Interstate Power & Light (IPLDP) 10-K 21 Feb 20 2019 FY Annual report Financial data

Document And Entity Information

Document And Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Jan. 31, 2020	Jun. 30, 2019
Entity Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2019
Document Transition Report	false
Entity Registrant Name	ALLIANT ENERGY CORP
Entity Central Index Key	0000352541
Entity Incorporation, State or Country Code	WI
Entity Address, Address Line One	4902 N. Biltmore Lane
Entity Address, City or Town	Madison
Entity Address, State or Province	WI
Entity Address, Postal Zip Code	53718
City Area Code	608
Local Phone Number	458-3311
Entity File Number	1-9894
Entity Tax Identification Number	39-1380265
Title of 12(b) Security	Common Stock, $0.01 Par Value
Trading Symbol	LNT
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 11,600
Entity Common Stock, Shares Outstanding		245,031,019
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Amendment Flag	false
Documents Incorporated by Reference	Portions of the Proxy Statement relating to Alliant Energy Corporation’s 2020 Annual Meeting of Shareowners are, or will be upon filing with the Securities and Exchange Commission, incorporated by reference into Part III hereof.
IPL [Member]
Entity Information [Line Items]
Entity Registrant Name	INTERSTATE POWER & LIGHT CO
Entity Central Index Key	0000052485
Entity Incorporation, State or Country Code	IA
Entity Address, Address Line One	Alliant Energy Tower
Entity Address, City or Town	Cedar Rapids
Entity Address, State or Province	IA
Entity Address, Postal Zip Code	52401
City Area Code	319
Local Phone Number	786-4411
Entity File Number	1-4117
Entity Tax Identification Number	42-0331370
Title of 12(b) Security	5.100% Series D Cumulative Perpetual Preferred Stock, $0.01 Par Value
Trading Symbol	IPLDP
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			0
Entity Common Stock, Shares Outstanding		13,370,788
WPL [Member]
Entity Information [Line Items]
Entity Registrant Name	WISCONSIN POWER & LIGHT CO
Entity Central Index Key	0000107832
Entity Incorporation, State or Country Code	WI
Entity Address, Address Line One	4902 N. Biltmore Lane
Entity Address, City or Town	Madison
Entity Address, State or Province	WI
Entity Address, Postal Zip Code	53718
City Area Code	608
Local Phone Number	458-3311
Entity File Number	0-337
Entity Tax Identification Number	39-0714890
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 0
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, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues:
Electric utility	$ 3,063.6	$ 3,000.3	$ 2,894.7
Gas utility	455.2	446.6	400.9
Other utility	46.5	48	47.5
Non-utility	82.4	39.6	39.1
Total revenues	3,647.7	3,534.5	3,382.2
Operating expenses:
Electric production fuel and purchased power	776.7	855	818.1
Electric transmission service	481.4	495.7	480.9
Cost of gas sold	221.7	232.3	211.4
Other operation and maintenance	712.2	645.8	633.2
Depreciation and amortization	567.2	506.9	461.8
Taxes other than income taxes	110.8	104.4	105.6
Total operating expenses	2,870	2,840.1	2,711
Operating income (loss)	777.7	694.4	671.2
Other (income) and deductions:
Interest expense	272.9	247	215.6
Equity income from unconsolidated investments, net	(53)	(54.6)	(44.8)
Allowance for funds used during construction	(92.7)	(75.6)	(49.7)
Other	14.4	7.6	17.3
Total other (income) and deductions	141.6	124.4	138.4
Income from continuing operations before income taxes	636.1	570	532.8
Income tax expense (benefit)	68.7	47.7	66.7
Income from continuing operations, net of tax	567.4	522.3	466.1
Income from discontinued operations, net of tax	0	0	1.4
Net income	567.4	522.3	467.5
Preferred dividend requirements of Interstate Power and Light Company	10.2	10.2	10.2
Net income attributable to common shareowners	$ 557.2	$ 512.1	$ 457.3
Weighted average number of common shares outstanding:
Basic (in shares)	238.5	233.6	229.7
Diluted (in shares)	239	233.6	229.7
Earnings per weighted average common share attributable to Alliant Energy common shareowners:
Basic (in dollars per share)	$ 2.34	$ 2.19	$ 1.99
Diluted (in dollars per share)	$ 2.33	$ 2.19	$ 1.99
Amounts attributable to common shareowners:
Income from continuing operations, net of tax	$ 557.2	$ 512.1	$ 455.9
Income from discontinued operations, net of tax	0	0	1.4
Net income	557.2	512.1	457.3
IPL [Member]
Revenues:
Electric utility	1,781.2	1,731.1	1,598.9
Gas utility	264.2	266.2	226
Other utility	44.2	45	45.4
Total revenues	2,089.6	2,042.3	1,870.3
Operating expenses:
Electric production fuel and purchased power	434.5	469	443.6
Electric transmission service	340.2	352.9	310.4
Cost of gas sold	119.9	129.6	115.6
Other operation and maintenance	404.6	402.6	396.6
Depreciation and amortization	326.7	283.5	245
Taxes other than income taxes	60.9	53.9	55
Total operating expenses	1,686.8	1,691.5	1,566.2
Operating income (loss)	402.8	350.8	304.1
Other (income) and deductions:
Interest expense	126.9	119.4	112.4
Allowance for funds used during construction	(49.4)	(42.2)	(31.4)
Other	6.9	2.6	7
Total other (income) and deductions	84.4	79.8	88
Income from continuing operations before income taxes	318.4	271	216.1
Income tax expense (benefit)	24.1	(3.2)	(10.9)
Net income	294.3	274.2	227
Preferred dividend requirements of Interstate Power and Light Company	10.2	10.2	10.2
Net income attributable to common shareowners	284.1	264	216.8
Amounts attributable to common shareowners:
Net income	284.1	264	216.8
WPL [Member]
Revenues:
Electric utility	1,282.4	1,269.2	1,295.8
Gas utility	191	180.4	174.9
Other utility	2.3	3	2.1
Total revenues	1,475.7	1,452.6	1,472.8
Operating expenses:
Electric production fuel and purchased power	342.2	386	374.5
Electric transmission service	141.2	142.8	170.5
Cost of gas sold	101.8	102.7	95.8
Other operation and maintenance	260.9	241.6	238.5
Depreciation and amortization	235.6	219.4	212.9
Taxes other than income taxes	46.8	47.2	46.9
Total operating expenses	1,128.5	1,139.7	1,139.1
Operating income (loss)	347.2	312.9	333.7
Other (income) and deductions:
Interest expense	102.2	97.8	93.8
Allowance for funds used during construction	(43.3)	(33.4)	(18.3)
Other	6	4.2	9.7
Total other (income) and deductions	64.9	68.6	85.2
Income from continuing operations before income taxes	282.3	244.3	248.5
Income tax expense (benefit)	49.3	36.2	61.9
Net income	$ 233	$ 208.1	$ 186.6

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents		$ 16.3	$ 20.9
Accounts receivable, less allowance for doubtful accounts		402.1	350.4
Production fuel, at weighted average cost		77.7	61.4
Gas stored underground, at weighted average cost		49.1	49
Materials and supplies, at weighted average cost		100.5	101.4
Regulatory assets		86.4	79.8
Prepaid gross receipts tax		41.7	42.2
Other		101.7	80
Total current assets		875.5	785.1
Property, plant and equipment, net		13,527.1	12,462.4
Investments:
ATC Holdings		320.1	293.6
Other		147.7	137.7
Total investments		467.8	431.3
Other assets:
Regulatory assets		1,758.3	1,657.5
Deferred charges and other		72	89.7
Total other assets		1,830.3	1,747.2
Total assets		16,700.7	15,426
Current liabilities:
Current maturities of long-term debt		657.2	256.5
Commercial paper		337.4	441.2
Accounts payable		422.3	543.3
Regulatory liabilities		212	142.7
Other		425.2	260.4
Total current liabilities		2,054.1	1,644.1
Long-term debt, net (excluding current portion)	[1]	5,533	5,246.3
Other liabilities:
Deferred tax liabilities		1,714	1,603.1
Regulatory liabilities		1,211.6	1,350.5
Pension and other benefit obligations		484	509.1
Other		298.9	287.2
Total other liabilities		3,708.5	3,749.9
Commitments and contingencies (Note 17)
Common equity:
Common stock		2.5	2.4
Additional paid-in capital		2,445.9	2,045.5
Retained earnings		2,765.4	2,545.9
Accumulated other comprehensive income		1.3	1.7
Shares in deferred compensation trust - 381,232 and 384,580 shares at a weighted average cost of $26.24 and $25.60 per share		(10)	(9.8)
Total common equity		5,205.1	4,585.7
Cumulative preferred stock of Interstate Power and Light Company		200	200
Total equity		5,405.1	4,785.7
Total liabilities and equity		16,700.7	15,426
IPL [Member]
Current assets:
Cash and cash equivalents		9.3	9.7
Accounts receivable, less allowance for doubtful accounts		202.8	153.5
Production fuel, at weighted average cost		47.1	44.8
Gas stored underground, at weighted average cost		21.7	26.1
Materials and supplies, at weighted average cost		55	55.4
Regulatory assets		43.5	39.2
Other		30	43.1
Total current assets		409.4	371.8
Property, plant and equipment, net		7,480.7	6,781.5
Other assets:
Regulatory assets		1,355.8	1,239.8
Deferred charges and other		31.6	18.3
Total other assets		1,387.4	1,258.1
Total assets		9,277.5	8,411.4
Current liabilities:
Current maturities of long-term debt		200	0
Commercial paper		0	50.4
Accounts payable		207	304.9
Regulatory liabilities		115.9	90
Accrued taxes		63.3	45.8
Accrued interest		36.6	31.2
Other		207.7	84.8
Total current liabilities		830.5	607.1
Long-term debt, net (excluding current portion)	[1]	2,947.3	2,552.3
Other liabilities:
Deferred tax liabilities		1,008	957.3
Regulatory liabilities		598.8	664.9
Pension and other benefit obligations		167.7	178.4
Other		253.4	220.7
Total other liabilities		2,027.9	2,021.3
Commitments and contingencies (Note 17)
Common equity:
Common stock		33.4	33.4
Additional paid-in capital		2,347.8	2,222.8
Retained earnings		890.6	774.5
Total common equity		3,271.8	3,030.7
Cumulative preferred stock of Interstate Power and Light Company		200	200
Total equity		3,471.8	3,230.7
Total liabilities and equity		9,277.5	8,411.4
WPL [Member]
Current assets:
Cash and cash equivalents		4.4	8.7
Accounts receivable, less allowance for doubtful accounts		190.9	190.1
Production fuel, at weighted average cost		30.6	16.6
Gas stored underground, at weighted average cost		27.4	22.9
Materials and supplies, at weighted average cost		43.1	42.9
Regulatory assets		42.9	40.6
Prepaid gross receipts tax		41.7	42.2
Other		61.7	20.6
Total current assets		442.7	384.6
Property, plant and equipment, net		5,638.3	5,287.3
Other assets:
Regulatory assets		402.5	417.7
Deferred charges and other		23	62.9
Total other assets		425.5	480.6
Total assets		6,506.5	6,152.5
Current liabilities:
Current maturities of long-term debt		150	250
Commercial paper		168.2	105.5
Accounts payable		159.9	180.9
Accounts payable to associated companies		41.8	31.8
Regulatory liabilities		96.1	52.7
Other		74.3	73.7
Total current liabilities		690.3	694.6
Long-term debt, net (excluding current portion)	[1]	1,782.7	1,584.9
Other liabilities:
Deferred tax liabilities		626.2	582
Regulatory liabilities		612.8	685.6
Finance lease obligations - Sheboygan Falls Energy Facility		51.4	60
Pension and other benefit obligations		210.8	217.7
Other		168.7	178.2
Total other liabilities		1,669.9	1,723.5
Commitments and contingencies (Note 17)
Common equity:
Common stock		66.2	66.2
Additional paid-in capital		1,434	1,309
Retained earnings		863.4	774.3
Total common equity		2,363.6	2,149.5
Total equity		2,363.6	2,149.5
Total liabilities and equity		$ 6,506.5	$ 6,152.5
[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, 2019	Dec. 31, 2018
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)	245,022,800	236,063,279
Shares in deferred compensation trust (in shares)	381,232	384,580
Shares in deferred compensation trust, weighted average cost per share (in dollars per share)	$ 26.24	$ 25.60
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, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from (used for) operating activities:
Net income	$ 567.4	$ 522.3	$ 467.5
Adjustments to reconcile net income to net cash flows from (used for) operating activities:
Depreciation and amortization	567.2	506.9	461.8
Deferred tax expense and tax credits	55.6	67	139.6
Equity component of allowance for funds used during construction	(65.5)	(51.4)	(33.6)
Other	19.9	7.7	21.7
Other changes in assets and liabilities:
Accounts receivable	(471.7)	(475.4)	(441.2)
Regulatory assets	(16.2)	(16.2)	(130.8)
Regulatory liabilities	(40.3)	1.3	(83.8)
Deferred income taxes	53.8	55.9	81.7
Other	(9.8)	(90.4)	38.7
Net cash flows from (used for) operating activities	660.4	527.7	521.6
Cash flows used for investing activities:
Utility construction and acquisition expenditures	(1,538.4)	(1,568.3)	(1,281.8)
Other construction and acquisition expenditures	(101.7)	(65.6)	(185.1)
Cash receipts on sold receivables	413.2	605.3	461.8
Other	(60.4)	(38.2)	(28.3)
Net cash flows used for investing activities	(1,287.3)	(1,066.8)	(1,033.4)
Cash flows from financing activities:
Common stock dividends	(337.7)	(312.2)	(288.3)
Proceeds from issuance of common stock, net	390.3	196.6	149.6
Proceeds from issuance of long-term debt	950	1,500	550
Payments to retire long-term debt	(256.5)	(855.7)	(4.6)
Net change in commercial paper and other short-term borrowings	(103.8)	26	171.1
Other	(23.2)	(24)	(45.2)
Net cash flows from (used for) financing activities	619.1	530.7	532.6
Net increase (decrease) in cash, cash equivalents and restricted cash	(7.8)	(8.4)	20.8
Cash, cash equivalents and restricted cash at beginning of period	25.5	33.9	13.1
Cash, cash equivalents and restricted cash at end of period	17.7	25.5	33.9
Supplemental cash flows information:
Interest	(267.9)	(247.5)	(212.6)
Income taxes, net	20.5	(5)	(11.3)
Significant non-cash investing and financing activities:
Accrued capital expenditures	195.8	299.5	196.5
Beneficial interest obtained in exchange for securitized accounts receivable	187.7	119.4	222.1
IPL [Member]
Cash flows from (used for) operating activities:
Net income	294.3	274.2	227
Adjustments to reconcile net income to net cash flows from (used for) operating activities:
Depreciation and amortization	326.7	283.5	245
Deferred tax expense and tax credits	15.3	2.2	55.8
Equity component of allowance for funds used during construction	(35.2)	(28.6)	(21.1)
Other	1	3.6	1.5
Other changes in assets and liabilities:
Accounts receivable	(466.6)	(494)	(478.7)
Regulatory assets	(11.5)	(20.2)	(126.2)
Accounts payable	(20.5)	(24.9)	24
Regulatory liabilities	2	0.6	(71.2)
Deferred income taxes	35.2	43.8	103.7
Other	32.2	(45.2)	18.4
Net cash flows from (used for) operating activities	172.9	(5)	(21.8)
Cash flows used for investing activities:
Utility construction and acquisition expenditures	(1,019.6)	(990.7)	(676)
Cash receipts on sold receivables	413.2	605.3	461.8
Other	(60.6)	(44)	(27.7)
Net cash flows used for investing activities	(667)	(429.4)	(241.9)
Cash flows from financing activities:
Common stock dividends	(168)	(168)	(156.1)
Capital contributions from parent	125	425	200
Proceeds from issuance of long-term debt	600	500	250
Payments to retire long-term debt	0	(350)	0
Net change in commercial paper and other short-term borrowings	(50.4)	50.4	0
Other	(15.6)	(17.8)	(27.2)
Net cash flows from (used for) financing activities	491	439.6	266.7
Net increase (decrease) in cash, cash equivalents and restricted cash	(3.1)	5.2	3
Cash, cash equivalents and restricted cash at beginning of period	12.4	7.2	4.2
Cash, cash equivalents and restricted cash at end of period	9.3	12.4	7.2
Supplemental cash flows information:
Interest	(121.6)	(120.3)	(111.8)
Income taxes, net	6.9	(23.8)	8.6
Significant non-cash investing and financing activities:
Accrued capital expenditures	111.6	186.6	76.4
Beneficial interest obtained in exchange for securitized accounts receivable	187.7	119.4	222.1
WPL [Member]
Cash flows from (used for) operating activities:
Net income	233	208.1	186.6
Adjustments to reconcile net income to net cash flows from (used for) operating activities:
Depreciation and amortization	235.6	219.4	212.9
Deferred tax expense and tax credits	23.6	49.8	53.9
Other	(11.5)	(17.5)	4.4
Other changes in assets and liabilities:
Regulatory liabilities	(42.3)	0.7	(12.6)
Other	(15.2)	(3.5)	20.5
Net cash flows from (used for) operating activities	423.2	457	465.7
Cash flows used for investing activities:
Utility construction and acquisition expenditures	(518.8)	(577.6)	(637.4)
Other	(38.4)	(29.9)	(29.9)
Net cash flows used for investing activities	(557.2)	(607.5)	(667.3)
Cash flows from financing activities:
Common stock dividends	(143.9)	(140.1)	(125.9)
Capital contributions from parent	125	200	90
Proceeds from issuance of long-term debt	350	0	300
Payments to retire long-term debt	(250)	0	0
Net change in commercial paper and other short-term borrowings	62.7	80.5	(27.3)
Other	(14.6)	(4.9)	(17.9)
Net cash flows from (used for) financing activities	129.2	135.5	218.9
Net increase (decrease) in cash, cash equivalents and restricted cash	(4.8)	(15)	17.3
Cash, cash equivalents and restricted cash at beginning of period	9.2	24.2	6.9
Cash, cash equivalents and restricted cash at end of period	4.4	9.2	24.2
Supplemental cash flows information:
Interest	(102.5)	(98.1)	(91.7)
Income taxes, net	(28.9)	14	(8.4)
Significant non-cash investing and financing activities:
Accrued capital expenditures	$ 81.5	$ 102.5	$ 114.5

