 10 years Pending with FERC; to be addressed with IUB WPL Nelson Dewey Units 1 and 2 and Edgewater Unit 3 December 2015 27.4 Return of and return on remaining net book value over 10 years PSCW and FERC WPL Edgewater Unit 4 September 2018 28.8 Return of and return on remaining net book value over 10 years PSCW and pending with FERC
Schedule of Regulatory Liabilities	At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2018 2017 2018 2017 2018 2017 Tax-related $890.6 $899.4 $390.1 $399.5 $500.5 $499.9 Cost of removal obligations 401.2 410.0 273.3 274.5 127.9 135.5 Electric transmission cost recovery 104.0 90.4 47.7 26.4 56.3 64.0 WPL earnings sharing mechanism 25.4 8.0 — — 25.4 8.0 Commodity cost recovery 16.8 21.0 11.9 14.6 4.9 6.4 IPL’s tax benefit riders 6.4 25.0 6.4 25.0 — — Other 48.8 43.4 25.5 15.4 23.3 28.0 $1,493.2 $1,497.2 $754.9 $755.4 $738.3 $741.8

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	12 Months Ended
	Dec. 31, 2018
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 2018 2017 2018 2017 2018 2017 Utility: Electric plant: Generation in service $6,800.6 $6,655.3 $3,610.4 $3,715.9 $3,190.2 $2,939.4 Distribution in service 5,452.2 5,123.5 3,023.7 2,820.9 2,428.5 2,302.6 Other in service 410.8 425.1 260.4 282.3 150.4 142.8 Anticipated to be retired early (a) — 93.0 — — — 93.0 Total electric plant 12,663.6 12,296.9 6,894.5 6,819.1 5,769.1 5,477.8 Gas plant in service 1,387.6 1,244.0 763.1 654.8 624.5 589.2 Other plant in service 513.2 571.9 322.4 333.4 190.8 238.5 Accumulated depreciation (4,314.6 ) (4,283.1 ) (2,294.7 ) (2,311.0 ) (2,019.9 ) (1,972.1 ) Net plant 10,249.8 9,829.7 5,685.3 5,496.3 4,564.5 4,333.4 Leased Sheboygan Falls Energy Facility, net (b) — — — — 38.1 46.2 Construction work in progress 1,774.8 962.2 1,091.2 424.4 683.6 537.8 Other, net 6.1 6.0 5.0 5.5 1.1 0.5 Total utility 12,030.7 10,797.9 6,781.5 5,926.2 5,287.3 4,917.9 Non-utility and other: Non-utility Generation, net (c) 86.9 90.9 — — — — Corporate Services and other, net (d) 344.8 345.7 — — — — Total non-utility and other 431.7 436.6 — — — — Total property, plant and equipment $12,462.4 $11,234.5 $6,781.5 $5,926.2 $5,287.3 $4,917.9 (a) In 2018, WPL retired Edgewater Unit 4 and reclassified the remaining net book value of this EGU from property, plant and equipment to a regulatory asset on Alliant Energy’s and WPL’s balance sheets. (b) Less accumulated amortization of $82.8 million and $77.6 million for WPL as of December 31, 2018 and 2017 , 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. (c) Less accumulated depreciation of $54.5 million and $50.5 million for Alliant Energy as of December 31, 2018 and 2017 , respectively. (d) Less accumulated depreciation of $167.5 million and $285.6 million for Alliant Energy as of December 31, 2018 and 2017 , 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 2018 2017 2016 2018 2017 2016 2018 2017 2016 Equity $51.4 $33.6 $42.3 $28.6 $21.1 $35.2 $22.8 $12.5 $7.1 Debt 24.2 16.1 20.2 13.6 10.3 16.8 10.6 5.8 3.4 $75.6 $49.7 $62.5 $42.2 $31.4 $52.0 $33.4 $18.3 $10.5
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 2018 2017 2018 2017 2018 2017 Utility: Electric plant: Generation in service $6,800.6 $6,655.3 $3,610.4 $3,715.9 $3,190.2 $2,939.4 Distribution in service 5,452.2 5,123.5 3,023.7 2,820.9 2,428.5 2,302.6 Other in service 410.8 425.1 260.4 282.3 150.4 142.8 Anticipated to be retired early (a) — 93.0 — — — 93.0 Total electric plant 12,663.6 12,296.9 6,894.5 6,819.1 5,769.1 5,477.8 Gas plant in service 1,387.6 1,244.0 763.1 654.8 624.5 589.2 Other plant in service 513.2 571.9 322.4 333.4 190.8 238.5 Accumulated depreciation (4,314.6 ) (4,283.1 ) (2,294.7 ) (2,311.0 ) (2,019.9 ) (1,972.1 ) Net plant 10,249.8 9,829.7 5,685.3 5,496.3 4,564.5 4,333.4 Leased Sheboygan Falls Energy Facility, net (b) — — — — 38.1 46.2 Construction work in progress 1,774.8 962.2 1,091.2 424.4 683.6 537.8 Other, net 6.1 6.0 5.0 5.5 1.1 0.5 Total utility 12,030.7 10,797.9 6,781.5 5,926.2 5,287.3 4,917.9 Non-utility and other: Non-utility Generation, net (c) 86.9 90.9 — — — — Corporate Services and other, net (d) 344.8 345.7 — — — — Total non-utility and other 431.7 436.6 — — — — Total property, plant and equipment $12,462.4 $11,234.5 $6,781.5 $5,926.2 $5,287.3 $4,917.9 (a) In 2018, WPL retired Edgewater Unit 4 and reclassified the remaining net book value of this EGU from property, plant and equipment to a regulatory asset on Alliant Energy’s and WPL’s balance sheets. (b) Less accumulated amortization of $82.8 million and $77.6 million for WPL as of December 31, 2018 and 2017 , 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. (c) Less accumulated depreciation of $54.5 million and $50.5 million for Alliant Energy as of December 31, 2018 and 2017 , respectively. (d) Less accumulated depreciation of $167.5 million and $285.6 million for Alliant Energy as of December 31, 2018 and 2017 , 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 2018 2017 2016 2018 2017 2016 2018 2017 2016 Equity $51.4 $33.6 $42.3 $28.6 $21.1 $35.2 $22.8 $12.5 $7.1 Debt 24.2 16.1 20.2 13.6 10.3 16.8 10.6 5.8 3.4 $75.6 $49.7 $62.5 $42.2 $31.4 $52.0 $33.4 $18.3 $10.5
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 2018 2017 2018 2017 2018 2017 Utility: Electric plant: Generation in service $6,800.6 $6,655.3 $3,610.4 $3,715.9 $3,190.2 $2,939.4 Distribution in service 5,452.2 5,123.5 3,023.7 2,820.9 2,428.5 2,302.6 Other in service 410.8 425.1 260.4 282.3 150.4 142.8 Anticipated to be retired early (a) — 93.0 — — — 93.0 Total electric plant 12,663.6 12,296.9 6,894.5 6,819.1 5,769.1 5,477.8 Gas plant in service 1,387.6 1,244.0 763.1 654.8 624.5 589.2 Other plant in service 513.2 571.9 322.4 333.4 190.8 238.5 Accumulated depreciation (4,314.6 ) (4,283.1 ) (2,294.7 ) (2,311.0 ) (2,019.9 ) (1,972.1 ) Net plant 10,249.8 9,829.7 5,685.3 5,496.3 4,564.5 4,333.4 Leased Sheboygan Falls Energy Facility, net (b) — — — — 38.1 46.2 Construction work in progress 1,774.8 962.2 1,091.2 424.4 683.6 537.8 Other, net 6.1 6.0 5.0 5.5 1.1 0.5 Total utility 12,030.7 10,797.9 6,781.5 5,926.2 5,287.3 4,917.9 Non-utility and other: Non-utility Generation, net (c) 86.9 90.9 — — — — Corporate Services and other, net (d) 344.8 345.7 — — — — Total non-utility and other 431.7 436.6 — — — — Total property, plant and equipment $12,462.4 $11,234.5 $6,781.5 $5,926.2 $5,287.3 $4,917.9 (a) In 2018, WPL retired Edgewater Unit 4 and reclassified the remaining net book value of this EGU from property, plant and equipment to a regulatory asset on Alliant Energy’s and WPL’s balance sheets. (b) Less accumulated amortization of $82.8 million and $77.6 million for WPL as of December 31, 2018 and 2017 , 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. (c) Less accumulated depreciation of $54.5 million and $50.5 million for Alliant Energy as of December 31, 2018 and 2017 , respectively. (d) Less accumulated depreciation of $167.5 million and $285.6 million for Alliant Energy as of December 31, 2018 and 2017 , 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 2018 2017 2016 2018 2017 2016 2018 2017 2016 Equity $51.4 $33.6 $42.3 $28.6 $21.1 $35.2 $22.8 $12.5 $7.1 Debt 24.2 16.1 20.2 13.6 10.3 16.8 10.6 5.8 3.4 $75.6 $49.7 $62.5 $42.2 $31.4 $52.0 $33.4 $18.3 $10.5
Forward Wind Energy Center [Member]
Property, Plant and Equipment [Line Items]
Assets Purchased and Liabilities Assumed	As of the closing date, the estimated fair value of the assets purchased and the liabilities assumed by WPL were as follows (in millions): Property, plant and equipment, net $81 Liabilities 7 Net assets acquired $74
Forward Wind Energy Center [Member] | WPL [Member]
Property, Plant and Equipment [Line Items]
Assets Purchased and Liabilities Assumed	As of the closing date, the estimated fair value of the assets purchased and the liabilities assumed by WPL were as follows (in millions): Property, plant and equipment, net $81 Liabilities 7 Net assets acquired $74
Franklin County Wind Farm [Member]
Property, Plant and Equipment [Line Items]
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
Franklin County Wind Farm [Member] | IPL [Member]
Property, Plant and Equipment [Line Items]
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

Jointly-Owned Electric Utilit_2

Jointly-Owned Electric Utility Plant (Tables)	12 Months Ended
	Dec. 31, 2018
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 EGUs at December 31, 2018 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0 % $507.1 $161.7 $65.3 George Neal Unit 4 25.7 % 187.3 89.8 1.2 George Neal Unit 3 28.0 % 156.7 57.8 1.4 Louisa Unit 1 4.0 % 39.2 23.9 0.8 890.3 333.2 68.7 WPL Columbia Units 1-2 52.5 % 779.4 247.9 9.8 FWEC 42.6 % 119.7 41.2 0.1 West Riverside (a) 91.8 % — — 538.4 899.1 289.1 548.3 Alliant Energy $1,789.4 $622.3 $617.0 (a) In January 2018, certain electric cooperatives, which currently have wholesale power supply agreements with WPL, acquired a partial ownership interest in West Riverside.
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 EGUs at December 31, 2018 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0 % $507.1 $161.7 $65.3 George Neal Unit 4 25.7 % 187.3 89.8 1.2 George Neal Unit 3 28.0 % 156.7 57.8 1.4 Louisa Unit 1 4.0 % 39.2 23.9 0.8 890.3 333.2 68.7 WPL Columbia Units 1-2 52.5 % 779.4 247.9 9.8 FWEC 42.6 % 119.7 41.2 0.1 West Riverside (a) 91.8 % — — 538.4 899.1 289.1 548.3 Alliant Energy $1,789.4 $622.3 $617.0 (a) In January 2018, certain electric cooperatives, which currently have wholesale power supply agreements with WPL, acquired a partial ownership interest in West Riverside.
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 EGUs at December 31, 2018 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0 % $507.1 $161.7 $65.3 George Neal Unit 4 25.7 % 187.3 89.8 1.2 George Neal Unit 3 28.0 % 156.7 57.8 1.4 Louisa Unit 1 4.0 % 39.2 23.9 0.8 890.3 333.2 68.7 WPL Columbia Units 1-2 52.5 % 779.4 247.9 9.8 FWEC 42.6 % 119.7 41.2 0.1 West Riverside (a) 91.8 % — — 538.4 899.1 289.1 548.3 Alliant Energy $1,789.4 $622.3 $617.0 (a) In January 2018, certain electric cooperatives, which currently have wholesale power supply agreements with WPL, acquired a partial ownership interest in West Riverside.

Receivables (Tables)

Receivables (Tables)	12 Months Ended
	Dec. 31, 2018
Accounts 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 2018 2017 2018 2017 2018 2017 Customer $91.0 $103.3 $— $— $84.8 $97.7 Unbilled utility revenues 74.2 85.1 — — 74.2 85.1 Deferred proceeds 119.4 222.1 119.4 222.1 — — Other 76.3 84.3 37.2 44.1 38.5 40.1 Allowance for doubtful accounts (10.5 ) (12.0 ) (3.1 ) (1.3 ) (7.4 ) (10.7 ) $350.4 $482.8 $153.5 $264.9 $190.1 $212.2
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 2018 2017 2016 2018 2017 2016 Outstanding aggregate cash proceeds $116.0 $112.0 $172.0 $53.4 $62.2 $73.2
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): 2018 2017 Customer accounts receivable $140.1 $133.8 Unbilled utility revenues 97.1 112.7 Other receivables 0.1 0.3 Receivables sold to third party 237.3 246.8 Less: cash proceeds 108.0 12.0 Deferred proceeds 129.3 234.8 Less: allowance for doubtful accounts 9.9 12.7 Fair value of deferred proceeds $119.4 $222.1 Outstanding receivables past due $35.5 $44.7
Additional Attributes Of Receivables Sold Under The Agreement	Additional attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2018 2017 2016 Collections $2,076.7 $1,647.1 $1,818.1 Write-offs, net of recoveries 21.3 17.7 4.8
IPL [Member]
Accounts 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 2018 2017 2018 2017 2018 2017 Customer $91.0 $103.3 $— $— $84.8 $97.7 Unbilled utility revenues 74.2 85.1 — — 74.2 85.1 Deferred proceeds 119.4 222.1 119.4 222.1 — — Other 76.3 84.3 37.2 44.1 38.5 40.1 Allowance for doubtful accounts (10.5 ) (12.0 ) (3.1 ) (1.3 ) (7.4 ) (10.7 ) $350.4 $482.8 $153.5 $264.9 $190.1 $212.2
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 2018 2017 2016 2018 2017 2016 Outstanding aggregate cash proceeds $116.0 $112.0 $172.0 $53.4 $62.2 $73.2
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): 2018 2017 Customer accounts receivable $140.1 $133.8 Unbilled utility revenues 97.1 112.7 Other receivables 0.1 0.3 Receivables sold to third party 237.3 246.8 Less: cash proceeds 108.0 12.0 Deferred proceeds 129.3 234.8 Less: allowance for doubtful accounts 9.9 12.7 Fair value of deferred proceeds $119.4 $222.1 Outstanding receivables past due $35.5 $44.7
Additional Attributes Of Receivables Sold Under The Agreement	Additional attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2018 2017 2016 Collections $2,076.7 $1,647.1 $1,818.1 Write-offs, net of recoveries 21.3 17.7 4.8
WPL [Member]
Accounts 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 2018 2017 2018 2017 2018 2017 Customer $91.0 $103.3 $— $— $84.8 $97.7 Unbilled utility revenues 74.2 85.1 — — 74.2 85.1 Deferred proceeds 119.4 222.1 119.4 222.1 — — Other 76.3 84.3 37.2 44.1 38.5 40.1 Allowance for doubtful accounts (10.5 ) (12.0 ) (3.1 ) (1.3 ) (7.4 ) (10.7 ) $350.4 $482.8 $153.5 $264.9 $190.1 $212.2

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 2018 2017 2018 2017 2016 Alliant Energy ATC Holdings (a) 16%-20% $293.6 $274.2 ($38.1 ) ($42.4 ) ($39.1 ) Non-utility wind farm in Oklahoma 50% 105.1 98.3 (15.6 ) (1.8 ) — Other Various 22.6 8.9 (0.9 ) (0.6 ) (0.5 ) $421.3 $381.4 ($54.6 ) ($44.8 ) ($39.6 ) WPL ATC —% $— $— $— $— ($39.1 ) Wisconsin River Power Company 50% 8.1 8.3 (0.9 ) (0.7 ) (0.7 ) $8.1 $8.3 ($0.9 ) ($0.7 ) ($39.8 ) (a) As of December 31, 2018 , Alliant Energy’s interest in ATC Holdings is comprised of a 16% ownership interest in ATC and a 20% ownership interest in ATC Holdco LLC, which are described below. Alliant Energy currently has the ability to exercise significant influence over ATC’s and ATC Holdco LLC’s financial and operating policies through its participation on ATC’s Board of Directors.
Summary Aggregate Financial Information	Summary aggregate financial information from the financial statements of these holdings is as follows (in millions): Alliant Energy WPL 2018 2017 2016 2018 2017 2016 Revenues $724 $741 $658 $8 $8 $658 Operating income 325 374 331 2 4 331 Net income 217 267 232 1 2 234 As of December 31: Current assets 144 104 4 7 Non-current assets 5,498 5,041 19 20 Current liabilities 644 770 1 2 Non-current liabilities 2,315 2,038 6 8 Noncontrolling interest 223 255 — —
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, 2018 2018 2017 2018 2017 2016 Alliant Energy ATC Holdings (a) 16%-20% $293.6 $274.2 ($38.1 ) ($42.4 ) ($39.1 ) Non-utility wind farm in Oklahoma 50% 105.1 98.3 (15.6 ) (1.8 ) — Other Various 22.6 8.9 (0.9 ) (0.6 ) (0.5 ) $421.3 $381.4 ($54.6 ) ($44.8 ) ($39.6 ) WPL ATC —% $— $— $— $— ($39.1 ) Wisconsin River Power Company 50% 8.1 8.3 (0.9 ) (0.7 ) (0.7 ) $8.1 $8.3 ($0.9 ) ($0.7 ) ($39.8 ) (a) As of December 31, 2018 , Alliant Energy’s interest in ATC Holdings is comprised of a 16% ownership interest in ATC and a 20% ownership interest in ATC Holdco LLC, which are described below. Alliant Energy currently has the ability to exercise significant influence over ATC’s and ATC Holdco LLC’s financial and operating policies through its participation on ATC’s Board of Directors.
Summary Aggregate Financial Information	Summary aggregate financial information from the financial statements of these holdings is as follows (in millions): Alliant Energy WPL 2018 2017 2016 2018 2017 2016 Revenues $724 $741 $658 $8 $8 $658 Operating income 325 374 331 2 4 331 Net income 217 267 232 1 2 234 As of December 31: Current assets 144 104 4 7 Non-current assets 5,498 5,041 19 20 Current liabilities 644 770 1 2 Non-current liabilities 2,315 2,038 6 8 Noncontrolling interest 223 255 — —

Common Equity (Tables)

Common Equity (Tables)	12 Months Ended
	Dec. 31, 2018
Schedule of Common Equity [Line Items]
Common Stock Activity	A summary of Alliant Energy’s common stock activity was as follows: 2018 2017 2016 Shares outstanding, January 1 231,348,646 227,673,654 226,918,432 At-the-market offering programs 4,171,013 3,074,931 — Shareowner Direct Plan issuances 576,965 640,723 732,814 Equity-based compensation plans ( Note 13(b) ) 5,078 5,185 22,408 Other (38,423 ) (45,847 ) — Shares outstanding, December 31 236,063,279 231,348,646 227,673,654

Preferred Stock (Tables)

Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2018
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 2018 2017 (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 2018 2017 (in millions) 5.1% $25 8,000,000 8,000,000 $200.0 $200.0

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2018
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 2018 2017 2018 2017 2018 2017 Commercial paper outstanding $441.2 $320.2 $50.4 $— $105.5 $25.0 Commercial paper weighted average interest rates 2.8% 2.0% 2.8% N/A 2.5% 1.5% Available credit facility capacity $558.8 $679.8 $199.6 $250.0 $244.5 $325.0 Alliant Energy IPL WPL For the year ended 2018 2017 2018 2017 2018 2017 Maximum amount outstanding (based on daily outstanding balances) $446.5 $424.4 $50.4 $20.0 $126.0 $271.2 Average amount outstanding (based on daily outstanding balances) $221.4 $294.3 $1.5 $0.5 $36.6 $118.2 Weighted average interest rates 2.2% 1.2% 2.3% 1.3% 2.1% 1.0%
Schedule of Debt-To-Capital Ratios	The required debt-to-capital ratios compared to the actual debt-to-capital ratios at December 31, 2018 were as follows: Alliant Energy IPL WPL Requirement, not to exceed 65% 65% 65% Actual 55% 45% 48%
Schedule of Long-term Debt	Long-term debt, net as of December 31 was as follows (dollars in millions): 2018 2017 Alliant Energy IPL WPL Alliant Energy IPL WPL Senior Debentures (a): 3.65%, due 2020 $200.0 $200.0 $— $200.0 $200.0 $— 3.25%, due 2024 500.0 500.0 — 500.0 500.0 — 3.4%, due 2025 250.0 250.0 — 250.0 250.0 — 5.5%, due 2025 50.0 50.0 — 50.0 50.0 — 4.1%, due 2028 (b) 500.0 500.0 — — — — 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 — 5.875% (Retired in 2018) — — — 100.0 100.0 — 7.25% (Retired in 2018) — — — 250.0 250.0 — 2,575.0 2,575.0 — 2,425.0 2,425.0 — 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 300.0 — 300.0 300.0 — 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 1,850.0 — 1,850.0 Other: AEF term loan credit agreement through April 2020, 3% at December 31, 2018 (with Alliant Energy as guarantor) (c) 300.0 — — — — — Corporate Services 3.45% senior notes, due 2022 (a) 75.0 — — 75.0 — — AEF 3.75% senior notes, due 2023 (with Alliant Energy as guarantor) (a)(d) 400.0 — — — — — AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a)(d) 300.0 — — — — — Sheboygan Power, LLC 5.06% senior secured notes, due 2019 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 44.3 — — 49.6 — — AEF term loan credit agreement, 2% at December 31, 2017 (with Alliant Energy as guarantor) (Retired in 2018) (d) — — — 500.0 — — Other, 1% at December 31, 2018, due 2019 to 2025 2.4 — — 2.9 — — 1,121.7 — — 627.5 — — Subtotal 5,546.7 2,575.0 1,850.0 4,902.5 2,425.0 1,850.0 Current maturities (256.5 ) — (250.0 ) (855.7 ) (350.0 ) — Unamortized debt issuance costs (32.1 ) (17.2 ) (9.6 ) (25.4 ) (14.3 ) (10.5 ) Unamortized debt (discount) and premium, net (11.8 ) (5.5 ) (5.5 ) (10.8 ) (4.7 ) (6.1 ) Long-term debt, net (e) $5,246.3 $2,552.3 $1,584.9 $4,010.6 $2,056.0 $1,833.4 (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 September 2018, IPL issued $500 million of 4.1% senior debentures due 2028. The senior debentures were issued as green bonds, and all of the net proceeds were allocated for the construction and development of wind and solar projects. (c) In April 2018, AEF entered into a $300 million variable-rate term loan credit agreement and used the proceeds from borrowings under this agreement for general corporate purposes. Refer to Note 9(a) for discussion of a financial covenant contained in AEF’s term loan credit agreement. (d) In June 2018, AEF issued $400 million of 3.75% senior notes due 2023 and $300 million of 4.25% senior notes due 2028. The proceeds from the issuances were used by AEF to retire its $500 million and $95 million variable-rate term loan credit agreements expiring in 2018, to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes. (e) There were no significant sinking fund requirements related to the outstanding long-term debt.
Schedule of Debt Maturities	At December 31, 2018 , long-term debt maturities for 2019 through 2023 were as follows (in millions): 2019 2020 2021 2022 2023 IPL $— $200 $— $— $— WPL 250 150 — 250 — Corporate Services — — — 75 — AEF 6 307 8 8 408 Alliant Energy $256 $657 $8 $333 $408
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 2018 2017 2018 2017 2018 2017 Commercial paper outstanding $441.2 $320.2 $50.4 $— $105.5 $25.0 Commercial paper weighted average interest rates 2.8% 2.0% 2.8% N/A 2.5% 1.5% Available credit facility capacity $558.8 $679.8 $199.6 $250.0 $244.5 $325.0 Alliant Energy IPL WPL For the year ended 2018 2017 2018 2017 2018 2017 Maximum amount outstanding (based on daily outstanding balances) $446.5 $424.4 $50.4 $20.0 $126.0 $271.2 Average amount outstanding (based on daily outstanding balances) $221.4 $294.3 $1.5 $0.5 $36.6 $118.2 Weighted average interest rates 2.2% 1.2% 2.3% 1.3% 2.1% 1.0%
Schedule of Debt-To-Capital Ratios	The required debt-to-capital ratios compared to the actual debt-to-capital ratios at December 31, 2018 were as follows: Alliant Energy IPL WPL Requirement, not to exceed 65% 65% 65% Actual 55% 45% 48%
Schedule of Long-term Debt	Long-term debt, net as of December 31 was as follows (dollars in millions): 2018 2017 Alliant Energy IPL WPL Alliant Energy IPL WPL Senior Debentures (a): 3.65%, due 2020 $200.0 $200.0 $— $200.0 $200.0 $— 3.25%, due 2024 500.0 500.0 — 500.0 500.0 — 3.4%, due 2025 250.0 250.0 — 250.0 250.0 — 5.5%, due 2025 50.0 50.0 — 50.0 50.0 — 4.1%, due 2028 (b) 500.0 500.0 — — — — 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 — 5.875% (Retired in 2018) — — — 100.0 100.0 — 7.25% (Retired in 2018) — — — 250.0 250.0 — 2,575.0 2,575.0 — 2,425.0 2,425.0 — 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 300.0 — 300.0 300.0 — 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 1,850.0 — 1,850.0 Other: AEF term loan credit agreement through April 2020, 3% at December 31, 2018 (with Alliant Energy as guarantor) (c) 300.0 — — — — — Corporate Services 3.45% senior notes, due 2022 (a) 75.0 — — 75.0 — — AEF 3.75% senior notes, due 2023 (with Alliant Energy as guarantor) (a)(d) 400.0 — — — — — AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a)(d) 300.0 — — — — — Sheboygan Power, LLC 5.06% senior secured notes, due 2019 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 44.3 — — 49.6 — — AEF term loan credit agreement, 2% at December 31, 2017 (with Alliant Energy as guarantor) (Retired in 2018) (d) — — — 500.0 — — Other, 1% at December 31, 2018, due 2019 to 2025 2.4 — — 2.9 — — 1,121.7 — — 627.5 — — Subtotal 5,546.7 2,575.0 1,850.0 4,902.5 2,425.0 1,850.0 Current maturities (256.5 ) — (250.0 ) (855.7 ) (350.0 ) — Unamortized debt issuance costs (32.1 ) (17.2 ) (9.6 ) (25.4 ) (14.3 ) (10.5 ) Unamortized debt (discount) and premium, net (11.8 ) (5.5 ) (5.5 ) (10.8 ) (4.7 ) (6.1 ) Long-term debt, net (e) $5,246.3 $2,552.3 $1,584.9 $4,010.6 $2,056.0 $1,833.4 (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 September 2018, IPL issued $500 million of 4.1% senior debentures due 2028. The senior debentures were issued as green bonds, and all of the net proceeds were allocated for the construction and development of wind and solar projects. (c) In April 2018, AEF entered into a $300 million variable-rate term loan credit agreement and used the proceeds from borrowings under this agreement for general corporate purposes. Refer to Note 9(a) for discussion of a financial covenant contained in AEF’s term loan credit agreement. (d) In June 2018, AEF issued $400 million of 3.75% senior notes due 2023 and $300 million of 4.25% senior notes due 2028. The proceeds from the issuances were used by AEF to retire its $500 million and $95 million variable-rate term loan credit agreements expiring in 2018, to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes. (e) There were no significant sinking fund requirements related to the outstanding long-term debt.
Schedule of Debt Maturities	At December 31, 2018 , long-term debt maturities for 2019 through 2023 were as follows (in millions): 2019 2020 2021 2022 2023 IPL $— $200 $— $— $— WPL 250 150 — 250 — Corporate Services — — — 75 — AEF 6 307 8 8 408 Alliant Energy $256 $657 $8 $333 $408
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 2018 2017 2018 2017 2018 2017 Commercial paper outstanding $441.2 $320.2 $50.4 $— $105.5 $25.0 Commercial paper weighted average interest rates 2.8% 2.0% 2.8% N/A 2.5% 1.5% Available credit facility capacity $558.8 $679.8 $199.6 $250.0 $244.5 $325.0 Alliant Energy IPL WPL For the year ended 2018 2017 2018 2017 2018 2017 Maximum amount outstanding (based on daily outstanding balances) $446.5 $424.4 $50.4 $20.0 $126.0 $271.2 Average amount outstanding (based on daily outstanding balances) $221.4 $294.3 $1.5 $0.5 $36.6 $118.2 Weighted average interest rates 2.2% 1.2% 2.3% 1.3% 2.1% 1.0%
Schedule of Debt-To-Capital Ratios	The required debt-to-capital ratios compared to the actual debt-to-capital ratios at December 31, 2018 were as follows: Alliant Energy IPL WPL Requirement, not to exceed 65% 65% 65% Actual 55% 45% 48%
Schedule of Long-term Debt	Long-term debt, net as of December 31 was as follows (dollars in millions): 2018 2017 Alliant Energy IPL WPL Alliant Energy IPL WPL Senior Debentures (a): 3.65%, due 2020 $200.0 $200.0 $— $200.0 $200.0 $— 3.25%, due 2024 500.0 500.0 — 500.0 500.0 — 3.4%, due 2025 250.0 250.0 — 250.0 250.0 — 5.5%, due 2025 50.0 50.0 — 50.0 50.0 — 4.1%, due 2028 (b) 500.0 500.0 — — — — 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 — 5.875% (Retired in 2018) — — — 100.0 100.0 — 7.25% (Retired in 2018) — — — 250.0 250.0 — 2,575.0 2,575.0 — 2,425.0 2,425.0 — 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 300.0 — 300.0 300.0 — 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 1,850.0 — 1,850.0 Other: AEF term loan credit agreement through April 2020, 3% at December 31, 2018 (with Alliant Energy as guarantor) (c) 300.0 — — — — — Corporate Services 3.45% senior notes, due 2022 (a) 75.0 — — 75.0 — — AEF 3.75% senior notes, due 2023 (with Alliant Energy as guarantor) (a)(d) 400.0 — — — — — AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a)(d) 300.0 — — — — — Sheboygan Power, LLC 5.06% senior secured notes, due 2019 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 44.3 — — 49.6 — — AEF term loan credit agreement, 2% at December 31, 2017 (with Alliant Energy as guarantor) (Retired in 2018) (d) — — — 500.0 — — Other, 1% at December 31, 2018, due 2019 to 2025 2.4 — — 2.9 — — 1,121.7 — — 627.5 — — Subtotal 5,546.7 2,575.0 1,850.0 4,902.5 2,425.0 1,850.0 Current maturities (256.5 ) — (250.0 ) (855.7 ) (350.0 ) — Unamortized debt issuance costs (32.1 ) (17.2 ) (9.6 ) (25.4 ) (14.3 ) (10.5 ) Unamortized debt (discount) and premium, net (11.8 ) (5.5 ) (5.5 ) (10.8 ) (4.7 ) (6.1 ) Long-term debt, net (e) $5,246.3 $2,552.3 $1,584.9 $4,010.6 $2,056.0 $1,833.4 (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 September 2018, IPL issued $500 million of 4.1% senior debentures due 2028. The senior debentures were issued as green bonds, and all of the net proceeds were allocated for the construction and development of wind and solar projects. (c) In April 2018, AEF entered into a $300 million variable-rate term loan credit agreement and used the proceeds from borrowings under this agreement for general corporate purposes. Refer to Note 9(a) for discussion of a financial covenant contained in AEF’s term loan credit agreement. (d) In June 2018, AEF issued $400 million of 3.75% senior notes due 2023 and $300 million of 4.25% senior notes due 2028. The proceeds from the issuances were used by AEF to retire its $500 million and $95 million variable-rate term loan credit agreements expiring in 2018, to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes. (e) There were no significant sinking fund requirements related to the outstanding long-term debt.
Schedule of Debt Maturities	At December 31, 2018 , long-term debt maturities for 2019 through 2023 were as follows (in millions): 2019 2020 2021 2022 2023 IPL $— $200 $— $— $— WPL 250 150 — 250 — Corporate Services — — — 75 — AEF 6 307 8 8 408 Alliant Energy $256 $657 $8 $333 $408