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]	Cumulative Preferred Stock [Member]	Cumulative Preferred Stock [Member]IPL [Member]
Beginning balance at Dec. 31, 2016				$ 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
Beginning balance at Dec. 31, 2016	$ 4,062	$ 2,449	$ 1,730.8
Increase (Decrease) in Shareowners' Equity [Roll Forward]
Net income attributable to common shareowners	457.3	216.8								457.3	216.8
Net income	467.5	227	186.6									186.6
Common stock dividends	(288.3)	(156.1)	(125.9)							(288.3)	(156.1)	(125.9)
Equity forward settlements, At-the-market offering program and Shareowner Direct Plan issuances	149.6						149.6
Capital contribution from parent		200	90					200	90
Equity-based compensation plans and 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				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
Ending balance at Dec. 31, 2017	4,382.2	2,709.7	1,881.5
Increase (Decrease) in Shareowners' Equity [Roll Forward]
Net income attributable to common shareowners	512.1	264								512.1	264
Net income	522.3	274.2	208.1									208.1
Common stock dividends	(312.2)	(168)	(140.1)							(312.2)	(168)	(140.1)
Equity forward settlements, At-the-market offering program and Shareowner Direct Plan issuances	196.6			0.1			196.5
Capital contribution from parent		425	200					425	200
Equity-based compensation plans and 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
Ending balance at Dec. 31, 2018	4,785.7	3,230.7	2,149.5
Increase (Decrease) in Shareowners' Equity [Roll Forward]
Net income attributable to common shareowners	557.2	284.1								557.2	284.1
Net income	567.4	294.3	233									233
Common stock dividends	(337.7)	(168)	(143.9)							(337.7)	(168)	(143.9)
Equity forward settlements, At-the-market offering program and Shareowner Direct Plan issuances	390.3			0.1			390.2
Capital contribution from parent		125	125					125	125
Equity-based compensation plans and other	10						10.2							(0.2)
Other comprehensive income (loss), net of tax	(0.4)	0	0										(0.4)
Ending balance at Dec. 31, 2019	5,205.1	3,271.8	2,363.6	$ 2.5	$ 33.4	$ 66.2	$ 2,445.9	$ 2,347.8	$ 1,434	$ 2,765.4	$ 890.6	$ 863.4	$ 1.3	$ (10)	$ 200	$ 200
Ending balance at Dec. 31, 2019	$ 5,405.1	$ 3,471.8	$ 2,363.6

Consolidated Statements of Co_2

Consolidated Statements of Common Equity (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Stockholders' Equity [Abstract]
Common stock dividends (in dollars per share)	$ 1.42	$ 1.34	$ 1.26

Summary Of Significant Accounti

Summary Of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2019
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 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; a barge terminal and hauling services on the Mississippi River; customized supply chain solution capabilities; freight and logistics brokering services; 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 that Alliant Energy and WPL do not control 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. 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 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. 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. 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 alternative minimum tax and allows unutilized alternative minimum tax 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 2019 , 2018 and 2017 . Cash and cash equivalents include short-term liquid investments that have original maturities of less than 90 days. At December 31, 2019 and 2018 , restricted cash primarily related to requirements in Sheboygan Power, LLC’s debt agreement. 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 2019 2018 2017 2019 2018 2017 Electric - generation 3.8% 3.6% 3.5% 3.6% 3.6% 3.5% Electric - distribution 2.9% 2.8% 2.4% 2.6% 2.6% 2.6% Electric - other 5.3% 4.7% 4.5% 5.8% 5.7% 6.9% Gas 3.3% 3.2% 3.4% 2.5% 2.5% 2.5% Other 5.9% 5.2% 4.0% 5.6% 5.8% 6.0% 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: 2019 2018 2017 IPL (Marshalltown CWIP) N/A N/A 7.8% IPL (Wind generation CWIP) 7.4% 7.5% 7.6% IPL (other CWIP) 7.5% 7.5% 7.6% WPL (retail jurisdiction) 6.8% 7.7% 7.6% WPL (wholesale jurisdiction) 6.9% 7.2% 6.0% 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. 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, 2019 , 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. 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 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. Renewable Energy Rider - Effective with the implementation of final rates covering the 2020 forward-looking Test Period, IPL will recover a return of, as well as earn a return on, its 1,000 MW of new wind generation placed in service in 2019 and 2020 from its retail electric customers through a renewable energy rider. Other applicable costs and tax benefits associated with the new wind generation, excluding operation and maintenance expenses, will also be included in the rider. This cost recovery mechanism provides for annual adjustments to electric rates charged to IPL’s retail electric customers for actual renewable energy costs and tax benefits. Changes in the under-/over-collection of these costs will be recognized in “Electric utility revenue” in Alliant Energy’s and IPL’s income statements. The cumulative effects of the under-/over-collection of these costs for IPL will be 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. 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. 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. The fair value of a legal obligation associated with the retirement of an asset is recorded as a liability when an asset is placed in service, when a legal obligation is subsequently identified or when sufficient information becomes available to determine a reasonable estimate of the fair value of future retirement costs. When an ARO is recorded as a liability, an equivalent amount is added to the asset cost. The fair value of AROs at inception is determined using discounted cash flows analyses. The liability is accreted to its present value and the capitalized cost is depreciated over the useful life of the related asset. Accretion and depreciation expenses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory assets on the balance sheets. Upon regulatory approval to recover IPL’s AROs expenditures, its regulatory assets are amortized to depreciation and amortization expenses in Alliant Energy’s and IPL’s income statements over the same time period the ARO expenditures are recovered from IPL’s customers. WPL’s regulatory assets related to AROs are being recovered as a component of depreciation rates pursuant to PSCW and FERC orders. Accretion and depreciation expenses related to AROs for Alliant Energy’s non-utility operations are recorded to depreciation and amortization expenses in Alliant Energy’s income statements. Upon settlement of the ARO liability, an entity settles the obligation for its recorded amount or incurs a gain or loss. Any gains or losses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory liabilities or regulatory assets on the balance sheets. Debt issuance 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. 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. 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. The determination of whether an arrangement qualifies as a lease occurs at the inception of the arrangement. Arrangements that qualify as leases are classified as either operating or finance. Operating and finance lease liabilities represent obligations to make payments arising from the lease. Operating and finance lease assets represent the right to use an underlying asset for the lease term and are recognized at the lease commencement date based on the present value of the lease payments over the lease term. Leases with initial terms less than 12 months are not recognized as leases. For operating leases, an incremental borrowing rate, as determined at the lease commencement date, is used to determine the present value of the lease payments. For finance leases, the rate implicit in the lease, if known, is used to determine the present value of the lease payments. Lease terms include options to extend or terminate the lease when it is reasonably certain that the option will be exercised. Operating lease expense is recognized on a straight-line basis over the expected lease term. Finance lease expense is comprised of depreciation and interest expenses. Finance lease assets are depreciated on a straight-line basis over the shorter of the useful life of the underlying asset or the lease term. Credit Losses - In June 2016, the FASB issued an accounting standard requiring use of a current expected credit loss model rather than an incurred loss method, which is intended to result in more timely recognition of credit losses on trade receivables and certain other assets. Alliant Energy, IPL and WPL adopted this standard on January 1, 2020 using a modified retrospective method of adoption, which requires cumulative effect adjustments to retained earnings on January 1, 2020. IPL and WPL did not record a cumulative effect adjustment to retained earnings and do not expect a material change to their financial condition or results of operations as a result of adopting this standard. Alliant Energy’s non-regulated entities continue to evaluate the final adoption impact for credit loss exposures related to their existing guarantees (described in Note 17(d) ); however, Alliant Energy does not expect a material change in its financial condition or results of operations as a result of adopting this standard. Cloud Computing Arrangements - In August 2018, the FASB issued an accounting standard that clarifies capitalization and presentation requirements of implementation costs incurred in cloud computing arrangements. Alliant Energy, IPL and WPL adopted this standard on January 1, 2020 and do not expect a material change to their financial condition or results of operations as a result
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 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; a barge terminal and hauling services on the Mississippi River; customized supply chain solution capabilities; freight and logistics brokering services; 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 that Alliant Energy and WPL do not control 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. 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 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. 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. 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 alternative minimum tax and allows unutilized alternative minimum tax 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 2019 , 2018 and 2017 . Cash and cash equivalents include short-term liquid investments that have original maturities of less than 90 days. At December 31, 2019 and 2018 , restricted cash primarily related to requirements in Sheboygan Power, LLC’s debt agreement. 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 2019 2018 2017 2019 2018 2017 Electric - generation 3.8% 3.6% 3.5% 3.6% 3.6% 3.5% Electric - distribution 2.9% 2.8% 2.4% 2.6% 2.6% 2.6% Electric - other 5.3% 4.7% 4.5% 5.8% 5.7% 6.9% Gas 3.3% 3.2% 3.4% 2.5% 2.5% 2.5% Other 5.9% 5.2% 4.0% 5.6% 5.8% 6.0% 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: 2019 2018 2017 IPL (Marshalltown CWIP) N/A N/A 7.8% IPL (Wind generation CWIP) 7.4% 7.5% 7.6% IPL (other CWIP) 7.5% 7.5% 7.6% WPL (retail jurisdiction) 6.8% 7.7% 7.6% WPL (wholesale jurisdiction) 6.9% 7.2% 6.0% 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. 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, 2019 , 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. 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 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. Renewable Energy Rider - Effective with the implementation of final rates covering the 2020 forward-looking Test Period, IPL will recover a return of, as well as earn a return on, its 1,000 MW of new wind generation placed in service in 2019 and 2020 from its retail electric customers through a renewable energy rider. Other applicable costs and tax benefits associated with the new wind generation, excluding operation and maintenance expenses, will also be included in the rider. This cost recovery mechanism provides for annual adjustments to electric rates charged to IPL’s retail electric customers for actual renewable energy costs and tax benefits. Changes in the under-/over-collection of these costs will be recognized in “Electric utility revenue” in Alliant Energy’s and IPL’s income statements. The cumulative effects of the under-/over-collection of these costs for IPL will be 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. 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. 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. The fair value of a legal obligation associated with the retirement of an asset is recorded as a liability when an asset is placed in service, when a legal obligation is subsequently identified or when sufficient information becomes available to determine a reasonable estimate of the fair value of future retirement costs. When an ARO is recorded as a liability, an equivalent amount is added to the asset cost. The fair value of AROs at inception is determined using discounted cash flows analyses. The liability is accreted to its present value and the capitalized cost is depreciated over the useful life of the related asset. Accretion and depreciation expenses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory assets on the balance sheets. Upon regulatory approval to recover IPL’s AROs expenditures, its regulatory assets are amortized to depreciation and amortization expenses in Alliant Energy’s and IPL’s income statements over the same time period the ARO expenditures are recovered from IPL’s customers. WPL’s regulatory assets related to AROs are being recovered as a component of depreciation rates pursuant to PSCW and FERC orders. Accretion and depreciation expenses related to AROs for Alliant Energy’s non-utility operations are recorded to depreciation and amortization expenses in Alliant Energy’s income statements. Upon settlement of the ARO liability, an entity settles the obligation for its recorded amount or incurs a gain or loss. Any gains or losses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory liabilities or regulatory assets on the balance sheets. Debt issuance 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. 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. 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. The determination of whether an arrangement qualifies as a lease occurs at the inception of the arrangement. Arrangements that qualify as leases are classified as either operating or finance. Operating and finance lease liabilities represent obligations to make payments arising from the lease. Operating and finance lease assets represent the right to use an underlying asset for the lease term and are recognized at the lease commencement date based on the present value of the lease payments over the lease term. Leases with initial terms less than 12 months are not recognized as leases. For operating leases, an incremental borrowing rate, as determined at the lease commencement date, is used to determine the present value of the lease payments. For finance leases, the rate implicit in the lease, if known, is used to determine the present value of the lease payments. Lease terms include options to extend or terminate the lease when it is reasonably certain that the option will be exercised. Operating lease expense is recognized on a straight-line basis over the expected lease term. Finance lease expense is comprised of depreciation and interest expenses. Finance lease assets are depreciated on a straight-line basis over the shorter of the useful life of the underlying asset or the lease term. Credit Losses - In June 2016, the FASB issued an accounting standard requiring use of a current expected credit loss model rather than an incurred loss method, which is intended to result in more timely recognition of credit losses on trade receivables and certain other assets. Alliant Energy, IPL and WPL adopted this standard on January 1, 2020 using a modified retrospective method of adoption, which requires cumulative effect adjustments to retained earnings on January 1, 2020. IPL and WPL did not record a cumulative effect adjustment to retained earnings and do not expect a material change to their financial condition or results of operations as a result of adopting this standard. Alliant Energy’s non-regulated entities continue to evaluate the final adoption impact for credit loss exposures related to their existing guarantees (described in Note 17(d) ); however, Alliant Energy does not expect a material change in its financial condition or results of operations as a result of adopting this standard. Cloud Computing Arrangements - In August 2018, the FASB issued an accounting standard that clarifies capitalization and presentation requirements of implementation costs incurred in cloud computing arrangements. Alliant Energy, IPL and WPL adopted this standard on January 1, 2020 and do not expect a material change to their financial condition or results of operations as a result
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 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; a barge terminal and hauling services on the Mississippi River; customized supply chain solution capabilities; freight and logistics brokering services; 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 that Alliant Energy and WPL do not control 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. 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 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. 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. 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 alternative minimum tax and allows unutilized alternative minimum tax 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 2019 , 2018 and 2017 . Cash and cash equivalents include short-term liquid investments that have original maturities of less than 90 days. At December 31, 2019 and 2018 , restricted cash primarily related to requirements in Sheboygan Power, LLC’s debt agreement. 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 2019 2018 2017 2019 2018 2017 Electric - generation 3.8% 3.6% 3.5% 3.6% 3.6% 3.5% Electric - distribution 2.9% 2.8% 2.4% 2.6% 2.6% 2.6% Electric - other 5.3% 4.7% 4.5% 5.8% 5.7% 6.9% Gas 3.3% 3.2% 3.4% 2.5% 2.5% 2.5% Other 5.9% 5.2% 4.0% 5.6% 5.8% 6.0% 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: 2019 2018 2017 IPL (Marshalltown CWIP) N/A N/A 7.8% IPL (Wind generation CWIP) 7.4% 7.5% 7.6% IPL (other CWIP) 7.5% 7.5% 7.6% WPL (retail jurisdiction) 6.8% 7.7% 7.6% WPL (wholesale jurisdiction) 6.9% 7.2% 6.0% 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. 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, 2019 , 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. 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 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. Renewable Energy Rider - Effective with the implementation of final rates covering the 2020 forward-looking Test Period, IPL will recover a return of, as well as earn a return on, its 1,000 MW of new wind generation placed in service in 2019 and 2020 from its retail electric customers through a renewable energy rider. Other applicable costs and tax benefits associated with the new wind generation, excluding operation and maintenance expenses, will also be included in the rider. This cost recovery mechanism provides for annual adjustments to electric rates charged to IPL’s retail electric customers for actual renewable energy costs and tax benefits. Changes in the under-/over-collection of these costs will be recognized in “Electric utility revenue” in Alliant Energy’s and IPL’s income statements. The cumulative effects of the under-/over-collection of these costs for IPL will be 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. 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. 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. The fair value of a legal obligation associated with the retirement of an asset is recorded as a liability when an asset is placed in service, when a legal obligation is subsequently identified or when sufficient information becomes available to determine a reasonable estimate of the fair value of future retirement costs. When an ARO is recorded as a liability, an equivalent amount is added to the asset cost. The fair value of AROs at inception is determined using discounted cash flows analyses. The liability is accreted to its present value and the capitalized cost is depreciated over the useful life of the related asset. Accretion and depreciation expenses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory assets on the balance sheets. Upon regulatory approval to recover IPL’s AROs expenditures, its regulatory assets are amortized to depreciation and amortization expenses in Alliant Energy’s and IPL’s income statements over the same time period the ARO expenditures are recovered from IPL’s customers. WPL’s regulatory assets related to AROs are being recovered as a component of depreciation rates pursuant to PSCW and FERC orders. Accretion and depreciation expenses related to AROs for Alliant Energy’s non-utility operations are recorded to depreciation and amortization expenses in Alliant Energy’s income statements. Upon settlement of the ARO liability, an entity settles the obligation for its recorded amount or incurs a gain or loss. Any gains or losses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory liabilities or regulatory assets on the balance sheets. Debt issuance 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. 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. 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. The determination of whether an arrangement qualifies as a lease occurs at the inception of the arrangement. Arrangements that qualify as leases are classified as either operating or finance. Operating and finance lease liabilities represent obligations to make payments arising from the lease. Operating and finance lease assets represent the right to use an underlying asset for the lease term and are recognized at the lease commencement date based on the present value of the lease payments over the lease term. Leases with initial terms less than 12 months are not recognized as leases. For operating leases, an incremental borrowing rate, as determined at the lease commencement date, is used to determine the present value of the lease payments. For finance leases, the rate implicit in the lease, if known, is used to determine the present value of the lease payments. Lease terms include options to extend or terminate the lease when it is reasonably certain that the option will be exercised. Operating lease expense is recognized on a straight-line basis over the expected lease term. Finance lease expense is comprised of depreciation and interest expenses. Finance lease assets are depreciated on a straight-line basis over the shorter of the useful life of the underlying asset or the lease term. Credit Losses - In June 2016, the FASB issued an accounting standard requiring use of a current expected credit loss model rather than an incurred loss method, which is intended to result in more timely recognition of credit losses on trade receivables and certain other assets. Alliant Energy, IPL and WPL adopted this standard on January 1, 2020 using a modified retrospective method of adoption, which requires cumulative effect adjustments to retained earnings on January 1, 2020. IPL and WPL did not record a cumulative effect adjustment to retained earnings and do not expect a material change to their financial condition or results of operations as a result of adopting this standard. Alliant Energy’s non-regulated entities continue to evaluate the final adoption impact for credit loss exposures related to their existing guarantees (described in Note 17(d) ); however, Alliant Energy does not expect a material change in its financial condition or results of operations as a result of adopting this standard. Cloud Computing Arrangements - In August 2018, the FASB issued an accounting standard that clarifies capitalization and presentation requirements of implementation costs incurred in cloud computing arrangements. Alliant Energy, IPL and WPL adopted this standard on January 1, 2020 and do not expect a material change to their financial condition or results of operations as a result

Regulatory Matters

Regulatory Matters	12 Months Ended
	Dec. 31, 2019
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, 2019 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 2019 2018 2019 2018 2019 2018 Tax-related $817.6 $820.6 $776.8 $783.1 $40.8 $37.5 Pension and OPEB costs 524.0 542.3 262.5 274.0 261.5 268.3 Assets retired early 134.0 111.6 87.9 55.4 46.1 56.2 AROs 111.8 110.8 76.2 76.3 35.6 34.5 IPL’s DAEC PPA amendment 108.2 — 108.2 — — — Derivatives 39.5 28.0 18.3 15.1 21.2 12.9 Emission allowances 21.1 23.6 21.1 23.6 — — Other 88.5 100.4 48.3 51.5 40.2 48.9 $1,844.7 $1,737.3 $1,399.3 $1,279.0 $445.4 $458.3 At December 31, 2019 , IPL and WPL had $78 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 assets retired early, emission allowances, debt redemption costs and costs for clean air compliance projects. WPL’s regulatory assets that were not earning a return consisted primarily of environmental-related costs and costs for future expansion projects. 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 income 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. 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. Assets retired early - IPL and WPL have retired various natural gas- and coal-fired EGUs, and IPL has retired certain analog electric meters. As a result, the remaining net book value of these assets was reclassified from property, plant and equipment to a regulatory asset on their respective balance sheets. Details regarding the recovery of the remaining net book value of these assets from IPL’s and WPL’s customers are as follows: Entity Asset Retirement Date Regulatory Asset Balance as of Dec. 31, 2019 Recovery Regulatory Approval IPL Sutherland Units 1 and 3 2017 $28.5 Return of and return on remaining net book value over 10 years (return on effective with new rates expected to be implemented by the end of the first quarter of 2020) IUB and FERC IPL M.L. Kapp Unit 2 2018 23.6 Return of and return on remaining net book value over 10 years IUB and FERC IPL Analog electric meters 2019 35.8 Return of remaining net book value over 10 years (effective with new rates expected to be implemented by the end of the first quarter of 2020) IUB WPL Nelson Dewey Units 1 and 2 and Edgewater Unit 3 2015 20.2 Return of and return on remaining net book value over 10 years PSCW and FERC WPL Edgewater Unit 4 2018 25.9 Return of and return on remaining net book value over 10 years PSCW and 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. IPL’s DAEC PPA Amendment - In January 2019, IPL incurred an obligation to make a September 2020 buyout payment of $110 million in exchange for shortening the term of IPL’s DAEC nuclear generation PPA by 5 years . The IUB approved recovery of the buyout payment with IPL’s 2020 Test Period retail electric rate review, which will be recovered from IPL’s retail customers over a 5 -year period following the payment. The offsetting obligation has been discounted and is recorded in “Other current liabilities” on Alliant Energy’s and IPL’s balance sheets. 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 sulfur dioxide 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 2019 2018 2019 2018 2019 2018 Tax-related $835.6 $890.6 $350.9 $390.1 $484.7 $500.5 Cost of removal obligations 387.7 401.2 257.0 273.3 130.7 127.9 Electric transmission cost recovery 88.6 104.0 51.3 47.7 37.3 56.3 Commodity cost recovery 24.2 16.8 8.8 11.9 15.4 4.9 WPL’s earnings sharing mechanism 21.9 25.4 — — 21.9 25.4 Derivatives 19.9 18.5 17.4 10.2 2.5 8.3 Other 45.7 36.7 29.3 21.7 16.4 15.0 $1,423.6 $1,493.2 $714.7 $754.9 $708.9 $738.3 Tax-related regulatory liabilities reduce revenue requirement calculations utilized in IPL’s and WPL’s respective rate proceedings. Cost of removal obligations, to the extent expensed through depreciation rates, reduce rate base. 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. The majority 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. Beginning 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 began providing these benefits back to its retail electric customers utilizing the transmission cost rider. WPL is currently reflecting these benefits for its retail electric customers in 2019 and 2020 through transmission expense amortizations as authorized in its retail electric rate review (2019/2020 Test Period). Commodity cost recovery - Refer to Note 1(g) for details of IPL’s and WPL’s commodity cost recovery mechanisms. WPL’s 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. The timing of the refund to customers for the majority of these regulatory liabilities will be determined in a future WPL regulatory proceeding. Utility Rate Reviews - IPL’s Retail Electric Rate Review (2020 Forward-looking Test Period) - In March 2019, IPL filed a request with the IUB to increase annual electric base rates for its Iowa retail electric customers based on a 2020 forward-looking Test Period. The key drivers for IPL’s request included recovery of capital projects, including new wind generation. IPL concurrently filed for interim retail electric rates based on 2018 historical data as adjusted for certain known and measurable changes occurring in the first quarter of 2019. An interim retail electric base rate increase of $90 million , on an annual basis, was implemented effective April 1, 2019. In October 2019, IPL reached a settlement agreement with certain intervenor groups for an annual retail electric base rate increase of $127 million . The agreement includes both the recovery of and a return on IPL’s early retired EGUs, and the recovery of IPL’s retired analog electric meters. In addition, as discussed in Note 1(g) , the net impact of certain costs and benefits resulting from IPL’s 1,000 MW expansion of wind generation in 2019 and 2020 will be recovered from its retail electric customers through a new renewable energy rider. The settlement agreement also includes IPL providing retail electric billing credits over a 12 -month period beginning with final rates, including $27 million of excess deferred tax benefits and $8 million from a partial refund of interim rates implemented in 2019. In January 2020, the IUB issued an order approving the settlement with final rates, which are expected to be effective by the end of the first quarter of 2020. IPL’s Retail Gas Rate Review (2020 Forward-looking Test Period) - In March 2019, IPL filed a request with the IUB to increase annual gas base rates for its Iowa retail gas customers based on a 2020 forward-looking Test Period. In October 2019, IPL reached a settlement agreement with intervenor groups for an annual retail gas base rate increase of $12 million . In December 2019, the IUB issued an order approving the settlement with final rates, which were effective January 10, 2020. 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 based on a 2016 historical Test Year. An interim retail electric base rate increase of $102 million , on an annual basis, was implemented effective April 13, 2017. In September 2017, IPL reached a partial, non-unanimous settlement agreement with intervenor groups for an annual retail electric base rate increase of $130 million . In February 2018, the IUB issued an order approving the settlement with final rates effective May 1, 2018. 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 intervenor groups. Under the settlement, WPL retail electric and gas base rates will not change from current levels through the end of 2020.
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, 2019 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 2019 2018 2019 2018 2019 2018 Tax-related $817.6 $820.6 $776.8 $783.1 $40.8 $37.5 Pension and OPEB costs 524.0 542.3 262.5 274.0 261.5 268.3 Assets retired early 134.0 111.6 87.9 55.4 46.1 56.2 AROs 111.8 110.8 76.2 76.3 35.6 34.5 IPL’s DAEC PPA amendment 108.2 — 108.2 — — — Derivatives 39.5 28.0 18.3 15.1 21.2 12.9 Emission allowances 21.1 23.6 21.1 23.6 — — Other 88.5 100.4 48.3 51.5 40.2 48.9 $1,844.7 $1,737.3 $1,399.3 $1,279.0 $445.4 $458.3 At December 31, 2019 , IPL and WPL had $78 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 assets retired early, emission allowances, debt redemption costs and costs for clean air compliance projects. WPL’s regulatory assets that were not earning a return consisted primarily of environmental-related costs and costs for future expansion projects. 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 income 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. 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. Assets retired early - IPL and WPL have retired various natural gas- and coal-fired EGUs, and IPL has retired certain analog electric meters. As a result, the remaining net book value of these assets was reclassified from property, plant and equipment to a regulatory asset on their respective balance sheets. Details regarding the recovery of the remaining net book value of these assets from IPL’s and WPL’s customers are as follows: Entity Asset Retirement Date Regulatory Asset Balance as of Dec. 31, 2019 Recovery Regulatory Approval IPL Sutherland Units 1 and 3 2017 $28.5 Return of and return on remaining net book value over 10 years (return on effective with new rates expected to be implemented by the end of the first quarter of 2020) IUB and FERC IPL M.L. Kapp Unit 2 2018 23.6 Return of and return on remaining net book value over 10 years IUB and FERC IPL Analog electric meters 2019 35.8 Return of remaining net book value over 10 years (effective with new rates expected to be implemented by the end of the first quarter of 2020) IUB WPL Nelson Dewey Units 1 and 2 and Edgewater Unit 3 2015 20.2 Return of and return on remaining net book value over 10 years PSCW and FERC WPL Edgewater Unit 4 2018 25.9 Return of and return on remaining net book value over 10 years PSCW and 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. IPL’s DAEC PPA Amendment - In January 2019, IPL incurred an obligation to make a September 2020 buyout payment of $110 million in exchange for shortening the term of IPL’s DAEC nuclear generation PPA by 5 years . The IUB approved recovery of the buyout payment with IPL’s 2020 Test Period retail electric rate review, which will be recovered from IPL’s retail customers over a 5 -year period following the payment. The offsetting obligation has been discounted and is recorded in “Other current liabilities” on Alliant Energy’s and IPL’s balance sheets. 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 sulfur dioxide 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 2019 2018 2019 2018 2019 2018 Tax-related $835.6 $890.6 $350.9 $390.1 $484.7 $500.5 Cost of removal obligations 387.7 401.2 257.0 273.3 130.7 127.9 Electric transmission cost recovery 88.6 104.0 51.3 47.7 37.3 56.3 Commodity cost recovery 24.2 16.8 8.8 11.9 15.4 4.9 WPL’s earnings sharing mechanism 21.9 25.4 — — 21.9 25.4 Derivatives 19.9 18.5 17.4 10.2 2.5 8.3 Other 45.7 36.7 29.3 21.7 16.4 15.0 $1,423.6 $1,493.2 $714.7 $754.9 $708.9 $738.3 Tax-related regulatory liabilities reduce revenue requirement calculations utilized in IPL’s and WPL’s respective rate proceedings. Cost of removal obligations, to the extent expensed through depreciation rates, reduce rate base. 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. The majority 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. Beginning 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 began providing these benefits back to its retail electric customers utilizing the transmission cost rider. WPL is currently reflecting these benefits for its retail electric customers in 2019 and 2020 through transmission expense amortizations as authorized in its retail electric rate review (2019/2020 Test Period). Commodity cost recovery - Refer to Note 1(g) for details of IPL’s and WPL’s commodity cost recovery mechanisms. WPL’s 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. The timing of the refund to customers for the majority of these regulatory liabilities will be determined in a future WPL regulatory proceeding. Utility Rate Reviews - IPL’s Retail Electric Rate Review (2020 Forward-looking Test Period) - In March 2019, IPL filed a request with the IUB to increase annual electric base rates for its Iowa retail electric customers based on a 2020 forward-looking Test Period. The key drivers for IPL’s request included recovery of capital projects, including new wind generation. IPL concurrently filed for interim retail electric rates based on 2018 historical data as adjusted for certain known and measurable changes occurring in the first quarter of 2019. An interim retail electric base rate increase of $90 million , on an annual basis, was implemented effective April 1, 2019. In October 2019, IPL reached a settlement agreement with certain intervenor groups for an annual retail electric base rate increase of $127 million . The agreement includes both the recovery of and a return on IPL’s early retired EGUs, and the recovery of IPL’s retired analog electric meters. In addition, as discussed in Note 1(g) , the net impact of certain costs and benefits resulting from IPL’s 1,000 MW expansion of wind generation in 2019 and 2020 will be recovered from its retail electric customers through a new renewable energy rider. The settlement agreement also includes IPL providing retail electric billing credits over a 12 -month period beginning with final rates, including $27 million of excess deferred tax benefits and $8 million from a partial refund of interim rates implemented in 2019. In January 2020, the IUB issued an order approving the settlement with final rates, which are expected to be effective by the end of the first quarter of 2020. IPL’s Retail Gas Rate Review (2020 Forward-looking Test Period) - In March 2019, IPL filed a request with the IUB to increase annual gas base rates for its Iowa retail gas customers based on a 2020 forward-looking Test Period. In October 2019, IPL reached a settlement agreement with intervenor groups for an annual retail gas base rate increase of $12 million . In December 2019, the IUB issued an order approving the settlement with final rates, which were effective January 10, 2020. 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 based on a 2016 historical Test Year. An interim retail electric base rate increase of $102 million , on an annual basis, was implemented effective April 13, 2017. In September 2017, IPL reached a partial, non-unanimous settlement agreement with intervenor groups for an annual retail electric base rate increase of $130 million . In February 2018, the IUB issued an order approving the settlement with final rates effective May 1, 2018. 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 intervenor groups. Under the settlement, WPL retail electric and gas base rates will not change from current levels through the end of 2020.
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, 2019 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 2019 2018 2019 2018 2019 2018 Tax-related $817.6 $820.6 $776.8 $783.1 $40.8 $37.5 Pension and OPEB costs 524.0 542.3 262.5 274.0 261.5 268.3 Assets retired early 134.0 111.6 87.9 55.4 46.1 56.2 AROs 111.8 110.8 76.2 76.3 35.6 34.5 IPL’s DAEC PPA amendment 108.2 — 108.2 — — — Derivatives 39.5 28.0 18.3 15.1 21.2 12.9 Emission allowances 21.1 23.6 21.1 23.6 — — Other 88.5 100.4 48.3 51.5 40.2 48.9 $1,844.7 $1,737.3 $1,399.3 $1,279.0 $445.4 $458.3 At December 31, 2019 , IPL and WPL had $78 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 assets retired early, emission allowances, debt redemption costs and costs for clean air compliance projects. WPL’s regulatory assets that were not earning a return consisted primarily of environmental-related costs and costs for future expansion projects. 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 income 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. 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. Assets retired early - IPL and WPL have retired various natural gas- and coal-fired EGUs, and IPL has retired certain analog electric meters. As a result, the remaining net book value of these assets was reclassified from property, plant and equipment to a regulatory asset on their respective balance sheets. Details regarding the recovery of the remaining net book value of these assets from IPL’s and WPL’s customers are as follows: Entity Asset Retirement Date Regulatory Asset Balance as of Dec. 31, 2019 Recovery Regulatory Approval IPL Sutherland Units 1 and 3 2017 $28.5 Return of and return on remaining net book value over 10 years (return on effective with new rates expected to be implemented by the end of the first quarter of 2020) IUB and FERC IPL M.L. Kapp Unit 2 2018 23.6 Return of and return on remaining net book value over 10 years IUB and FERC IPL Analog electric meters 2019 35.8 Return of remaining net book value over 10 years (effective with new rates expected to be implemented by the end of the first quarter of 2020) IUB WPL Nelson Dewey Units 1 and 2 and Edgewater Unit 3 2015 20.2 Return of and return on remaining net book value over 10 years PSCW and FERC WPL Edgewater Unit 4 2018 25.9 Return of and return on remaining net book value over 10 years PSCW and 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. IPL’s DAEC PPA Amendment - In January 2019, IPL incurred an obligation to make a September 2020 buyout payment of $110 million in exchange for shortening the term of IPL’s DAEC nuclear generation PPA by 5 years . The IUB approved recovery of the buyout payment with IPL’s 2020 Test Period retail electric rate review, which will be recovered from IPL’s retail customers over a 5 -year period following the payment. The offsetting obligation has been discounted and is recorded in “Other current liabilities” on Alliant Energy’s and IPL’s balance sheets. 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 sulfur dioxide 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 2019 2018 2019 2018 2019 2018 Tax-related $835.6 $890.6 $350.9 $390.1 $484.7 $500.5 Cost of removal obligations 387.7 401.2 257.0 273.3 130.7 127.9 Electric transmission cost recovery 88.6 104.0 51.3 47.7 37.3 56.3 Commodity cost recovery 24.2 16.8 8.8 11.9 15.4 4.9 WPL’s earnings sharing mechanism 21.9 25.4 — — 21.9 25.4 Derivatives 19.9 18.5 17.4 10.2 2.5 8.3 Other 45.7 36.7 29.3 21.7 16.4 15.0 $1,423.6 $1,493.2 $714.7 $754.9 $708.9 $738.3 Tax-related regulatory liabilities reduce revenue requirement calculations utilized in IPL’s and WPL’s respective rate proceedings. Cost of removal obligations, to the extent expensed through depreciation rates, reduce rate base. 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. The majority 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. Beginning 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 began providing these benefits back to its retail electric customers utilizing the transmission cost rider. WPL is currently reflecting these benefits for its retail electric customers in 2019 and 2020 through transmission expense amortizations as authorized in its retail electric rate review (2019/2020 Test Period). Commodity cost recovery - Refer to Note 1(g) for details of IPL’s and WPL’s commodity cost recovery mechanisms. WPL’s 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. The timing of the refund to customers for the majority of these regulatory liabilities will be determined in a future WPL regulatory proceeding. Utility Rate Reviews - IPL’s Retail Electric Rate Review (2020 Forward-looking Test Period) - In March 2019, IPL filed a request with the IUB to increase annual electric base rates for its Iowa retail electric customers based on a 2020 forward-looking Test Period. The key drivers for IPL’s request included recovery of capital projects, including new wind generation. IPL concurrently filed for interim retail electric rates based on 2018 historical data as adjusted for certain known and measurable changes occurring in the first quarter of 2019. An interim retail electric base rate increase of $90 million , on an annual basis, was implemented effective April 1, 2019. In October 2019, IPL reached a settlement agreement with certain intervenor groups for an annual retail electric base rate increase of $127 million . The agreement includes both the recovery of and a return on IPL’s early retired EGUs, and the recovery of IPL’s retired analog electric meters. In addition, as discussed in Note 1(g) , the net impact of certain costs and benefits resulting from IPL’s 1,000 MW expansion of wind generation in 2019 and 2020 will be recovered from its retail electric customers through a new renewable energy rider. The settlement agreement also includes IPL providing retail electric billing credits over a 12 -month period beginning with final rates, including $27 million of excess deferred tax benefits and $8 million from a partial refund of interim rates implemented in 2019. In January 2020, the IUB issued an order approving the settlement with final rates, which are expected to be effective by the end of the first quarter of 2020. IPL’s Retail Gas Rate Review (2020 Forward-looking Test Period) - In March 2019, IPL filed a request with the IUB to increase annual gas base rates for its Iowa retail gas customers based on a 2020 forward-looking Test Period. In October 2019, IPL reached a settlement agreement with intervenor groups for an annual retail gas base rate increase of $12 million . In December 2019, the IUB issued an order approving the settlement with final rates, which were effective January 10, 2020. 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 based on a 2016 historical Test Year. An interim retail electric base rate increase of $102 million , on an annual basis, was implemented effective April 13, 2017. In September 2017, IPL reached a partial, non-unanimous settlement agreement with intervenor groups for an annual retail electric base rate increase of $130 million . In February 2018, the IUB issued an order approving the settlement with final rates effective May 1, 2018. 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 intervenor groups. Under the settlement, WPL retail electric and gas base rates will not change from current levels through the end of 2020.