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2018
Schedule of Future Minimum Rental Payments for Operating Leases	At December 31, 2018 , future minimum operating lease payments, excluding contingent rentals, were as follows (in millions): 2019 2020 2021 2022 2023 Thereafter Total Alliant Energy $5 $5 $3 $3 $2 $12 $30 IPL 3 2 2 2 2 12 23 WPL 2 3 1 — — — 6
IPL [Member]
Schedule of Future Minimum Rental Payments for Operating Leases	At December 31, 2018 , future minimum operating lease payments, excluding contingent rentals, were as follows (in millions): 2019 2020 2021 2022 2023 Thereafter Total Alliant Energy $5 $5 $3 $3 $2 $12 $30 IPL 3 2 2 2 2 12 23 WPL 2 3 1 — — — 6
WPL [Member]
Schedule of Future Minimum Rental Payments for Operating Leases	At December 31, 2018 , future minimum operating lease payments, excluding contingent rentals, were as follows (in millions): 2019 2020 2021 2022 2023 Thereafter Total Alliant Energy $5 $5 $3 $3 $2 $12 $30 IPL 3 2 2 2 2 12 23 WPL 2 3 1 — — — 6
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): 2018 2017 2016 Interest expense $8 $9 $9 Depreciation and amortization 6 6 6 $14 $15 $15
Schedule of Future Minimum Lease Payments for Capital Leases	At December 31, 2018 , WPL’s estimated future minimum capital lease payments for the Sheboygan Falls Energy Facility were as follows (in millions): 2019 2020 2021 2022 2023 Thereafter Total Less: amount representing interest Present value of minimum capital lease payments Sheboygan Falls Energy Facility $15 $15 $15 $15 $15 $19 $94 $26 $68

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax [Line Items]
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 2018 2017 2016 2018 2017 2016 2018 2017 2016 Current tax expense (benefit): Federal ($1.0 ) ($41.0 ) $1.8 $14.9 ($27.9 ) ($12.8 ) ($9.2 ) $5.5 ($22.3 ) State (5.1 ) 8.5 17.2 (7.1 ) 1.6 15.5 (4.4 ) 2.5 1.1 IPL’s tax benefit riders (13.2 ) (40.4 ) (44.2 ) (13.2 ) (40.4 ) (44.2 ) — — — Deferred tax expense (benefit): Federal 67.9 159.5 112.8 9.5 72.5 59.1 43.8 55.0 112.3 State 29.8 12.3 4.9 7.3 (2.2 ) (9.0 ) 22.1 16.6 20.8 Production tax credits (29.5 ) (31.1 ) (31.8 ) (14.0 ) (14.1 ) (14.0 ) (15.5 ) (17.0 ) (17.8 ) Investment tax credits (1.2 ) (1.1 ) (1.3 ) (0.6 ) (0.4 ) (0.5 ) (0.6 ) (0.7 ) (0.8 ) $47.7 $66.7 $59.4 ($3.2 ) ($10.9 ) ($5.9 ) $36.2 $61.9 $93.3
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 2018 2017 2016 2018 2017 2016 2018 2017 2016 Statutory federal income tax rate 21.0 % 35.0 % 35.0 % 21.0 % 35.0 % 35.0 % 21.0 % 35.0 % 35.0 % State income taxes, net of federal benefits 7.0 5.5 5.4 7.7 6.5 6.4 6.2 5.1 5.1 Effect of rate-making on property-related differences (7.6 ) (8.5 ) (8.5 ) (14.0 ) (19.1 ) (16.2 ) (2.3 ) (1.7 ) (0.7 ) Production tax credits (5.2 ) (6.1 ) (7.2 ) (5.2 ) (6.7 ) (6.3 ) (6.4 ) (7.1 ) (6.2 ) IPL’s tax benefit riders (2.3 ) (7.6 ) (10.0 ) (4.9 ) (18.7 ) (20.1 ) — — — Adjustment for prior period taxes (2.3 ) (1.5 ) (0.8 ) (4.8 ) (3.4 ) (1.2 ) (0.2 ) — (0.1 ) Federal Tax Reform adjustments (1.0 ) (3.4 ) — (0.4 ) 1.7 — (2.3 ) (5.8 ) — Other items, net (1.2 ) (0.9 ) (0.5 ) (0.6 ) (0.3 ) (0.3 ) (1.2 ) (0.6 ) (0.5 ) Overall income tax rate 8.4 % 12.5 % 13.4 % (1.2 %) (5.0 %) (2.7 %) 14.8 % 24.9 % 32.6 %
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 2018 2017 2018 2017 2018 2017 Deferred tax liabilities: Property $1,975.5 $1,852.7 $1,158.4 $1,102.6 $735.2 $674.2 ATC Holdings 102.4 86.4 — — — — Other 80.5 75.9 69.4 63.4 34.4 36.5 Total deferred tax liabilities 2,158.4 2,015.0 1,227.8 1,166.0 769.6 710.7 Deferred tax assets: Federal credit carryforwards 299.1 260.7 133.2 113.1 147.4 131.0 Net operating losses carryforwards - federal 158.6 174.4 114.1 107.4 26.4 43.7 Net operating losses carryforwards - state 47.0 41.3 0.9 0.9 0.5 0.2 Other 59.8 69.2 22.8 34.5 14.1 14.7 Subtotal deferred tax assets 564.5 545.6 271.0 255.9 188.4 189.6 Valuation allowances (9.2 ) (9.0 ) (0.5 ) (0.6 ) (0.8 ) (1.3 ) Total deferred tax assets 555.3 536.6 270.5 255.3 187.6 188.3 Total deferred tax liabilities, net $1,603.1 $1,478.4 $957.3 $910.7 $582.0 $522.4
Summary Of Tax Credit Carryforwards	At December 31, 2018 , carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2031-2037 $766 $551 $126 State net operating losses 2018-2038 792 13 8 Federal tax credits 2022-2038 299 133 147
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) 2015 - 2017 Consolidated Iowa income tax returns (b) 2015 - 2017 Wisconsin combined tax returns (c) 2014 - 2017 (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 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 2018 2017 2016 2018 2017 2016 2018 2017 2016 Current tax expense (benefit): Federal ($1.0 ) ($41.0 ) $1.8 $14.9 ($27.9 ) ($12.8 ) ($9.2 ) $5.5 ($22.3 ) State (5.1 ) 8.5 17.2 (7.1 ) 1.6 15.5 (4.4 ) 2.5 1.1 IPL’s tax benefit riders (13.2 ) (40.4 ) (44.2 ) (13.2 ) (40.4 ) (44.2 ) — — — Deferred tax expense (benefit): Federal 67.9 159.5 112.8 9.5 72.5 59.1 43.8 55.0 112.3 State 29.8 12.3 4.9 7.3 (2.2 ) (9.0 ) 22.1 16.6 20.8 Production tax credits (29.5 ) (31.1 ) (31.8 ) (14.0 ) (14.1 ) (14.0 ) (15.5 ) (17.0 ) (17.8 ) Investment tax credits (1.2 ) (1.1 ) (1.3 ) (0.6 ) (0.4 ) (0.5 ) (0.6 ) (0.7 ) (0.8 ) $47.7 $66.7 $59.4 ($3.2 ) ($10.9 ) ($5.9 ) $36.2 $61.9 $93.3
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 2018 2017 2016 2018 2017 2016 2018 2017 2016 Statutory federal income tax rate 21.0 % 35.0 % 35.0 % 21.0 % 35.0 % 35.0 % 21.0 % 35.0 % 35.0 % State income taxes, net of federal benefits 7.0 5.5 5.4 7.7 6.5 6.4 6.2 5.1 5.1 Effect of rate-making on property-related differences (7.6 ) (8.5 ) (8.5 ) (14.0 ) (19.1 ) (16.2 ) (2.3 ) (1.7 ) (0.7 ) Production tax credits (5.2 ) (6.1 ) (7.2 ) (5.2 ) (6.7 ) (6.3 ) (6.4 ) (7.1 ) (6.2 ) IPL’s tax benefit riders (2.3 ) (7.6 ) (10.0 ) (4.9 ) (18.7 ) (20.1 ) — — — Adjustment for prior period taxes (2.3 ) (1.5 ) (0.8 ) (4.8 ) (3.4 ) (1.2 ) (0.2 ) — (0.1 ) Federal Tax Reform adjustments (1.0 ) (3.4 ) — (0.4 ) 1.7 — (2.3 ) (5.8 ) — Other items, net (1.2 ) (0.9 ) (0.5 ) (0.6 ) (0.3 ) (0.3 ) (1.2 ) (0.6 ) (0.5 ) Overall income tax rate 8.4 % 12.5 % 13.4 % (1.2 %) (5.0 %) (2.7 %) 14.8 % 24.9 % 32.6 %
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 2018 2017 2018 2017 2018 2017 Deferred tax liabilities: Property $1,975.5 $1,852.7 $1,158.4 $1,102.6 $735.2 $674.2 ATC Holdings 102.4 86.4 — — — — Other 80.5 75.9 69.4 63.4 34.4 36.5 Total deferred tax liabilities 2,158.4 2,015.0 1,227.8 1,166.0 769.6 710.7 Deferred tax assets: Federal credit carryforwards 299.1 260.7 133.2 113.1 147.4 131.0 Net operating losses carryforwards - federal 158.6 174.4 114.1 107.4 26.4 43.7 Net operating losses carryforwards - state 47.0 41.3 0.9 0.9 0.5 0.2 Other 59.8 69.2 22.8 34.5 14.1 14.7 Subtotal deferred tax assets 564.5 545.6 271.0 255.9 188.4 189.6 Valuation allowances (9.2 ) (9.0 ) (0.5 ) (0.6 ) (0.8 ) (1.3 ) Total deferred tax assets 555.3 536.6 270.5 255.3 187.6 188.3 Total deferred tax liabilities, net $1,603.1 $1,478.4 $957.3 $910.7 $582.0 $522.4
Summary Of Tax Credit Carryforwards	At December 31, 2018 , carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2031-2037 $766 $551 $126 State net operating losses 2018-2038 792 13 8 Federal tax credits 2022-2038 299 133 147
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) 2015 - 2017 Consolidated Iowa income tax returns (b) 2015 - 2017 Wisconsin combined tax returns (c) 2014 - 2017 (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 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 2018 2017 2016 2018 2017 2016 2018 2017 2016 Current tax expense (benefit): Federal ($1.0 ) ($41.0 ) $1.8 $14.9 ($27.9 ) ($12.8 ) ($9.2 ) $5.5 ($22.3 ) State (5.1 ) 8.5 17.2 (7.1 ) 1.6 15.5 (4.4 ) 2.5 1.1 IPL’s tax benefit riders (13.2 ) (40.4 ) (44.2 ) (13.2 ) (40.4 ) (44.2 ) — — — Deferred tax expense (benefit): Federal 67.9 159.5 112.8 9.5 72.5 59.1 43.8 55.0 112.3 State 29.8 12.3 4.9 7.3 (2.2 ) (9.0 ) 22.1 16.6 20.8 Production tax credits (29.5 ) (31.1 ) (31.8 ) (14.0 ) (14.1 ) (14.0 ) (15.5 ) (17.0 ) (17.8 ) Investment tax credits (1.2 ) (1.1 ) (1.3 ) (0.6 ) (0.4 ) (0.5 ) (0.6 ) (0.7 ) (0.8 ) $47.7 $66.7 $59.4 ($3.2 ) ($10.9 ) ($5.9 ) $36.2 $61.9 $93.3
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 2018 2017 2016 2018 2017 2016 2018 2017 2016 Statutory federal income tax rate 21.0 % 35.0 % 35.0 % 21.0 % 35.0 % 35.0 % 21.0 % 35.0 % 35.0 % State income taxes, net of federal benefits 7.0 5.5 5.4 7.7 6.5 6.4 6.2 5.1 5.1 Effect of rate-making on property-related differences (7.6 ) (8.5 ) (8.5 ) (14.0 ) (19.1 ) (16.2 ) (2.3 ) (1.7 ) (0.7 ) Production tax credits (5.2 ) (6.1 ) (7.2 ) (5.2 ) (6.7 ) (6.3 ) (6.4 ) (7.1 ) (6.2 ) IPL’s tax benefit riders (2.3 ) (7.6 ) (10.0 ) (4.9 ) (18.7 ) (20.1 ) — — — Adjustment for prior period taxes (2.3 ) (1.5 ) (0.8 ) (4.8 ) (3.4 ) (1.2 ) (0.2 ) — (0.1 ) Federal Tax Reform adjustments (1.0 ) (3.4 ) — (0.4 ) 1.7 — (2.3 ) (5.8 ) — Other items, net (1.2 ) (0.9 ) (0.5 ) (0.6 ) (0.3 ) (0.3 ) (1.2 ) (0.6 ) (0.5 ) Overall income tax rate 8.4 % 12.5 % 13.4 % (1.2 %) (5.0 %) (2.7 %) 14.8 % 24.9 % 32.6 %
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 2018 2017 2018 2017 2018 2017 Deferred tax liabilities: Property $1,975.5 $1,852.7 $1,158.4 $1,102.6 $735.2 $674.2 ATC Holdings 102.4 86.4 — — — — Other 80.5 75.9 69.4 63.4 34.4 36.5 Total deferred tax liabilities 2,158.4 2,015.0 1,227.8 1,166.0 769.6 710.7 Deferred tax assets: Federal credit carryforwards 299.1 260.7 133.2 113.1 147.4 131.0 Net operating losses carryforwards - federal 158.6 174.4 114.1 107.4 26.4 43.7 Net operating losses carryforwards - state 47.0 41.3 0.9 0.9 0.5 0.2 Other 59.8 69.2 22.8 34.5 14.1 14.7 Subtotal deferred tax assets 564.5 545.6 271.0 255.9 188.4 189.6 Valuation allowances (9.2 ) (9.0 ) (0.5 ) (0.6 ) (0.8 ) (1.3 ) Total deferred tax assets 555.3 536.6 270.5 255.3 187.6 188.3 Total deferred tax liabilities, net $1,603.1 $1,478.4 $957.3 $910.7 $582.0 $522.4
Summary Of Tax Credit Carryforwards	At December 31, 2018 , carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2031-2037 $766 $551 $126 State net operating losses 2018-2038 792 13 8 Federal tax credits 2022-2038 299 133 147
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) 2015 - 2017 Consolidated Iowa income tax returns (b) 2015 - 2017 Wisconsin combined tax returns (c) 2014 - 2017 (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.

Revenues (Tables)

Revenues (Tables)	12 Months Ended
	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Disaggregation of Revenues	Disaggregation of revenues from contracts with customers, which correlates to revenues for each reportable segment, was as follows (in millions): Alliant Energy IPL WPL 2018 2017 2016 2018 2017 2016 2018 2017 2016 Electric Utility: Retail - residential $1,063.4 $1,006.2 $1,001.1 $590.6 $535.6 $536.7 $472.8 $470.6 $464.4 Retail - commercial 735.6 710.3 712.6 486.8 452.7 445.4 248.8 257.6 267.2 Retail - industrial 888.8 853.1 851.1 500.8 459.7 460.4 388.0 393.4 390.7 Wholesale 188.4 238.4 256.6 71.2 95.5 94.2 117.2 142.9 162.4 Bulk power and other 124.1 86.7 54.1 81.7 55.4 33.0 42.4 31.3 21.1 Total Electric Utility 3,000.3 2,894.7 2,875.5 1,731.1 1,598.9 1,569.7 1,269.2 1,295.8 1,305.8 Gas Utility: Retail - residential 254.4 224.7 197.6 152.3 123.2 110.6 102.1 101.5 87.0 Retail - commercial 133.0 123.2 109.6 75.9 67.9 61.9 57.1 55.3 47.7 Retail - industrial 14.9 16.7 15.2 10.2 11.1 10.6 4.7 5.6 4.6 Transportation/other 44.3 36.3 33.0 27.8 23.8 20.9 16.5 12.5 12.1 Total Gas Utility 446.6 400.9 355.4 266.2 226.0 204.0 180.4 174.9 151.4 Other Utility: Steam 35.2 34.6 33.9 35.2 34.6 33.9 — — — Other utility 12.8 12.9 14.7 9.8 10.8 12.8 3.0 2.1 1.9 Total Other Utility 48.0 47.5 48.6 45.0 45.4 46.7 3.0 2.1 1.9 Non-Utility and Other: Transportation and other 39.6 39.1 40.5 — — — — — — Total Non-Utility and Other 39.6 39.1 40.5 — — — — — — Total revenues $3,534.5 $3,382.2 $3,320.0 $2,042.3 $1,870.3 $1,820.4 $1,452.6 $1,472.8 $1,459.1
IPL [Member]
Disaggregation of Revenue [Line Items]
Disaggregation of Revenues	Disaggregation of revenues from contracts with customers, which correlates to revenues for each reportable segment, was as follows (in millions): Alliant Energy IPL WPL 2018 2017 2016 2018 2017 2016 2018 2017 2016 Electric Utility: Retail - residential $1,063.4 $1,006.2 $1,001.1 $590.6 $535.6 $536.7 $472.8 $470.6 $464.4 Retail - commercial 735.6 710.3 712.6 486.8 452.7 445.4 248.8 257.6 267.2 Retail - industrial 888.8 853.1 851.1 500.8 459.7 460.4 388.0 393.4 390.7 Wholesale 188.4 238.4 256.6 71.2 95.5 94.2 117.2 142.9 162.4 Bulk power and other 124.1 86.7 54.1 81.7 55.4 33.0 42.4 31.3 21.1 Total Electric Utility 3,000.3 2,894.7 2,875.5 1,731.1 1,598.9 1,569.7 1,269.2 1,295.8 1,305.8 Gas Utility: Retail - residential 254.4 224.7 197.6 152.3 123.2 110.6 102.1 101.5 87.0 Retail - commercial 133.0 123.2 109.6 75.9 67.9 61.9 57.1 55.3 47.7 Retail - industrial 14.9 16.7 15.2 10.2 11.1 10.6 4.7 5.6 4.6 Transportation/other 44.3 36.3 33.0 27.8 23.8 20.9 16.5 12.5 12.1 Total Gas Utility 446.6 400.9 355.4 266.2 226.0 204.0 180.4 174.9 151.4 Other Utility: Steam 35.2 34.6 33.9 35.2 34.6 33.9 — — — Other utility 12.8 12.9 14.7 9.8 10.8 12.8 3.0 2.1 1.9 Total Other Utility 48.0 47.5 48.6 45.0 45.4 46.7 3.0 2.1 1.9 Non-Utility and Other: Transportation and other 39.6 39.1 40.5 — — — — — — Total Non-Utility and Other 39.6 39.1 40.5 — — — — — — Total revenues $3,534.5 $3,382.2 $3,320.0 $2,042.3 $1,870.3 $1,820.4 $1,452.6 $1,472.8 $1,459.1
WPL [Member]
Disaggregation of Revenue [Line Items]
Disaggregation of Revenues	Disaggregation of revenues from contracts with customers, which correlates to revenues for each reportable segment, was as follows (in millions): Alliant Energy IPL WPL 2018 2017 2016 2018 2017 2016 2018 2017 2016 Electric Utility: Retail - residential $1,063.4 $1,006.2 $1,001.1 $590.6 $535.6 $536.7 $472.8 $470.6 $464.4 Retail - commercial 735.6 710.3 712.6 486.8 452.7 445.4 248.8 257.6 267.2 Retail - industrial 888.8 853.1 851.1 500.8 459.7 460.4 388.0 393.4 390.7 Wholesale 188.4 238.4 256.6 71.2 95.5 94.2 117.2 142.9 162.4 Bulk power and other 124.1 86.7 54.1 81.7 55.4 33.0 42.4 31.3 21.1 Total Electric Utility 3,000.3 2,894.7 2,875.5 1,731.1 1,598.9 1,569.7 1,269.2 1,295.8 1,305.8 Gas Utility: Retail - residential 254.4 224.7 197.6 152.3 123.2 110.6 102.1 101.5 87.0 Retail - commercial 133.0 123.2 109.6 75.9 67.9 61.9 57.1 55.3 47.7 Retail - industrial 14.9 16.7 15.2 10.2 11.1 10.6 4.7 5.6 4.6 Transportation/other 44.3 36.3 33.0 27.8 23.8 20.9 16.5 12.5 12.1 Total Gas Utility 446.6 400.9 355.4 266.2 226.0 204.0 180.4 174.9 151.4 Other Utility: Steam 35.2 34.6 33.9 35.2 34.6 33.9 — — — Other utility 12.8 12.9 14.7 9.8 10.8 12.8 3.0 2.1 1.9 Total Other Utility 48.0 47.5 48.6 45.0 45.4 46.7 3.0 2.1 1.9 Non-Utility and Other: Transportation and other 39.6 39.1 40.5 — — — — — — Total Non-Utility and Other 39.6 39.1 40.5 — — — — — — Total revenues $3,534.5 $3,382.2 $3,320.0 $2,042.3 $1,870.3 $1,820.4 $1,452.6 $1,472.8 $1,459.1

Benefit Plans (Tables)

Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2018
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 2018 2017 2016 2018 2017 2016 Discount rate for benefit obligations 4.34% 3.66% 4.19% 4.24% 3.53% 3.98% Discount rate for net periodic cost 3.66% 4.19% 4.47% 3.53% 3.98% 4.30% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.44% 5.80% 6.30% Interest crediting rate for Alliant Energy Cash Balance Pension Plan 5.04% 4.64% 3.17% N/A N/A N/A 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 6.50% 6.75% 7.00% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans IPL 2018 2017 2016 2018 2017 2016 Discount rate for benefit obligations 4.35% 3.68% 4.22% 4.23% 3.51% 3.95% Discount rate for net periodic cost 3.68% 4.22% 4.50% 3.51% 3.95% 4.28% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.60% 6.20% 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 6.50% 6.75% 7.00% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans WPL 2018 2017 2016 2018 2017 2016 Discount rate for benefit obligations 4.35% 3.69% 4.23% 4.23% 3.51% 3.96% Discount rate for net periodic cost 3.69% 4.23% 4.51% 3.51% 3.96% 4.28% Expected rate of return on plan assets 7.60% 7.60% 7.60% 3.84% 3.50% 4.70% 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 6.50% 6.75% 7.00% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00%
Net Periodic Benefit Costs	The components of net periodic benefit costs for sponsored defined benefit pension and OPEB plans are included in the tables below (in millions). The service cost component of net periodic benefit costs is included in “Other operation and maintenance” expenses in the income statements and all other components of net periodic benefit costs are included in “Other (income) and deductions” in the income statements. Alliant Energy Defined Benefit Pension Plans OPEB Plans 2018 2017 2016 2018 2017 2016 Service cost $12.1 $12.5 $12.6 $4.2 $5.0 $5.3 Interest cost 46.8 51.0 53.0 7.7 8.6 9.4 Expected return on plan assets (a) (69.7 ) (65.5 ) (65.5 ) (6.0 ) (6.1 ) (6.1 ) Amortization of prior service credit (b) (0.7 ) (0.4 ) (0.3 ) (0.2 ) (0.2 ) (4.1 ) Amortization of actuarial loss (c) 35.2 37.6 37.4 3.4 3.8 4.7 Settlement losses (d) — 0.9 — — — — $23.7 $36.1 $37.2 $9.1 $11.1 $9.2 IPL Defined Benefit Pension Plans OPEB Plans 2018 2017 2016 2018 2017 2016 Service cost $7.4 $7.3 $7.5 $1.7 $2.1 $2.3 Interest cost 21.4 23.5 24.5 3.1 3.5 3.8 Expected return on plan assets (a) (32.6 ) (30.8 ) (30.9 ) (4.4 ) (4.3 ) (4.3 ) Amortization of prior service credit (b) (0.2 ) (0.2 ) (0.2 ) — — (2.6 ) Amortization of actuarial loss (c) 14.9 16.1 16.5 1.3 2.0 2.6 $10.9 $15.9 $17.4 $1.7 $3.3 $1.8 WPL Defined Benefit Pension Plans OPEB Plans 2018 2017 2016 2018 2017 2016 Service cost $4.4 $4.9 $4.9 $1.6 $1.9 $2.0 Interest cost 20.2 21.8 22.3 3.1 3.4 3.8 Expected return on plan assets (a) (30.4 ) (28.5 ) (28.3 ) (0.6 ) (0.8 ) (0.8 ) Amortization of prior service cost (credit) (b) (0.1 ) 0.1 0.2 (0.2 ) (0.2 ) (0.9 ) Amortization of actuarial loss (c) 17.2 18.5 17.6 2.0 1.6 1.8 $11.3 $16.8 $16.7 $5.9 $5.9 $5.9 (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.
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 2018 2017 2018 2017 Change in benefit obligation: Net benefit obligation at January 1 $1,303.1 $1,244.3 $222.3 $220.1 Service cost 12.1 12.5 4.2 5.0 Interest cost 46.8 51.0 7.7 8.6 Plan participants’ contributions — — 3.1 2.9 Actuarial (gain) loss (96.2 ) 83.6 (8.2 ) 5.4 Gross benefits paid (90.8 ) (88.3 ) (23.0 ) (19.7 ) Net benefit obligation at December 31 1,175.0 1,303.1 206.1 222.3 Change in plan assets: Fair value of plan assets at January 1 950.7 895.7 111.1 105.8 Actual return on plan assets (57.8 ) 136.7 (2.6 ) 12.9 Employer contributions 6.5 6.6 10.5 9.2 Plan participants’ contributions — — 3.1 2.9 Gross benefits paid (90.8 ) (88.3 ) (23.0 ) (19.7 ) Fair value of plan assets at December 31 808.6 950.7 99.1 111.1 Under funded status at December 31 ($366.4 ) ($352.4 ) ($107.0 ) ($111.2 ) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2018 2017 2018 2017 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $7.5 $8.8 Current liabilities (2.3 ) (2.2 ) (9.6 ) (9.1 ) Pension and other benefit obligations (364.1 ) (350.2 ) (104.9 ) (110.9 ) Net amounts recognized at December 31 ($366.4 ) ($352.4 ) ($107.0 ) ($111.2 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $505.2 $509.1 $44.5 $47.4 Prior service credit (5.8 ) (6.5 ) (1.1 ) (1.3 ) $499.4 $502.6 $43.4 $46.1 Defined Benefit Pension Plans OPEB Plans IPL 2018 2017 2018 2017 Change in benefit obligation: Net benefit obligation at January 1 $592.9 $570.4 $89.4 $90.1 Service cost 7.4 7.3 1.7 2.1 Interest cost 21.4 23.5 3.1 3.5 Plan participants’ contributions — — 1.0 1.0 Actuarial (gain) loss (44.4 ) 34.9 (4.3 ) (0.1 ) Gross benefits paid (43.4 ) (43.2 ) (8.4 ) (7.2 ) Net benefit obligation at December 31 533.9 592.9 82.5 89.4 Change in plan assets: Fair value of plan assets at January 1 443.7 422.0 72.9 68.2 Actual return on plan assets (26.8 ) 64.3 (1.4 ) 8.9 Employer contributions 4.3 0.6 2.6 2.0 Plan participants’ contributions — — 1.0 1.0 Gross benefits paid (43.4 ) (43.2 ) (8.4 ) (7.2 ) Fair value of plan assets at December 31 377.8 443.7 66.7 72.9 Under funded status at December 31 ($156.1 ) ($149.2 ) ($15.8 ) ($16.5 ) Defined Benefit Pension Plans OPEB Plans IPL 2018 2017 2018 2017 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $4.7 $5.9 Current liabilities (0.6 ) (0.5 ) (1.9 ) (2.0 ) Pension and other benefit obligations (155.5 ) (148.7 ) (18.6 ) (20.4 ) Net amounts recognized at December 31 ($156.1 ) ($149.2 ) ($15.8 ) ($16.5 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $218.8 $218.9 $18.8 $18.7 Prior service credit (1.8 ) (2.1 ) — — $217.0 $216.8 $18.8 $18.7 Defined Benefit Pension Plans OPEB Plans WPL 2018 2017 2018 2017 Change in benefit obligation: Net benefit obligation at January 1 $559.8 $529.2 $90.4 $88.9 Service cost 4.4 4.9 1.6 1.9 Interest cost 20.2 21.8 3.1 3.4 Plan participants’ contributions — — 1.4 1.4 Actuarial (gain) loss (40.0 ) 38.3 (2.5 ) 4.1 Gross benefits paid (37.8 ) (34.4 ) (10.8 ) (9.3 ) Net benefit obligation at December 31 506.6 559.8 83.2 90.4 Change in plan assets: Fair value of plan assets at January 1 415.0 389.7 18.7 18.6 Actual return on plan assets (25.1 ) 59.6 (0.1 ) 1.2 Employer contributions 0.1 0.1 7.5 6.8 Plan participants’ contributions — — 1.4 1.4 Gross benefits paid (37.8 ) (34.4 ) (10.8 ) (9.3 ) Fair value of plan assets at December 31 352.2 415.0 16.7 18.7 Under funded status at December 31 ($154.4 ) ($144.8 ) ($66.5 ) ($71.7 ) Defined Benefit Pension Plans OPEB Plans WPL 2018 2017 2018 2017 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $2.8 $2.9 Current liabilities (0.1 ) (0.1 ) (7.4 ) (6.8 ) Pension and other benefit obligations (154.3 ) (144.7 ) (61.9 ) (67.8 ) Net amounts recognized at December 31 ($154.4 ) ($144.8 ) ($66.5 ) ($71.7 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $223.0 $224.7 $19.9 $23.6 Prior service credit (1.3 ) (1.5 ) (1.1 ) (1.3 ) $221.7 $223.2 $18.8 $22.3
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 2018 2017 2018 2017 Accumulated benefit obligations $1,139.9 $1,269.0 $206.1 $222.3 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,139.9 1,269.0 206.1 222.3 Fair value of plan assets 808.6 950.7 99.1 111.1 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,175.0 1,303.1 N/A N/A Fair value of plan assets 808.6 950.7 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2018 2017 2018 2017 Accumulated benefit obligations $514.3 $573.1 $82.5 $89.4 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 514.3 573.1 82.5 89.4 Fair value of plan assets 377.8 443.7 66.7 72.9 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 533.9 592.9 N/A N/A Fair value of plan assets 377.8 443.7 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2018 2017 2018 2017 Accumulated benefit obligations $494.8 $548.1 $83.2 $90.4 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 494.8 548.1 83.2 90.4 Fair value of plan assets 352.2 415.0 16.7 18.7 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 506.6 559.8 N/A N/A Fair value of plan assets 352.2 415.0 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 2018 2017 2018 2017 Regulatory assets $38.2 $38.9 $27.7 $28.1
Estimated Future Employer Contributions	Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2019 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $33.8 $16.5 $15.6 OPEB plans 9.6 1.9 7.4 (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 2019 2020 2021 2022 2023 2024 - 2028 Defined benefit pension benefits $71.7 $86.1 $74.7 $76.3 $77.8 $388.3 OPEB 19.3 18.1 17.8 17.5 17.0 77.7 $91.0 $104.2 $92.5 $93.8 $94.8 $466.0 IPL 2019 2020 2021 2022 2023 2024 - 2028 Defined benefit pension benefits $33.0 $35.1 $35.1 $36.4 $37.5 $182.9 OPEB 7.3 7.3 7.2 7.1 6.9 31.3 $40.3 $42.4 $42.3 $43.5 $44.4 $214.2 WPL 2019 2020 2021 2022 2023 2024 - 2028 Defined benefit pension benefits $31.4 $31.8 $31.9 $32.0 $32.5 $165.3 OPEB 8.8 7.6 7.4 7.1 6.8 30.7 $40.2 $39.4 $39.3 $39.1 $39.3 $196.0
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 2018 2017 2016 2018 2017 2016 2018 2017 2016 Compensation expense $17.0 $15.1 $18.0 $9.4 $8.3 $9.5 $6.9 $6.4 $7.9 Income tax benefits 4.9 6.2 7.4 2.8 3.4 4.0 1.9 2.6 3.2
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): 2018 2017 Carrying value $9.8 $11.1 Fair market value 16.2 19.7
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 2018 2017 2016 2018 2017 2016 Discount rate for benefit obligations 4.34% 3.66% 4.19% 4.24% 3.53% 3.98% Discount rate for net periodic cost 3.66% 4.19% 4.47% 3.53% 3.98% 4.30% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.44% 5.80% 6.30% Interest crediting rate for Alliant Energy Cash Balance Pension Plan 5.04% 4.64% 3.17% N/A N/A N/A 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 6.50% 6.75% 7.00% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans IPL 2018 2017 2016 2018 2017 2016 Discount rate for benefit obligations 4.35% 3.68% 4.22% 4.23% 3.51% 3.95% Discount rate for net periodic cost 3.68% 4.22% 4.50% 3.51% 3.95% 4.28% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.60% 6.20% 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 6.50% 6.75% 7.00% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans WPL 2018 2017 2016 2018 2017 2016 Discount rate for benefit obligations 4.35% 3.69% 4.23% 4.23% 3.51% 3.96% Discount rate for net periodic cost 3.69% 4.23% 4.51% 3.51% 3.96% 4.28% Expected rate of return on plan assets 7.60% 7.60% 7.60% 3.84% 3.50% 4.70% 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 6.50% 6.75% 7.00% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00%
Net Periodic Benefit Costs	The components of net periodic benefit costs for sponsored defined benefit pension and OPEB plans are included in the tables below (in millions). The service cost component of net periodic benefit costs is included in “Other operation and maintenance” expenses in the income statements and all other components of net periodic benefit costs are included in “Other (income) and deductions” in the income statements. Alliant Energy Defined Benefit Pension Plans OPEB Plans 2018 2017 2016 2018 2017 2016 Service cost $12.1 $12.5 $12.6 $4.2 $5.0 $5.3 Interest cost 46.8 51.0 53.0 7.7 8.6 9.4 Expected return on plan assets (a) (69.7 ) (65.5 ) (65.5 ) (6.0 ) (6.1 ) (6.1 ) Amortization of prior service credit (b) (0.7 ) (0.4 ) (0.3 ) (0.2 ) (0.2 ) (4.1 ) Amortization of actuarial loss (c) 35.2 37.6 37.4 3.4 3.8 4.7 Settlement losses (d) — 0.9 — — — — $23.7 $36.1 $37.2 $9.1 $11.1 $9.2 IPL Defined Benefit Pension Plans OPEB Plans 2018 2017 2016 2018 2017 2016 Service cost $7.4 $7.3 $7.5 $1.7 $2.1 $2.3 Interest cost 21.4 23.5 24.5 3.1 3.5 3.8 Expected return on plan assets (a) (32.6 ) (30.8 ) (30.9 ) (4.4 ) (4.3 ) (4.3 ) Amortization of prior service credit (b) (0.2 ) (0.2 ) (0.2 ) — — (2.6 ) Amortization of actuarial loss (c) 14.9 16.1 16.5 1.3 2.0 2.6 $10.9 $15.9 $17.4 $1.7 $3.3 $1.8 WPL Defined Benefit Pension Plans OPEB Plans 2018 2017 2016 2018 2017 2016 Service cost $4.4 $4.9 $4.9 $1.6 $1.9 $2.0 Interest cost 20.2 21.8 22.3 3.1 3.4 3.8 Expected return on plan assets (a) (30.4 ) (28.5 ) (28.3 ) (0.6 ) (0.8 ) (0.8 ) Amortization of prior service cost (credit) (b) (0.1 ) 0.1 0.2 (0.2 ) (0.2 ) (0.9 ) Amortization of actuarial loss (c) 17.2 18.5 17.6 2.0 1.6 1.8 $11.3 $16.8 $16.7 $5.9 $5.9 $5.9 (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.
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 2018 2017 2018 2017 Change in benefit obligation: Net benefit obligation at January 1 $1,303.1 $1,244.3 $222.3 $220.1 Service cost 12.1 12.5 4.2 5.0 Interest cost 46.8 51.0 7.7 8.6 Plan participants’ contributions — — 3.1 2.9 Actuarial (gain) loss (96.2 ) 83.6 (8.2 ) 5.4 Gross benefits paid (90.8 ) (88.3 ) (23.0 ) (19.7 ) Net benefit obligation at December 31 1,175.0 1,303.1 206.1 222.3 Change in plan assets: Fair value of plan assets at January 1 950.7 895.7 111.1 105.8 Actual return on plan assets (57.8 ) 136.7 (2.6 ) 12.9 Employer contributions 6.5 6.6 10.5 9.2 Plan participants’ contributions — — 3.1 2.9 Gross benefits paid (90.8 ) (88.3 ) (23.0 ) (19.7 ) Fair value of plan assets at December 31 808.6 950.7 99.1 111.1 Under funded status at December 31 ($366.4 ) ($352.4 ) ($107.0 ) ($111.2 ) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2018 2017 2018 2017 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $7.5 $8.8 Current liabilities (2.3 ) (2.2 ) (9.6 ) (9.1 ) Pension and other benefit obligations (364.1 ) (350.2 ) (104.9 ) (110.9 ) Net amounts recognized at December 31 ($366.4 ) ($352.4 ) ($107.0 ) ($111.2 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $505.2 $509.1 $44.5 $47.4 Prior service credit (5.8 ) (6.5 ) (1.1 ) (1.3 ) $499.4 $502.6 $43.4 $46.1 Defined Benefit Pension Plans OPEB Plans IPL 2018 2017 2018 2017 Change in benefit obligation: Net benefit obligation at January 1 $592.9 $570.4 $89.4 $90.1 Service cost 7.4 7.3 1.7 2.1 Interest cost 21.4 23.5 3.1 3.5 Plan participants’ contributions — — 1.0 1.0 Actuarial (gain) loss (44.4 ) 34.9 (4.3 ) (0.1 ) Gross benefits paid (43.4 ) (43.2 ) (8.4 ) (7.2 ) Net benefit obligation at December 31 533.9 592.9 82.5 89.4 Change in plan assets: Fair value of plan assets at January 1 443.7 422.0 72.9 68.2 Actual return on plan assets (26.8 ) 64.3 (1.4 ) 8.9 Employer contributions 4.3 0.6 2.6 2.0 Plan participants’ contributions — — 1.0 1.0 Gross benefits paid (43.4 ) (43.2 ) (8.4 ) (7.2 ) Fair value of plan assets at December 31 377.8 443.7 66.7 72.9 Under funded status at December 31 ($156.1 ) ($149.2 ) ($15.8 ) ($16.5 ) Defined Benefit Pension Plans OPEB Plans IPL 2018 2017 2018 2017 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $4.7 $5.9 Current liabilities (0.6 ) (0.5 ) (1.9 ) (2.0 ) Pension and other benefit obligations (155.5 ) (148.7 ) (18.6 ) (20.4 ) Net amounts recognized at December 31 ($156.1 ) ($149.2 ) ($15.8 ) ($16.5 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $218.8 $218.9 $18.8 $18.7 Prior service credit (1.8 ) (2.1 ) — — $217.0 $216.8 $18.8 $18.7 Defined Benefit Pension Plans OPEB Plans WPL 2018 2017 2018 2017 Change in benefit obligation: Net benefit obligation at January 1 $559.8 $529.2 $90.4 $88.9 Service cost 4.4 4.9 1.6 1.9 Interest cost 20.2 21.8 3.1 3.4 Plan participants’ contributions — — 1.4 1.4 Actuarial (gain) loss (40.0 ) 38.3 (2.5 ) 4.1 Gross benefits paid (37.8 ) (34.4 ) (10.8 ) (9.3 ) Net benefit obligation at December 31 506.6 559.8 83.2 90.4 Change in plan assets: Fair value of plan assets at January 1 415.0 389.7 18.7 18.6 Actual return on plan assets (25.1 ) 59.6 (0.1 ) 1.2 Employer contributions 0.1 0.1 7.5 6.8 Plan participants’ contributions — — 1.4 1.4 Gross benefits paid (37.8 ) (34.4 ) (10.8 ) (9.3 ) Fair value of plan assets at December 31 352.2 415.0 16.7 18.7 Under funded status at December 31 ($154.4 ) ($144.8 ) ($66.5 ) ($71.7 ) Defined Benefit Pension Plans OPEB Plans WPL 2018 2017 2018 2017 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $2.8 $2.9 Current liabilities (0.1 ) (0.1 ) (7.4 ) (6.8 ) Pension and other benefit obligations (154.3 ) (144.7 ) (61.9 ) (67.8 ) Net amounts recognized at December 31 ($154.4 ) ($144.8 ) ($66.5 ) ($71.7 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $223.0 $224.7 $19.9 $23.6 Prior service credit (1.3 ) (1.5 ) (1.1 ) (1.3 ) $221.7 $223.2 $18.8 $22.3
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 2018 2017 2018 2017 Accumulated benefit obligations $1,139.9 $1,269.0 $206.1 $222.3 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,139.9 1,269.0 206.1 222.3 Fair value of plan assets 808.6 950.7 99.1 111.1 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,175.0 1,303.1 N/A N/A Fair value of plan assets 808.6 950.7 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2018 2017 2018 2017 Accumulated benefit obligations $514.3 $573.1 $82.5 $89.4 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 514.3 573.1 82.5 89.4 Fair value of plan assets 377.8 443.7 66.7 72.9 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 533.9 592.9 N/A N/A Fair value of plan assets 377.8 443.7 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2018 2017 2018 2017 Accumulated benefit obligations $494.8 $548.1 $83.2 $90.4 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 494.8 548.1 83.2 90.4 Fair value of plan assets 352.2 415.0 16.7 18.7 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 506.6 559.8 N/A N/A Fair value of plan assets 352.2 415.0 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 2018 2017 2018 2017 Regulatory assets $38.2 $38.9 $27.7 $28.1
Estimated Future Employer Contributions	Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2019 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $33.8 $16.5 $15.6 OPEB plans 9.6 1.9 7.4 (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 2019 2020 2021 2022 2023 2024 - 2028 Defined benefit pension benefits $71.7 $86.1 $74.7 $76.3 $77.8 $388.3 OPEB 19.3 18.1 17.8 17.5 17.0 77.7 $91.0 $104.2 $92.5 $93.8 $94.8 $466.0 IPL 2019 2020 2021 2022 2023 2024 - 2028 Defined benefit pension benefits $33.0 $35.1 $35.1 $36.4 $37.5 $182.9 OPEB 7.3 7.3 7.2 7.1 6.9 31.3 $40.3 $42.4 $42.3 $43.5 $44.4 $214.2 WPL 2019 2020 2021 2022 2023 2024 - 2028 Defined benefit pension benefits $31.4 $31.8 $31.9 $32.0 $32.5 $165.3 OPEB 8.8 7.6 7.4 7.1 6.8 30.7 $40.2 $39.4 $39.3 $39.1 $39.3 $196.0
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 2018 2017 2016 2018 2017 2016 2018 2017 2016 Compensation expense $17.0 $15.1 $18.0 $9.4 $8.3 $9.5 $6.9 $6.4 $7.9 Income tax benefits 4.9 6.2 7.4 2.8 3.4 4.0 1.9 2.6 3.2
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 2018 2017 2016 2018 2017 2016 Discount rate for benefit obligations 4.34% 3.66% 4.19% 4.24% 3.53% 3.98% Discount rate for net periodic cost 3.66% 4.19% 4.47% 3.53% 3.98% 4.30% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.44% 5.80% 6.30% Interest crediting rate for Alliant Energy Cash Balance Pension Plan 5.04% 4.64% 3.17% N/A N/A N/A 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 6.50% 6.75% 7.00% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans IPL 2018 2017 2016 2018 2017 2016 Discount rate for benefit obligations 4.35% 3.68% 4.22% 4.23% 3.51% 3.95% Discount rate for net periodic cost 3.68% 4.22% 4.50% 3.51% 3.95% 4.28% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.60% 6.20% 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 6.50% 6.75% 7.00% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00% Qualified Defined Benefit Pension Plan OPEB Plans WPL 2018 2017 2016 2018 2017 2016 Discount rate for benefit obligations 4.35% 3.69% 4.23% 4.23% 3.51% 3.96% Discount rate for net periodic cost 3.69% 4.23% 4.51% 3.51% 3.96% 4.28% Expected rate of return on plan assets 7.60% 7.60% 7.60% 3.84% 3.50% 4.70% 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 6.50% 6.75% 7.00% Ultimate trend rate N/A N/A N/A 5.00% 5.00% 5.00%
Net Periodic Benefit Costs	The components of net periodic benefit costs for sponsored defined benefit pension and OPEB plans are included in the tables below (in millions). The service cost component of net periodic benefit costs is included in “Other operation and maintenance” expenses in the income statements and all other components of net periodic benefit costs are included in “Other (income) and deductions” in the income statements. Alliant Energy Defined Benefit Pension Plans OPEB Plans 2018 2017 2016 2018 2017 2016 Service cost $12.1 $12.5 $12.6 $4.2 $5.0 $5.3 Interest cost 46.8 51.0 53.0 7.7 8.6 9.4 Expected return on plan assets (a) (69.7 ) (65.5 ) (65.5 ) (6.0 ) (6.1 ) (6.1 ) Amortization of prior service credit (b) (0.7 ) (0.4 ) (0.3 ) (0.2 ) (0.2 ) (4.1 ) Amortization of actuarial loss (c) 35.2 37.6 37.4 3.4 3.8 4.7 Settlement losses (d) — 0.9 — — — — $23.7 $36.1 $37.2 $9.1 $11.1 $9.2 IPL Defined Benefit Pension Plans OPEB Plans 2018 2017 2016 2018 2017 2016 Service cost $7.4 $7.3 $7.5 $1.7 $2.1 $2.3 Interest cost 21.4 23.5 24.5 3.1 3.5 3.8 Expected return on plan assets (a) (32.6 ) (30.8 ) (30.9 ) (4.4 ) (4.3 ) (4.3 ) Amortization of prior service credit (b) (0.2 ) (0.2 ) (0.2 ) — — (2.6 ) Amortization of actuarial loss (c) 14.9 16.1 16.5 1.3 2.0 2.6 $10.9 $15.9 $17.4 $1.7 $3.3 $1.8 WPL Defined Benefit Pension Plans OPEB Plans 2018 2017 2016 2018 2017 2016 Service cost $4.4 $4.9 $4.9 $1.6 $1.9 $2.0 Interest cost 20.2 21.8 22.3 3.1 3.4 3.8 Expected return on plan assets (a) (30.4 ) (28.5 ) (28.3 ) (0.6 ) (0.8 ) (0.8 ) Amortization of prior service cost (credit) (b) (0.1 ) 0.1 0.2 (0.2 ) (0.2 ) (0.9 ) Amortization of actuarial loss (c) 17.2 18.5 17.6 2.0 1.6 1.8 $11.3 $16.8 $16.7 $5.9 $5.9 $5.9 (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.
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 2018 2017 2018 2017 Change in benefit obligation: Net benefit obligation at January 1 $1,303.1 $1,244.3 $222.3 $220.1 Service cost 12.1 12.5 4.2 5.0 Interest cost 46.8 51.0 7.7 8.6 Plan participants’ contributions — — 3.1 2.9 Actuarial (gain) loss (96.2 ) 83.6 (8.2 ) 5.4 Gross benefits paid (90.8 ) (88.3 ) (23.0 ) (19.7 ) Net benefit obligation at December 31 1,175.0 1,303.1 206.1 222.3 Change in plan assets: Fair value of plan assets at January 1 950.7 895.7 111.1 105.8 Actual return on plan assets (57.8 ) 136.7 (2.6 ) 12.9 Employer contributions 6.5 6.6 10.5 9.2 Plan participants’ contributions — — 3.1 2.9 Gross benefits paid (90.8 ) (88.3 ) (23.0 ) (19.7 ) Fair value of plan assets at December 31 808.6 950.7 99.1 111.1 Under funded status at December 31 ($366.4 ) ($352.4 ) ($107.0 ) ($111.2 ) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2018 2017 2018 2017 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $7.5 $8.8 Current liabilities (2.3 ) (2.2 ) (9.6 ) (9.1 ) Pension and other benefit obligations (364.1 ) (350.2 ) (104.9 ) (110.9 ) Net amounts recognized at December 31 ($366.4 ) ($352.4 ) ($107.0 ) ($111.2 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $505.2 $509.1 $44.5 $47.4 Prior service credit (5.8 ) (6.5 ) (1.1 ) (1.3 ) $499.4 $502.6 $43.4 $46.1 Defined Benefit Pension Plans OPEB Plans IPL 2018 2017 2018 2017 Change in benefit obligation: Net benefit obligation at January 1 $592.9 $570.4 $89.4 $90.1 Service cost 7.4 7.3 1.7 2.1 Interest cost 21.4 23.5 3.1 3.5 Plan participants’ contributions — — 1.0 1.0 Actuarial (gain) loss (44.4 ) 34.9 (4.3 ) (0.1 ) Gross benefits paid (43.4 ) (43.2 ) (8.4 ) (7.2 ) Net benefit obligation at December 31 533.9 592.9 82.5 89.4 Change in plan assets: Fair value of plan assets at January 1 443.7 422.0 72.9 68.2 Actual return on plan assets (26.8 ) 64.3 (1.4 ) 8.9 Employer contributions 4.3 0.6 2.6 2.0 Plan participants’ contributions — — 1.0 1.0 Gross benefits paid (43.4 ) (43.2 ) (8.4 ) (7.2 ) Fair value of plan assets at December 31 377.8 443.7 66.7 72.9 Under funded status at December 31 ($156.1 ) ($149.2 ) ($15.8 ) ($16.5 ) Defined Benefit Pension Plans OPEB Plans IPL 2018 2017 2018 2017 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $4.7 $5.9 Current liabilities (0.6 ) (0.5 ) (1.9 ) (2.0 ) Pension and other benefit obligations (155.5 ) (148.7 ) (18.6 ) (20.4 ) Net amounts recognized at December 31 ($156.1 ) ($149.2 ) ($15.8 ) ($16.5 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $218.8 $218.9 $18.8 $18.7 Prior service credit (1.8 ) (2.1 ) — — $217.0 $216.8 $18.8 $18.7 Defined Benefit Pension Plans OPEB Plans WPL 2018 2017 2018 2017 Change in benefit obligation: Net benefit obligation at January 1 $559.8 $529.2 $90.4 $88.9 Service cost 4.4 4.9 1.6 1.9 Interest cost 20.2 21.8 3.1 3.4 Plan participants’ contributions — — 1.4 1.4 Actuarial (gain) loss (40.0 ) 38.3 (2.5 ) 4.1 Gross benefits paid (37.8 ) (34.4 ) (10.8 ) (9.3 ) Net benefit obligation at December 31 506.6 559.8 83.2 90.4 Change in plan assets: Fair value of plan assets at January 1 415.0 389.7 18.7 18.6 Actual return on plan assets (25.1 ) 59.6 (0.1 ) 1.2 Employer contributions 0.1 0.1 7.5 6.8 Plan participants’ contributions — — 1.4 1.4 Gross benefits paid (37.8 ) (34.4 ) (10.8 ) (9.3 ) Fair value of plan assets at December 31 352.2 415.0 16.7 18.7 Under funded status at December 31 ($154.4 ) ($144.8 ) ($66.5 ) ($71.7 ) Defined Benefit Pension Plans OPEB Plans WPL 2018 2017 2018 2017 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $2.8 $2.9 Current liabilities (0.1 ) (0.1 ) (7.4 ) (6.8 ) Pension and other benefit obligations (154.3 ) (144.7 ) (61.9 ) (67.8 ) Net amounts recognized at December 31 ($154.4 ) ($144.8 ) ($66.5 ) ($71.7 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $223.0 $224.7 $19.9 $23.6 Prior service credit (1.3 ) (1.5 ) (1.1 ) (1.3 ) $221.7 $223.2 $18.8 $22.3
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 2018 2017 2018 2017 Accumulated benefit obligations $1,139.9 $1,269.0 $206.1 $222.3 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,139.9 1,269.0 206.1 222.3 Fair value of plan assets 808.6 950.7 99.1 111.1 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,175.0 1,303.1 N/A N/A Fair value of plan assets 808.6 950.7 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2018 2017 2018 2017 Accumulated benefit obligations $514.3 $573.1 $82.5 $89.4 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 514.3 573.1 82.5 89.4 Fair value of plan assets 377.8 443.7 66.7 72.9 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 533.9 592.9 N/A N/A Fair value of plan assets 377.8 443.7 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2018 2017 2018 2017 Accumulated benefit obligations $494.8 $548.1 $83.2 $90.4 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 494.8 548.1 83.2 90.4 Fair value of plan assets 352.2 415.0 16.7 18.7 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 506.6 559.8 N/A N/A Fair value of plan assets 352.2 415.0 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 2018 2017 2018 2017 Regulatory assets $38.2 $38.9 $27.7 $28.1
Estimated Future Employer Contributions	Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2019 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $33.8 $16.5 $15.6 OPEB plans 9.6 1.9 7.4 (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 2019 2020 2021 2022 2023 2024 - 2028 Defined benefit pension benefits $71.7 $86.1 $74.7 $76.3 $77.8 $388.3 OPEB 19.3 18.1 17.8 17.5 17.0 77.7 $91.0 $104.2 $92.5 $93.8 $94.8 $466.0 IPL 2019 2020 2021 2022 2023 2024 - 2028 Defined benefit pension benefits $33.0 $35.1 $35.1 $36.4 $37.5 $182.9 OPEB 7.3 7.3 7.2 7.1 6.9 31.3 $40.3 $42.4 $42.3 $43.5 $44.4 $214.2 WPL 2019 2020 2021 2022 2023 2024 - 2028 Defined benefit pension benefits $31.4 $31.8 $31.9 $32.0 $32.5 $165.3 OPEB 8.8 7.6 7.4 7.1 6.8 30.7 $40.2 $39.4 $39.3 $39.1 $39.3 $196.0
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 2018 2017 2016 2018 2017 2016 2018 2017 2016 Compensation expense $17.0 $15.1 $18.0 $9.4 $8.3 $9.5 $6.9 $6.4 $7.9 Income tax benefits 4.9 6.2 7.4 2.8 3.4 4.0 1.9 2.6 3.2
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, 2018 , 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% 4% Equity securities - U.S. 11 % - 41% 25% Equity securities - international 14 % - 34% 22% Global asset securities 5 % - 15% 9% 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): 2018 2017 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $36.4 $3.0 $33.4 $— $28.2 $4.5 $23.7 $— Equity securities - U.S. 130.2 130.2 — — 158.3 158.3 — — Equity securities - international 116.0 116.0 — — 137.5 137.5 — — Global asset securities 42.1 42.1 — — 49.4 49.4 — — Fixed income securities 127.8 52.5 75.3 — 135.9 55.8 80.1 — Total assets in fair value hierarchy 452.5 $343.8 $108.7 $— 509.3 $405.5 $103.8 $— Assets measured at net asset value 355.4 441.1 Accrued investment income 1.2 1.0 Due to brokers, net (pending trades with brokers) (0.5 ) (0.7 ) Total pension plan assets $808.6 $950.7 2018 2017 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $17.0 $1.4 $15.6 $— $13.2 $2.2 $11.0 $— Equity securities - U.S. 60.8 60.8 — — 73.9 73.9 — — Equity securities - international 54.2 54.2 — — 64.2 64.2 — — Global asset securities 19.7 19.7 — — 23.0 23.0 — — Fixed income securities 59.7 24.5 35.2 — 63.4 26.0 37.4 — Total assets in fair value hierarchy 211.4 $160.6 $50.8 $— 237.7 $189.3 $48.4 $— Assets measured at net asset value 166.1 205.8 Accrued investment income 0.6 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.3 ) Total pension plan assets $377.8 $443.7 2018 2017 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $15.9 $1.3 $14.6 $— $12.3 $2.0 $10.3 $— Equity securities - U.S. 56.7 56.7 — — 69.1 69.1 — — Equity securities - international 50.5 50.5 — — 60.0 60.0 — — Global asset securities 18.4 18.4 — — 21.6 21.6 — — Fixed income securities 55.6 22.8 32.8 — 59.3 24.3 35.0 — Total assets in fair value hierarchy 197.1 $149.7 $47.4 $— 222.3 $177.0 $45.3 $— Assets measured at net asset value 154.8 192.5 Accrued investment income 0.5 0.5 Due to brokers, net (pending trades with brokers) (0.2 ) (0.3 ) Total pension plan assets $352.2 $415.0
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): 2018 2017 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $36.4 $3.0 $33.4 $— $28.2 $4.5 $23.7 $— Equity securities - U.S. 130.2 130.2 — — 158.3 158.3 — — Equity securities - international 116.0 116.0 — — 137.5 137.5 — — Global asset securities 42.1 42.1 — — 49.4 49.4 — — Fixed income securities 127.8 52.5 75.3 — 135.9 55.8 80.1 — Total assets in fair value hierarchy 452.5 $343.8 $108.7 $— 509.3 $405.5 $103.8 $— Assets measured at net asset value 355.4 441.1 Accrued investment income 1.2 1.0 Due to brokers, net (pending trades with brokers) (0.5 ) (0.7 ) Total pension plan assets $808.6 $950.7 2018 2017 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $17.0 $1.4 $15.6 $— $13.2 $2.2 $11.0 $— Equity securities - U.S. 60.8 60.8 — — 73.9 73.9 — — Equity securities - international 54.2 54.2 — — 64.2 64.2 — — Global asset securities 19.7 19.7 — — 23.0 23.0 — — Fixed income securities 59.7 24.5 35.2 — 63.4 26.0 37.4 — Total assets in fair value hierarchy 211.4 $160.6 $50.8 $— 237.7 $189.3 $48.4 $— Assets measured at net asset value 166.1 205.8 Accrued investment income 0.6 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.3 ) Total pension plan assets $377.8 $443.7 2018 2017 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $15.9 $1.3 $14.6 $— $12.3 $2.0 $10.3 $— Equity securities - U.S. 56.7 56.7 — — 69.1 69.1 — — Equity securities - international 50.5 50.5 — — 60.0 60.0 — — Global asset securities 18.4 18.4 — — 21.6 21.6 — — Fixed income securities 55.6 22.8 32.8 — 59.3 24.3 35.0 — Total assets in fair value hierarchy 197.1 $149.7 $47.4 $— 222.3 $177.0 $45.3 $— Assets measured at net asset value 154.8 192.5 Accrued investment income 0.5 0.5 Due to brokers, net (pending trades with brokers) (0.2 ) (0.3 ) Total pension plan assets $352.2 $415.0 At December 31, 2018 , 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% 4% Equity securities - U.S. 11 % - 41% 25% Equity securities - international 14 % - 34% 22% Global asset securities 5 % - 15% 9% 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, the fair values of qualified and non-qualified defined benefit pension plan assets were as follows (in millions): 2018 2017 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $36.4 $3.0 $33.4 $— $28.2 $4.5 $23.7 $— Equity securities - U.S. 130.2 130.2 — — 158.3 158.3 — — Equity securities - international 116.0 116.0 — — 137.5 137.5 — — Global asset securities 42.1 42.1 — — 49.4 49.4 — — Fixed income securities 127.8 52.5 75.3 — 135.9 55.8 80.1 — Total assets in fair value hierarchy 452.5 $343.8 $108.7 $— 509.3 $405.5 $103.8 $— Assets measured at net asset value 355.4 441.1 Accrued investment income 1.2 1.0 Due to brokers, net (pending trades with brokers) (0.5 ) (0.7 ) Total pension plan assets $808.6 $950.7 2018 2017 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $17.0 $1.4 $15.6 $— $13.2 $2.2 $11.0 $— Equity securities - U.S. 60.8 60.8 — — 73.9 73.9 — — Equity securities - international 54.2 54.2 — — 64.2 64.2 — — Global asset securities 19.7 19.7 — — 23.0 23.0 — — Fixed income securities 59.7 24.5 35.2 — 63.4 26.0 37.4 — Total assets in fair value hierarchy 211.4 $160.6 $50.8 $— 237.7 $189.3 $48.4 $— Assets measured at net asset value 166.1 205.8 Accrued investment income 0.6 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.3 ) Total pension plan assets $377.8 $443.7 2018 2017 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $15.9 $1.3 $14.6 $— $12.3 $2.0 $10.3 $— Equity securities - U.S. 56.7 56.7 — — 69.1 69.1 — — Equity securities - international 50.5 50.5 — — 60.0 60.0 — — Global asset securities 18.4 18.4 — — 21.6 21.6 — — Fixed income securities 55.6 22.8 32.8 — 59.3 24.3 35.0 — Total assets in fair value hierarchy 197.1 $149.7 $47.4 $— 222.3 $177.0 $45.3 $— Assets measured at net asset value 154.8 192.5 Accrued investment income 0.5 0.5 Due to brokers, net (pending trades with brokers) (0.2 ) (0.3 ) Total pension plan assets $352.2 $415.0 At December 31, 2018 , 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% 4% Equity securities - U.S. 11 % - 41% 25% Equity securities - international 14 % - 34% 22% Global asset securities 5 % - 15% 9% Risk parity securities 5 % - 15% 10% Fixed income securities 20 % - 40% 30%
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, 2018 , 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 % - 46% 24% Equity securities - international 0 % - 34% 2% Fixed income securities 20 % - 100% 73% At December 31, the fair values of OPEB plan assets were as follows (in millions): 2018 2017 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $1.4 $1.1 $0.3 $— $1.2 $0.7 $0.5 $— Equity securities - U.S. 3.9 3.9 — — 27.9 27.9 — — Equity securities - international 2.9 2.9 — — 11.4 11.4 — — Global asset securities 0.4 0.4 — — 0.4 0.4 — — Fixed income securities 68.2 67.5 0.7 — 66.6 66.0 0.6 — Total assets in fair value hierarchy 76.8 $75.8 $1.0 $— 107.5 $106.4 $1.1 $— Assets measured at net asset value 22.3 3.6 Total OPEB plan assets $99.1 $111.1 2018 2017 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.7 $0.7 $— $— $0.3 $0.3 $— $— Equity securities - U.S. — — — — 22.3 22.3 — — Equity securities - international — — — — 7.5 7.5 — — Fixed income securities 47.0 47.0 — — 42.8 42.8 — — Total assets in fair value hierarchy 47.7 $47.7 $— $— 72.9 $72.9 $— $— Assets measured at net asset value 19.0 — Total OPEB plan assets $66.7 $72.9 2018 2017 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.1 $0.1 $— $— $0.6 $0.3 $0.3 $— Fixed income securities 16.6 16.6 — — 18.1 18.1 — — Total OPEB plan assets $16.7 $16.7 $— $— $18.7 $18.4 $0.3 $—
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, 2018 , 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 % - 46% 24% Equity securities - international 0 % - 34% 2% Fixed income securities 20 % - 100% 73% At December 31, the fair values of OPEB plan assets were as follows (in millions): 2018 2017 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $1.4 $1.1 $0.3 $— $1.2 $0.7 $0.5 $— Equity securities - U.S. 3.9 3.9 — — 27.9 27.9 — — Equity securities - international 2.9 2.9 — — 11.4 11.4 — — Global asset securities 0.4 0.4 — — 0.4 0.4 — — Fixed income securities 68.2 67.5 0.7 — 66.6 66.0 0.6 — Total assets in fair value hierarchy 76.8 $75.8 $1.0 $— 107.5 $106.4 $1.1 $— Assets measured at net asset value 22.3 3.6 Total OPEB plan assets $99.1 $111.1 2018 2017 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.7 $0.7 $— $— $0.3 $0.3 $— $— Equity securities - U.S. — — — — 22.3 22.3 — — Equity securities - international — — — — 7.5 7.5 — — Fixed income securities 47.0 47.0 — — 42.8 42.8 — — Total assets in fair value hierarchy 47.7 $47.7 $— $— 72.9 $72.9 $— $— Assets measured at net asset value 19.0 — Total OPEB plan assets $66.7 $72.9 2018 2017 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.1 $0.1 $— $— $0.6 $0.3 $0.3 $— Fixed income securities 16.6 16.6 — — 18.1 18.1 — — Total OPEB plan assets $16.7 $16.7 $— $— $18.7 $18.4 $0.3 $—
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, 2018 , 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 % - 46% 24% Equity securities - international 0 % - 34% 2% Fixed income securities 20 % - 100% 73% At December 31, the fair values of OPEB plan assets were as follows (in millions): 2018 2017 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $1.4 $1.1 $0.3 $— $1.2 $0.7 $0.5 $— Equity securities - U.S. 3.9 3.9 — — 27.9 27.9 — — Equity securities - international 2.9 2.9 — — 11.4 11.4 — — Global asset securities 0.4 0.4 — — 0.4 0.4 — — Fixed income securities 68.2 67.5 0.7 — 66.6 66.0 0.6 — Total assets in fair value hierarchy 76.8 $75.8 $1.0 $— 107.5 $106.4 $1.1 $— Assets measured at net asset value 22.3 3.6 Total OPEB plan assets $99.1 $111.1 2018 2017 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.7 $0.7 $— $— $0.3 $0.3 $— $— Equity securities - U.S. — — — — 22.3 22.3 — — Equity securities - international — — — — 7.5 7.5 — — Fixed income securities 47.0 47.0 — — 42.8 42.8 — — Total assets in fair value hierarchy 47.7 $47.7 $— $— 72.9 $72.9 $— $— Assets measured at net asset value 19.0 — Total OPEB plan assets $66.7 $72.9 2018 2017 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.1 $0.1 $— $— $0.6 $0.3 $0.3 $— Fixed income securities 16.6 16.6 — — 18.1 18.1 — — Total OPEB plan assets $16.7 $16.7 $— $— $18.7 $18.4 $0.3 $—
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 2018 2017 2016 2018 2017 2016 2018 2017 2016 401(k) costs $25.1 $24.8 $23.6 $13.0 $12.8 $12.0 $11.2 $11.1 $10.7
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 2018 2017 2016 2018 2017 2016 2018 2017 2016 401(k) costs $25.1 $24.8 $23.6 $13.0 $12.8 $12.0 $11.2 $11.1 $10.7
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 2018 2017 2016 2018 2017 2016 2018 2017 2016 401(k) costs $25.1 $24.8 $23.6 $13.0 $12.8 $12.0 $11.2 $11.1 $10.7
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 2018 2017 2016 2018 2017 2016 Nonvested awards, January 1 223,511 257,599 288,430 71,737 93,320 116,412 Granted 74,163 65,350 68,585 19,840 21,558 23,918 Vested (90,806 ) (99,438 ) (98,186 ) (31,910 ) (37,395 ) (42,760 ) Forfeited (3,680 ) — (1,230 ) (1,906 ) (5,746 ) (4,250 ) Nonvested awards, December 31 203,188 223,511 257,599 57,761 71,737 93,320 Granted Awards - Each performance share’s and performance unit’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. 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 2018 2017 2016 2018 2017 2016 2015 Grant 2014 Grant 2013 Grant 2015 Grant 2014 Grant 2013 Grant Performance awards vested 90,806 99,438 98,186 31,910 37,395 42,760 Percentage of target number of performance awards 137.5% 147.5% 165.0% 137.5% 147.5% 165.0% Aggregate payout value (in millions) $5.3 $5.6 $5.1 $1.4 $1.5 $1.7 Payout - cash (in millions) $4.9 $5.1 $2.9 $1.4 $1.5 $1.7 Payout - common stock shares issued 5,078 5,185 22,408 N/A N/A N/A Fair Value of Awards - At December 31, 2018 , Alliant Energy’s common stock closing price was $42.25 . Additio nal i nformation related to fair values of nonvested performance shares and performance units at December 31, 2018 , by year of grant, was as follows: Performance Shares Performance Units 2018 Grant 2017 Grant 2016 Grant 2018 Grant 2017 Grant 2016 Grant Nonvested awards at target 70,483 65,350 67,355 19,196 18,062 20,503 Estimated payout percentage based on performance criteria 85 % 85 % 143 % 85 % 85 % 143 % Fair values of each nonvested award $35.91 $35.91 $60.42 $35.91 $35.91 $60.42
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: 2018 2017 2016 Units Weighted Average Grant Date Fair Value Units Weighted Average Grant Date Fair Value Units Weighted Average Grant Date Fair Value Nonvested units, January 1 132,705 $36.50 67,355 $33.96 — $— Granted 74,163 38.60 65,350 39.12 68,585 33.96 Forfeited (3,680 ) 38.60 — — (1,230 ) 33.90 Nonvested units, December 31 203,188 37.23 132,705 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: 2018 2017 2016 Nonvested units, January 1 113,749 57,736 — Granted 63,568 56,013 58,790 Forfeited (3,154 ) — (1,054 ) Nonvested units, December 31 174,163 113,749 57,736