Property, Plant and Equipment

Property, Plant and Equipment	12 Months Ended
	Dec. 31, 2019
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 2019 2018 2019 2018 2019 2018 Utility: Electric plant: Generation in service $7,625.3 $6,800.6 $4,432.4 $3,610.4 $3,192.9 $3,190.2 Distribution in service 5,783.3 5,452.2 3,190.0 3,023.7 2,593.3 2,428.5 Other in service 456.0 410.8 298.6 260.4 157.4 150.4 Total electric plant 13,864.6 12,663.6 7,921.0 6,894.5 5,943.6 5,769.1 Gas plant in service 1,462.7 1,387.6 801.5 763.1 661.2 624.5 Other plant in service 519.7 513.2 344.7 322.4 175.0 190.8 Accumulated depreciation (4,601.4 ) (4,314.6 ) (2,498.3 ) (2,294.7 ) (2,103.1 ) (2,019.9 ) Net plant 11,245.6 10,249.8 6,568.9 5,685.3 4,676.7 4,564.5 Leased Sheboygan Falls Energy Facility, net (a) — — — — 32.3 38.1 Construction work in progress 1,835.0 1,774.8 906.8 1,091.2 928.2 683.6 Other, net 6.1 6.1 5.0 5.0 1.1 1.1 Total utility 13,086.7 12,030.7 7,480.7 6,781.5 5,638.3 5,287.3 Non-utility and other: Non-utility Generation, net (b) 82.9 86.9 — — — — Corporate Services and other, net (c) 357.5 344.8 — — — — Total non-utility and other 440.4 431.7 — — — — Total property, plant and equipment $13,527.1 $12,462.4 $7,480.7 $6,781.5 $5,638.3 $5,287.3 (a) Less accumulated amortization of $88.7 million and $82.8 million for WPL as of December 31, 2019 and 2018 , 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. (b) Less accumulated depreciation of $58.5 million and $54.5 million for Alliant Energy as of December 31, 2019 and 2018 , respectively. (c) Less accumulated depreciation of $172.4 million and $167.5 million for Alliant Energy as of December 31, 2019 and 2018 , respectively. 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 2019 2018 2017 2019 2018 2017 2019 2018 2017 Equity $65.5 $51.4 $33.6 $35.2 $28.6 $21.1 $30.3 $22.8 $12.5 Debt 27.2 24.2 16.1 14.2 13.6 10.3 13.0 10.6 5.8 $92.7 $75.6 $49.7 $49.4 $42.2 $31.4 $43.3 $33.4 $18.3 Non-utility and Other - The non-utility and other property, plant and equipment recorded on Alliant Energy’s balance sheets include 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. Corporate Services and Other - Property, plant and equipment related to Corporate Services include 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. Other property, plant and equipment include Transportation assets (a short-line railway in Iowa and a barge terminal on the Mississippi River). All Corporate Services and Other property, plant and equipment are depreciated using the straight-line method over periods ranging from 5 to 30 years. Transportation Acquisitions - In the first quarter of 2019, Alliant Energy, through its wholly-owned non-utility subsidiaries, completed acquisitions of freight management companies located in Cedar Rapids, Iowa and Stoughton, Wisconsin. These acquisitions were purchased for $21 million , including contingent consideration of $8 million , which was paid in January 2020. The purchase price was largely allocated to intangibles and the remainder was allocated to working capital and property.
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 2019 2018 2019 2018 2019 2018 Utility: Electric plant: Generation in service $7,625.3 $6,800.6 $4,432.4 $3,610.4 $3,192.9 $3,190.2 Distribution in service 5,783.3 5,452.2 3,190.0 3,023.7 2,593.3 2,428.5 Other in service 456.0 410.8 298.6 260.4 157.4 150.4 Total electric plant 13,864.6 12,663.6 7,921.0 6,894.5 5,943.6 5,769.1 Gas plant in service 1,462.7 1,387.6 801.5 763.1 661.2 624.5 Other plant in service 519.7 513.2 344.7 322.4 175.0 190.8 Accumulated depreciation (4,601.4 ) (4,314.6 ) (2,498.3 ) (2,294.7 ) (2,103.1 ) (2,019.9 ) Net plant 11,245.6 10,249.8 6,568.9 5,685.3 4,676.7 4,564.5 Leased Sheboygan Falls Energy Facility, net (a) — — — — 32.3 38.1 Construction work in progress 1,835.0 1,774.8 906.8 1,091.2 928.2 683.6 Other, net 6.1 6.1 5.0 5.0 1.1 1.1 Total utility 13,086.7 12,030.7 7,480.7 6,781.5 5,638.3 5,287.3 Non-utility and other: Non-utility Generation, net (b) 82.9 86.9 — — — — Corporate Services and other, net (c) 357.5 344.8 — — — — Total non-utility and other 440.4 431.7 — — — — Total property, plant and equipment $13,527.1 $12,462.4 $7,480.7 $6,781.5 $5,638.3 $5,287.3 (a) Less accumulated amortization of $88.7 million and $82.8 million for WPL as of December 31, 2019 and 2018 , 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. (b) Less accumulated depreciation of $58.5 million and $54.5 million for Alliant Energy as of December 31, 2019 and 2018 , respectively. (c) Less accumulated depreciation of $172.4 million and $167.5 million for Alliant Energy as of December 31, 2019 and 2018 , respectively. 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 2019 2018 2017 2019 2018 2017 2019 2018 2017 Equity $65.5 $51.4 $33.6 $35.2 $28.6 $21.1 $30.3 $22.8 $12.5 Debt 27.2 24.2 16.1 14.2 13.6 10.3 13.0 10.6 5.8 $92.7 $75.6 $49.7 $49.4 $42.2 $31.4 $43.3 $33.4 $18.3
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 2019 2018 2019 2018 2019 2018 Utility: Electric plant: Generation in service $7,625.3 $6,800.6 $4,432.4 $3,610.4 $3,192.9 $3,190.2 Distribution in service 5,783.3 5,452.2 3,190.0 3,023.7 2,593.3 2,428.5 Other in service 456.0 410.8 298.6 260.4 157.4 150.4 Total electric plant 13,864.6 12,663.6 7,921.0 6,894.5 5,943.6 5,769.1 Gas plant in service 1,462.7 1,387.6 801.5 763.1 661.2 624.5 Other plant in service 519.7 513.2 344.7 322.4 175.0 190.8 Accumulated depreciation (4,601.4 ) (4,314.6 ) (2,498.3 ) (2,294.7 ) (2,103.1 ) (2,019.9 ) Net plant 11,245.6 10,249.8 6,568.9 5,685.3 4,676.7 4,564.5 Leased Sheboygan Falls Energy Facility, net (a) — — — — 32.3 38.1 Construction work in progress 1,835.0 1,774.8 906.8 1,091.2 928.2 683.6 Other, net 6.1 6.1 5.0 5.0 1.1 1.1 Total utility 13,086.7 12,030.7 7,480.7 6,781.5 5,638.3 5,287.3 Non-utility and other: Non-utility Generation, net (b) 82.9 86.9 — — — — Corporate Services and other, net (c) 357.5 344.8 — — — — Total non-utility and other 440.4 431.7 — — — — Total property, plant and equipment $13,527.1 $12,462.4 $7,480.7 $6,781.5 $5,638.3 $5,287.3 (a) Less accumulated amortization of $88.7 million and $82.8 million for WPL as of December 31, 2019 and 2018 , 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. (b) Less accumulated depreciation of $58.5 million and $54.5 million for Alliant Energy as of December 31, 2019 and 2018 , respectively. (c) Less accumulated depreciation of $172.4 million and $167.5 million for Alliant Energy as of December 31, 2019 and 2018 , respectively. 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 2019 2018 2017 2019 2018 2017 2019 2018 2017 Equity $65.5 $51.4 $33.6 $35.2 $28.6 $21.1 $30.3 $22.8 $12.5 Debt 27.2 24.2 16.1 14.2 13.6 10.3 13.0 10.6 5.8 $92.7 $75.6 $49.7 $49.4 $42.2 $31.4 $43.3 $33.4 $18.3

Jointly-Owned Electric Utility

Jointly-Owned Electric Utility Plant	12 Months Ended
	Dec. 31, 2019
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, 2019 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0 % $571.5 $180.8 $18.8 George Neal Unit 4 25.7 % 191.8 93.5 0.8 George Neal Unit 3 28.0 % 163.7 63.2 0.9 Louisa Unit 1 4.0 % 39.9 24.9 0.2 966.9 362.4 20.7 WPL Columbia Units 1-2 53.3 % 777.0 264.7 5.5 FWEC 42.6 % 120.2 43.6 — West Riverside 91.8 % — — 647.5 897.2 308.3 653.0 Alliant Energy $1,864.1 $670.7 $673.7
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, 2019 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0 % $571.5 $180.8 $18.8 George Neal Unit 4 25.7 % 191.8 93.5 0.8 George Neal Unit 3 28.0 % 163.7 63.2 0.9 Louisa Unit 1 4.0 % 39.9 24.9 0.2 966.9 362.4 20.7 WPL Columbia Units 1-2 53.3 % 777.0 264.7 5.5 FWEC 42.6 % 120.2 43.6 — West Riverside 91.8 % — — 647.5 897.2 308.3 653.0 Alliant Energy $1,864.1 $670.7 $673.7
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, 2019 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0 % $571.5 $180.8 $18.8 George Neal Unit 4 25.7 % 191.8 93.5 0.8 George Neal Unit 3 28.0 % 163.7 63.2 0.9 Louisa Unit 1 4.0 % 39.9 24.9 0.2 966.9 362.4 20.7 WPL Columbia Units 1-2 53.3 % 777.0 264.7 5.5 FWEC 42.6 % 120.2 43.6 — West Riverside 91.8 % — — 647.5 897.2 308.3 653.0 Alliant Energy $1,864.1 $670.7 $673.7

Receivables

Receivables	12 Months Ended
	Dec. 31, 2019
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 2019 2018 2019 2018 2019 2018 Customer $91.6 $91.0 $— $— $83.5 $84.8 Unbilled utility revenues 82.1 74.2 — — 82.1 74.2 Deferred proceeds 187.7 119.4 187.7 119.4 — — Other 48.0 76.3 16.3 37.2 31.4 38.5 Allowance for doubtful accounts (7.3 ) (10.5 ) (1.2 ) (3.1 ) (6.1 ) (7.4 ) $402.1 $350.4 $202.8 $153.5 $190.9 $190.1 IPL maintains a Receivables Agreement whereby it may sell its customer accounts receivables, unbilled revenues and certain other accounts receivables to a third party through wholly-owned and consolidated special purpose entities. The purchase commitment from the third party to which it sells its receivables expires in March 2021. 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. 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, 2019 , IPL had $83 million of 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 2019 2018 2017 2019 2018 2017 Outstanding aggregate cash proceeds $108.0 $116.0 $112.0 $35.9 $53.4 $62.2 As of December 31, the attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2019 2018 Customer accounts receivable $124.7 $140.1 Unbilled utility revenues 95.5 97.1 Other receivables 0.9 0.1 Receivables sold to third party 221.1 237.3 Less: cash proceeds 27.0 108.0 Deferred proceeds 194.1 129.3 Less: allowance for doubtful accounts 6.4 9.9 Fair value of deferred proceeds $187.7 $119.4 Outstanding receivables past due $26.0 $35.5 Additional attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2019 2018 2017 Collections $2,192.8 $2,076.7 $1,647.1 Write-offs, net of recoveries 19.2 21.3 17.7
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 2019 2018 2019 2018 2019 2018 Customer $91.6 $91.0 $— $— $83.5 $84.8 Unbilled utility revenues 82.1 74.2 — — 82.1 74.2 Deferred proceeds 187.7 119.4 187.7 119.4 — — Other 48.0 76.3 16.3 37.2 31.4 38.5 Allowance for doubtful accounts (7.3 ) (10.5 ) (1.2 ) (3.1 ) (6.1 ) (7.4 ) $402.1 $350.4 $202.8 $153.5 $190.9 $190.1 IPL maintains a Receivables Agreement whereby it may sell its customer accounts receivables, unbilled revenues and certain other accounts receivables to a third party through wholly-owned and consolidated special purpose entities. The purchase commitment from the third party to which it sells its receivables expires in March 2021. 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. 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, 2019 , IPL had $83 million of 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 2019 2018 2017 2019 2018 2017 Outstanding aggregate cash proceeds $108.0 $116.0 $112.0 $35.9 $53.4 $62.2 As of December 31, the attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2019 2018 Customer accounts receivable $124.7 $140.1 Unbilled utility revenues 95.5 97.1 Other receivables 0.9 0.1 Receivables sold to third party 221.1 237.3 Less: cash proceeds 27.0 108.0 Deferred proceeds 194.1 129.3 Less: allowance for doubtful accounts 6.4 9.9 Fair value of deferred proceeds $187.7 $119.4 Outstanding receivables past due $26.0 $35.5 Additional attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2019 2018 2017 Collections $2,192.8 $2,076.7 $1,647.1 Write-offs, net of recoveries 19.2 21.3 17.7
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 2019 2018 2019 2018 2019 2018 Customer $91.6 $91.0 $— $— $83.5 $84.8 Unbilled utility revenues 82.1 74.2 — — 82.1 74.2 Deferred proceeds 187.7 119.4 187.7 119.4 — — Other 48.0 76.3 16.3 37.2 31.4 38.5 Allowance for doubtful accounts (7.3 ) (10.5 ) (1.2 ) (3.1 ) (6.1 ) (7.4 ) $402.1 $350.4 $202.8 $153.5 $190.9 $190.1

Investments

Investments	12 Months Ended
	Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]
Investments	INVESTMENTS Unconsolidated Equity Investments - Alliant Energy’s 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, 2019 2019 2018 2019 2018 2017 ATC Holdings 16%, 20% $320.1 $293.6 ($45.3 ) ($38.1 ) ($42.4 ) Non-utility wind farm in Oklahoma 50% 105.3 105.1 (5.2 ) (15.6 ) (1.8 ) Other Various 32.9 22.6 (2.5 ) (0.9 ) (0.6 ) $458.3 $421.3 ($53.0 ) ($54.6 ) ($44.8 ) ATC Holdings - As of December 31, 2019 , Alliant Energy has a 16% ownership interest in ATC and a 20% ownership interest in ATC Holdco LLC, collectively referred to as ATC Holdings. ATC is an independent, for-profit, transmission-only company. ATC Holdco LLC holds Duke-American Transmission Company, LLC, a joint venture between Duke Energy Corporation and ATC, that owns electric transmission infrastructure in North America. Non-utility Wind Farm in Oklahoma - The non-utility wind farm located in Oklahoma 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.

Common Equity

Common Equity	12 Months Ended
	Dec. 31, 2019
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: 2019 2018 2017 Shares outstanding, January 1 236,063,279 231,348,646 227,673,654 Equity forward agreements 8,358,973 — — At-the-market offering programs — 4,171,013 3,074,931 Shareowner Direct Plan 501,808 576,965 640,723 Equity-based compensation plans 101,478 5,078 5,185 Other (2,738 ) (38,423 ) (45,847 ) Shares outstanding, December 31 245,022,800 236,063,279 231,348,646 At December 31, 2019 , Alliant Energy had a total of 12.1 million shares available for issuance in the aggregate, pursuant to its Amended and Restated OIP, Shareowner Direct Plan and 401(k) Savings Plan. 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 2019, Alliant Energy settled $366 million under the forward sale agreements by delivering 8,358,973 shares of newly issued Alliant Energy common stock at a weighted average forward sale price of $43.75 per share. Alliant Energy used the net proceeds from the settlements for general corporate purposes. In November 2019, Alliant Energy entered into forward sale agreements with a counterparty in connection with a public offering of 4,275,127 shares of Alliant Energy common stock. In the forward sale transaction, the counterparty, or its affiliates, borrowed an aggregate of 4,275,127 shares of Alliant Energy common stock from third parties and sold such shares to the related underwriter. Alliant Energy has not yet received any proceeds from this 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, 2020 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 $52.235 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. Alliant Energy has concluded that the forward sale agreements meet the derivative scope exception for certain contracts involving an entity’s own equity. 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, 2019 , 67,689 incremental shares were included in the calculation of diluted EPS related to the securities under the forward sale agreements. 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 and 2018 at-the-market offering programs have expired. 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. Comprehensive Income (Loss) - In 2019 , 2018 and 2017 , Alliant Energy’s other comprehensive income (loss) was ($0.4) million , $2.2 million and ($0.1) 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 2019 , 2018 and 2017 , IPL and WPL had no other comprehensive income; therefore their comprehensive income was equal to their net income, and IPL’s comprehensive income available for common stock was equal to its net income 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: 2019 2018 2017 Shares outstanding, January 1 236,063,279 231,348,646 227,673,654 Equity forward agreements 8,358,973 — — At-the-market offering programs — 4,171,013 3,074,931 Shareowner Direct Plan 501,808 576,965 640,723 Equity-based compensation plans 101,478 5,078 5,185 Other (2,738 ) (38,423 ) (45,847 ) Shares outstanding, December 31 245,022,800 236,063,279 231,348,646 At December 31, 2019 , Alliant Energy had a total of 12.1 million shares available for issuance in the aggregate, pursuant to its Amended and Restated OIP, Shareowner Direct Plan and 401(k) Savings Plan. 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 2019, Alliant Energy settled $366 million under the forward sale agreements by delivering 8,358,973 shares of newly issued Alliant Energy common stock at a weighted average forward sale price of $43.75 per share. Alliant Energy used the net proceeds from the settlements for general corporate purposes. In November 2019, Alliant Energy entered into forward sale agreements with a counterparty in connection with a public offering of 4,275,127 shares of Alliant Energy common stock. In the forward sale transaction, the counterparty, or its affiliates, borrowed an aggregate of 4,275,127 shares of Alliant Energy common stock from third parties and sold such shares to the related underwriter. Alliant Energy has not yet received any proceeds from this 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, 2020 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 $52.235 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. Alliant Energy has concluded that the forward sale agreements meet the derivative scope exception for certain contracts involving an entity’s own equity. 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, 2019 , 67,689 incremental shares were included in the calculation of diluted EPS related to the securities under the forward sale agreements. 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 and 2018 at-the-market offering programs have expired. 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. Comprehensive Income (Loss) - In 2019 , 2018 and 2017 , Alliant Energy’s other comprehensive income (loss) was ($0.4) million , $2.2 million and ($0.1) 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 2019 , 2018 and 2017 , IPL and WPL had no other comprehensive income; therefore their comprehensive income was equal to their net income, and IPL’s comprehensive income available for common stock was equal to its net income 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: 2019 2018 2017 Shares outstanding, January 1 236,063,279 231,348,646 227,673,654 Equity forward agreements 8,358,973 — — At-the-market offering programs — 4,171,013 3,074,931 Shareowner Direct Plan 501,808 576,965 640,723 Equity-based compensation plans 101,478 5,078 5,185 Other (2,738 ) (38,423 ) (45,847 ) Shares outstanding, December 31 245,022,800 236,063,279 231,348,646 At December 31, 2019 , Alliant Energy had a total of 12.1 million shares available for issuance in the aggregate, pursuant to its Amended and Restated OIP, Shareowner Direct Plan and 401(k) Savings Plan. 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 2019, Alliant Energy settled $366 million under the forward sale agreements by delivering 8,358,973 shares of newly issued Alliant Energy common stock at a weighted average forward sale price of $43.75 per share. Alliant Energy used the net proceeds from the settlements for general corporate purposes. In November 2019, Alliant Energy entered into forward sale agreements with a counterparty in connection with a public offering of 4,275,127 shares of Alliant Energy common stock. In the forward sale transaction, the counterparty, or its affiliates, borrowed an aggregate of 4,275,127 shares of Alliant Energy common stock from third parties and sold such shares to the related underwriter. Alliant Energy has not yet received any proceeds from this 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, 2020 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 $52.235 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. Alliant Energy has concluded that the forward sale agreements meet the derivative scope exception for certain contracts involving an entity’s own equity. 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, 2019 , 67,689 incremental shares were included in the calculation of diluted EPS related to the securities under the forward sale agreements. 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 and 2018 at-the-market offering programs have expired. 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. Comprehensive Income (Loss) - In 2019 , 2018 and 2017 , Alliant Energy’s other comprehensive income (loss) was ($0.4) million , $2.2 million and ($0.1) 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 2019 , 2018 and 2017 , IPL and WPL had no other comprehensive income; therefore their comprehensive income was equal to their net income, and IPL’s comprehensive income available for common stock was equal to its net income available for common stock, for such periods.

Preferred Stock

Preferred Stock	12 Months Ended
	Dec. 31, 2019
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 2019 2018 (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]
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 2019 2018 (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, 2019
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, 2019 , the short-term borrowing capacity under a single credit facility agreement, which expires in August 2023, totaled $1 billion ( $450 million for Alliant Energy at the parent company level, $250 million for IPL and $300 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 2019 2018 2019 2018 2019 2018 Commercial paper outstanding $337.4 $441.2 $— $50.4 $168.2 $105.5 Commercial paper weighted average interest rates 1.9% 2.8% N/A 2.8% 1.8% 2.5% Available credit facility capacity $662.6 $558.8 $250.0 $199.6 $131.8 $244.5 Alliant Energy IPL WPL For the year ended 2019 2018 2019 2018 2019 2018 Maximum amount outstanding (based on daily outstanding balances) $600.6 $446.5 $50.4 $50.4 $195.1 $126.0 Average amount outstanding (based on daily outstanding balances) $453.5 $221.4 $0.1 $1.5 $92.6 $36.6 Weighted average interest rates 2.5% 2.2% 2.8% 2.3% 2.4% 2.1% 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, 2019 were as follows: Alliant Energy IPL WPL Requirement, not to exceed 65% 65% 65% Actual 55% 48% 48% The debt component of the capital ratios includes, when applicable, 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), finance 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). Long-term debt, net as of December 31 was as follows (dollars in millions): 2019 2018 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 500.0 500.0 — 500.0 500.0 — 3.6%, due 2029 (b) 300.0 300.0 — — — — 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 — 3.5%, due 2049 (b) 300.0 300.0 — — — — 3,175.0 3,175.0 — 2,575.0 2,575.0 — Debentures (a): 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 3%, due 2029 (c) 350.0 — 350.0 — — — 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 5% (Retired in 2019) — — — 250.0 — 250.0 1,950.0 — 1,950.0 1,850.0 — 1,850.0 Other: AEF term loan credit agreement through April 2020, 2% at December 31, 2019 (with Alliant Energy as guarantor) 300.0 — — 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) 400.0 — — 400.0 — — AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a) 300.0 — — 300.0 — — Sheboygan Power, LLC 5.06% senior secured notes, due 2020 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 38.2 — — 44.3 — — Other, 1% at December 31, 2019, due 2020 to 2025 2.1 — — 2.4 — — 1,115.3 — — 1,121.7 — — Subtotal 6,240.3 3,175.0 1,950.0 5,546.7 2,575.0 1,850.0 Current maturities (657.2 ) (200.0 ) (150.0 ) (256.5 ) — (250.0 ) Unamortized debt issuance costs (36.9 ) (21.0 ) (11.3 ) (32.1 ) (17.2 ) (9.6 ) Unamortized debt (discount) and premium, net (13.2 ) (6.7 ) (6.0 ) (11.8 ) (5.5 ) (5.5 ) Long-term debt, net (d) $5,533.0 $2,947.3 $1,782.7 $5,246.3 $2,552.3 $1,584.9 (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 April 2019, IPL issued $300 million of 3.6% senior debentures due 2029. In September 2019, IPL issued $300 million of 3.5% senior debentures due 2049. The senior debentures were issued as green bonds, and all of the net proceeds were allocated for the construction and development of IPL’s wind projects. (c) In June 2019, WPL issued $350 million of 3% debentures due 2029. The net proceeds from the issuance were used by WPL to reduce its outstanding commercial paper and retire its $250 million 5% debentures that matured in July 2019. (d) 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, 2019 , long-term debt maturities for 2020 through 2024 were as follows (in millions): 2020 2021 2022 2023 2024 IPL $200 $— $— $— $500 WPL 150 — 250 — — Corporate Services — — 75 — — AEF 307 8 8 408 9 Alliant Energy $657 $8 $333 $408 $509 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, 2019 , the short-term borrowing capacity under a single credit facility agreement, which expires in August 2023, totaled $1 billion ( $450 million for Alliant Energy at the parent company level, $250 million for IPL and $300 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 2019 2018 2019 2018 2019 2018 Commercial paper outstanding $337.4 $441.2 $— $50.4 $168.2 $105.5 Commercial paper weighted average interest rates 1.9% 2.8% N/A 2.8% 1.8% 2.5% Available credit facility capacity $662.6 $558.8 $250.0 $199.6 $131.8 $244.5 Alliant Energy IPL WPL For the year ended 2019 2018 2019 2018 2019 2018 Maximum amount outstanding (based on daily outstanding balances) $600.6 $446.5 $50.4 $50.4 $195.1 $126.0 Average amount outstanding (based on daily outstanding balances) $453.5 $221.4 $0.1 $1.5 $92.6 $36.6 Weighted average interest rates 2.5% 2.2% 2.8% 2.3% 2.4% 2.1% 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, 2019 were as follows: Alliant Energy IPL WPL Requirement, not to exceed 65% 65% 65% Actual 55% 48% 48% The debt component of the capital ratios includes, when applicable, 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), finance 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). Long-term debt, net as of December 31 was as follows (dollars in millions): 2019 2018 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 500.0 500.0 — 500.0 500.0 — 3.6%, due 2029 (b) 300.0 300.0 — — — — 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 — 3.5%, due 2049 (b) 300.0 300.0 — — — — 3,175.0 3,175.0 — 2,575.0 2,575.0 — Debentures (a): 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 3%, due 2029 (c) 350.0 — 350.0 — — — 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 5% (Retired in 2019) — — — 250.0 — 250.0 1,950.0 — 1,950.0 1,850.0 — 1,850.0 Other: AEF term loan credit agreement through April 2020, 2% at December 31, 2019 (with Alliant Energy as guarantor) 300.0 — — 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) 400.0 — — 400.0 — — AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a) 300.0 — — 300.0 — — Sheboygan Power, LLC 5.06% senior secured notes, due 2020 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 38.2 — — 44.3 — — Other, 1% at December 31, 2019, due 2020 to 2025 2.1 — — 2.4 — — 1,115.3 — — 1,121.7 — — Subtotal 6,240.3 3,175.0 1,950.0 5,546.7 2,575.0 1,850.0 Current maturities (657.2 ) (200.0 ) (150.0 ) (256.5 ) — (250.0 ) Unamortized debt issuance costs (36.9 ) (21.0 ) (11.3 ) (32.1 ) (17.2 ) (9.6 ) Unamortized debt (discount) and premium, net (13.2 ) (6.7 ) (6.0 ) (11.8 ) (5.5 ) (5.5 ) Long-term debt, net (d) $5,533.0 $2,947.3 $1,782.7 $5,246.3 $2,552.3 $1,584.9 (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 April 2019, IPL issued $300 million of 3.6% senior debentures due 2029. In September 2019, IPL issued $300 million of 3.5% senior debentures due 2049. The senior debentures were issued as green bonds, and all of the net proceeds were allocated for the construction and development of IPL’s wind projects. (c) In June 2019, WPL issued $350 million of 3% debentures due 2029. The net proceeds from the issuance were used by WPL to reduce its outstanding commercial paper and retire its $250 million 5% debentures that matured in July 2019. (d) 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, 2019 , long-term debt maturities for 2020 through 2024 were as follows (in millions): 2020 2021 2022 2023 2024 IPL $200 $— $— $— $500 WPL 150 — 250 — — Corporate Services — — 75 — — AEF 307 8 8 408 9 Alliant Energy $657 $8 $333 $408 $509 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, 2019 , the short-term borrowing capacity under a single credit facility agreement, which expires in August 2023, totaled $1 billion ( $450 million for Alliant Energy at the parent company level, $250 million for IPL and $300 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 2019 2018 2019 2018 2019 2018 Commercial paper outstanding $337.4 $441.2 $— $50.4 $168.2 $105.5 Commercial paper weighted average interest rates 1.9% 2.8% N/A 2.8% 1.8% 2.5% Available credit facility capacity $662.6 $558.8 $250.0 $199.6 $131.8 $244.5 Alliant Energy IPL WPL For the year ended 2019 2018 2019 2018 2019 2018 Maximum amount outstanding (based on daily outstanding balances) $600.6 $446.5 $50.4 $50.4 $195.1 $126.0 Average amount outstanding (based on daily outstanding balances) $453.5 $221.4 $0.1 $1.5 $92.6 $36.6 Weighted average interest rates 2.5% 2.2% 2.8% 2.3% 2.4% 2.1% 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, 2019 were as follows: Alliant Energy IPL WPL Requirement, not to exceed 65% 65% 65% Actual 55% 48% 48% The debt component of the capital ratios includes, when applicable, 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), finance 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). Long-term debt, net as of December 31 was as follows (dollars in millions): 2019 2018 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 500.0 500.0 — 500.0 500.0 — 3.6%, due 2029 (b) 300.0 300.0 — — — — 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 — 3.5%, due 2049 (b) 300.0 300.0 — — — — 3,175.0 3,175.0 — 2,575.0 2,575.0 — Debentures (a): 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 3%, due 2029 (c) 350.0 — 350.0 — — — 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 5% (Retired in 2019) — — — 250.0 — 250.0 1,950.0 — 1,950.0 1,850.0 — 1,850.0 Other: AEF term loan credit agreement through April 2020, 2% at December 31, 2019 (with Alliant Energy as guarantor) 300.0 — — 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) 400.0 — — 400.0 — — AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a) 300.0 — — 300.0 — — Sheboygan Power, LLC 5.06% senior secured notes, due 2020 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 38.2 — — 44.3 — — Other, 1% at December 31, 2019, due 2020 to 2025 2.1 — — 2.4 — — 1,115.3 — — 1,121.7 — — Subtotal 6,240.3 3,175.0 1,950.0 5,546.7 2,575.0 1,850.0 Current maturities (657.2 ) (200.0 ) (150.0 ) (256.5 ) — (250.0 ) Unamortized debt issuance costs (36.9 ) (21.0 ) (11.3 ) (32.1 ) (17.2 ) (9.6 ) Unamortized debt (discount) and premium, net (13.2 ) (6.7 ) (6.0 ) (11.8 ) (5.5 ) (5.5 ) Long-term debt, net (d) $5,533.0 $2,947.3 $1,782.7 $5,246.3 $2,552.3 $1,584.9 (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 April 2019, IPL issued $300 million of 3.6% senior debentures due 2029. In September 2019, IPL issued $300 million of 3.5% senior debentures due 2049. The senior debentures were issued as green bonds, and all of the net proceeds were allocated for the construction and development of IPL’s wind projects. (c) In June 2019, WPL issued $350 million of 3% debentures due 2029. The net proceeds from the issuance were used by WPL to reduce its outstanding commercial paper and retire its $250 million 5% debentures that matured in July 2019. (d) 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, 2019 , long-term debt maturities for 2020 through 2024 were as follows (in millions): 2020 2021 2022 2023 2024 IPL $200 $— $— $— $500 WPL 150 — 250 — — Corporate Services — — 75 — — AEF 307 8 8 408 9 Alliant Energy $657 $8 $333 $408 $509 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, 2019
Leases [Line Items]
Operating Leases	LEASES Operating Leases - Alliant Energy’s, IPL’s and WPL’s operating leases primarily include leases of space on telecommunication towers and leases of property. Operating lease details are as follows (dollars in millions): December 31, 2019 Alliant Energy IPL WPL Property, plant and equipment, net $16 $10 $6 Other current liabilities $2 $1 $1 Other liabilities 14 9 5 Total operating lease liabilities $16 $10 $6 Weighted average remaining lease term 11 years 12 years 10 years Weighted average discount rate 4 % 4 % 4 % 2019 Alliant Energy IPL WPL Operating lease cost $2 $1 $1 Finance Lease - WPL is currently leasing the Sheboygan Falls Energy Facility from AEF’s Non-utility Generation business through 2025, the initial lease term. WPL is responsible for the operation of the EGU and has exclusive rights to its output. This finance lease contains two lease renewal periods, which are not included in the finance lease obligation, as well as an option to purchase the facility at the end of the initial lease term. WPL’s finance lease details are as follows (dollars in millions): December 31, 2019 Property, plant and equipment, net $32 Other current liabilities $9 Finance lease obligations - Sheboygan Falls Energy Facility 51 Total finance lease liabilities $60 Remaining lease term 5 years Discount rate 11 % 2019 Depreciation expense $6 Interest expense 7 Total finance lease expense $13 Expected Maturities - As of December 31, 2019 , expected maturities of lease liabilities were as follows (in millions): 2020 2021 2022 2023 2024 Thereafter Total Less: amount representing interest Present value of minimum lease payments Operating Leases: Alliant Energy $2 $2 $2 $2 $2 $10 $20 $4 $16 IPL 1 1 1 1 1 7 12 2 10 WPL 1 1 1 1 1 3 8 2 6 WPL’s Finance Lease: Sheboygan Falls Energy Facility 15 15 15 15 15 5 80 20 60 Prior period amounts have not been restated to reflect the adoption of the new lease accounting standard and continue to be reported under the accounting standards in effect for those periods. As of December 31, 2018 , future minimum operating (excluding contingent rentals) and capital lease payments were as follows (in millions): 2019 2020 2021 2022 2023 Thereafter Total Less: amount representing interest Present value of minimum capital lease payments Operating Leases: Alliant Energy $5 $5 $3 $3 $2 $12 $30 IPL 3 2 2 2 2 12 23 WPL 2 3 1 — — — 6 WPL’s Capital Lease: Sheboygan Falls Energy Facility $15 $15 $15 $15 $15 $19 $94 $26 $68
IPL [Member]
Leases [Line Items]
Operating Leases	LEASES Operating Leases - Alliant Energy’s, IPL’s and WPL’s operating leases primarily include leases of space on telecommunication towers and leases of property. Operating lease details are as follows (dollars in millions): December 31, 2019 Alliant Energy IPL WPL Property, plant and equipment, net $16 $10 $6 Other current liabilities $2 $1 $1 Other liabilities 14 9 5 Total operating lease liabilities $16 $10 $6 Weighted average remaining lease term 11 years 12 years 10 years Weighted average discount rate 4 % 4 % 4 % 2019 Alliant Energy IPL WPL Operating lease cost $2 $1 $1 Finance Lease - WPL is currently leasing the Sheboygan Falls Energy Facility from AEF’s Non-utility Generation business through 2025, the initial lease term. WPL is responsible for the operation of the EGU and has exclusive rights to its output. This finance lease contains two lease renewal periods, which are not included in the finance lease obligation, as well as an option to purchase the facility at the end of the initial lease term. WPL’s finance lease details are as follows (dollars in millions): December 31, 2019 Property, plant and equipment, net $32 Other current liabilities $9 Finance lease obligations - Sheboygan Falls Energy Facility 51 Total finance lease liabilities $60 Remaining lease term 5 years Discount rate 11 % 2019 Depreciation expense $6 Interest expense 7 Total finance lease expense $13 Expected Maturities - As of December 31, 2019 , expected maturities of lease liabilities were as follows (in millions): 2020 2021 2022 2023 2024 Thereafter Total Less: amount representing interest Present value of minimum lease payments Operating Leases: Alliant Energy $2 $2 $2 $2 $2 $10 $20 $4 $16 IPL 1 1 1 1 1 7 12 2 10 WPL 1 1 1 1 1 3 8 2 6 WPL’s Finance Lease: Sheboygan Falls Energy Facility 15 15 15 15 15 5 80 20 60 Prior period amounts have not been restated to reflect the adoption of the new lease accounting standard and continue to be reported under the accounting standards in effect for those periods. As of December 31, 2018 , future minimum operating (excluding contingent rentals) and capital lease payments were as follows (in millions): 2019 2020 2021 2022 2023 Thereafter Total Less: amount representing interest Present value of minimum capital lease payments Operating Leases: Alliant Energy $5 $5 $3 $3 $2 $12 $30 IPL 3 2 2 2 2 12 23 WPL 2 3 1 — — — 6 WPL’s Capital Lease: Sheboygan Falls Energy Facility $15 $15 $15 $15 $15 $19 $94 $26 $68
WPL [Member]
Leases [Line Items]
Operating Leases	LEASES Operating Leases - Alliant Energy’s, IPL’s and WPL’s operating leases primarily include leases of space on telecommunication towers and leases of property. Operating lease details are as follows (dollars in millions): December 31, 2019 Alliant Energy IPL WPL Property, plant and equipment, net $16 $10 $6 Other current liabilities $2 $1 $1 Other liabilities 14 9 5 Total operating lease liabilities $16 $10 $6 Weighted average remaining lease term 11 years 12 years 10 years Weighted average discount rate 4 % 4 % 4 % 2019 Alliant Energy IPL WPL Operating lease cost $2 $1 $1 Finance Lease - WPL is currently leasing the Sheboygan Falls Energy Facility from AEF’s Non-utility Generation business through 2025, the initial lease term. WPL is responsible for the operation of the EGU and has exclusive rights to its output. This finance lease contains two lease renewal periods, which are not included in the finance lease obligation, as well as an option to purchase the facility at the end of the initial lease term. WPL’s finance lease details are as follows (dollars in millions): December 31, 2019 Property, plant and equipment, net $32 Other current liabilities $9 Finance lease obligations - Sheboygan Falls Energy Facility 51 Total finance lease liabilities $60 Remaining lease term 5 years Discount rate 11 % 2019 Depreciation expense $6 Interest expense 7 Total finance lease expense $13 Expected Maturities - As of December 31, 2019 , expected maturities of lease liabilities were as follows (in millions): 2020 2021 2022 2023 2024 Thereafter Total Less: amount representing interest Present value of minimum lease payments Operating Leases: Alliant Energy $2 $2 $2 $2 $2 $10 $20 $4 $16 IPL 1 1 1 1 1 7 12 2 10 WPL 1 1 1 1 1 3 8 2 6 WPL’s Finance Lease: Sheboygan Falls Energy Facility 15 15 15 15 15 5 80 20 60 Prior period amounts have not been restated to reflect the adoption of the new lease accounting standard and continue to be reported under the accounting standards in effect for those periods. As of December 31, 2018 , future minimum operating (excluding contingent rentals) and capital lease payments were as follows (in millions): 2019 2020 2021 2022 2023 Thereafter Total Less: amount representing interest Present value of minimum capital lease payments Operating Leases: Alliant Energy $5 $5 $3 $3 $2 $12 $30 IPL 3 2 2 2 2 12 23 WPL 2 3 1 — — — 6 WPL’s Capital Lease: Sheboygan Falls Energy Facility $15 $15 $15 $15 $15 $19 $94 $26 $68
Finance Leases	LEASES Operating Leases - Alliant Energy’s, IPL’s and WPL’s operating leases primarily include leases of space on telecommunication towers and leases of property. Operating lease details are as follows (dollars in millions): December 31, 2019 Alliant Energy IPL WPL Property, plant and equipment, net $16 $10 $6 Other current liabilities $2 $1 $1 Other liabilities 14 9 5 Total operating lease liabilities $16 $10 $6 Weighted average remaining lease term 11 years 12 years 10 years Weighted average discount rate 4 % 4 % 4 % 2019 Alliant Energy IPL WPL Operating lease cost $2 $1 $1 Finance Lease - WPL is currently leasing the Sheboygan Falls Energy Facility from AEF’s Non-utility Generation business through 2025, the initial lease term. WPL is responsible for the operation of the EGU and has exclusive rights to its output. This finance lease contains two lease renewal periods, which are not included in the finance lease obligation, as well as an option to purchase the facility at the end of the initial lease term. WPL’s finance lease details are as follows (dollars in millions): December 31, 2019 Property, plant and equipment, net $32 Other current liabilities $9 Finance lease obligations - Sheboygan Falls Energy Facility 51 Total finance lease liabilities $60 Remaining lease term 5 years Discount rate 11 % 2019 Depreciation expense $6 Interest expense 7 Total finance lease expense $13 Expected Maturities - As of December 31, 2019 , expected maturities of lease liabilities were as follows (in millions): 2020 2021 2022 2023 2024 Thereafter Total Less: amount representing interest Present value of minimum lease payments Operating Leases: Alliant Energy $2 $2 $2 $2 $2 $10 $20 $4 $16 IPL 1 1 1 1 1 7 12 2 10 WPL 1 1 1 1 1 3 8 2 6 WPL’s Finance Lease: Sheboygan Falls Energy Facility 15 15 15 15 15 5 80 20 60 Prior period amounts have not been restated to reflect the adoption of the new lease accounting standard and continue to be reported under the accounting standards in effect for those periods. As of December 31, 2018 , future minimum operating (excluding contingent rentals) and capital lease payments were as follows (in millions): 2019 2020 2021 2022 2023 Thereafter Total Less: amount representing interest Present value of minimum capital lease payments Operating Leases: Alliant Energy $5 $5 $3 $3 $2 $12 $30 IPL 3 2 2 2 2 12 23 WPL 2 3 1 — — — 6 WPL’s Capital Lease: Sheboygan Falls Energy Facility $15 $15 $15 $15 $15 $19 $94 $26 $68