Asset Retirement Obligations (T

Asset Retirement Obligations (Tables)	12 Months Ended
	Dec. 31, 2018
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 2018 2017 2018 2017 2018 2017 Balance, January 1 $184.5 $195.7 $134.1 $124.7 $50.4 $61.4 Revisions in estimated cash flows (10.1 ) 4.3 (10.1 ) 7.0 — (2.7 ) Liabilities settled (10.4 ) (23.5 ) (9.7 ) (13.1 ) (0.7 ) (10.4 ) Liabilities incurred 7.3 2.0 — 11.7 7.3 — Accretion expense 6.2 6.0 4.0 3.8 2.2 2.1 Balance, December 31 $177.5 $184.5 $118.3 $134.1 $59.2 $50.4
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 2018 2017 2018 2017 2018 2017 Balance, January 1 $184.5 $195.7 $134.1 $124.7 $50.4 $61.4 Revisions in estimated cash flows (10.1 ) 4.3 (10.1 ) 7.0 — (2.7 ) Liabilities settled (10.4 ) (23.5 ) (9.7 ) (13.1 ) (0.7 ) (10.4 ) Liabilities incurred 7.3 2.0 — 11.7 7.3 — Accretion expense 6.2 6.0 4.0 3.8 2.2 2.1 Balance, December 31 $177.5 $184.5 $118.3 $134.1 $59.2 $50.4
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 2018 2017 2018 2017 2018 2017 Balance, January 1 $184.5 $195.7 $134.1 $124.7 $50.4 $61.4 Revisions in estimated cash flows (10.1 ) 4.3 (10.1 ) 7.0 — (2.7 ) Liabilities settled (10.4 ) (23.5 ) (9.7 ) (13.1 ) (0.7 ) (10.4 ) Liabilities incurred 7.3 2.0 — 11.7 7.3 — Accretion expense 6.2 6.0 4.0 3.8 2.2 2.1 Balance, December 31 $177.5 $184.5 $118.3 $134.1 $59.2 $50.4

Derivative Instruments (Tables)

Derivative Instruments (Tables)	12 Months Ended
	Dec. 31, 2018
Derivative [Line Items]
Notional Amounts of Derivative Instruments	As of December 31, 2018 , 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): FTRs Natural Gas Coal Diesel Fuel MWhs Years Dths Years Tons Years Gallons Years Alliant Energy 10,399 2019 181,694 2019-2026 10,467 2019-2021 3,024 2019 IPL 5,954 2019 80,150 2019-2026 4,410 2019-2021 — WPL 4,445 2019 101,544 2019-2026 6,057 2019-2021 3,024 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 2018 2017 2018 2017 2018 2017 Current derivative assets $24.6 $21.1 $16.1 $15.8 $8.5 $5.3 Non-current derivative assets 3.7 4.0 1.6 1.3 2.1 2.7 Current derivative liabilities 5.6 18.7 3.1 5.0 2.5 13.7 Non-current derivative liabilities 17.7 23.0 8.1 14.4 9.6 8.6
IPL [Member]
Derivative [Line Items]
Notional Amounts of Derivative Instruments	As of December 31, 2018 , 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): FTRs Natural Gas Coal Diesel Fuel MWhs Years Dths Years Tons Years Gallons Years Alliant Energy 10,399 2019 181,694 2019-2026 10,467 2019-2021 3,024 2019 IPL 5,954 2019 80,150 2019-2026 4,410 2019-2021 — WPL 4,445 2019 101,544 2019-2026 6,057 2019-2021 3,024 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 2018 2017 2018 2017 2018 2017 Current derivative assets $24.6 $21.1 $16.1 $15.8 $8.5 $5.3 Non-current derivative assets 3.7 4.0 1.6 1.3 2.1 2.7 Current derivative liabilities 5.6 18.7 3.1 5.0 2.5 13.7 Non-current derivative liabilities 17.7 23.0 8.1 14.4 9.6 8.6
WPL [Member]
Derivative [Line Items]
Notional Amounts of Derivative Instruments	As of December 31, 2018 , 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): FTRs Natural Gas Coal Diesel Fuel MWhs Years Dths Years Tons Years Gallons Years Alliant Energy 10,399 2019 181,694 2019-2026 10,467 2019-2021 3,024 2019 IPL 5,954 2019 80,150 2019-2026 4,410 2019-2021 — WPL 4,445 2019 101,544 2019-2026 6,057 2019-2021 3,024 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 2018 2017 2018 2017 2018 2017 Current derivative assets $24.6 $21.1 $16.1 $15.8 $8.5 $5.3 Non-current derivative assets 3.7 4.0 1.6 1.3 2.1 2.7 Current derivative liabilities 5.6 18.7 3.1 5.0 2.5 13.7 Non-current derivative liabilities 17.7 23.0 8.1 14.4 9.6 8.6

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2018
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 2018 2017 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $28.3 $— $8.9 $19.4 $28.3 $25.1 $— $4.1 $21.0 $25.1 Deferred proceeds 119.4 — — 119.4 119.4 222.1 — — 222.1 222.1 Liabilities and equity: Derivatives 23.3 — 16.1 7.2 23.3 41.7 — 8.5 33.2 41.7 Long-term debt (incl. current maturities) 5,502.8 — 5,858.4 2.4 5,860.8 4,866.3 — 5,444.6 2.9 5,447.5 IPL 2018 2017 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $17.7 $— $4.0 $13.7 $17.7 $17.1 $— $2.0 $15.1 $17.1 Deferred proceeds 119.4 — — 119.4 119.4 222.1 — — 222.1 222.1 Liabilities and equity: Derivatives 11.2 — 6.5 4.7 11.2 19.4 — 2.9 16.5 19.4 Long-term debt (incl. current maturities) 2,552.3 — 2,691.2 — 2,691.2 2,406.0 — 2,665.7 — 2,665.7 WPL 2018 2017 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $10.6 $— $4.9 $5.7 $10.6 $8.0 $— $2.1 $5.9 $8.0 Liabilities and equity: Derivatives 12.1 — 9.6 2.5 12.1 22.3 — 5.6 16.7 22.3 Long-term debt (incl. current maturities) 1,834.9 — 2,043.7 — 2,043.7 1,833.4 — 2,147.9 — 2,147.9
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 2018 2017 2018 2017 Beginning balance, January 1 ($12.2 ) $8.7 $222.1 $211.1 Total net gains (losses) included in changes in net assets (realized/unrealized) 9.1 (32.9 ) — — Transfers out of Level 3 (a) 16.1 12.2 — — Purchases 26.7 28.3 — — Sales (0.5 ) (0.3 ) — — Settlements (b) (27.0 ) (28.2 ) (102.7 ) 11.0 Ending balance, December 31 $12.2 ($12.2 ) $119.4 $222.1 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 $10.7 ($31.0 ) $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2018 2017 2018 2017 Beginning balance, January 1 ($1.4 ) $10.1 $222.1 $211.1 Total net losses included in changes in net assets (realized/unrealized) (0.5 ) (14.8 ) — — Transfers out of Level 3 (a) 11.0 3.1 — — Purchases 22.5 24.6 — — Sales (0.4 ) (0.2 ) — — Settlements (b) (22.2 ) (24.2 ) (102.7 ) 11.0 Ending balance, December 31 $9.0 ($1.4 ) $119.4 $222.1 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 $0.2 ($13.5 ) $— $— WPL Commodity Contract Derivative Assets and (Liabilities), net 2018 2017 Beginning balance, January 1 ($10.8 ) ($1.4 ) Total net gains (losses) included in changes in net assets (realized/unrealized) 9.6 (18.1 ) Transfers out of Level 3 (a) 5.1 9.1 Purchases 4.2 3.7 Sales (0.1 ) (0.1 ) Settlements (4.8 ) (4.0 ) Ending balance, December 31 $3.2 ($10.8 ) 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 $10.5 ($17.5 ) (a) Observable market inputs became available for certain commodity contracts previously classified as Level 3 for transfers out of Level 3. (b) 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, FTR and natural gas 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 2018 $3.2 $9.0 $1.8 $7.2 $1.4 $1.8 2017 (23.5 ) 11.3 (11.5 ) 10.1 (12.0 ) 1.2
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 2018 2017 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $28.3 $— $8.9 $19.4 $28.3 $25.1 $— $4.1 $21.0 $25.1 Deferred proceeds 119.4 — — 119.4 119.4 222.1 — — 222.1 222.1 Liabilities and equity: Derivatives 23.3 — 16.1 7.2 23.3 41.7 — 8.5 33.2 41.7 Long-term debt (incl. current maturities) 5,502.8 — 5,858.4 2.4 5,860.8 4,866.3 — 5,444.6 2.9 5,447.5 IPL 2018 2017 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $17.7 $— $4.0 $13.7 $17.7 $17.1 $— $2.0 $15.1 $17.1 Deferred proceeds 119.4 — — 119.4 119.4 222.1 — — 222.1 222.1 Liabilities and equity: Derivatives 11.2 — 6.5 4.7 11.2 19.4 — 2.9 16.5 19.4 Long-term debt (incl. current maturities) 2,552.3 — 2,691.2 — 2,691.2 2,406.0 — 2,665.7 — 2,665.7 WPL 2018 2017 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $10.6 $— $4.9 $5.7 $10.6 $8.0 $— $2.1 $5.9 $8.0 Liabilities and equity: Derivatives 12.1 — 9.6 2.5 12.1 22.3 — 5.6 16.7 22.3 Long-term debt (incl. current maturities) 1,834.9 — 2,043.7 — 2,043.7 1,833.4 — 2,147.9 — 2,147.9
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 2018 2017 2018 2017 Beginning balance, January 1 ($12.2 ) $8.7 $222.1 $211.1 Total net gains (losses) included in changes in net assets (realized/unrealized) 9.1 (32.9 ) — — Transfers out of Level 3 (a) 16.1 12.2 — — Purchases 26.7 28.3 — — Sales (0.5 ) (0.3 ) — — Settlements (b) (27.0 ) (28.2 ) (102.7 ) 11.0 Ending balance, December 31 $12.2 ($12.2 ) $119.4 $222.1 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 $10.7 ($31.0 ) $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2018 2017 2018 2017 Beginning balance, January 1 ($1.4 ) $10.1 $222.1 $211.1 Total net losses included in changes in net assets (realized/unrealized) (0.5 ) (14.8 ) — — Transfers out of Level 3 (a) 11.0 3.1 — — Purchases 22.5 24.6 — — Sales (0.4 ) (0.2 ) — — Settlements (b) (22.2 ) (24.2 ) (102.7 ) 11.0 Ending balance, December 31 $9.0 ($1.4 ) $119.4 $222.1 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 $0.2 ($13.5 ) $— $— WPL Commodity Contract Derivative Assets and (Liabilities), net 2018 2017 Beginning balance, January 1 ($10.8 ) ($1.4 ) Total net gains (losses) included in changes in net assets (realized/unrealized) 9.6 (18.1 ) Transfers out of Level 3 (a) 5.1 9.1 Purchases 4.2 3.7 Sales (0.1 ) (0.1 ) Settlements (4.8 ) (4.0 ) Ending balance, December 31 $3.2 ($10.8 ) 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 $10.5 ($17.5 ) (a) Observable market inputs became available for certain commodity contracts previously classified as Level 3 for transfers out of Level 3. (b) 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, FTR and natural gas 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 2018 $3.2 $9.0 $1.8 $7.2 $1.4 $1.8 2017 (23.5 ) 11.3 (11.5 ) 10.1 (12.0 ) 1.2
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 2018 2017 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $28.3 $— $8.9 $19.4 $28.3 $25.1 $— $4.1 $21.0 $25.1 Deferred proceeds 119.4 — — 119.4 119.4 222.1 — — 222.1 222.1 Liabilities and equity: Derivatives 23.3 — 16.1 7.2 23.3 41.7 — 8.5 33.2 41.7 Long-term debt (incl. current maturities) 5,502.8 — 5,858.4 2.4 5,860.8 4,866.3 — 5,444.6 2.9 5,447.5 IPL 2018 2017 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $17.7 $— $4.0 $13.7 $17.7 $17.1 $— $2.0 $15.1 $17.1 Deferred proceeds 119.4 — — 119.4 119.4 222.1 — — 222.1 222.1 Liabilities and equity: Derivatives 11.2 — 6.5 4.7 11.2 19.4 — 2.9 16.5 19.4 Long-term debt (incl. current maturities) 2,552.3 — 2,691.2 — 2,691.2 2,406.0 — 2,665.7 — 2,665.7 WPL 2018 2017 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $10.6 $— $4.9 $5.7 $10.6 $8.0 $— $2.1 $5.9 $8.0 Liabilities and equity: Derivatives 12.1 — 9.6 2.5 12.1 22.3 — 5.6 16.7 22.3 Long-term debt (incl. current maturities) 1,834.9 — 2,043.7 — 2,043.7 1,833.4 — 2,147.9 — 2,147.9
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 2018 2017 2018 2017 Beginning balance, January 1 ($12.2 ) $8.7 $222.1 $211.1 Total net gains (losses) included in changes in net assets (realized/unrealized) 9.1 (32.9 ) — — Transfers out of Level 3 (a) 16.1 12.2 — — Purchases 26.7 28.3 — — Sales (0.5 ) (0.3 ) — — Settlements (b) (27.0 ) (28.2 ) (102.7 ) 11.0 Ending balance, December 31 $12.2 ($12.2 ) $119.4 $222.1 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 $10.7 ($31.0 ) $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2018 2017 2018 2017 Beginning balance, January 1 ($1.4 ) $10.1 $222.1 $211.1 Total net losses included in changes in net assets (realized/unrealized) (0.5 ) (14.8 ) — — Transfers out of Level 3 (a) 11.0 3.1 — — Purchases 22.5 24.6 — — Sales (0.4 ) (0.2 ) — — Settlements (b) (22.2 ) (24.2 ) (102.7 ) 11.0 Ending balance, December 31 $9.0 ($1.4 ) $119.4 $222.1 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 $0.2 ($13.5 ) $— $— WPL Commodity Contract Derivative Assets and (Liabilities), net 2018 2017 Beginning balance, January 1 ($10.8 ) ($1.4 ) Total net gains (losses) included in changes in net assets (realized/unrealized) 9.6 (18.1 ) Transfers out of Level 3 (a) 5.1 9.1 Purchases 4.2 3.7 Sales (0.1 ) (0.1 ) Settlements (4.8 ) (4.0 ) Ending balance, December 31 $3.2 ($10.8 ) 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 $10.5 ($17.5 ) (a) Observable market inputs became available for certain commodity contracts previously classified as Level 3 for transfers out of Level 3. (b) 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, FTR and natural gas 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 2018 $3.2 $9.0 $1.8 $7.2 $1.4 $1.8 2017 (23.5 ) 11.3 (11.5 ) 10.1 (12.0 ) 1.2

Commitments And Contingencies (

Commitments And Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Long-term Purchase Commitment [Line Items]
Other Purchase Commitments	At December 31, 2018 , the related minimum future commitments were as follows (in millions): Alliant Energy 2019 2020 2021 2022 2023 Thereafter Total Purchased power (a) $159 $135 $149 $140 $155 $307 $1,045 Natural gas 254 151 119 94 67 196 881 Coal (b) 102 49 26 9 9 — 195 Other (c) 60 4 3 2 2 — 71 $575 $339 $297 $245 $233 $503 $2,192 IPL 2019 2020 2021 2022 2023 Thereafter Total Purchased power (a) $144 $135 $149 $140 $155 $307 $1,030 Natural gas 124 55 42 33 26 83 363 Coal (b) 52 29 20 9 9 — 119 Other (c) 24 3 3 2 2 — 34 $344 $222 $214 $184 $192 $390 $1,546 WPL 2019 2020 2021 2022 2023 Thereafter Total Purchased power $15 $— $— $— $— $— $15 Natural gas 130 96 77 61 41 113 518 Coal (b) 50 20 6 — — — 76 Other (c) 23 — — — — — 23 $218 $116 $83 $61 $41 $113 $632 (a) Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. In July 2018, IPL entered into an amendment to shorten the term of the DAEC PPA by five years in exchange for a $110 million buyout payment by IPL in September 2020, subject to IUB approval. In December 2018, IPL received an order from the IUB, which was effective in January 2019, approving recovery of the buyout payment. As a result, Alliant Energy’s and IPL’s amounts in the above table do not include the September 2020 buyout payment, and the minimum future commitments reflect IPL’s purchase of capacity and the resulting energy from DAEC through December 2025, the original term of the PPA prior to the amendment. Amounts in the above table for 2021 and beyond relate to the DAEC PPA. (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, 2018 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, 2018 .
Schedule of Environmental Liabilities	At December 31, 2018 , 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, 2018 , such amounts for WPL were not material. Alliant Energy IPL Range of estimated future costs $11 - $29 $8 - $24 Current and non-current environmental liabilities 15 12
IPL [Member]
Long-term Purchase Commitment [Line Items]
Other Purchase Commitments	At December 31, 2018 , the related minimum future commitments were as follows (in millions): Alliant Energy 2019 2020 2021 2022 2023 Thereafter Total Purchased power (a) $159 $135 $149 $140 $155 $307 $1,045 Natural gas 254 151 119 94 67 196 881 Coal (b) 102 49 26 9 9 — 195 Other (c) 60 4 3 2 2 — 71 $575 $339 $297 $245 $233 $503 $2,192 IPL 2019 2020 2021 2022 2023 Thereafter Total Purchased power (a) $144 $135 $149 $140 $155 $307 $1,030 Natural gas 124 55 42 33 26 83 363 Coal (b) 52 29 20 9 9 — 119 Other (c) 24 3 3 2 2 — 34 $344 $222 $214 $184 $192 $390 $1,546 WPL 2019 2020 2021 2022 2023 Thereafter Total Purchased power $15 $— $— $— $— $— $15 Natural gas 130 96 77 61 41 113 518 Coal (b) 50 20 6 — — — 76 Other (c) 23 — — — — — 23 $218 $116 $83 $61 $41 $113 $632 (a) Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. In July 2018, IPL entered into an amendment to shorten the term of the DAEC PPA by five years in exchange for a $110 million buyout payment by IPL in September 2020, subject to IUB approval. In December 2018, IPL received an order from the IUB, which was effective in January 2019, approving recovery of the buyout payment. As a result, Alliant Energy’s and IPL’s amounts in the above table do not include the September 2020 buyout payment, and the minimum future commitments reflect IPL’s purchase of capacity and the resulting energy from DAEC through December 2025, the original term of the PPA prior to the amendment. Amounts in the above table for 2021 and beyond relate to the DAEC PPA. (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, 2018 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, 2018 .
Schedule of Environmental Liabilities	At December 31, 2018 , 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, 2018 , such amounts for WPL were not material. Alliant Energy IPL Range of estimated future costs $11 - $29 $8 - $24 Current and non-current environmental liabilities 15 12
WPL [Member]
Long-term Purchase Commitment [Line Items]
Other Purchase Commitments	At December 31, 2018 , the related minimum future commitments were as follows (in millions): Alliant Energy 2019 2020 2021 2022 2023 Thereafter Total Purchased power (a) $159 $135 $149 $140 $155 $307 $1,045 Natural gas 254 151 119 94 67 196 881 Coal (b) 102 49 26 9 9 — 195 Other (c) 60 4 3 2 2 — 71 $575 $339 $297 $245 $233 $503 $2,192 IPL 2019 2020 2021 2022 2023 Thereafter Total Purchased power (a) $144 $135 $149 $140 $155 $307 $1,030 Natural gas 124 55 42 33 26 83 363 Coal (b) 52 29 20 9 9 — 119 Other (c) 24 3 3 2 2 — 34 $344 $222 $214 $184 $192 $390 $1,546 WPL 2019 2020 2021 2022 2023 Thereafter Total Purchased power $15 $— $— $— $— $— $15 Natural gas 130 96 77 61 41 113 518 Coal (b) 50 20 6 — — — 76 Other (c) 23 — — — — — 23 $218 $116 $83 $61 $41 $113 $632 (a) Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. In July 2018, IPL entered into an amendment to shorten the term of the DAEC PPA by five years in exchange for a $110 million buyout payment by IPL in September 2020, subject to IUB approval. In December 2018, IPL received an order from the IUB, which was effective in January 2019, approving recovery of the buyout payment. As a result, Alliant Energy’s and IPL’s amounts in the above table do not include the September 2020 buyout payment, and the minimum future commitments reflect IPL’s purchase of capacity and the resulting energy from DAEC through December 2025, the original term of the PPA prior to the amendment. Amounts in the above table for 2021 and beyond relate to the DAEC PPA. (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, 2018 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, 2018 .