Revenues

Revenues	12 Months Ended
	Dec. 31, 2019
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; a barge terminal and hauling services on the Mississippi River; customized supply chain solution capabilities; freight and logistics brokering services; and other transfer and storage services. Disaggregation of revenues from contracts with customers, which correlates to revenues for each reportable segment, was as follows (in millions): Alliant Energy IPL WPL 2019 2018 2017 2019 2018 2017 2019 2018 2017 Electric Utility: Retail - residential $1,092.4 $1,063.4 $1,006.2 $601.5 $590.6 $535.6 $490.9 $472.8 $470.6 Retail - commercial 769.7 735.6 710.3 510.3 486.8 452.7 259.4 248.8 257.6 Retail - industrial 889.1 888.8 853.1 503.9 500.8 459.7 385.2 388.0 393.4 Wholesale 176.4 188.4 238.4 63.5 71.2 95.5 112.9 117.2 142.9 Bulk power and other 136.0 124.1 86.7 102.0 81.7 55.4 34.0 42.4 31.3 Total Electric Utility 3,063.6 3,000.3 2,894.7 1,781.2 1,731.1 1,598.9 1,282.4 1,269.2 1,295.8 Gas Utility: Retail - residential 259.4 254.4 224.7 149.3 152.3 123.2 110.1 102.1 101.5 Retail - commercial 133.0 133.0 123.2 74.9 75.9 67.9 58.1 57.1 55.3 Retail - industrial 16.0 14.9 16.7 11.7 10.2 11.1 4.3 4.7 5.6 Transportation/other 46.8 44.3 36.3 28.3 27.8 23.8 18.5 16.5 12.5 Total Gas Utility 455.2 446.6 400.9 264.2 266.2 226.0 191.0 180.4 174.9 Other Utility: Steam 37.2 35.2 34.6 37.2 35.2 34.6 — — — Other utility 9.3 12.8 12.9 7.0 9.8 10.8 2.3 3.0 2.1 Total Other Utility 46.5 48.0 47.5 44.2 45.0 45.4 2.3 3.0 2.1 Non-Utility and Other: Transportation and other 82.4 39.6 39.1 — — — — — — Total Non-Utility and Other 82.4 39.6 39.1 — — — — — — Total revenues $3,647.7 $3,534.5 $3,382.2 $2,089.6 $2,042.3 $1,870.3 $1,475.7 $1,452.6 $1,472.8
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; a barge terminal and hauling services on the Mississippi River; customized supply chain solution capabilities; freight and logistics brokering services; and other transfer and storage services. Disaggregation of revenues from contracts with customers, which correlates to revenues for each reportable segment, was as follows (in millions): Alliant Energy IPL WPL 2019 2018 2017 2019 2018 2017 2019 2018 2017 Electric Utility: Retail - residential $1,092.4 $1,063.4 $1,006.2 $601.5 $590.6 $535.6 $490.9 $472.8 $470.6 Retail - commercial 769.7 735.6 710.3 510.3 486.8 452.7 259.4 248.8 257.6 Retail - industrial 889.1 888.8 853.1 503.9 500.8 459.7 385.2 388.0 393.4 Wholesale 176.4 188.4 238.4 63.5 71.2 95.5 112.9 117.2 142.9 Bulk power and other 136.0 124.1 86.7 102.0 81.7 55.4 34.0 42.4 31.3 Total Electric Utility 3,063.6 3,000.3 2,894.7 1,781.2 1,731.1 1,598.9 1,282.4 1,269.2 1,295.8 Gas Utility: Retail - residential 259.4 254.4 224.7 149.3 152.3 123.2 110.1 102.1 101.5 Retail - commercial 133.0 133.0 123.2 74.9 75.9 67.9 58.1 57.1 55.3 Retail - industrial 16.0 14.9 16.7 11.7 10.2 11.1 4.3 4.7 5.6 Transportation/other 46.8 44.3 36.3 28.3 27.8 23.8 18.5 16.5 12.5 Total Gas Utility 455.2 446.6 400.9 264.2 266.2 226.0 191.0 180.4 174.9 Other Utility: Steam 37.2 35.2 34.6 37.2 35.2 34.6 — — — Other utility 9.3 12.8 12.9 7.0 9.8 10.8 2.3 3.0 2.1 Total Other Utility 46.5 48.0 47.5 44.2 45.0 45.4 2.3 3.0 2.1 Non-Utility and Other: Transportation and other 82.4 39.6 39.1 — — — — — — Total Non-Utility and Other 82.4 39.6 39.1 — — — — — — Total revenues $3,647.7 $3,534.5 $3,382.2 $2,089.6 $2,042.3 $1,870.3 $1,475.7 $1,452.6 $1,472.8
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; a barge terminal and hauling services on the Mississippi River; customized supply chain solution capabilities; freight and logistics brokering services; and other transfer and storage services. Disaggregation of revenues from contracts with customers, which correlates to revenues for each reportable segment, was as follows (in millions): Alliant Energy IPL WPL 2019 2018 2017 2019 2018 2017 2019 2018 2017 Electric Utility: Retail - residential $1,092.4 $1,063.4 $1,006.2 $601.5 $590.6 $535.6 $490.9 $472.8 $470.6 Retail - commercial 769.7 735.6 710.3 510.3 486.8 452.7 259.4 248.8 257.6 Retail - industrial 889.1 888.8 853.1 503.9 500.8 459.7 385.2 388.0 393.4 Wholesale 176.4 188.4 238.4 63.5 71.2 95.5 112.9 117.2 142.9 Bulk power and other 136.0 124.1 86.7 102.0 81.7 55.4 34.0 42.4 31.3 Total Electric Utility 3,063.6 3,000.3 2,894.7 1,781.2 1,731.1 1,598.9 1,282.4 1,269.2 1,295.8 Gas Utility: Retail - residential 259.4 254.4 224.7 149.3 152.3 123.2 110.1 102.1 101.5 Retail - commercial 133.0 133.0 123.2 74.9 75.9 67.9 58.1 57.1 55.3 Retail - industrial 16.0 14.9 16.7 11.7 10.2 11.1 4.3 4.7 5.6 Transportation/other 46.8 44.3 36.3 28.3 27.8 23.8 18.5 16.5 12.5 Total Gas Utility 455.2 446.6 400.9 264.2 266.2 226.0 191.0 180.4 174.9 Other Utility: Steam 37.2 35.2 34.6 37.2 35.2 34.6 — — — Other utility 9.3 12.8 12.9 7.0 9.8 10.8 2.3 3.0 2.1 Total Other Utility 46.5 48.0 47.5 44.2 45.0 45.4 2.3 3.0 2.1 Non-Utility and Other: Transportation and other 82.4 39.6 39.1 — — — — — — Total Non-Utility and Other 82.4 39.6 39.1 — — — — — — Total revenues $3,647.7 $3,534.5 $3,382.2 $2,089.6 $2,042.3 $1,870.3 $1,475.7 $1,452.6 $1,472.8

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
Income Tax [Line Items]
Income Taxes	INCOME TAXES Income Tax Expense (Benefit) - The components of “Income tax expense (benefit)” in the income statements were as follows (in millions): Alliant Energy IPL WPL 2019 2018 2017 2019 2018 2017 2019 2018 2017 Current tax expense (benefit): Federal ($6.6 ) ($1.0 ) ($41.0 ) ($11.0 ) $14.9 ($27.9 ) $12.0 ($9.2 ) $5.5 State 24.2 (5.1 ) 8.5 24.3 (7.1 ) 1.6 13.7 (4.4 ) 2.5 IPL’s tax benefit riders (4.5 ) (13.2 ) (40.4 ) (4.5 ) (13.2 ) (40.4 ) — — — Deferred tax expense (benefit): Federal 69.7 67.9 159.5 26.4 9.5 72.5 30.7 43.8 55.0 State 41.4 29.8 12.3 30.9 7.3 (2.2 ) 6.3 22.1 16.6 Production tax credits (54.7 ) (29.5 ) (31.1 ) (41.8 ) (14.0 ) (14.1 ) (12.8 ) (15.5 ) (17.0 ) Investment tax credits (0.8 ) (1.2 ) (1.1 ) (0.2 ) (0.6 ) (0.4 ) (0.6 ) (0.6 ) (0.7 ) $68.7 $47.7 $66.7 $24.1 ($3.2 ) ($10.9 ) $49.3 $36.2 $61.9 Income Tax Rates - The overall income tax rates shown in the following table were computed by dividing income tax expense (benefit) by income from continuing operations before income taxes. Alliant Energy IPL WPL 2019 2018 2017 2019 2018 2017 2019 2018 2017 Statutory federal income tax rate 21.0 % 21.0 % 35.0 % 21.0 % 21.0 % 35.0 % 21.0 % 21.0 % 35.0 % State income taxes, net of federal benefits 6.8 7.0 5.5 8.5 7.7 6.5 6.2 6.2 5.1 Production tax credits (8.6 ) (5.2 ) (6.1 ) (13.1 ) (5.2 ) (6.7 ) (4.5 ) (6.4 ) (7.1 ) Effect of rate-making on property-related differences (6.1 ) (7.6 ) (8.5 ) (9.8 ) (14.0 ) (19.1 ) (2.7 ) (2.3 ) (1.7 ) Amortization of excess deferred taxes (1.0 ) (0.3 ) (0.1 ) (0.3 ) — (0.1 ) (1.8 ) (0.2 ) (0.1 ) IPL’s tax benefit riders (0.7 ) (2.3 ) (7.6 ) (1.4 ) (4.9 ) (18.7 ) — — — Adjustment for prior period taxes 0.6 (2.3 ) (1.5 ) 3.1 (4.8 ) (3.4 ) (0.3 ) (0.2 ) — Federal Tax Reform adjustments — (1.0 ) (3.4 ) — (0.4 ) 1.7 — (2.3 ) (5.8 ) Other items, net (1.2 ) (0.9 ) (0.8 ) (0.4 ) (0.6 ) (0.2 ) (0.4 ) (1.0 ) (0.5 ) Overall income tax rate 10.8 % 8.4 % 12.5 % 7.6 % (1.2 %) (5.0 %) 17.5 % 14.8 % 24.9 % 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 2019 2018 2019 2018 2019 2018 Deferred tax liabilities: Property $2,021.5 $1,975.5 $1,183.5 $1,158.4 $770.0 $735.2 ATC Holdings 110.8 102.4 — — — — Other 85.1 80.5 76.5 69.4 32.1 34.4 Total deferred tax liabilities 2,217.4 2,158.4 1,260.0 1,227.8 802.1 769.6 Deferred tax assets: Federal credit carryforwards 355.4 299.1 175.0 133.2 160.3 147.4 Net operating losses carryforwards - federal 60.5 158.6 56.3 114.1 0.5 26.4 Net operating losses carryforwards - state 36.6 47.0 0.6 0.9 0.1 0.5 Other 60.5 59.8 20.6 22.8 15.8 14.1 Subtotal deferred tax assets 513.0 564.5 252.5 271.0 176.7 188.4 Valuation allowances (9.6 ) (9.2 ) (0.5 ) (0.5 ) (0.8 ) (0.8 ) Total deferred tax assets 503.4 555.3 252.0 270.5 175.9 187.6 Total deferred tax liabilities, net $1,714.0 $1,603.1 $1,008.0 $957.3 $626.2 $582.0 Carryforwards - At December 31, 2019 , carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2037 $297 $276 $2 State net operating losses 2020-2039 615 9 2 Federal tax credits 2022-2039 355 175 160 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 federal 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, 2019. Uncertain Tax Positions - At December 31, 2019 , 2018 and 2017 , there were no uncertain tax positions or penalties accrued related to uncertain tax positions. As of December 31, 2019 , 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) 2016 - 2018 Consolidated Iowa income tax returns (b) 2016 - 2018 Wisconsin combined tax returns (c) 2015 - 2018 (a) The 2016 and 2017 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. 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 impacts 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 a 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.
IPL [Member]
Income Tax [Line Items]
Income Taxes	INCOME TAXES Income Tax Expense (Benefit) - The components of “Income tax expense (benefit)” in the income statements were as follows (in millions): Alliant Energy IPL WPL 2019 2018 2017 2019 2018 2017 2019 2018 2017 Current tax expense (benefit): Federal ($6.6 ) ($1.0 ) ($41.0 ) ($11.0 ) $14.9 ($27.9 ) $12.0 ($9.2 ) $5.5 State 24.2 (5.1 ) 8.5 24.3 (7.1 ) 1.6 13.7 (4.4 ) 2.5 IPL’s tax benefit riders (4.5 ) (13.2 ) (40.4 ) (4.5 ) (13.2 ) (40.4 ) — — — Deferred tax expense (benefit): Federal 69.7 67.9 159.5 26.4 9.5 72.5 30.7 43.8 55.0 State 41.4 29.8 12.3 30.9 7.3 (2.2 ) 6.3 22.1 16.6 Production tax credits (54.7 ) (29.5 ) (31.1 ) (41.8 ) (14.0 ) (14.1 ) (12.8 ) (15.5 ) (17.0 ) Investment tax credits (0.8 ) (1.2 ) (1.1 ) (0.2 ) (0.6 ) (0.4 ) (0.6 ) (0.6 ) (0.7 ) $68.7 $47.7 $66.7 $24.1 ($3.2 ) ($10.9 ) $49.3 $36.2 $61.9 Income Tax Rates - The overall income tax rates shown in the following table were computed by dividing income tax expense (benefit) by income from continuing operations before income taxes. Alliant Energy IPL WPL 2019 2018 2017 2019 2018 2017 2019 2018 2017 Statutory federal income tax rate 21.0 % 21.0 % 35.0 % 21.0 % 21.0 % 35.0 % 21.0 % 21.0 % 35.0 % State income taxes, net of federal benefits 6.8 7.0 5.5 8.5 7.7 6.5 6.2 6.2 5.1 Production tax credits (8.6 ) (5.2 ) (6.1 ) (13.1 ) (5.2 ) (6.7 ) (4.5 ) (6.4 ) (7.1 ) Effect of rate-making on property-related differences (6.1 ) (7.6 ) (8.5 ) (9.8 ) (14.0 ) (19.1 ) (2.7 ) (2.3 ) (1.7 ) Amortization of excess deferred taxes (1.0 ) (0.3 ) (0.1 ) (0.3 ) — (0.1 ) (1.8 ) (0.2 ) (0.1 ) IPL’s tax benefit riders (0.7 ) (2.3 ) (7.6 ) (1.4 ) (4.9 ) (18.7 ) — — — Adjustment for prior period taxes 0.6 (2.3 ) (1.5 ) 3.1 (4.8 ) (3.4 ) (0.3 ) (0.2 ) — Federal Tax Reform adjustments — (1.0 ) (3.4 ) — (0.4 ) 1.7 — (2.3 ) (5.8 ) Other items, net (1.2 ) (0.9 ) (0.8 ) (0.4 ) (0.6 ) (0.2 ) (0.4 ) (1.0 ) (0.5 ) Overall income tax rate 10.8 % 8.4 % 12.5 % 7.6 % (1.2 %) (5.0 %) 17.5 % 14.8 % 24.9 % 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 2019 2018 2019 2018 2019 2018 Deferred tax liabilities: Property $2,021.5 $1,975.5 $1,183.5 $1,158.4 $770.0 $735.2 ATC Holdings 110.8 102.4 — — — — Other 85.1 80.5 76.5 69.4 32.1 34.4 Total deferred tax liabilities 2,217.4 2,158.4 1,260.0 1,227.8 802.1 769.6 Deferred tax assets: Federal credit carryforwards 355.4 299.1 175.0 133.2 160.3 147.4 Net operating losses carryforwards - federal 60.5 158.6 56.3 114.1 0.5 26.4 Net operating losses carryforwards - state 36.6 47.0 0.6 0.9 0.1 0.5 Other 60.5 59.8 20.6 22.8 15.8 14.1 Subtotal deferred tax assets 513.0 564.5 252.5 271.0 176.7 188.4 Valuation allowances (9.6 ) (9.2 ) (0.5 ) (0.5 ) (0.8 ) (0.8 ) Total deferred tax assets 503.4 555.3 252.0 270.5 175.9 187.6 Total deferred tax liabilities, net $1,714.0 $1,603.1 $1,008.0 $957.3 $626.2 $582.0 Carryforwards - At December 31, 2019 , carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2037 $297 $276 $2 State net operating losses 2020-2039 615 9 2 Federal tax credits 2022-2039 355 175 160 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 federal 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, 2019. Uncertain Tax Positions - At December 31, 2019 , 2018 and 2017 , there were no uncertain tax positions or penalties accrued related to uncertain tax positions. As of December 31, 2019 , 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) 2016 - 2018 Consolidated Iowa income tax returns (b) 2016 - 2018 Wisconsin combined tax returns (c) 2015 - 2018 (a) The 2016 and 2017 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. 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 impacts 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 a 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.
WPL [Member]
Income Tax [Line Items]
Income Taxes	INCOME TAXES Income Tax Expense (Benefit) - The components of “Income tax expense (benefit)” in the income statements were as follows (in millions): Alliant Energy IPL WPL 2019 2018 2017 2019 2018 2017 2019 2018 2017 Current tax expense (benefit): Federal ($6.6 ) ($1.0 ) ($41.0 ) ($11.0 ) $14.9 ($27.9 ) $12.0 ($9.2 ) $5.5 State 24.2 (5.1 ) 8.5 24.3 (7.1 ) 1.6 13.7 (4.4 ) 2.5 IPL’s tax benefit riders (4.5 ) (13.2 ) (40.4 ) (4.5 ) (13.2 ) (40.4 ) — — — Deferred tax expense (benefit): Federal 69.7 67.9 159.5 26.4 9.5 72.5 30.7 43.8 55.0 State 41.4 29.8 12.3 30.9 7.3 (2.2 ) 6.3 22.1 16.6 Production tax credits (54.7 ) (29.5 ) (31.1 ) (41.8 ) (14.0 ) (14.1 ) (12.8 ) (15.5 ) (17.0 ) Investment tax credits (0.8 ) (1.2 ) (1.1 ) (0.2 ) (0.6 ) (0.4 ) (0.6 ) (0.6 ) (0.7 ) $68.7 $47.7 $66.7 $24.1 ($3.2 ) ($10.9 ) $49.3 $36.2 $61.9 Income Tax Rates - The overall income tax rates shown in the following table were computed by dividing income tax expense (benefit) by income from continuing operations before income taxes. Alliant Energy IPL WPL 2019 2018 2017 2019 2018 2017 2019 2018 2017 Statutory federal income tax rate 21.0 % 21.0 % 35.0 % 21.0 % 21.0 % 35.0 % 21.0 % 21.0 % 35.0 % State income taxes, net of federal benefits 6.8 7.0 5.5 8.5 7.7 6.5 6.2 6.2 5.1 Production tax credits (8.6 ) (5.2 ) (6.1 ) (13.1 ) (5.2 ) (6.7 ) (4.5 ) (6.4 ) (7.1 ) Effect of rate-making on property-related differences (6.1 ) (7.6 ) (8.5 ) (9.8 ) (14.0 ) (19.1 ) (2.7 ) (2.3 ) (1.7 ) Amortization of excess deferred taxes (1.0 ) (0.3 ) (0.1 ) (0.3 ) — (0.1 ) (1.8 ) (0.2 ) (0.1 ) IPL’s tax benefit riders (0.7 ) (2.3 ) (7.6 ) (1.4 ) (4.9 ) (18.7 ) — — — Adjustment for prior period taxes 0.6 (2.3 ) (1.5 ) 3.1 (4.8 ) (3.4 ) (0.3 ) (0.2 ) — Federal Tax Reform adjustments — (1.0 ) (3.4 ) — (0.4 ) 1.7 — (2.3 ) (5.8 ) Other items, net (1.2 ) (0.9 ) (0.8 ) (0.4 ) (0.6 ) (0.2 ) (0.4 ) (1.0 ) (0.5 ) Overall income tax rate 10.8 % 8.4 % 12.5 % 7.6 % (1.2 %) (5.0 %) 17.5 % 14.8 % 24.9 % 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 2019 2018 2019 2018 2019 2018 Deferred tax liabilities: Property $2,021.5 $1,975.5 $1,183.5 $1,158.4 $770.0 $735.2 ATC Holdings 110.8 102.4 — — — — Other 85.1 80.5 76.5 69.4 32.1 34.4 Total deferred tax liabilities 2,217.4 2,158.4 1,260.0 1,227.8 802.1 769.6 Deferred tax assets: Federal credit carryforwards 355.4 299.1 175.0 133.2 160.3 147.4 Net operating losses carryforwards - federal 60.5 158.6 56.3 114.1 0.5 26.4 Net operating losses carryforwards - state 36.6 47.0 0.6 0.9 0.1 0.5 Other 60.5 59.8 20.6 22.8 15.8 14.1 Subtotal deferred tax assets 513.0 564.5 252.5 271.0 176.7 188.4 Valuation allowances (9.6 ) (9.2 ) (0.5 ) (0.5 ) (0.8 ) (0.8 ) Total deferred tax assets 503.4 555.3 252.0 270.5 175.9 187.6 Total deferred tax liabilities, net $1,714.0 $1,603.1 $1,008.0 $957.3 $626.2 $582.0 Carryforwards - At December 31, 2019 , carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2037 $297 $276 $2 State net operating losses 2020-2039 615 9 2 Federal tax credits 2022-2039 355 175 160 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 federal 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, 2019. Uncertain Tax Positions - At December 31, 2019 , 2018 and 2017 , there were no uncertain tax positions or penalties accrued related to uncertain tax positions. As of December 31, 2019 , 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) 2016 - 2018 Consolidated Iowa income tax returns (b) 2016 - 2018 Wisconsin combined tax returns (c) 2015 - 2018 (a) The 2016 and 2017 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. 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 impacts 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 a 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.