Segments Of Business (Tables)

Segments Of Business (Tables)	12 Months Ended
	Dec. 31, 2018
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 Holdings, Utility Non-utility, Alliant Energy 2018 Electric Gas Other Total Parent and Other Consolidated Revenues $3,000.3 $446.6 $48.0 $3,494.9 $39.6 $3,534.5 Depreciation and amortization 457.3 42.0 3.6 502.9 4.0 506.9 Operating income 610.2 53.2 0.3 663.7 30.7 694.4 Interest expense 217.2 29.8 247.0 Equity income from unconsolidated investments, net (0.9 ) — — (0.9 ) (53.7 ) (54.6 ) Income taxes 33.0 14.7 47.7 Net income attributable to Alliant Energy common shareowners 472.1 40.0 512.1 Total assets 12,486.3 1,184.4 893.2 14,563.9 862.1 15,426.0 Investments in equity method subsidiaries 8.1 — — 8.1 413.2 421.3 Construction and acquisition expenditures 1,421.1 146.8 0.4 1,568.3 65.6 1,633.9 ATC Holdings, Utility Non-utility, Alliant Energy 2017 Electric Gas Other Total Parent and Other Consolidated 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) 601.7 47.7 (11.6 ) 637.8 33.4 671.2 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 Holdings, Utility Non-utility, Alliant Energy 2016 Electric Gas Other Total Parent and Other Consolidated 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) 586.5 33.0 (4.7 ) 614.8 (60.7 ) 554.1 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
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): 2018 Electric Gas Other Total Revenues $1,731.1 $266.2 $45.0 $2,042.3 Depreciation and amortization 254.7 25.2 3.6 283.5 Operating income 318.2 28.3 4.3 350.8 Interest expense 119.4 Income tax benefit (3.2 ) Earnings available for common stock 264.0 Total assets 7,219.9 687.5 504.0 8,411.4 Construction and acquisition expenditures 890.6 99.7 0.4 990.7 2017 Electric Gas Other Total 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) 287.3 21.7 (4.9 ) 304.1 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 Revenues $1,569.7 $204.0 $46.7 $1,820.4 Depreciation and amortization 189.4 19.3 2.1 210.8 Operating income 257.8 16.4 3.4 277.6 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
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): 2018 Electric Gas Other Total Revenues $1,269.2 $180.4 $3.0 $1,452.6 Depreciation and amortization 202.6 16.8 — 219.4 Operating income (loss) 292.0 24.9 (4.0 ) 312.9 Interest expense 97.8 Income taxes 36.2 Earnings available for common stock 208.1 Total assets 5,266.4 496.9 389.2 6,152.5 Construction and acquisition expenditures 530.5 47.1 — 577.6 2017 Electric Gas Other Total Revenues $1,295.8 $174.9 $2.1 $1,472.8 Depreciation and amortization 196.9 16.0 — 212.9 Operating income (loss) 314.4 26.0 (6.7 ) 333.7 Interest expense 93.8 Income taxes 61.9 Earnings available for common stock 186.6 Total assets 4,871.8 471.9 412.8 5,756.5 Construction and acquisition expenditures 592.4 44.5 0.5 637.4 2016 Electric Gas Other Total Revenues $1,305.8 $151.4 $1.9 $1,459.1 Depreciation and amortization 177.6 14.9 — 192.5 Operating income (loss) 328.7 16.6 (8.1 ) 337.2 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 Construction and acquisition expenditures 395.9 45.6 11.5 453.0

Related Parties (Tables)

Related Parties (Tables)	12 Months Ended
	Dec. 31, 2018
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 2018 2017 2016 2018 2017 2016 Corporate Services billings $170 $177 $161 $132 $135 $133 Sales credited 48 23 8 28 13 7 Purchases billed 358 364 433 81 115 102
Net Intercompany Payables	As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2018 2017 IPL $95 $114 WPL 71 61
Related Amounts Billed Between Parties	The related amounts billed between the parties were as follows (in millions): 2018 2017 2016 ATC billings to WPL $106 $105 $110 WPL billings to ATC 11 10 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 2018 2017 2016 2018 2017 2016 Corporate Services billings $170 $177 $161 $132 $135 $133 Sales credited 48 23 8 28 13 7 Purchases billed 358 364 433 81 115 102
Net Intercompany Payables	As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2018 2017 IPL $95 $114 WPL 71 61
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 2018 2017 2016 2018 2017 2016 Corporate Services billings $170 $177 $161 $132 $135 $133 Sales credited 48 23 8 28 13 7 Purchases billed 358 364 433 81 115 102
Net Intercompany Payables	As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2018 2017 IPL $95 $114 WPL 71 61
Related Amounts Billed Between Parties	The related amounts billed between the parties were as follows (in millions): 2018 2017 2016 ATC billings to WPL $106 $105 $110 WPL billings to ATC 11 10 13

Selected Consolidated Quarter_2

Selected Consolidated Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2018
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. 2018 2017 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions, except per share data) Revenues $916.3 $816.1 $928.6 $873.5 $853.9 $765.3 $906.9 $856.1 Operating income 165.7 151.2 256.1 121.4 147.2 153.7 236.3 134.0 Amounts attributable to Alliant Energy common shareowners: Income from continuing operations, net of tax 120.9 100.4 205.5 85.3 99.0 94.3 168.8 93.8 Income from discontinued operations, net of tax — — — — 1.4 — — — Net income 120.9 100.4 205.5 85.3 100.4 94.3 168.8 93.8 Earnings per weighted average common share attributable to Alliant Energy common shareowners: Income from continuing operations, net of tax 0.52 0.43 0.87 0.36 0.43 0.41 0.73 0.41 Income from discontinued operations, net of tax — — — — 0.01 — — — Net income 0.52 0.43 0.87 0.36 0.44 0.41 0.73 0.41
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. 2018 2017 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Revenues $525.8 $474.8 $547.6 $494.1 $450.5 $420.2 $527.4 $472.2 Operating income 75.6 77.7 143.1 54.4 51.3 68.1 133.8 50.9 Net income 49.3 54.2 129.1 41.6 39.8 45.3 123.0 18.9 Earnings available for common stock 46.7 51.7 126.5 39.1 37.2 42.8 120.4 16.4
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. 2018 2017 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Revenues $381.7 $330.8 $370.7 $369.4 $393.1 $334.8 $370.2 $374.7 Operating income 84.0 63.4 104.2 61.3 88.6 76.2 93.5 75.4 Net income 54.0 39.8 76.3 38.0 45.5 38.1 49.8 53.2 Earnings available for common stock 54.0 39.8 76.3 38.0 45.5 38.1 49.8 53.2

Condensed Parent Company Fina_2

Condensed Parent Company Financial Statements (Tables)	12 Months Ended
	Dec. 31, 2018
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Condensed Statements of Income	ALLIANT ENERGY CORPORATION (Parent Company Only) Year Ended December 31, CONDENSED STATEMENTS OF INCOME 2018 2017 2016 (in millions) Revenues $— $— $1 Operating expenses 5 2 3 Operating loss (5 ) (2 ) (2 ) Other (income) and deductions: Equity earnings from consolidated subsidiaries (523 ) (457 ) (374 ) Interest expense 4 3 3 Other 2 — (2 ) Total other (income) and deductions (517 ) (454 ) (373 ) Income before income taxes 512 452 371 Income tax benefit (1 ) (6 ) (1 ) Net income $513 $458 $372 Refer to accompanying Notes to Condensed Financial Statements.
Condensed Balance Sheets	ALLIANT ENERGY CORPORATION (Parent Company Only) December 31, CONDENSED BALANCE SHEETS 2018 2017 (in millions) ASSETS Current assets: Notes receivable from affiliated companies $23 $50 Other 4 7 Total current assets 27 57 Investments: Investments in consolidated subsidiaries 5,518 4,676 Other 1 2 Total investments 5,519 4,678 Other assets 81 78 Total assets $5,627 $4,813 LIABILITIES AND EQUITY Current liabilities: Commercial paper $285 $295 Notes payable to affiliated companies 719 305 Other 21 12 Total current liabilities 1,025 612 Other liabilities 17 20 Common equity: Common stock and additional paid-in capital 2,048 1,848 Retained earnings 2,545 2,344 Accumulated other comprehensive income 2 — Shares in deferred compensation trust (10 ) (11 ) Total common equity 4,585 4,181 Total liabilities and equity $5,627 $4,813 Refer to accompanying Notes to Condensed Financial Statements.
Condensed Statements of Cash Flows	ALLIANT ENERGY CORPORATION (Parent Company Only) Year Ended December 31, CONDENSED STATEMENTS OF CASH FLOWS 2018 2017 2016 (in millions) Net cash flows from operating activities $311 $273 $254 Cash flows from (used for) investing activities: Capital contributions to consolidated subsidiaries (625 ) (290 ) (250 ) Capital repayments from consolidated subsidiaries — — 130 Net change in notes receivable from and payable to affiliates 441 54 294 Other — — 10 Net cash flows from (used for) investing activities (184 ) (236 ) 184 Cash flows used for financing activities: Common stock dividends (312 ) (288 ) (267 ) Proceeds from issuance of common stock, net 197 150 27 Payments to retire long-term debt — — (250 ) Net change in commercial paper (10 ) 103 52 Other (2 ) (2 ) — Net cash flows used for financing activities (127 ) (37 ) (438 ) Net increase (decrease) in cash, cash equivalents and restricted cash — — — Cash, cash equivalents and restricted cash at beginning of period — — — Cash, cash equivalents and restricted cash at end of period $— $— $— Supplemental cash flows information: Cash (paid) refunded during the period for: Interest, net of capitalized interest ($4 ) ($3 ) ($3 ) Income taxes, net 5 — (37 ) Refer to accompanying Notes to Condensed Financial Statements.

Summary Of Significant Accoun_4

Summary Of Significant Accounting Policies (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)customerMW	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Jan. 01, 2019USD ($)
New Accounting Standards:
Adjustment To Net Periodic Benefit Costs, Percentage	40.00%
Income statement impact of adopting new pension and other postretirement plans accounting standard		$ 17.8	$ 17.1
Cash receipts on sold receivables	$ 605.3	461.8	466.8
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%
New Accounting Standards:
Income statement impact of adopting new pension and other postretirement plans accounting standard		7.2	6.8
Cash receipts on sold receivables	$ 605.3	461.8	466.8
WPL [Member]
Property, Plant and Equipment:
AFUDC accrual recorded, percentage of estimated CWIP	50.00%
AFUDC rates, projects with approval	100.00%
New Accounting Standards:
Income statement impact of adopting new pension and other postretirement plans accounting standard		10.5	10.2
Alliant Energy Finance, LLC [Member]
General:
Ownership interest	50.00%
Electric capacity of wind farm (in megawatts) | MW	225
Alliant Energy and IPL [Member]
New Accounting Standards:
Cash receipts on sold receivables		$ 461.8	$ 466.8
Sheboygan Falls Energy Facility [Member]
General:
Fossil-fueled EGU capacity (in megawatts) | MW	347
Capital lease, lease term	20 years
Scenario, Forecast [Member]
New Accounting Standards:
Balance sheet impact of adopting new lease accounting standard				$ 18
Scenario, Forecast [Member] | IPL [Member]
New Accounting Standards:
Balance sheet impact of adopting new lease accounting standard				11
Scenario, Forecast [Member] | WPL [Member]
New Accounting Standards:
Balance sheet impact of adopting new lease accounting standard				$ 7

Summary Of Significant Accoun_5

Summary Of Significant Accounting Policies (Schedule Of Average Rates Of Depreciation) (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Electric - generation [Member] | IPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	3.60%	3.50%	3.50%
Electric - generation [Member] | WPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	3.60%	3.50%	3.10%
Electric - distribution [Member] | IPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	2.80%	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%	2.60%
Electric - other [Member] | IPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	4.70%	4.50%	4.20%
Electric - other [Member] | WPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	5.70%	6.90%	4.70%
Gas [Member] | IPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	3.20%	3.40%	3.30%
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	5.20%	4.00%	3.90%
Other [Member] | WPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	5.80%	6.00%	5.90%

Summary Of Significant Accoun_6

Summary Of Significant Accounting Policies (Schedule Of Allowance For Funds Used During Construction Recovery Rate) (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
IPL [Member] | IPL (Marshalltown CWIP) [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Recovery rates		7.80%	7.90%
IPL [Member] | IPL (Wind generation CWIP) [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Recovery rates	7.50%	7.60%
IPL [Member] | IPL (other CWIP) [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Recovery rates	7.50%	7.60%	7.70%
WPL [Member] | WPL (retail jurisdiction) [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Recovery rates	7.70%	7.60%	8.20%
WPL [Member] | WPL (wholesale jurisdiction) [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Recovery rates	7.20%	6.00%	6.70%

Regulatory Matters (Narrative)

Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Regulatory Matters [Line Items]
Regulatory liabilities	$ 1,493.2	$ 1,497.2
Refunded Federal Tax Reform benefits	66
Depreciation and amortization	506.9	461.8	$ 411.6
Revenue Subject to Refund [Member]
Regulatory Matters [Line Items]
Regulatory liabilities	20
IPL [Member]
Regulatory Matters [Line Items]
Regulatory assets not earning a return	77
Regulatory liabilities	754.9	755.4
Refunded Federal Tax Reform benefits	25
Other operation and maintenance	402.6	396.6	376.9
Depreciation and amortization	283.5	245	210.8
IPL [Member] | Revenue Subject to Refund [Member]
Regulatory Matters [Line Items]
Regulatory liabilities	15
WPL [Member]
Regulatory Matters [Line Items]
Regulatory assets not earning a return	6
Regulatory liabilities	738.3	741.8
Refunded Federal Tax Reform benefits	41
Other operation and maintenance	241.6	238.5	209.6
Depreciation and amortization	219.4	212.9	$ 192.5
WPL [Member] | Revenue Subject to Refund [Member]
Regulatory Matters [Line Items]
Regulatory liabilities	5
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%
Authorized increase (decrease) in final rates, amount	$ 130
Authorized increase (decrease) in final rates, percentage	9.00%
Write-down of regulatory assets		$ 9
2017/2018 Test Period Retail Electric [Member] | WPL [Member]
Regulatory Matters [Line Items]
Authorized increase (decrease) in final rates, amount		9
Increase in base rates		60
Reduction in fuel-related costs		51
2017/2018 Test Period Retail Electric [Member] | WPL [Member] | Revenue Subject to Refund [Member]
Regulatory Matters [Line Items]
Regulatory liabilities	$ 20
2017/2018 Test Period Retail Gas [Member] | WPL [Member]
Regulatory Matters [Line Items]
Authorized increase (decrease) in final rates, amount		9
Forward Contracts [Member] | IUB [Member] | IPL [Member]
Regulatory Matters [Line Items]
Recovery period, authorized	10 years
Sutherland Units 1 And 3 [Member] | 2016 Test Year Retail Electric [Member] | IPL [Member]
Regulatory Matters [Line Items]
Other operation and maintenance		4
Sutherland Units 1 And 3 [Member] | IUB [Member] | IPL [Member]
Regulatory Matters [Line Items]
Recovery period, authorized	10 years
AROs [Member] | 2016 Test Year Retail Electric [Member] | IPL [Member]
Regulatory Matters [Line Items]
Depreciation and amortization		$ 5
AROs [Member] | IUB [Member] | IPL [Member]
Regulatory Matters [Line Items]
Recovery period, authorized	4 years

Regulatory Matters (Regulatory

Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Regulatory assets	$ 1,737.3	$ 1,666.7
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	820.6	778.2
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	542.3	548
EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	111.6	63.8
AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	110.8	109.3
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	28	45.3
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	23.6	25.5
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	100.4	96.6
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,279	1,231.6
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	783.1	750.5
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	274	274.4
IPL [Member] | EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	55.4	31.6
IPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	76.3	72.5
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	15.1	21.8
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	23.6	25.5
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	51.5	55.3
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	458.3	435.1
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	37.5	27.7
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	268.3	273.6
WPL [Member] | EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	56.2	32.2
WPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	34.5	36.8
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	12.9	23.5
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 48.9	$ 41.3

Regulatory Matters (Electric Ge

Regulatory Matters (Electric Generating Units Retired Early (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Regulatory assets	$ 1,737.3	$ 1,666.7
EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	111.6	63.8
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,279	1,231.6
IPL [Member] | EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	55.4	31.6
IPL [Member] | EGUs retired early [Member] | Sutherland Units 1 And 3 [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 27.6
Recovery period, authorized	10 years
IPL [Member] | EGUs retired early [Member] | ML Kapp [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 27.8
Recovery period, authorized	10 years
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 458.3	435.1
WPL [Member] | EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	56.2	$ 32.2
WPL [Member] | EGUs retired early [Member] | Nelson Dewey Units 1 and 2 and Edgewater Unit 3 [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 27.4
Recovery period, authorized	10 years
WPL [Member] | EGUs retired early [Member] | Edgewater Unit 4 [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 28.8
Recovery period, authorized	10 years

Regulatory Matters (Regulator_2

Regulatory Matters (Regulatory Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 1,493.2	$ 1,497.2
Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	890.6	899.4
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	401.2	410
Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	104	90.4
WPL earnings sharing mechanism [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	25.4	8
Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	16.8	21
IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	6.4	25
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	48.8	43.4
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	754.9	755.4
IPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	390.1	399.5
IPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	273.3	274.5
IPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	47.7	26.4
IPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	11.9	14.6
IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	6.4	25
IPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	25.5	15.4
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	738.3	741.8
WPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	500.5	499.9
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	127.9	135.5
WPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	56.3	64
WPL [Member] | WPL earnings sharing mechanism [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	25.4	8
WPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	4.9	6.4
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 23.3	$ 28

Regulatory Matters (IPL's Tax B

Regulatory Matters (IPL's Tax Benefit Riders) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	$ (1.3)	$ 83.8	$ 63
Alliant Energy and IPL [Member] | Electric tax benefit rider credits [Member]
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	17
Alliant Energy and IPL [Member] | Gas tax benefit rider credits [Member]
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	2
Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	$ 19

Property, Plant and Equipment_2

Property, Plant and Equipment (Narrative) (Details) $ / shares in Units, $ in Millions		1 Months Ended	12 Months Ended
		Apr. 30, 2018USD ($)MW	Dec. 31, 2018USD ($)MW	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)$ / shares	Apr. 30, 2017USD ($)
Property, Plant and Equipment [Line Items]
Asset valuation charges, pre-tax			$ 0	$ 0	$ 86.4
Non-utility Generation, net	[1]		$ 86.9	$ 90.9
IPL [Member]
Property, Plant and Equipment [Line Items]
Net assets acquired						$ 32
WPL [Member]
Property, Plant and Equipment [Line Items]
Net assets acquired		$ 74
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
Expansion of Wind Generation [Member]
Property, Plant and Equipment [Line Items]
Electric capacity of wind farm (in megawatts) | MW			129
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
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
Sheboygan Falls Energy Facility [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life			35 years
Non-utility Generation, net			$ 87
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
[1]	Less accumulated depreciation of $54.5 million and $50.5 million for Alliant Energy as of December 31, 2018 and 2017, respectively.

Property, Plant and Equipment_3

Property, Plant and Equipment (Property, Plant and Equipment) (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Electric plant anticipated to be retired early	[1]	$ 0	$ 93
Total electric plant		12,663.6	12,296.9
Gas plant in service		1,387.6	1,244
Other plant in service		513.2	571.9
Accumulated depreciation		(4,314.6)	(4,283.1)
Net plant		10,249.8	9,829.7
Construction work in progress		1,774.8	962.2
Other, net		6.1	6
Total utility		12,030.7	10,797.9
Non-utility Generation, net	[2]	86.9	90.9
Corporate Services and other, net	[3]	344.8	345.7
Total non-utility and other		431.7	436.6
Total property, plant and equipment		12,462.4	11,234.5
Non-utility Generation, accumulated depreciation		54.5	50.5
Corporate Services and other, accumulated depreciation		167.5	285.6
Generation [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		6,800.6	6,655.3
Distribution [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		5,452.2	5,123.5
Other [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		410.8	425.1
IPL [Member]
Property, Plant and Equipment [Line Items]
Electric plant anticipated to be retired early		0	0
Total electric plant		6,894.5	6,819.1
Gas plant in service		763.1	654.8
Other plant in service		322.4	333.4
Accumulated depreciation		(2,294.7)	(2,311)
Net plant		5,685.3	5,496.3
Construction work in progress		1,091.2	424.4
Other, net		5	5.5
Total utility		6,781.5	5,926.2
Total property, plant and equipment		6,781.5	5,926.2
IPL [Member] | Generation [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		3,610.4	3,715.9
IPL [Member] | Distribution [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		3,023.7	2,820.9
IPL [Member] | Other [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		260.4	282.3
WPL [Member]
Property, Plant and Equipment [Line Items]
Electric plant anticipated to be retired early	[1]	0	93
Total electric plant		5,769.1	5,477.8
Gas plant in service		624.5	589.2
Other plant in service		190.8	238.5
Accumulated depreciation		(2,019.9)	(1,972.1)
Net plant		4,564.5	4,333.4
Leased Sheboygan Falls Energy Facility, net	[4]	38.1	46.2
Construction work in progress		683.6	537.8
Other, net		1.1	0.5
Total utility		5,287.3	4,917.9
Total property, plant and equipment		5,287.3	4,917.9
Leased Sheboygan Falls Energy Facility, accumulated depreciation		82.8	77.6
WPL [Member] | Generation [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		3,190.2	2,939.4
WPL [Member] | Distribution [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		2,428.5	2,302.6
WPL [Member] | Other [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		$ 150.4	$ 142.8
[1]	In 2018, WPL retired Edgewater Unit 4 and reclassified the remaining net book value of this EGU from property, plant and equipment to a regulatory asset on Alliant Energy’s and WPL’s balance sheets.
[2]	Less accumulated depreciation of $54.5 million and $50.5 million for Alliant Energy as of December 31, 2018 and 2017, respectively.
[3]	Less accumulated depreciation of $167.5 million and $285.6 million for Alliant Energy as of December 31, 2018 and 2017, respectively.
[4]	Less accumulated amortization of $82.8 million and $77.6 million for WPL as of December 31, 2018 and 2017, 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_4

Property, Plant and Equipment (Estimated Fair Value of Forward Wind Energy Center Assets and LIabilities Transferred) (Details) - WPL [Member] $ in Millions	Apr. 30, 2018USD ($)
Property, Plant and Equipment [Line Items]
Property, plant and equipment, net	$ 81
Liabilities	7
Net assets acquired	$ 74

Property, Plant and Equipment_5

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 Equipment_6

Property, Plant and Equipment (Equity and Debt AFUDC) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 75.6	$ 49.7	$ 62.5
Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	51.4	33.6	42.3
Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	24.2	16.1	20.2
IPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	42.2	31.4	52
IPL [Member] | Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	28.6	21.1	35.2
IPL [Member] | Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	13.6	10.3	16.8
WPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	33.4	18.3	10.5
WPL [Member] | Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	22.8	12.5	7.1
WPL [Member] | Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 10.6	$ 5.8	$ 3.4

Jointly-Owned Electric Utilit_3

Jointly-Owned Electric Utility Plant (Details) $ in Millions	Dec. 31, 2018USD ($)
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	$ 1,789.4
Accumulated Provision for Depreciation	622.3
Construction Work in Progress	617
IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	890.3
Accumulated Provision for Depreciation	333.2
Construction Work in Progress	68.7
WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	899.1
Accumulated Provision for Depreciation	289.1
Construction Work in Progress	$ 548.3
Ottumwa Unit 1 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	48.00%
Electric Plant	$ 507.1
Accumulated Provision for Depreciation	161.7
Construction Work in Progress	$ 65.3
George Neal Unit 4 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	25.70%
Electric Plant	$ 187.3
Accumulated Provision for Depreciation	89.8
Construction Work in Progress	$ 1.2
George Neal Unit 3 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	28.00%
Electric Plant	$ 156.7
Accumulated Provision for Depreciation	57.8
Construction Work in Progress	$ 1.4
Louisa Unit 1 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	4.00%
Electric Plant	$ 39.2
Accumulated Provision for Depreciation	23.9
Construction Work in Progress	$ 0.8
Columbia Units 1-2 [Member] | WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	52.50%
Electric Plant	$ 779.4
Accumulated Provision for Depreciation	247.9
Construction Work in Progress	$ 9.8
Forward Wind Energy Center [Member] | WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	42.60%
Electric Plant	$ 119.7
Accumulated Provision for Depreciation	41.2
Construction Work in Progress	$ 0.1
West Riverside [Member] | WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	91.80%	[1]
Electric Plant	$ 0	[1]
Accumulated Provision for Depreciation	0	[1]
Construction Work in Progress	$ 538.4	[1]
[1]	In January 2018, certain electric cooperatives, which currently have wholesale power supply agreements with WPL, acquired a partial ownership interest in West Riverside.

Receivables (Narrative) (Detail

Receivables (Narrative) (Details) $ in Millions	Dec. 31, 2018USD ($)
Accounts Receivable [Line Items]
Limit on cash proceeds to be received from third-party	$ 90
Maximum [Member]
Accounts Receivable [Line Items]
Limit on cash proceeds to be received from third-party	110
Receivables Sold [Member] | IPL [Member]
Accounts Receivable [Line Items]
Available capacity	$ 0

Receivables (Details of Account

Receivables (Details of Accounts Receivable) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Accounts Receivable [Line Items]
Customer	$ 91	$ 103.3
Unbilled utility revenues	74.2	85.1
Deferred proceeds	119.4	222.1
Other	76.3	84.3
Allowance for doubtful accounts	(10.5)	(12)
Accounts receivable, less allowance for doubtful accounts	350.4	482.8
IPL [Member]
Accounts Receivable [Line Items]
Customer	0	0
Unbilled utility revenues	0	0
Deferred proceeds	119.4	222.1
Other	37.2	44.1
Allowance for doubtful accounts	(3.1)	(1.3)
Accounts receivable, less allowance for doubtful accounts	153.5	264.9
WPL [Member]
Accounts Receivable [Line Items]
Customer	84.8	97.7
Unbilled utility revenues	74.2	85.1
Other	38.5	40.1
Allowance for doubtful accounts	(7.4)	(10.7)
Accounts receivable, less allowance for doubtful accounts	$ 190.1	$ 212.2

Receivables (Maximum And Averag

Receivables (Maximum And Average Outstanding Cash Proceeds) (Details) - IPL [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Maximum [Member]
Accounts Receivable [Line Items]
Outstanding aggregate cash proceeds (based on daily outstanding balances)	$ 116	$ 112	$ 172
Average [Member]
Accounts Receivable [Line Items]
Outstanding aggregate cash proceeds (based on daily outstanding balances)	$ 53.4	$ 62.2	$ 73.2

Receivables (Receivables Sold U

Receivables (Receivables Sold Under The Agreement) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Accounts Receivable [Line Items]
Fair value of deferred proceeds	$ 119.4	$ 222.1
IPL [Member]
Accounts Receivable [Line Items]
Fair value of deferred proceeds	119.4	222.1
Receivables Sold [Member] | IPL [Member]
Accounts Receivable [Line Items]
Customer accounts receivable	140.1	133.8
Unbilled utility revenues	97.1	112.7
Other receivables	0.1	0.3
Receivables sold to third party	237.3	246.8
Less: cash proceeds	108	12
Deferred proceeds	129.3	234.8
Less: allowance for doubtful accounts	9.9	12.7
Fair value of deferred proceeds	119.4	222.1
Outstanding receivables past due	$ 35.5	$ 44.7

Receivables (Additional Attribu

Receivables (Additional Attributes Of Receivables Sold Under The Agreement) (Details) - IPL [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts Receivable [Line Items]
Collections	$ 2,076.7	$ 1,647.1	$ 1,818.1
Write-offs, net of recoveries	$ 21.3	$ 17.7	$ 4.8

Investments (Narrative) (Detail

Investments (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018MW	Dec. 31, 2016USD ($)	Dec. 31, 2013
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
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

Investments (Unconsolidated Equ

Investments (Unconsolidated Equity Investments) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Carrying value		$ 421.3	$ 381.4	$ 326
Equity (income) / loss		(54.6)	(44.8)	(39.6)
WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity (income) / loss		(0.9)	(0.7)	(39.8)
ATC Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value	[1]	293.6	274.2
Equity (income) / loss	[1]	$ (38.1)	(42.4)	(39.1)
Non-utility wind farm in Oklahoma [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest		50.00%
Carrying value		$ 105.1	98.3
Equity (income) / loss		(15.6)	(1.8)	0
Other [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value		22.6	8.9
Equity (income) / loss		$ (0.9)	(0.6)	(0.5)
ATC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	[1]	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	0	(39.1)
Wisconsin River Power Company [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest		50.00%
Carrying value		$ 8.1	8.3
Equity (income) / loss		(0.9)	(0.7)	(0.7)
Totals [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value		421.3	381.4
Equity (income) / loss		(54.6)	(44.8)	(39.6)
Totals [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value		8.1	8.3
Equity (income) / loss		$ (0.9)	$ (0.7)	$ (39.8)
ATC Holdco LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	[1]	20.00%
[1]	As of December 31, 2018, Alliant Energy’s interest in ATC Holdings is comprised of a 16% ownership interest in ATC and a 20% ownership interest in ATC Holdco LLC, which are described below. Alliant Energy currently has the ability to exercise significant influence over ATC’s and ATC Holdco LLC’s financial and operating policies through its participation on ATC’s Board of Directors.

Investments (Summary Financial

Investments (Summary Financial Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues	$ 724	$ 741	$ 658
Operating income	325	374	331
Net income	217	267	232
Current assets	144	104
Non-current assets	5,498	5,041
Current liabilities	644	770
Non-current liabilities	2,315	2,038
Noncontrolling interest	223	255
WPL [Member]
Revenues	8	8	658
Operating income	2	4	331
Net income	1	2	$ 234
Current assets	4	7
Non-current assets	19	20
Current liabilities	1	2
Non-current liabilities	6	8
Noncontrolling interest	$ 0	$ 0

Common Equity (Narrative) (Deta

Common Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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)		11,200,000
Proceeds from issuance of common stock, net		$ 196.6	$ 149.6	$ 26.6
Initial forward sale price (in dollars per share)		$ 44.33
Ownership percentage		15.00%
Other comprehensive income (loss)		$ 2.2	(0.1)	0
IPL [Member]
Schedule of Common Equity [Line Items]
Other comprehensive income (loss)		0	0	0
WPL [Member]
Schedule of Common Equity [Line Items]
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		$ 175	$ 125
Common stock issued during the period, At-the-market offering programs (in shares)		4,171,013	3,074,931	0
Proceeds from issuance of common stock, net		$ 173	$ 124
Fees and commissions from issuance of common stock		$ 2	$ 1
Scenario, Forecast [Member]
Schedule of Common Equity [Line Items]
Common stock issued during the period, At-the-market offering programs (in shares)	8,358,973

Common Equity (Common Share Act

Common Equity (Common Share Activity) (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Common Stock Oustanding [Roll Forward]
Shares outstanding, January 1 (in shares)	231,348,646	227,673,654	226,918,432
Common stock issued during the period, Shareowner Direct Plan (in shares)	576,965	640,723	732,814
Equity-based compensation plans (in shares)	5,078	5,185	22,408
Other (in shares)	(38,423)	(45,847)	0
Shares outstanding, December 31 (in shares)	236,063,279	231,348,646	227,673,654
At The Market Offering Program [Member]
Common Stock Oustanding [Roll Forward]
Common stock issued during the period, At-the-market offering programs (in shares)	4,171,013	3,074,931	0

Preferred Stock (Narrative) (De

Preferred Stock (Narrative) (Details) - IPL [Member]	12 Months Ended
	Dec. 31, 2018Qboard_member$ / sharesshares	Dec. 31, 2017shares
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

Preferred Stock (Carrying Value

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, 2018	Dec. 31, 2017
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
	Jun. 30, 2018	Dec. 31, 2019	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]
Repayments 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, 2018	Dec. 31, 2017
Commercial paper:
Amount outstanding	$ 441.2	$ 320.2
Weighted average interest rates	2.80%	2.00%
Available credit facility capacity	$ 558.8	$ 679.8
IPL [Member]
Commercial paper:
Amount outstanding	$ 50.4	0
Weighted average interest rates	2.80%
Available credit facility capacity	$ 199.6	250
WPL [Member]
Commercial paper:
Amount outstanding	$ 105.5	$ 25
Weighted average interest rates	2.50%	1.50%
Available credit facility capacity	$ 244.5	$ 325

Debt (Other Short-Term Borrowin

Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 446.5	$ 424.4
Average amount outstanding (based on daily outstanding balances)	$ 221.4	$ 294.3
Weighted average interest rates	2.20%	1.20%
IPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 50.4	$ 20
Average amount outstanding (based on daily outstanding balances)	$ 1.5	$ 0.5
Weighted average interest rates	2.30%	1.30%
WPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 126	$ 271.2
Average amount outstanding (based on daily outstanding balances)	$ 36.6	$ 118.2
Weighted average interest rates	2.10%	1.00%

Debt (Schedule Of Debt-To-Capit

Debt (Schedule Of Debt-To-Capital Ratios) (Details)	Dec. 31, 2018
Actual debt-to-capital ratio	55.00%
IPL [Member]
Actual debt-to-capital ratio	45.00%
WPL [Member]
Actual debt-to-capital ratio	48.00%
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		Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term debt		$ 5,546.7	$ 4,902.5
Current maturities		(256.5)	(855.7)
Unamortized debt issuance costs		(32.1)	(25.4)
Unamortized debt (discount) and premium, net		(11.8)	(10.8)
Long-term debt, net	[1]	5,246.3	4,010.6
Senior Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	2,575	2,425
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]	$ 500	$ 500
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] | 4.1% senior debenture, due 2028 [Member]
Debt Instrument [Line Items]
Long-term debt	[2],[3]	$ 500
Interest rate		4.10%
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%
Senior Debentures [Member] | 5.875% senior debenture, retired in 2018 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 0	$ 100
Interest rate		0.00%	5.875%
Senior Debentures [Member] | 7.25% senior debenture, retired in 2018 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 0	$ 250
Interest rate		0.00%	7.25%
Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 1,850	$ 1,850
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]	$ 300	$ 300
Interest rate		3.05%	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		$ 1,121.7	$ 627.5
Other [Member] | Other, 1% at December 31, 2018, due 2019 to 2025 [Member]
Debt Instrument [Line Items]
Long-term debt		$ 2.4	2.9
Interest rate		1.00%
IPL [Member]
Debt Instrument [Line Items]
Long-term debt		$ 2,575	2,425
Current maturities		0	(350)
Unamortized debt issuance costs		(17.2)	(14.3)
Unamortized debt (discount) and premium, net		(5.5)	(4.7)
Long-term debt, net	[1]	2,552.3	2,056
IPL [Member] | Senior Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	2,575	2,425
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]	$ 500	$ 500
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] | 4.1% senior debenture, due 2028 [Member]
Debt Instrument [Line Items]
Long-term debt	[2],[3]	$ 500
Interest rate		4.10%
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%
IPL [Member] | Senior Debentures [Member] | 5.875% senior debenture, retired in 2018 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 0	$ 100
Interest rate		0.00%	5.875%
IPL [Member] | Senior Debentures [Member] | 7.25% senior debenture, retired in 2018 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 0	$ 250
Interest rate		0.00%	7.25%
WPL [Member]
Debt Instrument [Line Items]
Long-term debt		$ 1,850	$ 1,850
Current maturities		(250)	0
Unamortized debt issuance costs		(9.6)	(10.5)
Unamortized debt (discount) and premium, net		(5.5)	(6.1)
Long-term debt, net	[1]	1,584.9	1,833.4
WPL [Member] | Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	1,850	1,850
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]	$ 300	$ 300
Interest rate		3.05%	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 April 2020 [Member]
Debt Instrument [Line Items]
Long-term debt	[4]	$ 300
Interest rate		3.00%
Alliant Energy Finance, LLC [Member] | Term Loan Credit Agreement [Member] | Term loan credit agreement, retired in 2018, 2% at December 31, 2017 [Member]
Debt Instrument [Line Items]
Long-term debt	[5]	$ 0	$ 500
Interest rate			2.00%
Alliant Energy Finance, LLC [Member] | Senior Notes [Member] | 3.75% senior notes, due 2023 [Member]
Debt Instrument [Line Items]
Long-term debt	[2],[5]	$ 400
Interest rate		3.75%
Alliant Energy Finance, LLC [Member] | Senior Notes [Member] | 4.25% senior notes, due 2028 [Member]
Debt Instrument [Line Items]
Long-term debt	[2],[5]	$ 300
Interest rate		4.25%
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 2019 to 2024 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 44.3	$ 49.6
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 September 2018, IPL issued $500 million of 4.1% senior debentures due 2028. The senior debentures were issued as green bonds, and all of the net proceeds were allocated for the construction and development of wind and solar projects.
[4]	In April 2018, AEF entered into a $300 million variable-rate term loan credit agreement and used the proceeds from borrowings under this agreement for general corporate purposes. Refer to Note 9(a) for discussion of a financial covenant contained in AEF’s term loan credit agreement.
[5]	In June 2018, AEF issued $400 million of 3.75% senior notes due 2023 and $300 million of 4.25% senior notes due 2028. The proceeds from the issuances were used by AEF to retire its $500 million and $95 million variable-rate term loan credit agreements expiring in 2018, to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes.

Debt (Schedule Of Debt Maturiti

Debt (Schedule Of Debt Maturities) (Details) $ in Millions	Dec. 31, 2018USD ($)
2,019	$ 256
2,020	657
2,021	8
2,022	333
2,023	408
IPL [Member]
2,019	0
2,020	200
2,021	0
2,022	0
2,023	0
WPL [Member]
2,019	250
2,020	150
2,021	0
2,022	250
2,023	0
Corporate Services [Member]
2,019	0
2,020	0
2,021	0
2,022	75
2,023	0
Alliant Energy Finance, LLC [Member]
2,019	6
2,020	307
2,021	8
2,022	8
2,023	$ 408

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, 2018USD ($)
Operating Leased Assets [Line Items]
2,019	$ 5
2,020	5
2,021	3
2,022	3
2,023	2
Thereafter	12
Total future minimum operating lease payments, excluding contingent rentals	30
IPL [Member]
Operating Leased Assets [Line Items]
2,019	3
2,020	2
2,021	2
2,022	2
2,023	2
Thereafter	12
Total future minimum operating lease payments, excluding contingent rentals	23
WPL [Member]
Operating Leased Assets [Line Items]
2,019	2
2,020	3
2,021	1
2,022	0
2,023	0
Thereafter	0
Total future minimum operating lease payments, excluding contingent rentals	$ 6

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Interest expense	$ 8	$ 9	$ 9
Depreciation and amortization	6	6	6
Total lease expenses included in income statements	$ 14	$ 15	$ 15

Leases (Future Minimum Capital

Leases (Future Minimum Capital Lease Payments) (Details) - WPL [Member] - Sheboygan Falls Energy Facility [Member] $ in Millions	Dec. 31, 2018USD ($)
Schedule of Future Lease Payments for Capital Leases [Line Items]
2,019	$ 15
2,020	15
2,021	15
2,022	15
2,023	15
Thereafter	19
Total estimated future minimum capital lease payments	94
Less: amount representing interest	26
Present value of minimum capital lease payments	$ 68

Disaggregation of Revenues (Det

Disaggregation of Revenues (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenues	$ 3,534.5	$ 3,382.2	$ 3,320
IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,042.3	1,870.3	1,820.4
WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,452.6	1,472.8	1,459.1
Electric [Member]
Disaggregation of Revenue [Line Items]
Revenues	3,000.3	2,894.7	2,875.5
Electric [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,731.1	1,598.9	1,569.7
Electric [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,269.2	1,295.8	1,305.8
Electric [Member] | Retail - Residential [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,063.4	1,006.2	1,001.1
Electric [Member] | Retail - Residential [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	590.6	535.6	536.7
Electric [Member] | Retail - Residential [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	472.8	470.6	464.4
Electric [Member] | Retail - Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenues	735.6	710.3	712.6
Electric [Member] | Retail - Commercial [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	486.8	452.7	445.4
Electric [Member] | Retail - Commercial [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	248.8	257.6	267.2
Electric [Member] | Retail - Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenues	888.8	853.1	851.1
Electric [Member] | Retail - Industrial [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	500.8	459.7	460.4
Electric [Member] | Retail - Industrial [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	388	393.4	390.7
Electric [Member] | Wholesale [Member]
Disaggregation of Revenue [Line Items]
Revenues	188.4	238.4	256.6
Electric [Member] | Wholesale [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	71.2	95.5	94.2
Electric [Member] | Wholesale [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	117.2	142.9	162.4
Electric [Member] | Other Customer [Member]
Disaggregation of Revenue [Line Items]
Revenues	124.1	86.7	54.1
Electric [Member] | Other Customer [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	81.7	55.4	33
Electric [Member] | Other Customer [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	42.4	31.3	21.1
Gas [Member]
Disaggregation of Revenue [Line Items]
Revenues	446.6	400.9	355.4
Gas [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	266.2	226	204
Gas [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	180.4	174.9	151.4
Gas [Member] | Retail - Residential [Member]
Disaggregation of Revenue [Line Items]
Revenues	254.4	224.7	197.6
Gas [Member] | Retail - Residential [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	152.3	123.2	110.6
Gas [Member] | Retail - Residential [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	102.1	101.5	87
Gas [Member] | Retail - Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenues	133	123.2	109.6
Gas [Member] | Retail - Commercial [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	75.9	67.9	61.9
Gas [Member] | Retail - Commercial [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	57.1	55.3	47.7
Gas [Member] | Retail - Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenues	14.9	16.7	15.2
Gas [Member] | Retail - Industrial [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	10.2	11.1	10.6
Gas [Member] | Retail - Industrial [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	4.7	5.6	4.6
Gas [Member] | Other Customer [Member]
Disaggregation of Revenue [Line Items]
Revenues	44.3	36.3	33
Gas [Member] | Other Customer [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	27.8	23.8	20.9
Gas [Member] | Other Customer [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	16.5	12.5	12.1
Other Utility [Member]
Disaggregation of Revenue [Line Items]
Revenues	48	47.5	48.6
Other Utility [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	45	45.4	46.7
Other Utility [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	3	2.1	1.9
Other Utility [Member] | Other Customer [Member]
Disaggregation of Revenue [Line Items]
Revenues	12.8	12.9	14.7
Other Utility [Member] | Other Customer [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	9.8	10.8	12.8
Other Utility [Member] | Other Customer [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	3	2.1	1.9
Other Utility [Member] | Steam [Member]
Disaggregation of Revenue [Line Items]
Revenues	35.2	34.6	33.9
Other Utility [Member] | Steam [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	35.2	34.6	33.9
Other Segments [Member]
Disaggregation of Revenue [Line Items]
Revenues	39.6	39.1	40.5
Other Segments [Member] | Other Customer [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 39.6	$ 39.1	$ 40.5

Income Taxes (Narrative) (Detai

Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2021	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax [Line Items]
Statutory federal income tax rate			21.00%	35.00%	35.00%
Income tax expense (benefits) Federal Tax Reform			$ (5.6)	$ (18.1)
Overall income tax rate			8.40%	12.50%	13.40%
Decrease in regulatory assets			$ (16.2)	$ (130.8)	$ (3.6)
Decrease in regulatory liabilities			$ 1.3	$ (83.8)	$ (63)
IPL [Member]
Income Tax [Line Items]
Statutory federal income tax rate			21.00%	35.00%	35.00%
Income tax expense (benefits) Federal Tax Reform			$ (1.1)	$ 3.8
Overall income tax rate			(1.20%)	(5.00%)	(2.70%)
Decrease in regulatory assets			$ (20.2)	$ (126.2)	$ (54.7)
Decrease in regulatory liabilities			$ 0.6	$ (71.2)	$ (67.3)
WPL [Member]
Income Tax [Line Items]
Statutory federal income tax rate			21.00%	35.00%	35.00%
Income tax expense (benefits) Federal Tax Reform			$ (5.5)	$ (14.5)
Overall income tax rate			14.80%	24.90%	32.60%
Decrease in regulatory assets			$ 4	$ (4.7)	$ 51.1
Iowa Tax Reform [Member] | Alliant Energy and IPL [Member]
Income Tax [Line Items]
Decrease in regulatory assets	$ 33.7
Decrease in regulatory liabilities	$ 7.3
State [Member] | IPL [Member]
Income Tax [Line Items]
Overall income tax rate			12.00%
State [Member] | IPL [Member] | Scenario, Forecast [Member]
Income Tax [Line Items]
Overall income tax rate		9.80%

Income Taxes (Schedule Of Compo

Income Taxes (Schedule Of Components Of Income Tax) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current tax expense (benefit):
Federal	$ (1)	$ (41)	$ 1.8
State	(5.1)	8.5	17.2
Deferred tax expense (benefit):
Federal	67.9	159.5	112.8
State	29.8	12.3	4.9
Production tax credits	(29.5)	(31.1)	(31.8)
Investment tax credits	(1.2)	(1.1)	(1.3)
Income tax expense (benefit)	47.7	66.7	59.4
IPL's tax benefit riders [Member]
Current tax expense (benefit):
IPL's tax benefit riders	(13.2)	(40.4)	(44.2)
IPL [Member]
Current tax expense (benefit):
Federal	14.9	(27.9)	(12.8)
State	(7.1)	1.6	15.5
Deferred tax expense (benefit):
Federal	9.5	72.5	59.1
State	7.3	(2.2)	(9)
Production tax credits	(14)	(14.1)	(14)
Investment tax credits	(0.6)	(0.4)	(0.5)
Income tax expense (benefit)	(3.2)	(10.9)	(5.9)
IPL [Member] | IPL's tax benefit riders [Member]
Current tax expense (benefit):
IPL's tax benefit riders	(13.2)	(40.4)	(44.2)
WPL [Member]
Current tax expense (benefit):
Federal	(9.2)	5.5	(22.3)
State	(4.4)	2.5	1.1
Deferred tax expense (benefit):
Federal	43.8	55	112.3
State	22.1	16.6	20.8
Production tax credits	(15.5)	(17)	(17.8)
Investment tax credits	(0.6)	(0.7)	(0.8)
Income tax expense (benefit)	$ 36.2	$ 61.9	$ 93.3

Income Taxes (Schedule Of Effec

Income Taxes (Schedule Of Effective Income Tax Rates) (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective Tax Rate [Line Items]
Statutory federal income tax rate	21.00%	35.00%	35.00%
State income taxes, net of federal benefits	7.00%	5.50%	5.40%
Effect of rate-making on property-related differences	(7.60%)	(8.50%)	(8.50%)
Production tax credits	(5.20%)	(6.10%)	(7.20%)
Adjustment for prior period taxes	(2.30%)	(1.50%)	(0.80%)
Federal Tax Reform Adjustments	(1.00%)	(3.40%)	(0.00%)
Other items, net	(1.20%)	(0.90%)	(0.50%)
Overall income tax rate	8.40%	12.50%	13.40%
IPL [Member]
Effective Tax Rate [Line Items]
Statutory federal income tax rate	21.00%	35.00%	35.00%
State income taxes, net of federal benefits	7.70%	6.50%	6.40%
Effect of rate-making on property-related differences	(14.00%)	(19.10%)	(16.20%)
Production tax credits	(5.20%)	(6.70%)	(6.30%)
Adjustment for prior period taxes	(4.80%)	(3.40%)	(1.20%)
Federal Tax Reform Adjustments	(0.40%)	1.70%	(0.00%)
Other items, net	(0.60%)	(0.30%)	(0.30%)
Overall income tax rate	(1.20%)	(5.00%)	(2.70%)
WPL [Member]
Effective Tax Rate [Line Items]
Statutory federal income tax rate	21.00%	35.00%	35.00%
State income taxes, net of federal benefits	6.20%	5.10%	5.10%
Effect of rate-making on property-related differences	(2.30%)	(1.70%)	(0.70%)
Production tax credits	(6.40%)	(7.10%)	(6.20%)
Adjustment for prior period taxes	(0.20%)	(0.00%)	(0.10%)
Federal Tax Reform Adjustments	(2.30%)	(5.80%)	(0.00%)
Other items, net	(1.20%)	(0.60%)	(0.50%)
Overall income tax rate	14.80%	24.90%	32.60%
IPL's tax benefit riders [Member]
Effective Tax Rate [Line Items]
IPL’s tax benefit riders	(2.30%)	(7.60%)	(10.00%)
IPL's tax benefit riders [Member] | IPL [Member]
Effective Tax Rate [Line Items]
IPL’s tax benefit riders	(4.90%)	(18.70%)	(20.10%)

Income Taxes (Schedule Of Defer

Income Taxes (Schedule Of Deferred Tax Assets And Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Deferred tax liabilities, property	$ 1,975.5	$ 1,852.7
Deferred tax liabilities, other	80.5	75.9
Total deferred tax liabilities	2,158.4	2,015
Deferred tax assets, federal credit carryforwards	299.1	260.7
Deferred tax assets, net operating losses carryforwards - federal	158.6	174.4
Deferred tax assets, net operating losses carryforwards - state	47	41.3
Deferred tax assets, other	59.8	69.2
Subtotal deferred tax assets	564.5	545.6
Deferred tax assets, valuation allowances	(9.2)	(9)
Total deferred tax assets	555.3	536.6
Total deferred tax liabilities, net	1,603.1	1,478.4
IPL [Member]
Deferred tax liabilities, property	1,158.4	1,102.6
Deferred tax liabilities, other	69.4	63.4
Total deferred tax liabilities	1,227.8	1,166
Deferred tax assets, federal credit carryforwards	133.2	113.1
Deferred tax assets, net operating losses carryforwards - federal	114.1	107.4
Deferred tax assets, net operating losses carryforwards - state	0.9	0.9
Deferred tax assets, other	22.8	34.5
Subtotal deferred tax assets	271	255.9
Deferred tax assets, valuation allowances	(0.5)	(0.6)
Total deferred tax assets	270.5	255.3
Total deferred tax liabilities, net	957.3	910.7
WPL [Member]
Deferred tax liabilities, property	735.2	674.2
Deferred tax liabilities, other	34.4	36.5
Total deferred tax liabilities	769.6	710.7
Deferred tax assets, federal credit carryforwards	147.4	131
Deferred tax assets, net operating losses carryforwards - federal	26.4	43.7
Deferred tax assets, net operating losses carryforwards - state	0.5	0.2
Deferred tax assets, other	14.1	14.7
Subtotal deferred tax assets	188.4	189.6
Deferred tax assets, valuation allowances	(0.8)	(1.3)
Total deferred tax assets	187.6	188.3
Total deferred tax liabilities, net	582	522.4
ATC Holdings [Member]
Deferred tax liabilities, ATC Holdings	$ 102.4	$ 86.4

Income Taxes (Summary Of Tax Cr

Income Taxes (Summary Of Tax Credit Carryforwards) (Details) $ in Millions	Dec. 31, 2018USD ($)
Federal [Member]
Tax carryforwards, net operating losses	$ 766
Tax carryforwards, tax credits	299
Federal [Member] | IPL [Member]
Tax carryforwards, net operating losses	551
Tax carryforwards, tax credits	133
Federal [Member] | WPL [Member]
Tax carryforwards, net operating losses	126
Tax carryforwards, tax credits	147
State [Member]
Tax carryforwards, net operating losses	792
State [Member] | IPL [Member]
Tax carryforwards, net operating losses	13
State [Member] | WPL [Member]
Tax carryforwards, net operating losses	$ 8

Income Taxes (Schedule Of Uncer

Income Taxes (Schedule Of Uncertain Tax Positions) (Details)	12 Months Ended
	Dec. 31, 2018
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 ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
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%	11.50%
Unrecognized compensation cost	$ 4.9
Alliant Energy common stock closing price on December 31, 2018 (in dollars per share)	$ 42.25
Percentage of base salary and performance-based compensation	100.00%
Carrying value of deferred compensation obligations	$ 21	$ 21.8
Performance Shares and Performance Units [Member]
Benefit Plans [Line Items]
Performance period	3 years
Instrument valuation based on shares of common stock, number of shares	1
Actual number of shares paid out upon vesting, minimum percentage of target shares	0.00%
Actual number of shares paid out upon vesting, maximum percentage of target shares	200.00%
Performance Restricted Stock Unit [Member]
Benefit Plans [Line Items]
Performance period	3 years
Instrument valuation based on shares of common stock, number of shares	1
Actual number of shares paid out upon vesting, minimum percentage of target shares	0.00%
Actual number of shares paid out upon vesting, maximum percentage of target shares	200.00%
Restricted Stock Units [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	$ 2.7
WPL [Member]
Benefit Plans [Line Items]
Unrecognized compensation cost	2.1
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	6,800,000

Benefit Plans (Assumptions Used

Benefit Plans (Assumptions Used To Measure Benefit Plans) (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate for benefit obligations	4.34%	3.66%	4.19%
Discount rate for net periodic cost	3.66%	4.19%	4.47%
Expected rate of return on plan assets	7.60%	7.60%	7.60%
Interest crediting rate for Alliant Energy Cash Balance Pension Plan	5.04%	4.64%	3.17%
Pension Plans, Defined Benefit [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate for benefit obligations	4.35%	3.68%	4.22%
Discount rate for net periodic cost	3.68%	4.22%	4.50%
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	4.35%	3.69%	4.23%
Discount rate for net periodic cost	3.69%	4.23%	4.51%
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	4.24%	3.53%	3.98%
Discount rate for net periodic cost	3.53%	3.98%	4.30%
Expected rate of return on plan assets	5.44%	5.80%	6.30%
Medical cost trend on covered charges, initial trend rate (end of year)	6.50%	6.75%	7.00%
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	4.23%	3.51%	3.95%
Discount rate for net periodic cost	3.51%	3.95%	4.28%
Expected rate of return on plan assets	5.60%	6.20%	6.60%
Medical cost trend on covered charges, initial trend rate (end of year)	6.50%	6.75%	7.00%
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	4.23%	3.51%	3.96%
Discount rate for net periodic cost	3.51%	3.96%	4.28%
Expected rate of return on plan assets	3.84%	3.50%	4.70%
Medical cost trend on covered charges, initial trend rate (end of year)	6.50%	6.75%	7.00%
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, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 12.1	$ 12.5	$ 12.6
Interest cost		46.8	51	53
Expected return on plan assets	[1]	(69.7)	(65.5)	(65.5)
Amortization of prior service cost (credit)		(0.7)	(0.4)	(0.3)
Amortization of actuarial loss	[2]	35.2	37.6	37.4
Settlement losses	[3]	0	0.9	0
Total		23.7	36.1	37.2
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		4.2	5	5.3
Interest cost		7.7	8.6	9.4
Expected return on plan assets	[1]	(6)	(6.1)	(6.1)
Amortization of prior service cost (credit)	[4]	(0.2)	(0.2)	(4.1)
Amortization of actuarial loss	[2]	3.4	3.8	4.7
Settlement losses		0	0	0
Total		9.1	11.1	9.2
IPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		7.4	7.3	7.5
Interest cost		21.4	23.5	24.5
Expected return on plan assets	[1]	(32.6)	(30.8)	(30.9)
Amortization of prior service cost (credit)		(0.2)	(0.2)	(0.2)
Amortization of actuarial loss	[2]	14.9	16.1	16.5
Total		10.9	15.9	17.4
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		1.7	2.1	2.3
Interest cost		3.1	3.5	3.8
Expected return on plan assets	[1]	(4.4)	(4.3)	(4.3)
Amortization of prior service cost (credit)	[4]	0	0	(2.6)
Amortization of actuarial loss	[2]	1.3	2	2.6
Total		1.7	3.3	1.8
WPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		4.4	4.9	4.9
Interest cost		20.2	21.8	22.3
Expected return on plan assets	[1]	(30.4)	(28.5)	(28.3)
Amortization of prior service cost (credit)		(0.1)	0.1	0.2
Amortization of actuarial loss	[2]	17.2	18.5	17.6
Total		11.3	16.8	16.7
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		1.6	1.9	2
Interest cost		3.1	3.4	3.8
Expected return on plan assets	[1]	(0.6)	(0.8)	(0.8)
Amortization of prior service cost (credit)	[4]	(0.2)	(0.2)	(0.9)
Amortization of actuarial loss	[2]	2	1.6	1.8
Total		$ 5.9	$ 5.9	$ 5.9
[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 (Funded Status of

Benefit Plans (Funded Status of Benefits Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plans, Defined Benefit [Member]
Change in benefit obligation:
Net benefit obligation at January 1	$ 1,303.1	$ 1,244.3
Service cost	12.1	12.5	$ 12.6
Interest cost	46.8	51	53
Plan participants' contributions	0	0
Actuarial (gain) loss	(96.2)	83.6
Gross benefits paid	(90.8)	(88.3)
Net benefit obligation at December 31	1,175	1,303.1	1,244.3
Change in plan assets:
Fair value of plan assets at January 1	950.7	895.7
Actual return on plan assets	(57.8)	136.7
Employer contributions	6.5	6.6
Plan participants' contributions	0	0
Gross benefits paid	(90.8)	(88.3)
Fair value of plan assets at December 31	808.6	950.7	895.7
Under funded status at December 31	(366.4)	(352.4)
Pension Plans, Defined Benefit [Member] | IPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	592.9	570.4
Service cost	7.4	7.3	7.5
Interest cost	21.4	23.5	24.5
Plan participants' contributions	0	0
Actuarial (gain) loss	(44.4)	34.9
Gross benefits paid	(43.4)	(43.2)
Net benefit obligation at December 31	533.9	592.9	570.4
Change in plan assets:
Fair value of plan assets at January 1	443.7	422
Actual return on plan assets	(26.8)	64.3
Employer contributions	4.3	0.6
Plan participants' contributions	0	0
Gross benefits paid	(43.4)	(43.2)
Fair value of plan assets at December 31	377.8	443.7	422
Under funded status at December 31	(156.1)	(149.2)
Pension Plans, Defined Benefit [Member] | WPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	559.8	529.2
Service cost	4.4	4.9	4.9
Interest cost	20.2	21.8	22.3
Plan participants' contributions	0	0
Actuarial (gain) loss	(40)	38.3
Gross benefits paid	(37.8)	(34.4)
Net benefit obligation at December 31	506.6	559.8	529.2
Change in plan assets:
Fair value of plan assets at January 1	415	389.7
Actual return on plan assets	(25.1)	59.6
Employer contributions	0.1	0.1
Plan participants' contributions	0	0
Gross benefits paid	(37.8)	(34.4)
Fair value of plan assets at December 31	352.2	415	389.7
Under funded status at December 31	(154.4)	(144.8)
Other Postretirement Benefits Plans [Member]
Change in benefit obligation:
Net benefit obligation at January 1	222.3	220.1
Service cost	4.2	5	5.3
Interest cost	7.7	8.6	9.4
Plan participants' contributions	3.1	2.9
Actuarial (gain) loss	(8.2)	5.4
Gross benefits paid	(23)	(19.7)
Net benefit obligation at December 31	206.1	222.3	220.1
Change in plan assets:
Fair value of plan assets at January 1	111.1	105.8
Actual return on plan assets	(2.6)	12.9
Employer contributions	10.5	9.2
Plan participants' contributions	3.1	2.9
Gross benefits paid	(23)	(19.7)
Fair value of plan assets at December 31	99.1	111.1	105.8
Under funded status at December 31	(107)	(111.2)
Other Postretirement Benefits Plans [Member] | IPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	89.4	90.1
Service cost	1.7	2.1	2.3
Interest cost	3.1	3.5	3.8
Plan participants' contributions	1	1
Actuarial (gain) loss	(4.3)	(0.1)
Gross benefits paid	(8.4)	(7.2)
Net benefit obligation at December 31	82.5	89.4	90.1
Change in plan assets:
Fair value of plan assets at January 1	72.9	68.2
Actual return on plan assets	(1.4)	8.9
Employer contributions	2.6	2
Plan participants' contributions	1	1
Gross benefits paid	(8.4)	(7.2)
Fair value of plan assets at December 31	66.7	72.9	68.2
Under funded status at December 31	(15.8)	(16.5)
Other Postretirement Benefits Plans [Member] | WPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	90.4	88.9
Service cost	1.6	1.9	2
Interest cost	3.1	3.4	3.8
Plan participants' contributions	1.4	1.4
Actuarial (gain) loss	(2.5)	4.1
Gross benefits paid	(10.8)	(9.3)
Net benefit obligation at December 31	83.2	90.4	88.9
Change in plan assets:
Fair value of plan assets at January 1	18.7	18.6
Actual return on plan assets	(0.1)	1.2
Employer contributions	7.5	6.8
Plan participants' contributions	1.4	1.4
Gross benefits paid	(10.8)	(9.3)
Fair value of plan assets at December 31	16.7	18.7	$ 18.6
Under funded status at December 31	$ (66.5)	$ (71.7)

Benefit Plans (Amounts Recogniz

Benefit Plans (Amounts Recognized On The Consolidated Balance Sheets) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Amounts recognized on the balance sheets consist of:
Pension and other benefit obligations	$ (509.1)	$ (504)
Pension Plans, Defined Benefit [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	0	0
Current liabilities	(2.3)	(2.2)
Pension and other benefit obligations	(364.1)	(350.2)
Net amounts recognized at December 31	(366.4)	(352.4)
Amounts recognized in Regulatory Assets consist of:
Net actuarial loss	505.2	509.1
Prior service credit	(5.8)	(6.5)
Net amount recognized at December 31	499.4	502.6
Other Postretirement Benefits Plans [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	7.5	8.8
Current liabilities	(9.6)	(9.1)
Pension and other benefit obligations	(104.9)	(110.9)
Net amounts recognized at December 31	(107)	(111.2)
Amounts recognized in Regulatory Assets consist of:
Net actuarial loss	44.5	47.4
Prior service credit	(1.1)	(1.3)
Net amount recognized at December 31	43.4	46.1
IPL [Member]
Amounts recognized on the balance sheets consist of:
Pension and other benefit obligations	(178.4)	(173.8)
IPL [Member] | Pension Plans, Defined Benefit [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	0	0
Current liabilities	(0.6)	(0.5)
Pension and other benefit obligations	(155.5)	(148.7)
Net amounts recognized at December 31	(156.1)	(149.2)
Amounts recognized in Regulatory Assets consist of:
Net actuarial loss	218.8	218.9
Prior service credit	(1.8)	(2.1)
Net amount recognized at December 31	217	216.8
IPL [Member] | Other Postretirement Benefits Plans [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	4.7	5.9
Current liabilities	(1.9)	(2)
Pension and other benefit obligations	(18.6)	(20.4)
Net amounts recognized at December 31	(15.8)	(16.5)
Amounts recognized in Regulatory Assets consist of:
Net actuarial loss	18.8	18.7
Prior service credit	0	0
Net amount recognized at December 31	18.8	18.7
WPL [Member]
Amounts recognized on the balance sheets consist of:
Pension and other benefit obligations	(217.7)	(213.7)
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	(154.3)	(144.7)
Net amounts recognized at December 31	(154.4)	(144.8)
Amounts recognized in Regulatory Assets consist of:
Net actuarial loss	223	224.7
Prior service credit	(1.3)	(1.5)
Net amount recognized at December 31	221.7	223.2
WPL [Member] | Other Postretirement Benefits Plans [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	2.8	2.9
Current liabilities	(7.4)	(6.8)
Pension and other benefit obligations	(61.9)	(67.8)
Net amounts recognized at December 31	(66.5)	(71.7)
Amounts recognized in Regulatory Assets consist of:
Net actuarial loss	19.9	23.6
Prior service credit	(1.1)	(1.3)
Net amount recognized at December 31	$ 18.8	$ 22.3

Benefit Plans (Accumulated Bene

Benefit Plans (Accumulated Benefit Obligations) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	$ 1,139.9	$ 1,269
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	1,139.9	1,269
Fair value of plan assets	808.6	950.7
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	1,175	1,303.1
Fair value of plan assets	808.6	950.7
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	206.1	222.3
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	206.1	222.3
Fair value of plan assets	99.1	111.1
Plans with projected benefit obligations in excess of plan assets:
Fair value of plan assets	76.8	107.5
IPL [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	514.3	573.1
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	514.3	573.1
Fair value of plan assets	377.8	443.7
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	533.9	592.9
Fair value of plan assets	377.8	443.7
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	82.5	89.4
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	82.5	89.4
Fair value of plan assets	66.7	72.9
Plans with projected benefit obligations in excess of plan assets:
Fair value of plan assets	47.7	72.9
WPL [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	494.8	548.1
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	494.8	548.1
Fair value of plan assets	352.2	415
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	506.6	559.8
Fair value of plan assets	352.2	415
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	83.2	90.4
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	83.2	90.4
Fair value of plan assets	$ 16.7	$ 18.7

Benefit Plans (Regulatory Asset

Benefit Plans (Regulatory Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	$ 38.2	$ 38.9
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	$ 27.7	$ 28.1

Benefit Plans (Estimated Future

Benefit Plans (Estimated Future Funding) (Details) $ in Millions	Dec. 31, 2018USD ($)
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2019	$ 33.8	[1]
Defined Benefit Pension Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2019	16.5	[1]
Defined Benefit Pension Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2019	15.6	[1]
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2019	9.6
Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2019	1.9
Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2019	$ 7.4
[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, 2018USD ($)
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2019	$ 91
Expected benefit payments, 2020	104.2
Expected benefit payments, 2021	92.5
Expected benefit payments, 2022	93.8
Expected benefit payments, 2023	94.8
Expected benefit payments, 2024 - 2028	466
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2019	71.7
Expected benefit payments, 2020	86.1
Expected benefit payments, 2021	74.7
Expected benefit payments, 2022	76.3
Expected benefit payments, 2023	77.8
Expected benefit payments, 2024 - 2028	388.3
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2019	19.3
Expected benefit payments, 2020	18.1
Expected benefit payments, 2021	17.8
Expected benefit payments, 2022	17.5
Expected benefit payments, 2023	17
Expected benefit payments, 2024 - 2028	77.7
IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2019	40.3
Expected benefit payments, 2020	42.4
Expected benefit payments, 2021	42.3
Expected benefit payments, 2022	43.5
Expected benefit payments, 2023	44.4
Expected benefit payments, 2024 - 2028	214.2
IPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2019	33
Expected benefit payments, 2020	35.1
Expected benefit payments, 2021	35.1
Expected benefit payments, 2022	36.4
Expected benefit payments, 2023	37.5
Expected benefit payments, 2024 - 2028	182.9
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2019	7.3
Expected benefit payments, 2020	7.3
Expected benefit payments, 2021	7.2
Expected benefit payments, 2022	7.1
Expected benefit payments, 2023	6.9
Expected benefit payments, 2024 - 2028	31.3
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2019	40.2
Expected benefit payments, 2020	39.4
Expected benefit payments, 2021	39.3
Expected benefit payments, 2022	39.1
Expected benefit payments, 2023	39.3
Expected benefit payments, 2024 - 2028	196
WPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2019	31.4
Expected benefit payments, 2020	31.8
Expected benefit payments, 2021	31.9
Expected benefit payments, 2022	32
Expected benefit payments, 2023	32.5
Expected benefit payments, 2024 - 2028	165.3
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2019	8.8
Expected benefit payments, 2020	7.6
Expected benefit payments, 2021	7.4
Expected benefit payments, 2022	7.1
Expected benefit payments, 2023	6.8
Expected benefit payments, 2024 - 2028	$ 30.7

Benefit Plans (Allocation Of Pl

Benefit Plans (Allocation Of Plan Assets) (Details)	Dec. 31, 2018
Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	4.00%
Pension Plans, Defined Benefit [Member] | Equity securites - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	25.00%
Pension Plans, Defined Benefit [Member] | Equity securities - international [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	22.00%
Pension Plans, Defined Benefit [Member] | Global asset securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	9.00%
Pension Plans, Defined Benefit [Member] | Risk parity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	10.00%
Pension Plans, Defined Benefit [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	30.00%
Other Postretirement Benefits Plans [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	1.00%
Other Postretirement Benefits Plans [Member] | Equity securites - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	24.00%
Other Postretirement Benefits Plans [Member] | Equity securities - international [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	2.00%
Other Postretirement Benefits Plans [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	73.00%
Minimum [Member] | Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	0.00%
Minimum [Member] | Pension Plans, Defined Benefit [Member] | Equity securites - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	11.00%
Minimum [Member] | Pension Plans, Defined Benefit [Member] | Equity securities - international [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	14.00%
Minimum [Member] | Pension Plans, Defined Benefit [Member] | Global asset securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	5.00%
Minimum [Member] | Pension Plans, Defined Benefit [Member] | Risk parity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	5.00%
Minimum [Member] | Pension Plans, Defined Benefit [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	20.00%
Minimum [Member] | Other Postretirement Benefits Plans [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	0.00%
Minimum [Member] | Other Postretirement Benefits Plans [Member] | Equity securites - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	0.00%
Minimum [Member] | Other Postretirement Benefits Plans [Member] | Equity securities - international [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	0.00%
Minimum [Member] | Other Postretirement Benefits Plans [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	20.00%
Maximum [Member] | Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	5.00%
Maximum [Member] | Pension Plans, Defined Benefit [Member] | Equity securites - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	41.00%
Maximum [Member] | Pension Plans, Defined Benefit [Member] | Equity securities - international [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	34.00%
Maximum [Member] | Pension Plans, Defined Benefit [Member] | Global asset securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	15.00%
Maximum [Member] | Pension Plans, Defined Benefit [Member] | Risk parity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	15.00%
Maximum [Member] | Pension Plans, Defined Benefit [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	40.00%
Maximum [Member] | Other Postretirement Benefits Plans [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	5.00%
Maximum [Member] | Other Postretirement Benefits Plans [Member] | Equity securites - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	46.00%
Maximum [Member] | Other Postretirement Benefits Plans [Member] | Equity securities - international [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	34.00%
Maximum [Member] | Other Postretirement Benefits Plans [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	100.00%

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 808.6	$ 950.7	$ 895.7
Assets measured at net asset value	355.4	441.1
Total assets in fair value hierarchy	808.6	950.7
Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	36.4	28.2
Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	4.5
Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	33.4	23.7
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	130.2	158.3
Pension Plans, Defined Benefit [Member] | Equity securites - U.S. [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	130.2	158.3
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	116	137.5
Pension Plans, Defined Benefit [Member] | Equity securities - international [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	116	137.5
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	42.1	49.4
Pension Plans, Defined Benefit [Member] | Global asset securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	42.1	49.4
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	127.8	135.9
Pension Plans, Defined Benefit [Member] | Fixed income securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	52.5	55.8
Pension Plans, Defined Benefit [Member] | Fixed income securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	75.3	80.1
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	452.5	509.3
Pension Plans, Defined Benefit [Member] | Subtotal [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	343.8	405.5
Pension Plans, Defined Benefit [Member] | Subtotal [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	108.7	103.8
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	1.2	1
Pension Plans, Defined Benefit [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	377.8	443.7	422
Assets measured at net asset value	166.1	205.8
Total assets in fair value hierarchy	377.8	443.7
Pension Plans, Defined Benefit [Member] | IPL [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	17	13.2
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	1.4	2.2
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	15.6	11
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	60.8	73.9
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	60.8	73.9
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	54.2	64.2
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	54.2	64.2
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	19.7	23
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	19.7	23
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	59.7	63.4
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	24.5	26
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	35.2	37.4
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	211.4	237.7
Pension Plans, Defined Benefit [Member] | IPL [Member] | Subtotal [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	160.6	189.3
Pension Plans, Defined Benefit [Member] | IPL [Member] | Subtotal [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	50.8	48.4
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	0.6	0.5
Pension Plans, Defined Benefit [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	352.2	415	389.7
Assets measured at net asset value	154.8	192.5
Total assets in fair value hierarchy	352.2	415
Pension Plans, Defined Benefit [Member] | WPL [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15.9	12.3
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	1.3	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	14.6	10.3
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	56.7	69.1
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	56.7	69.1
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	50.5	60
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	50.5	60
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	18.4	21.6
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	18.4	21.6
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	55.6	59.3
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	22.8	24.3
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	32.8	35
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	197.1	222.3
Pension Plans, Defined Benefit [Member] | WPL [Member] | Subtotal [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	149.7	177
Pension Plans, Defined Benefit [Member] | WPL [Member] | Subtotal [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	47.4	45.3
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	99.1	111.1	105.8
Assets measured at net asset value	22.3	3.6
Total assets in fair value hierarchy	76.8	107.5
Other Postretirement Benefit Plans, Defined Benefit [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	75.8	106.4
Other Postretirement Benefit Plans, Defined Benefit [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	1.1
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	1.4	1.2
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	1.1	0.7
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	0.3	0.5
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	3.9	27.9
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	3.9	27.9
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	2.9	11.4
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	2.9	11.4
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	0.4
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	0.4
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	68.2	66.6
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	67.5	66
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	0.7	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	66.7	72.9	68.2
Assets measured at net asset value	19	0
Total assets in fair value hierarchy	47.7	72.9
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	47.7	72.9
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	0.7	0.3
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	0.7	0.3
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	0	22.3
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	0	22.3
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	0	7.5
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	0	7.5
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] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	47	42.8
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	47	42.8
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	16.7	18.7	$ 18.6
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16.7	18.4
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0.3
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	0.1	0.6
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	0.1	0.3
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	0.3
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] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16.6	18.1
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	16.6	18.1
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	(0.5)	(0.7)
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)	(0.3)
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	$ (0.2)	$ (0.3)

Benefit Plans (Employees Partic

Benefit Plans (Employees Participate In Defined Contribution Retirement Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
401(k) costs	$ 25.1	$ 24.8	$ 23.6
IPL [Member]
401(k) costs	13	12.8	12
WPL [Member]
401(k) costs	$ 11.2	$ 11.1	$ 10.7

Benefit Plans (Recognized Compe

Benefit Plans (Recognized Compensation Expense And Income Tax Benefits) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Compensation expense	$ 17	$ 15.1	$ 18
Income tax benefits	4.9	6.2	7.4
IPL [Member]
Compensation expense	9.4	8.3	9.5
Income tax benefits	2.8	3.4	4
WPL [Member]
Compensation expense	6.9	6.4	7.9
Income tax benefits	$ 1.9	$ 2.6	$ 3.2

Benefit Plans (Summary Of Perfo

Benefit Plans (Summary Of Performance Shares and Performance Units Activity) (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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)	223,511	257,599	288,430
Granted (in shares/awards)	74,163	65,350	68,585
Vested (in shares/awards)	(90,806)	(99,438)	(98,186)
Forfeited (in shares/awards)	(3,680)	0	(1,230)
Nonvested, December 31 (in shares/awards)	203,188	223,511	257,599
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)	71,737	93,320	116,412
Granted (in shares/awards)	19,840	21,558	23,918
Vested (in shares/awards)	(31,910)	(37,395)	(42,760)
Forfeited (in shares/awards)	(1,906)	(5,746)	(4,250)
Nonvested, December 31 (in shares/awards)	57,761	71,737	93,320

Benefit Plans (Summary Of Per_2

Benefit Plans (Summary Of Performance Shares and Performance Units Vested Awards) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares/awards)	90,806	99,438	98,186
Performance Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares/awards)	31,910	37,395	42,760
2015 Grant [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares/awards)	90,806
Vested percentage of the target	137.50%
Cash and stock payout value	$ 5.3
Cash payout value	$ 4.9
Common stock shares from vested performance shares (in shares)	5,078
2015 Grant [Member] | Performance Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares/awards)	31,910
Vested percentage of the target	137.50%
Cash and stock payout value	$ 1.4
Cash payout value	$ 1.4
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

Benefit Plans (Fair Values Of N

Benefit Plans (Fair Values Of Nonvested Performance Shares And Performance Units) (Details)	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Performance Shares [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards)	203,188
Performance Units [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards)	57,761
2018 Grant [Member] | Performance Shares [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards)	70,483
Estimated payout percentage based on performance criteria	85.00%
Fair values of each nonvested award (in dollars per share) | $ / shares	$ 35.91
2018 Grant [Member] | Performance Units [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards)	19,196
Estimated payout percentage based on performance criteria	85.00%
Fair values of each nonvested award (in dollars per share) | $ / shares	$ 35.91
2017 Grant [Member] | Performance Shares [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards)	65,350
Estimated payout percentage based on performance criteria	85.00%
Fair values of each nonvested award (in dollars per share) | $ / shares	$ 35.91
2017 Grant [Member] | Performance Units [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards)	18,062
Estimated payout percentage based on performance criteria	85.00%
Fair values of each nonvested award (in dollars per share) | $ / shares	$ 35.91
2016 Grant [Member] | Performance Shares [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards)	67,355
Estimated payout percentage based on performance criteria	143.00%
Fair values of each nonvested award (in dollars per share) | $ / shares	$ 60.42
2016 Grant [Member] | Performance Units [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards)	20,503
Estimated payout percentage based on performance criteria	143.00%
Fair values of each nonvested award (in dollars per share) | $ / shares	$ 60.42

Benefit Plans (Summary of Per_3

Benefit Plans (Summary of Performance Restricted Stock Units) (Details) - Performance Restricted Stock Unit [Member] - $ / shares	12 Months Ended
	Dec. 31, 2018	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)	132,705	67,355	0
Nonvested shares, January 1, weighted average grant date fair value (in dollars per share)	$ 36.50	$ 33.96	$ 0
Granted (in shares/awards)	74,163	65,350	68,585
Granted, weighted average grant date fair value (in dollars per share)	$ 38.60	$ 39.12	$ 33.96
Forfeited (in shares/awards)	(3,680)	0	(1,230)
Forfeited, weighted average grant date fair value (in dollars per share)	$ 38.60	$ 0	$ 33.90
Nonvested, December 31 (in shares/awards)	203,188	132,705	67,355
Nonvested shares, December 31, weighted average grant date fair value (in dollars per share)	$ 37.23	$ 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, 2018	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)	113,749	57,736	0
Granted (in shares/awards)	63,568	56,013	58,790
Forfeited (in shares/awards)	(3,154)	0	(1,054)
Nonvested, December 31 (in shares/awards)	174,163	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, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Carrying value of deferred compensation obligation	$ 9.8	$ 11.1
Fair market value of deferred compensation obligation	$ 16.2	$ 19.7

Asset Retirement Obligations (R

Asset Retirement Obligations (Reconciliation Of Changes In Asset Retirement Obligations) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1	$ 184.5	$ 195.7
Revisions in estimated cash flows	(10.1)	4.3
Liabilities settled	(10.4)	(23.5)
Liabilities incurred	7.3	2
Accretion expense	6.2	6
Balance, December 31	177.5	184.5
IPL [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1	134.1	124.7
Revisions in estimated cash flows	(10.1)	7
Liabilities settled	(9.7)	(13.1)
Liabilities incurred	0	11.7
Accretion expense	4	3.8
Balance, December 31	118.3	134.1
WPL [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1	50.4	61.4
Revisions in estimated cash flows	0	(2.7)
Liabilities settled	(0.7)	(10.4)
Liabilities incurred	7.3	0
Accretion expense	2.2	2.1
Balance, December 31	$ 59.2	$ 50.4

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, 2018DekathermsTMWhgal
FTRs (MWhs) [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs/Dths/tons) | MWh	10,399
FTRs (MWhs) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs/Dths/tons) | MWh	5,954
FTRs (MWhs) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs/Dths/tons) | MWh	4,445
Natural Gas (Dths) [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs/Dths/tons) | Dekatherms	181,694
Natural Gas (Dths) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs/Dths/tons) | Dekatherms	80,150
Natural Gas (Dths) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs/Dths/tons) | Dekatherms	101,544
Coal (tons) [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs/Dths/tons) | T	10,467
Coal (tons) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs/Dths/tons) | T	4,410
Coal (tons) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs/Dths/tons) | T	6,057
Diesel Fuel (gallons) [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in gallons) | gal	3,024
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	3,024

Derivative Instruments (Fair Va

Derivative Instruments (Fair Value Of Financial Instruments) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Current derivative assets	$ 24.6	$ 21.1
Non-current derivative assets	3.7	4
Current derivative liabilities	5.6	18.7
Non-current derivative liabilities	17.7	23
IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	16.1	15.8
Non-current derivative assets	1.6	1.3
Current derivative liabilities	3.1	5
Non-current derivative liabilities	8.1	14.4
WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	8.5	5.3
Non-current derivative assets	2.1	2.7
Current derivative liabilities	2.5	13.7
Non-current derivative liabilities	$ 9.6	$ 8.6

Fair Value Measurements (Recurr

Fair Value Measurements (Recurring Fair Value Measurements) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Assets:
Deferred proceeds	$ 119.4	$ 222.1
Liabilities and equity:
Long-term debt (including current maturities)	5,860.8	5,447.5
Commodity Contracts [Member]
Assets:
Derivatives	28.3	25.1
Liabilities and equity:
Derivatives	23.3	41.7
Level 1 [Member]
Assets:
Deferred proceeds	0	0
Liabilities and equity:
Long-term debt (including current maturities)	0	0
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)	5,858.4	5,444.6
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	8.9	4.1
Liabilities and equity:
Derivatives	16.1	8.5
Level 3 [Member]
Assets:
Deferred proceeds	119.4	222.1
Liabilities and equity:
Long-term debt (including current maturities)	2.4	2.9
Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	19.4	21
Liabilities and equity:
Derivatives	7.2	33.2
Carrying Amount [Member]
Assets:
Deferred proceeds	119.4	222.1
Liabilities and equity:
Long-term debt (including current maturities)	5,502.8	4,866.3
Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	28.3	25.1
Liabilities and equity:
Derivatives	23.3	41.7
IPL [Member]
Assets:
Deferred proceeds	119.4	222.1
Liabilities and equity:
Long-term debt (including current maturities)	2,691.2	2,665.7
IPL [Member] | Commodity Contracts [Member]
Assets:
Derivatives	17.7	17.1
Liabilities and equity:
Derivatives	11.2	19.4
IPL [Member] | Level 1 [Member]
Assets:
Deferred proceeds	0	0
Liabilities and equity:
Long-term debt (including current maturities)	0	0
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)	2,691.2	2,665.7
IPL [Member] | Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	4	2
Liabilities and equity:
Derivatives	6.5	2.9
IPL [Member] | Level 3 [Member]
Assets:
Deferred proceeds	119.4	222.1
Liabilities and equity:
Long-term debt (including current maturities)	0	0
IPL [Member] | Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	13.7	15.1
Liabilities and equity:
Derivatives	4.7	16.5
IPL [Member] | Carrying Amount [Member]
Assets:
Deferred proceeds	119.4	222.1
Liabilities and equity:
Long-term debt (including current maturities)	2,552.3	2,406
IPL [Member] | Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	17.7	17.1
Liabilities and equity:
Derivatives	11.2	19.4
WPL [Member]
Liabilities and equity:
Long-term debt (including current maturities)	2,043.7	2,147.9
WPL [Member] | Commodity Contracts [Member]
Assets:
Derivatives	10.6	8
Liabilities and equity:
Derivatives	12.1	22.3
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)	2,043.7	2,147.9
WPL [Member] | Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	4.9	2.1
Liabilities and equity:
Derivatives	9.6	5.6
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	5.7	5.9
Liabilities and equity:
Derivatives	2.5	16.7
WPL [Member] | Carrying Amount [Member]
Liabilities and equity:
Long-term debt (including current maturities)	1,834.9	1,833.4
WPL [Member] | Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	10.6	8
Liabilities and equity:
Derivatives	$ 12.1	$ 22.3

Fair Value Measurements (Fair V

Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
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		$ (12.2)	$ 8.7
Total net gains (losses) included in changes in net assets (realized/unrealized)		9.1	(32.9)
Transfers out of Level 3	[1]	16.1	12.2
Purchases		26.7	28.3
Sales		(0.5)	(0.3)
Settlements	[2]	(27)	(28.2)
Ending balance, December 31		12.2	(12.2)
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		10.7	(31)
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		(1.4)	10.1
Total net gains (losses) included in changes in net assets (realized/unrealized)		(0.5)	(14.8)
Transfers out of Level 3	[1]	11	3.1
Purchases		22.5	24.6
Sales		(0.4)	(0.2)
Settlements	[2]	(22.2)	(24.2)
Ending balance, December 31		9	(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		0.2	(13.5)
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		(10.8)	(1.4)
Total net gains (losses) included in changes in net assets (realized/unrealized)		9.6	(18.1)
Transfers out of Level 3	[1]	5.1	9.1
Purchases		4.2	3.7
Sales		(0.1)	(0.1)
Settlements		(4.8)	(4)
Ending balance, December 31		3.2	(10.8)
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		10.5	(17.5)
Deferred Proceeds [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance, January 1		222.1	211.1
Total net gains (losses) included in changes in net assets (realized/unrealized)		0	0
Transfers out of Level 3		0	0
Purchases		0	0
Sales		0	0
Settlements	[2]	(102.7)	11
Ending balance, December 31		119.4	222.1
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		222.1	211.1
Total net gains (losses) included in changes in net assets (realized/unrealized)		0	0
Transfers out of Level 3		0	0
Purchases		0	0
Sales		0	0
Settlements	[2]	(102.7)	11
Ending balance, December 31		119.4	222.1
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]	Observable market inputs became available for certain commodity contracts previously classified as Level 3 for transfers out of Level 3.
[2]	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 (Fair_2

Fair Value Measurements (Fair Value Of Net Derivative Assets (Liabilities)) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	$ 12.2	$ (12.2)	$ 8.7
Excluding FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	3.2
Fair value, net derivative liabilities		23.5
FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	9	11.3
IPL [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	9	(1.4)	10.1
IPL [Member] | Excluding FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	1.8
Fair value, net derivative liabilities		11.5
IPL [Member] | FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	7.2	10.1
WPL [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	3.2	(10.8)	$ (1.4)
WPL [Member] | Excluding FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	1.4
Fair value, net derivative liabilities		12
WPL [Member] | FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	$ 1.8	$ 1.2

Commitments And Contingencies_2

Commitments And Contingencies (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)
Long-term Purchase Commitment [Line Items]
Minimum future commitments	$ 2,192
Present value abandonment obligation	$ 36
Workforce Subject to Collective Bargaining Arrangements [Member]
Long-term Purchase Commitment [Line Items]
Employees covered by collective bargaining agreement	55.00%
Workforce Subject to Collective Bargaining Arrangements Expiring within One Year [Member]
Long-term Purchase Commitment [Line Items]
Employees covered by collective bargaining agreement	26.00%
Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	$ 1,045	[1]
Capital Purchase Commitment [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	40
IPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	1,546
Maximum indemnification limit	$ 17
IPL [Member] | Workforce Subject to Collective Bargaining Arrangements [Member]
Long-term Purchase Commitment [Line Items]
Employees covered by collective bargaining agreement	63.00%
IPL [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	$ 1,030	[1]
IPL [Member] | Capital Purchase Commitment [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	14
WPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	$ 632
WPL [Member] | Workforce Subject to Collective Bargaining Arrangements [Member]
Long-term Purchase Commitment [Line Items]
Employees covered by collective bargaining agreement	82.00%
WPL [Member] | Workforce Subject to Collective Bargaining Arrangements Expiring within One Year [Member]
Long-term Purchase Commitment [Line Items]
Employees covered by collective bargaining agreement	82.00%
WPL [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	$ 15
WPL [Member] | Capital Purchase Commitment [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	26
Indemnification Agreement [Member]
Long-term Purchase Commitment [Line Items]
Maximum indemnification limit	90
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. In July 2018, IPL entered into an amendment to shorten the term of the DAEC PPA by five years in exchange for a $110 million buyout payment by IPL in September 2020, subject to IUB approval. In December 2018, IPL received an order from the IUB, which was effective in January 2019, approving recovery of the buyout payment. As a result, Alliant Energy’s and IPL’s amounts in the above table do not include the September 2020 buyout payment, and the minimum future commitments reflect IPL’s purchase of capacity and the resulting energy from DAEC through December 2025, the original term of the PPA prior to the amendment. Amounts in the above table for 2021 and beyond relate to the DAEC PPA.

Commitments And Contingencies_3

Commitments And Contingencies (Other Purchase Obligations) (Details) - USD ($) $ in Millions		1 Months Ended		12 Months Ended
		Sep. 30, 2020	Jul. 31, 2018	Dec. 31, 2018
Long-term Purchase Commitment [Line Items]
Minimum future commitments				$ 2,192
Individual commitments incurred				1
Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[1]			1,045
Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				881
Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[2]			195
Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[3]			71
2019 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				575
2019 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[1]			159
2019 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				254
2019 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[2]			102
2019 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[3]			60
2020 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				339
2020 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[1]			135
2020 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				151
2020 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[2]			49
2020 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[3]			4
2021 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				297
2021 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[1]			149
2021 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				119
2021 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[2]			26
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				245
2022 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[1]			140
2022 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				94
2022 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[2]			9
2022 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[3]			2
2023 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				233
2023 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[1]			155
2023 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				67
2023 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[2]			9
2023 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[3]			2
Thereafter [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				503
Thereafter [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[1]			307
Thereafter [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				196
Thereafter [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[2]			0
Thereafter [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[3]			0
IPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				1,546
IPL [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[1]			1,030
IPL [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				363
IPL [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[2]			119
IPL [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[3]			34
IPL [Member] | 2019 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				344
IPL [Member] | 2019 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[1]			144
IPL [Member] | 2019 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				124
IPL [Member] | 2019 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[2]			52
IPL [Member] | 2019 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[3]			24
IPL [Member] | 2020 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				222
IPL [Member] | 2020 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[1]			135
IPL [Member] | 2020 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				55
IPL [Member] | 2020 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[2]			29
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				214
IPL [Member] | 2021 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[1]			149
IPL [Member] | 2021 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				42
IPL [Member] | 2021 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[2]			20
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				184
IPL [Member] | 2022 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[1]			140
IPL [Member] | 2022 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				33
IPL [Member] | 2022 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[2]			9
IPL [Member] | 2022 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[3]			2
IPL [Member] | 2023 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				192
IPL [Member] | 2023 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[1]			155
IPL [Member] | 2023 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				26
IPL [Member] | 2023 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[2]			9
IPL [Member] | 2023 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[3]			2
IPL [Member] | Thereafter [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				390
IPL [Member] | Thereafter [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[1]			307
IPL [Member] | Thereafter [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				83
IPL [Member] | Thereafter [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[2]			0
IPL [Member] | Thereafter [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[3]			0
WPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				632
WPL [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				15
WPL [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				518
WPL [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[2]			76
WPL [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[3]			23
WPL [Member] | 2019 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				218
WPL [Member] | 2019 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				15
WPL [Member] | 2019 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				130
WPL [Member] | 2019 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[2]			50
WPL [Member] | 2019 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[3]			23
WPL [Member] | 2020 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				116
WPL [Member] | 2020 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				0
WPL [Member] | 2020 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				96
WPL [Member] | 2020 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[2]			20
WPL [Member] | 2020 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[3]			0
WPL [Member] | 2021 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				83
WPL [Member] | 2021 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				0
WPL [Member] | 2021 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				77
WPL [Member] | 2021 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[2]			6
WPL [Member] | 2021 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[3]			0
WPL [Member] | 2022 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				61
WPL [Member] | 2022 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				0
WPL [Member] | 2022 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				61
WPL [Member] | 2022 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[2]			0
WPL [Member] | 2022 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[3]			0
WPL [Member] | 2023 [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				41
WPL [Member] | 2023 [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				0
WPL [Member] | 2023 [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				41
WPL [Member] | 2023 [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[2]			0
WPL [Member] | 2023 [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[3]			0
WPL [Member] | Thereafter [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				113
WPL [Member] | Thereafter [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				0
WPL [Member] | Thereafter [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments				113
WPL [Member] | Thereafter [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[2]			0
WPL [Member] | Thereafter [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	[3]			$ 0
DAEC PPA [Member]
Long-term Purchase Commitment [Line Items]
Long-term Contract for Purchase of Electric Power, Contract Term Amendment			5 years
Scenario, Forecast [Member] | DAEC PPA [Member]
Long-term Purchase Commitment [Line Items]
Long-term Contract for Purchase of Electric Power, Buyout Payment		$ 110
[1]	Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. In July 2018, IPL entered into an amendment to shorten the term of the DAEC PPA by five years in exchange for a $110 million buyout payment by IPL in September 2020, subject to IUB approval. In December 2018, IPL received an order from the IUB, which was effective in January 2019, approving recovery of the buyout payment. As a result, Alliant Energy’s and IPL’s amounts in the above table do not include the September 2020 buyout payment, and the minimum future commitments reflect IPL’s purchase of capacity and the resulting energy from DAEC through December 2025, the original term of the PPA prior to the amendment. Amounts in the above table for 2021 and beyond relate to the DAEC PPA.
[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, 2018 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, 2018.

Commitments And Contingencies_4

Commitments And Contingencies (Schedule Of Environmental Liabilities) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)
Minimum range of estimated future cost incurred for investigation, remediation and monitoring	$ 11
Maximum remaining estimated cost incurred for investigation, remediation and monitoring	29
IPL [Member]
Minimum range of estimated future cost incurred for investigation, remediation and monitoring	8
Maximum remaining estimated cost incurred for investigation, remediation and monitoring	24
Manufactured Gas Plant [Member]
Total environmental liabilities	15
Manufactured Gas Plant [Member] | IPL [Member]
Total environmental liabilities	$ 12

Segments Of Business (Narrative

Segments Of Business (Narrative) (Details)	12 Months Ended
	Dec. 31, 2018segment
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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues	$ 873.5	$ 928.6	$ 816.1	$ 916.3	$ 856.1	$ 906.9	$ 765.3	$ 853.9	$ 3,534.5	$ 3,382.2	$ 3,320
Depreciation and amortization									506.9	461.8	411.6
Operating income (loss)	121.4	256.1	151.2	165.7	134	236.3	153.7	147.2	694.4	671.2	554.1
Interest expense									247	215.6	196.2
Equity (income) loss from unconsolidated investments, net									(54.6)	(44.8)	(39.6)
Income tax expense (benefit)									47.7	66.7	59.4
Net income (loss) attributable to common shareowners	85.3	205.5	100.4	120.9	93.8	168.8	94.3	100.4	512.1	457.3	371.5
Total assets	15,426				14,187.8				15,426	14,187.8	13,373.8
Investments in equity method subsidiaries	421.3				381.4				421.3	381.4	326
Construction and acquisition expenditures									1,633.9	1,466.9	1,196.8
IPL [Member]
Segment Reporting Information [Line Items]
Revenues	494.1	547.6	474.8	525.8	472.2	527.4	420.2	450.5	2,042.3	1,870.3	1,820.4
Depreciation and amortization									283.5	245	210.8
Operating income (loss)	54.4	143.1	77.7	75.6	50.9	133.8	68.1	51.3	350.8	304.1	277.6
Interest expense									119.4	112.4	103.2
Income tax expense (benefit)									(3.2)	(10.9)	(5.9)
Net income (loss) attributable to common shareowners	39.1	126.5	51.7	46.7	16.4	120.4	42.8	37.2	264	216.8	215.6
Total assets	8,411.4				7,606				8,411.4	7,606
WPL [Member]
Segment Reporting Information [Line Items]
Revenues	369.4	370.7	330.8	381.7	374.7	370.2	334.8	393.1	1,452.6	1,472.8	1,459.1
Depreciation and amortization									219.4	212.9	192.5
Operating income (loss)	61.3	104.2	63.4	84	75.4	93.5	76.2	88.6	312.9	333.7	337.2
Interest expense									97.8	93.8	91.4
Equity (income) loss from unconsolidated investments, net									(0.9)	(0.7)	(39.8)
Income tax expense (benefit)									36.2	61.9	93.3
Net income (loss) attributable to common shareowners	38	$ 76.3	$ 39.8	$ 54	53.2	$ 49.8	$ 38.1	$ 45.5	208.1	186.6	190.4
Total assets	6,152.5				5,756.5				6,152.5	5,756.5
Electric [Member]
Segment Reporting Information [Line Items]
Revenues									3,000.3	2,894.7	2,875.5
Depreciation and amortization									457.3	412	367
Operating income (loss)									610.2	601.7	586.5
Equity (income) loss from unconsolidated investments, net									(0.9)	(0.7)	(0.7)
Total assets	12,486.3				11,396.2				12,486.3	11,396.2	10,722.9
Investments in equity method subsidiaries	8.1				8.3				8.1	8.3	7.7
Construction and acquisition expenditures									1,421.1	1,154.9	994
Electric [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Revenues									1,731.1	1,598.9	1,569.7
Depreciation and amortization									254.7	215.1	189.4
Operating income (loss)									318.2	287.3	257.8
Total assets	7,219.9				6,524.4				7,219.9	6,524.4	6,278.2
Construction and acquisition expenditures									890.6	594.1	598.1
Electric [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Revenues									1,269.2	1,295.8	1,305.8
Depreciation and amortization									202.6	196.9	177.6
Operating income (loss)									292	314.4	328.7
Equity (income) loss from unconsolidated investments, net											(0.7)
Total assets	5,266.4				4,871.8				5,266.4	4,871.8	4,444.7
Construction and acquisition expenditures									530.5	592.4	395.9
Gas [Member]
Segment Reporting Information [Line Items]
Revenues									446.6	400.9	355.4
Depreciation and amortization									42	38.2	34.2
Operating income (loss)									53.2	47.7	33
Equity (income) loss from unconsolidated investments, net									0	0	0
Total assets	1,184.4				1,199.8				1,184.4	1,199.8	1,091.1
Investments in equity method subsidiaries	0				0				0	0	0
Construction and acquisition expenditures									146.8	125.2	137.1
Gas [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Revenues									266.2	226	204
Depreciation and amortization									25.2	22.2	19.3
Operating income (loss)									28.3	21.7	16.4
Total assets	687.5				727.9				687.5	727.9	653.3
Construction and acquisition expenditures									99.7	80.7	91.5
Gas [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Revenues									180.4	174.9	151.4
Depreciation and amortization									16.8	16	14.9
Operating income (loss)									24.9	26	16.6
Equity (income) loss from unconsolidated investments, net											0
Total assets	496.9				471.9				496.9	471.9	437.8
Construction and acquisition expenditures									47.1	44.5	45.6
Other Utility [Member]
Segment Reporting Information [Line Items]
Revenues									48	47.5	48.6
Depreciation and amortization									3.6	7.7	2.1
Operating income (loss)									0.3	(11.6)	(4.7)
Equity (income) loss from unconsolidated investments, net									0	0	0
Total assets	893.2				766.5				893.2	766.5	781
Investments in equity method subsidiaries	0				0				0	0	0
Construction and acquisition expenditures									0.4	1.7	0.1
Other Utility [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Revenues									45	45.4	46.7
Depreciation and amortization									3.6	7.7	2.1
Operating income (loss)									4.3	(4.9)	3.4
Total assets	504				353.7				504	353.7	373.2
Construction and acquisition expenditures									0.4	1.2	0.1
Other Utility [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Revenues									3	2.1	1.9
Depreciation and amortization									0	0	0
Operating income (loss)									(4)	(6.7)	(8.1)
Equity (income) loss from unconsolidated investments, net											(39.1)
Total assets	389.2				412.8				389.2	412.8	407.8
Construction and acquisition expenditures									0	0.5	11.5
Utility Business [Member]
Segment Reporting Information [Line Items]
Revenues									3,494.9	3,343.1	3,279.5
Depreciation and amortization									502.9	457.9	403.3
Operating income (loss)									663.7	637.8	614.8
Interest expense									217.2	206.2	194.6
Equity (income) loss from unconsolidated investments, net									(0.9)	(0.7)	(0.7)
Income tax expense (benefit)									33	51	71.4
Net income (loss) attributable to common shareowners									472.1	403.4	385.2
Total assets	14,563.9				13,362.5				14,563.9	13,362.5	12,595
Investments in equity method subsidiaries	8.1				8.3				8.1	8.3	7.7
Construction and acquisition expenditures									1,568.3	1,281.8	1,131.2
Utility Business [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Revenues									2,042.3	1,870.3	1,820.4
Depreciation and amortization									283.5	245	210.8
Operating income (loss)									350.8	304.1	277.6
Interest expense									119.4	112.4	103.2
Income tax expense (benefit)									(3.2)	(10.9)	(5.9)
Net income (loss) attributable to common shareowners									264	216.8	215.6
Total assets	8,411.4				7,606				8,411.4	7,606	7,304.7
Construction and acquisition expenditures									990.7	676	689.7
Utility Business [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Revenues									1,452.6	1,472.8	1,459.1
Depreciation and amortization									219.4	212.9	192.5
Operating income (loss)									312.9	333.7	337.2
Interest expense									97.8	93.8	91.4
Equity (income) loss from unconsolidated investments, net											(39.8)
Income tax expense (benefit)									36.2	61.9	93.3
Net income (loss) attributable to common shareowners									208.1	186.6	190.4
Total assets	6,152.5				5,756.5				6,152.5	5,756.5	5,290.3
Construction and acquisition expenditures									577.6	637.4	453
Non-Utility [Member]
Segment Reporting Information [Line Items]
Revenues									39.6	39.1	40.5
Depreciation and amortization									4	3.9	8.3
Operating income (loss)									30.7	33.4	(60.7)
Interest expense									29.8	9.4	1.6
Equity (income) loss from unconsolidated investments, net									(53.7)	(44.1)	(38.9)
Income tax expense (benefit)									14.7	15.7	(12)
Net income (loss) attributable to common shareowners									40	53.9	(13.7)
Total assets	862.1				825.3				862.1	825.3	778.8
Investments in equity method subsidiaries	$ 413.2				$ 373.1				413.2	373.1	318.3
Construction and acquisition expenditures									$ 65.6	$ 185.1	$ 65.6

Related Parties (Narrative) (De

Related Parties (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
WPL [Member] | WPL Owed ATC [Member]
Related Party Transaction [Line Items]
Net amounts owed	$ 8	$ 9

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Administrative and General Services Billings [Member] | IPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	$ 170	$ 177	$ 161
Administrative and General Services Billings [Member] | WPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	132	135	133
Transmission Sales Credited [Member] | IPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	48	23	8
Transmission Sales Credited [Member] | WPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	28	13	7
Transmission Purchases Billed [Member] | IPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	358	364	433
Transmission Purchases Billed [Member] | WPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	$ 81	$ 115	$ 102

Related Parties (Net Intercompa

Related Parties (Net Intercompany Payables) (Details) - Corporate Services [Member] - Subsidiary of Common Parent [Member] - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
IPL [Member]
Related Party Transaction [Line Items]
Intercompany payables to Corporate Services	$ 95	$ 114
WPL [Member]
Related Party Transaction [Line Items]
Intercompany payables to Corporate Services	$ 71	$ 61

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, 2018	Dec. 31, 2017	Dec. 31, 2016
ATC Billings To WPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	$ 106	$ 105	$ 110
WPL Billings To ATC [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	$ 11	$ 10	$ 13

Selected Consolidated Quarter_3

Selected Consolidated Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues	$ 873.5	$ 928.6	$ 816.1	$ 916.3	$ 856.1	$ 906.9	$ 765.3	$ 853.9	$ 3,534.5	$ 3,382.2	$ 3,320
Operating income	121.4	256.1	151.2	165.7	134	236.3	153.7	147.2	694.4	671.2	554.1
Income from continuing operations, net of tax	85.3	205.5	100.4	120.9	93.8	168.8	94.3	99	512.1	455.9	373.8
Income (loss) from discontinued operations, net of tax	0	0	0	0	0	0	0	1.4	0	1.4	(2.3)
Net income									522.3	467.5	381.7
Net income attributable to common shareowners	$ 85.3	$ 205.5	$ 100.4	$ 120.9	$ 93.8	$ 168.8	$ 94.3	$ 100.4	512.1	457.3	371.5
Income from continuing operations, net of tax (in dollars per share)	$ 0.36	$ 0.87	$ 0.43	$ 0.52	$ 0.41	$ 0.73	$ 0.41	$ 0.43
Income from discontinued operations, net of tax (in dollars per share)	0	0	0	0	0	0	0	0.01
Net income (in dollars per share)	$ 0.36	$ 0.87	$ 0.43	$ 0.52	$ 0.41	$ 0.73	$ 0.41	$ 0.44
IPL [Member]
Revenues	$ 494.1	$ 547.6	$ 474.8	$ 525.8	$ 472.2	$ 527.4	$ 420.2	$ 450.5	2,042.3	1,870.3	1,820.4
Operating income	54.4	143.1	77.7	75.6	50.9	133.8	68.1	51.3	350.8	304.1	277.6
Net income	41.6	129.1	54.2	49.3	18.9	123	45.3	39.8	274.2	227	225.8
Net income attributable to common shareowners	39.1	126.5	51.7	46.7	16.4	120.4	42.8	37.2	264	216.8	215.6
WPL [Member]
Revenues	369.4	370.7	330.8	381.7	374.7	370.2	334.8	393.1	1,452.6	1,472.8	1,459.1
Operating income	61.3	104.2	63.4	84	75.4	93.5	76.2	88.6	312.9	333.7	337.2
Net income	38	76.3	39.8	54	53.2	49.8	38.1	45.5	208.1	186.6	192.8
Net income attributable to common shareowners	$ 38	$ 76.3	$ 39.8	$ 54	$ 53.2	$ 49.8	$ 38.1	$ 45.5	$ 208.1	$ 186.6	$ 190.4

Condensed Parent Company Fina_3

Condensed Parent Company Financial Statements (Condensed Statements Of Income) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues	$ 873.5	$ 928.6	$ 816.1	$ 916.3	$ 856.1	$ 906.9	$ 765.3	$ 853.9	$ 3,534.5	$ 3,382.2	$ 3,320
Operating expenses									2,840.1	2,711	2,765.9
Operating income (loss)	$ 121.4	$ 256.1	$ 151.2	$ 165.7	$ 134	$ 236.3	$ 153.7	$ 147.2	694.4	671.2	554.1
Other (income) and deductions:
Equity earnings from consolidated subsidiaries									(54.6)	(44.8)	(39.6)
Interest expense									247	215.6	196.2
Total other (income) and deductions									124.4	138.4	110.7
Income from continuing operations before income taxes									570	532.8	443.4
Income tax benefit									47.7	66.7	59.4
Net income									522.3	467.5	381.7
Parent Company [Member]
Revenues									0	0	1
Operating expenses									5	2	3
Operating income (loss)									(5)	(2)	(2)
Other (income) and deductions:
Equity earnings from consolidated subsidiaries									(523)	(457)	(374)
Interest expense									4	3	3
Other									2	0	(2)
Total other (income) and deductions									(517)	(454)	(373)
Income from continuing operations before income taxes									512	452	371
Income tax benefit									(1)	(6)	(1)
Net income									$ 513	$ 458	$ 372

Condensed Parent Company Fina_4

Condensed Parent Company Financial Statements (Condensed Balance Sheet) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Other	$ 80	$ 46.4
Total current assets	785.1	905.1
Investments:
Other	137.7	121.9
Total investments	431.3	396.1
Other assets	89.7	69.7
Total assets	15,426	14,187.8	$ 13,373.8
Current liabilities:
Commercial paper	441.2	320.2
Other	260.4	260.8
Total current liabilities	1,644.1	2,149
Other liabilities	287.2	306.4
Common Equity:
Retained earnings	2,545.9	2,346
Accumulated other comprehensive income (loss)	1.7	(0.5)
Shares in deferred compensation trust	(9.8)	(11.1)
Total common equity	4,585.7	4,182.2	$ 4,062
Total liabilities and equity	15,426	14,187.8
Parent Company [Member]
Current assets:
Notes receivable from affiliated companies	23	50
Other	4	7
Total current assets	27	57
Investments:
Investments in consolidated subsidiaries	5,518	4,676
Other	1	2
Total investments	5,519	4,678
Other assets	81	78
Total assets	5,627	4,813
Current liabilities:
Commercial paper	285	295
Notes payable to affiliated companies	719	305
Other	21	12
Total current liabilities	1,025	612
Other liabilities	17	20
Common Equity:
Common stock and additional paid-in capital	2,048	1,848
Retained earnings	2,545	2,344
Accumulated other comprehensive income (loss)	2	0
Shares in deferred compensation trust	(10)	(11)
Total common equity	4,585	4,181
Total liabilities and equity	$ 5,627	$ 4,813

Condensed Parent Company Fina_5

Condensed Parent Company Financial Statements (Condensed Statements of Cash Flow) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net cash flows from operating activities	$ 527.7	$ 521.6	$ 392.8
Cash flows from (used for) investing activities:
Other	(38.2)	(28.3)	9.8
Net cash flows from (used for) investing activities	(1,066.8)	(1,033.4)	(720.2)
Cash flows used for financing activities:
Common stock dividends	(312.2)	(288.3)	(266.5)
Proceeds from issuance of common stock, net	196.6	149.6	26.6
Payments to retire long-term debt	(855.7)	(4.6)	(313.4)
Net change in commercial paper	26	171.1	84.3
Other	(24)	(45.2)	(1.7)
Net cash flows from (used for) financing activities	530.7	532.6	329.3
Net increase (decrease) in cash, cash equivalents and restricted cash	(8.4)	20.8	1.9
Cash, cash equivalents and restricted cash at beginning of period	33.9	13.1	11.2
Cash, cash equivalents and restricted cash at end of period	25.5	33.9	13.1
Supplemental cash flows information:
Interest, net of capitalized interest	(247.5)	(212.6)	(192.4)
Income taxes, net	(5)	(11.3)	(9.8)
Parent Company [Member]
Net cash flows from operating activities	311	273	254
Cash flows from (used for) investing activities:
Capital contributions to consolidated subsidiaries	(625)	(290)	(250)
Captial repayments from consolidated subsidiaries	0	0	130
Net change in notes receivable from and payable to affiliates	441	54	294
Other	0	0	10
Net cash flows from (used for) investing activities	(184)	(236)	184
Cash flows used for financing activities:
Common stock dividends	(312)	(288)	(267)
Proceeds from issuance of common stock, net	197	150	27
Payments to retire long-term debt	0	0	(250)
Net change in commercial paper	(10)	103	52
Other	(2)	(2)	0
Net cash flows from (used for) financing activities	(127)	(37)	(438)
Net increase (decrease) in cash, cash equivalents and restricted cash	0	0	0
Cash, cash equivalents and restricted cash at beginning of period	0	0	0
Cash, cash equivalents and restricted cash at end of period	0	0	0
Supplemental cash flows information:
Interest, net of capitalized interest	(4)	(3)	(3)
Income taxes, net	$ 5	$ 0	$ (37)

Valuation And Qualifying Acco_2

Valuation And Qualifying Accounts and Reserves (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Provision for Uncollectible Accounts [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	[1]	$ 12	$ 8.7	$ 4.8
Charged to Expense	[1]	21.2	15.1	17.4
Charged to Other Accounts	[1],[2]	1	5.4	8.8
Deductions	[1],[3]	23.7	17.2	22.3
Ending Balance	[1]	10.5	12	8.7
Accumulated Provision for Uncollectible Accounts [Member] | IPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	[1]	1.3	1.1	0.6
Charged to Expense	[1]	20.9	14.9	17.2
Charged to Other Accounts	[1],[2]	0	0	0
Deductions	[1],[3]	19.1	14.7	16.7
Ending Balance	[1]	3.1	1.3	1.1
Accumulated Provision for Uncollectible Accounts [Member] | WPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance		10.7	7.1	3.7
Charged to Expense		0.3	0.2	0.1
Charged to Other Accounts	[2]	1	5.4	8.8
Deductions	[3]	4.6	2	5.5
Ending Balance		7.4	10.7	7.1
Accumulated Provision for Other Reserves [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	[4]	23	25.1	27.1
Charged to Expense	[4]	1.4	3.3	6.1
Charged to Other Accounts	[2],[4]	0	5.1	0
Deductions	[3],[4]	9.5	10.5	8.1
Ending Balance	[4]	14.9	23	25.1
Accumulated Provision for Other Reserves [Member] | IPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	[4]	7.6	8.7	9.4
Charged to Expense	[4]	0.9	0.3	1
Charged to Other Accounts	[2],[4]	0	0	0
Deductions	[3],[4]	2.1	1.4	1.7
Ending Balance	[4]	6.4	7.6	8.7
Accumulated Provision for Other Reserves [Member] | WPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	[4]	6.4	8.1	11.4
Charged to Expense	[4]	0.5	0.1	1.8
Charged to Other Accounts	[2],[4]	0	0	0
Deductions	[3],[4]	2.3	1.8	5.1
Ending Balance	[4]	$ 4.6	$ 6.4	$ 8.1
[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 Federal Tax Reform.
[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 Federal Tax Reform.