Benefit Plans

Benefit Plans	12 Months Ended
	Dec. 31, 2019
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. For IPL and WPL, amounts below represent the amounts for their plan participants covered under plans they sponsor, as well as amounts directly assigned to them related to certain participants in the Alliant Energy and Corporate Services sponsored plans. 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 2019 2018 2017 2019 2018 2017 Discount rate for benefit obligations 3.48% 4.34% 3.66% 3.40% 4.24% 3.53% Discount rate for net periodic cost 4.34% 3.66% 4.19% 4.24% 3.53% 3.98% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.44% 5.44% 5.80% Interest crediting rate for Alliant Energy Cash Balance Pension Plan 5.52% 5.04% 4.64% 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 Qualified Defined Benefit Pension Plan OPEB Plans IPL 2019 2018 2017 2019 2018 2017 Discount rate for benefit obligations 3.51% 4.35% 3.68% 3.39% 4.23% 3.51% Discount rate for net periodic cost 4.35% 3.68% 4.22% 4.23% 3.51% 3.95% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.60% 5.60% 6.20% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Qualified Defined Benefit Pension Plan OPEB Plans WPL 2019 2018 2017 2019 2018 2017 Discount rate for benefit obligations 3.50% 4.35% 3.69% 3.39% 4.23% 3.51% Discount rate for net periodic cost 4.35% 3.69% 4.23% 4.23% 3.51% 3.96% Expected rate of return on plan assets 7.60% 7.60% 7.60% 3.81% 3.84% 3.50% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A 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 2019 by the Society of Actuaries. Net Periodic Benefit Costs - The components of net periodic benefit costs for sponsored defined benefit pension and OPEB plans are included 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 2019 2018 2017 2019 2018 2017 Service cost $9.7 $12.1 $12.5 $3.3 $4.2 $5.0 Interest cost 49.8 46.8 51.0 8.5 7.7 8.6 Expected return on plan assets (a) (60.1 ) (69.7 ) (65.5 ) (5.0 ) (6.0 ) (6.1 ) Amortization of prior service credit (b) (0.7 ) (0.7 ) (0.4 ) (0.2 ) (0.2 ) (0.2 ) Amortization of actuarial loss (c) 36.4 35.2 37.6 3.3 3.4 3.8 Settlement losses (d) — — 0.9 — — — $35.1 $23.7 $36.1 $9.9 $9.1 $11.1 IPL Defined Benefit Pension Plans OPEB Plans 2019 2018 2017 2019 2018 2017 Service cost $6.0 $7.4 $7.3 $1.3 $1.7 $2.1 Interest cost 22.7 21.4 23.5 3.4 3.1 3.5 Expected return on plan assets (a) (28.1 ) (32.6 ) (30.8 ) (3.6 ) (4.4 ) (4.3 ) Amortization of prior service credit (b) (0.2 ) (0.2 ) (0.2 ) — — — Amortization of actuarial loss (c) 15.7 14.9 16.1 1.5 1.3 2.0 $16.1 $10.9 $15.9 $2.6 $1.7 $3.3 WPL Defined Benefit Pension Plans OPEB Plans 2019 2018 2017 2019 2018 2017 Service cost $3.5 $4.4 $4.9 $1.3 $1.6 $1.9 Interest cost 21.5 20.2 21.8 3.3 3.1 3.4 Expected return on plan assets (a) (26.2 ) (30.4 ) (28.5 ) (0.6 ) (0.6 ) (0.8 ) Amortization of prior service cost (credit) (b) (0.2 ) (0.1 ) 0.1 (0.2 ) (0.2 ) (0.2 ) Amortization of actuarial loss (c) 17.6 17.2 18.5 1.6 2.0 1.6 $16.2 $11.3 $16.8 $5.4 $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 2019 2018 2019 2018 Change in benefit obligation: Net benefit obligation at January 1 $1,175.0 $1,303.1 $206.1 $222.3 Service cost 9.7 12.1 3.3 4.2 Interest cost 49.8 46.8 8.5 7.7 Plan participants’ contributions — — 3.4 3.1 Actuarial (gain) loss 124.6 (96.2 ) 13.6 (8.2 ) Gross benefits paid (79.4 ) (90.8 ) (21.3 ) (23.0 ) Net benefit obligation at December 31 1,279.7 1,175.0 213.6 206.1 Change in plan assets: Fair value of plan assets at January 1 808.6 950.7 99.1 111.1 Actual return on plan assets 168.1 (57.8 ) 14.6 (2.6 ) Employer contributions 33.1 6.5 9.1 10.5 Plan participants’ contributions — — 3.4 3.1 Gross benefits paid (79.4 ) (90.8 ) (21.3 ) (23.0 ) Fair value of plan assets at December 31 930.4 808.6 104.9 99.1 Under funded status at December 31 ($349.3 ) ($366.4 ) ($108.7 ) ($107.0 ) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2019 2018 2019 2018 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $9.8 $7.5 Current liabilities (14.3 ) (2.3 ) (8.1 ) (9.6 ) Pension and other benefit obligations (335.0 ) (364.1 ) (110.4 ) (104.9 ) Net amounts recognized at December 31 ($349.3 ) ($366.4 ) ($108.7 ) ($107.0 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $485.3 $505.2 $45.2 $44.5 Prior service credit (5.1 ) (5.8 ) (0.9 ) (1.1 ) $480.2 $499.4 $44.3 $43.4 Defined Benefit Pension Plans OPEB Plans IPL 2019 2018 2019 2018 Change in benefit obligation: Net benefit obligation at January 1 $533.9 $592.9 $82.5 $89.4 Service cost 6.0 7.4 1.3 1.7 Interest cost 22.7 21.4 3.4 3.1 Plan participants’ contributions — — 1.3 1.0 Actuarial (gain) loss 55.9 (44.4 ) 5.7 (4.3 ) Gross benefits paid (37.3 ) (43.4 ) (8.5 ) (8.4 ) Net benefit obligation at December 31 581.2 533.9 85.7 82.5 Change in plan assets: Fair value of plan assets at January 1 377.8 443.7 66.7 72.9 Actual return on plan assets 78.9 (26.8 ) 10.1 (1.4 ) Employer contributions 16.6 4.3 2.3 2.6 Plan participants’ contributions — — 1.3 1.0 Gross benefits paid (37.3 ) (43.4 ) (8.5 ) (8.4 ) Fair value of plan assets at December 31 436.0 377.8 71.9 66.7 Under funded status at December 31 ($145.2 ) ($156.1 ) ($13.8 ) ($15.8 ) Defined Benefit Pension Plans OPEB Plans IPL 2019 2018 2019 2018 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $6.6 $4.7 Current liabilities (0.5 ) (0.6 ) (1.8 ) (1.9 ) Pension and other benefit obligations (144.7 ) (155.5 ) (18.6 ) (18.6 ) Net amounts recognized at December 31 ($145.2 ) ($156.1 ) ($13.8 ) ($15.8 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $208.2 $218.8 $16.6 $18.8 Prior service credit (1.7 ) (1.8 ) — — $206.5 $217.0 $16.6 $18.8 Defined Benefit Pension Plans OPEB Plans WPL 2019 2018 2019 2018 Change in benefit obligation: Net benefit obligation at January 1 $506.6 $559.8 $83.2 $90.4 Service cost 3.5 4.4 1.3 1.6 Interest cost 21.5 20.2 3.3 3.1 Plan participants’ contributions — — 1.5 1.4 Actuarial (gain) loss 53.8 (40.0 ) 5.0 (2.5 ) Gross benefits paid (35.3 ) (37.8 ) (9.0 ) (10.8 ) Net benefit obligation at December 31 550.1 506.6 85.3 83.2 Change in plan assets: Fair value of plan assets at January 1 352.2 415.0 16.7 18.7 Actual return on plan assets 73.4 (25.1 ) 1.7 (0.1 ) Employer contributions 15.7 0.1 6.2 7.5 Plan participants’ contributions — — 1.5 1.4 Gross benefits paid (35.3 ) (37.8 ) (9.0 ) (10.8 ) Fair value of plan assets at December 31 406.0 352.2 17.1 16.7 Under funded status at December 31 ($144.1 ) ($154.4 ) ($68.2 ) ($66.5 ) Defined Benefit Pension Plans OPEB Plans WPL 2019 2018 2019 2018 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $3.2 $2.8 Current liabilities (0.1 ) (0.1 ) (6.0 ) (7.4 ) Pension and other benefit obligations (144.0 ) (154.3 ) (65.4 ) (61.9 ) Net amounts recognized at December 31 ($144.1 ) ($154.4 ) ($68.2 ) ($66.5 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $212.0 $223.0 $22.2 $19.9 Prior service credit (1.2 ) (1.3 ) (0.9 ) (1.1 ) $210.8 $221.7 $21.3 $18.8 Actuarial losses related to benefit obligations in 2019 for defined benefit pension and OPEB plans were primarily due to decreases in the discount rates and experience losses, partially offset by the impact of the updated base mortality table. 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. 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 2019 2018 2019 2018 Accumulated benefit obligations $1,234.4 $1,139.9 $213.6 $206.1 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,234.4 1,139.9 213.6 206.1 Fair value of plan assets 930.4 808.6 104.9 99.1 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,279.7 1,175.0 N/A N/A Fair value of plan assets 930.4 808.6 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2019 2018 2019 2018 Accumulated benefit obligations $557.8 $514.3 $85.7 $82.5 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 557.8 514.3 85.7 82.5 Fair value of plan assets 436.0 377.8 71.9 66.7 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 581.2 533.9 N/A N/A Fair value of plan assets 436.0 377.8 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2019 2018 2019 2018 Accumulated benefit obligations $535.9 $494.8 $85.3 $83.2 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 535.9 494.8 85.3 83.2 Fair value of plan assets 406.0 352.2 17.1 16.7 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 550.1 506.6 N/A N/A Fair value of plan assets 406.0 352.2 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 2019 2018 2019 2018 Regulatory assets $39.4 $38.2 $29.0 $27.7 Estimated Future Employer Contributions and Benefit Payments - Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2020 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $59.8 $19.4 $21.8 OPEB plans 8.1 1.8 6.0 (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 2020 2021 2022 2023 2024 2025 - 2029 Defined benefit pension benefits $86.4 $74.5 $76.6 $77.5 $78.2 $390.2 OPEB 17.7 17.7 17.5 17.1 16.8 76.6 $104.1 $92.2 $94.1 $94.6 $95.0 $466.8 IPL 2020 2021 2022 2023 2024 2025 - 2029 Defined benefit pension benefits $35.1 $35.3 $36.6 $37.7 $36.6 $182.5 OPEB 7.2 7.1 7.1 7.0 6.9 30.6 $42.3 $42.4 $43.7 $44.7 $43.5 $213.1 WPL 2020 2021 2022 2023 2024 2025 - 2029 Defined benefit pension benefits $32.0 $32.0 $32.1 $32.4 $33.1 $165.2 OPEB 7.3 7.4 7.1 6.8 6.7 30.2 $39.3 $39.4 $39.2 $39.2 $39.8 $195.4 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 U.S. and international equity, fixed income, 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. 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, 2019 , 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% 5% Equity securities - U.S. 6 % - 56% 25% Equity securities - international 14 % - 34% 22% Global asset securities 5 % - 15% 9% Risk parity securities 5 % - 15% 10% Fixed income securities 20 % - 40% 29% 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, 2019 , 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% 3% Equity securities - U.S. 0 % - 46% 25% Equity securities - international 0 % - 34% 2% Fixed income securities 20 % - 100% 70% 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 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): 2019 2018 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $45.7 $3.7 $42.0 $— $36.4 $3.0 $33.4 $— Equity securities - U.S. 160.6 160.6 — — 130.2 130.2 — — Equity securities - international 135.9 135.9 — — 116.0 116.0 — — Global asset securities 47.2 47.2 — — 42.1 42.1 — — Fixed income securities 130.7 58.0 72.7 — 127.8 52.5 75.3 — Total assets in fair value hierarchy 520.1 $405.4 $114.7 $— 452.5 $343.8 $108.7 $— Assets measured at net asset value 410.1 355.4 Accrued investment income 0.9 1.2 Due to brokers, net (pending trades with brokers) (0.7 ) (0.5 ) Total pension plan assets $930.4 $808.6 2019 2018 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $21.4 $1.7 $19.7 $— $17.0 $1.4 $15.6 $— Equity securities - U.S. 75.2 75.2 — — 60.8 60.8 — — Equity securities - international 63.7 63.7 — — 54.2 54.2 — — Global asset securities 22.1 22.1 — — 19.7 19.7 — — Fixed income securities 61.3 27.2 34.1 — 59.7 24.5 35.2 — Total assets in fair value hierarchy 243.7 $189.9 $53.8 $— 211.4 $160.6 $50.8 $— Assets measured at net asset value 192.2 166.1 Accrued investment income 0.4 0.6 Due to brokers, net (pending trades with brokers) (0.3 ) (0.3 ) Total pension plan assets $436.0 $377.8 2019 2018 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $19.9 $1.6 $18.3 $— $15.9 $1.3 $14.6 $— Equity securities - U.S. 70.1 70.1 — — 56.7 56.7 — — Equity securities - international 59.3 59.3 — — 50.5 50.5 — — Global asset securities 20.6 20.6 — — 18.4 18.4 — — Fixed income securities 57.0 25.3 31.7 — 55.6 22.8 32.8 — Total assets in fair value hierarchy 226.9 $176.9 $50.0 $— 197.1 $149.7 $47.4 $— Assets measured at net asset value 179.0 154.8 Accrued investment income 0.4 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.2 ) Total pension plan assets $406.0 $352.2 At December 31, the fair values of OPEB plan assets were as follows (in millions): 2019 2018 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $3.4 $3.0 $0.4 $— $1.4 $1.1 $0.3 $— Equity securities - U.S. 3.7 3.7 — — 3.9 3.9 — — Equity securities - international 2.8 2.8 — — 2.9 2.9 — — Global asset securities 0.5 0.5 — — 0.4 0.4 — — Fixed income securities 68.5 67.8 0.7 — 68.2 67.5 0.7 — Total assets in fair value hierarchy 78.9 $77.8 $1.1 $— 76.8 $75.8 $1.0 $— Assets measured at net asset value 26.0 22.3 Total OPEB plan assets $104.9 $99.1 2019 2018 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $1.5 $1.5 $— $— $0.7 $0.7 $— $— Fixed income securities 48.3 48.3 — — 47.0 47.0 — — Total assets in fair value hierarchy 49.8 $49.8 $— $— 47.7 $47.7 $— $— Assets measured at net asset value 22.1 19.0 Total OPEB plan assets $71.9 $66.7 2019 2018 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.4 $0.4 $— $— $0.1 $0.1 $— $— Fixed income securities 16.7 16.7 — — 16.6 16.6 — — Total OPEB plan assets $17.1 $17.1 $— $— $16.7 $16.7 $— $— 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, 2019 and 2018 . 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, 2019 and 2018 , 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 2019 2018 2017 2019 2018 2017 2019 2018 2017 401(k) costs $25.5 $25.1 $24.8 $13.0 $13.0 $12.8 $11.5 $11.2 $11.1 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, 2019 , performance shares and restricted stock units (performance- and time-vesting) were outstanding under the Amended and Restated OIP, and 6.4 million shares of Alliant Energy 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, 2019 , performance units and restricted units (performance- and time-vesting) were outstanding under the DLIP, and 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 2019 2018 2017 2019 2018 2017 2019 2018 2017 Compensation expense $26.0 $17.0 $15.1 $14.5 $9.4 $8.3 $10.3 $6.9 $6.4 Income tax benefits 7.3 4.9 6.2 4.2 2.8 3.4 2.8 1.9 2.6 As of December 31, 2019 , Alliant Energy’s, IPL’s and WPL’s total unrecognized compensation cost related to share-based compensation awards was $5.4 million , $3.0 million and $2.2 million , respectively, which is expected to be recognized over a weighted average period of between one and two years . Share-based compensation expense is recognized on a straight-line basis over the requisite service periods and is recorded in “Other operation and maintenance” in the income statements. As of December 31, 2019 , 343,396 shares were included in the calculation of diluted EPS related to the nonvested equity awards. Performance Shares - Equity Awards - Payouts of 2019 performance shares under the Amended and Restated OIP are contingent upon achievement over a three-year period of specified performance criteria, which currently is total shareowner return relative to an investor-owned utility peer group. Performance shares granted in 2019 are to be paid out in shares of Alliant Energy common stock and are accounted for as equity awards. The fair value of each performance share granted in 2019 is based on the fair value of the underlying common stock on the grant date and the probability of satisfying the market condition contained in the agreement during a three-year performance period. The actual number of 2019 performance shares that will be paid out upon vesting is dependent upon actual performance and may range from zero to 200% of the target number of shares. Compensation expense is recorded ratably over the performance period based on the fair value of the awards at the grant date. A summary of the 2019 performance shares activity, with amounts representing the target number of awards, was as follows: 2019 Performance Shares - Equity Weighted Average Grant Date Fair Value Nonvested awards, January 1 — $— Granted 91,816 47.23 Forfeited (17,623 ) 46.35 Nonvested awards, December 31 74,193 47.44 Performance Shares and Performance Units - Liability Awards - Payouts of performance shares granted prior to 2019 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 is total shareowner return relative to an investor-owned utility peer group. Performance shares granted prior to 2019 can be paid out in shares of Alliant Energy 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 granted prior to 2019 and performance units in cash; therefore, these performance shares and performance units are accounted for as liability awards. Compensation expense for these performance shares and performance units is recorded ratably over the performance period based on the fair value of the awards at each reporting period. A summary of these performance shares and performance units activity, with amounts representing the target number of awards, was as follows: Performance Shares (granted prior to 2019) Performance Units (granted prior to 2019) 2019 2018 2017 2019 2018 2017 Nonvested awards, January 1 203,188 223,511 257,599 57,761 71,737 93,320 Granted — 74,163 65,350 — 19,840 21,558 Vested (66,322 ) (90,806 ) (99,438 ) (20,131 ) (31,910 ) (37,395 ) Forfeited (4,994 ) (3,680 ) — (3,056 ) (1,906 ) (5,746 ) Nonvested awards, December 31 131,872 203,188 223,511 34,574 57,761 71,737 Fair Value of Awards - The fair value of each performance share and performance unit is based on the closing market price of one share of Alliant Energy 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. At December 31, 2019 , the Alliant Energy common stock closing price was $54.72 . Additional information related to the fair value of nonvested performance shares and performance units at December 31, 2019 , by year of grant, was as follows: Performance Shares (granted prior to 2019) Performance Units (granted prior to 2019) 2018 Grant 2017 Grant 2018 Grant 2017 Grant Nonvested awards at target 68,307 63,565 17,622 16,952 Estimated payout percentage based on performance criteria 118 % 155 % 118 % 155 % Fair values of each nonvested award $64.57 $84.82 $64.57 $84.82 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 (granted prior to 2019) Performance Units (granted prior to 2019) 2019 2018 2017 2019 2018 2017 2016 Grant 2015 Grant 2014 Grant 2016 Grant 2015 Grant 2014 Grant Performance awards vested 66,322 90,806 99,438 20,131 31,910 37,395 Percentage of target number of performance awards 142.5% 137.5% 147.5% 142.5% 137.5% 147.5% Aggregate payout value (in millions) $4.3 $5.3 $5.6 $1.3 $1.4 $1.5 Payout - cash (in millions) $3.7 $4.9 $5.1 $1.3 $1.4 $1.5 Payout - common stock shares issued 6,447 5,078 5,185 N/A N/A N/A Restricted Stock Units - Equity Awards - Payouts of 2019 restricted stock units under the Amended and Restated OIP are based on the expiration of a three-year time-vesting period. Restricted stock units granted in 2019 are to be paid out in shares of Alliant Energy common stock and are accounted for as equity awards. The fair value of each restricted stock unit granted in 2019 is based on the closing market price of one share of Alliant Energy common stock on the grant date of the award. Compensation expense is recorded ratably over the performance period based on the fair value of the awards on the grant date. A summary of the 2019 restricted stock units activity was as follows: 2019 Restricted Stock Units - Equity Weighted Average Grant Date Fair Value Nonvested units, January 1 — $— Granted 105,348 45.98 Forfeited (16,067 ) 45.63 Nonvested units, December 31 89,281 46.04 Restricted Stock Units - Liability Awards - Payouts of restricted stock units granted prior to 2019 under the Amended and Restated OIP are based on the expiration of a three-year time-vesting period. Restricted stock units granted prior to 2019 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 granted prior to 2019 in cash; therefore, restricted stock units granted prior to 2019 are accounted for as liability awards. The fair value of each restricted stock unit granted prior to 2019 is based on the closing market price of one share of Alliant Energy 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. A summary of the restricted stock units granted prior to 2019 activity was as follows: 2019 2018 2017 Nonvested units, January 1 174,163 113,749 57,736 Granted — 63,568 56,013 Vested (56,849 ) — — Forfeited (4,281 ) (3,154 ) — Nonvested units, December 31 113,033 174,163 113,749 Performance Restricted Stock Units - Equity Awards - Payouts of performance restricted stock units under the Amended and Restated OIP are based upon achievement of certain performance targets during a three-year performance period, which currently includes specified growth of consolidated net income from continuing operations. 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. The fair value of each performance restricted stock unit is based on the closing market price of one share of Alliant Energy 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: 2019 2018 2017 Units Weighted Average Grant Date Fair Value Units Weighted Average Grant Date Fair Value Units Weighted Average Grant Date Fair Value Nonvested units, January 1 203,188 $37.23 132,705 $36.50 67,355 $33.96 Granted 91,816 46.10 74,163 38.60 65,350 39.12 Vested (66,322 ) 33.93 — — — — Forfeited (22,617 ) 44.00 (3,680 ) 38.60 — — Nonvested units, December 31 206,065 41.50 203,188 37
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. For IPL and WPL, amounts below represent the amounts for their plan participants covered under plans they sponsor, as well as amounts directly assigned to them related to certain participants in the Alliant Energy and Corporate Services sponsored plans. 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 2019 2018 2017 2019 2018 2017 Discount rate for benefit obligations 3.48% 4.34% 3.66% 3.40% 4.24% 3.53% Discount rate for net periodic cost 4.34% 3.66% 4.19% 4.24% 3.53% 3.98% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.44% 5.44% 5.80% Interest crediting rate for Alliant Energy Cash Balance Pension Plan 5.52% 5.04% 4.64% 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 Qualified Defined Benefit Pension Plan OPEB Plans IPL 2019 2018 2017 2019 2018 2017 Discount rate for benefit obligations 3.51% 4.35% 3.68% 3.39% 4.23% 3.51% Discount rate for net periodic cost 4.35% 3.68% 4.22% 4.23% 3.51% 3.95% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.60% 5.60% 6.20% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Qualified Defined Benefit Pension Plan OPEB Plans WPL 2019 2018 2017 2019 2018 2017 Discount rate for benefit obligations 3.50% 4.35% 3.69% 3.39% 4.23% 3.51% Discount rate for net periodic cost 4.35% 3.69% 4.23% 4.23% 3.51% 3.96% Expected rate of return on plan assets 7.60% 7.60% 7.60% 3.81% 3.84% 3.50% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A 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 2019 by the Society of Actuaries. Net Periodic Benefit Costs - The components of net periodic benefit costs for sponsored defined benefit pension and OPEB plans are included 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 2019 2018 2017 2019 2018 2017 Service cost $9.7 $12.1 $12.5 $3.3 $4.2 $5.0 Interest cost 49.8 46.8 51.0 8.5 7.7 8.6 Expected return on plan assets (a) (60.1 ) (69.7 ) (65.5 ) (5.0 ) (6.0 ) (6.1 ) Amortization of prior service credit (b) (0.7 ) (0.7 ) (0.4 ) (0.2 ) (0.2 ) (0.2 ) Amortization of actuarial loss (c) 36.4 35.2 37.6 3.3 3.4 3.8 Settlement losses (d) — — 0.9 — — — $35.1 $23.7 $36.1 $9.9 $9.1 $11.1 IPL Defined Benefit Pension Plans OPEB Plans 2019 2018 2017 2019 2018 2017 Service cost $6.0 $7.4 $7.3 $1.3 $1.7 $2.1 Interest cost 22.7 21.4 23.5 3.4 3.1 3.5 Expected return on plan assets (a) (28.1 ) (32.6 ) (30.8 ) (3.6 ) (4.4 ) (4.3 ) Amortization of prior service credit (b) (0.2 ) (0.2 ) (0.2 ) — — — Amortization of actuarial loss (c) 15.7 14.9 16.1 1.5 1.3 2.0 $16.1 $10.9 $15.9 $2.6 $1.7 $3.3 WPL Defined Benefit Pension Plans OPEB Plans 2019 2018 2017 2019 2018 2017 Service cost $3.5 $4.4 $4.9 $1.3 $1.6 $1.9 Interest cost 21.5 20.2 21.8 3.3 3.1 3.4 Expected return on plan assets (a) (26.2 ) (30.4 ) (28.5 ) (0.6 ) (0.6 ) (0.8 ) Amortization of prior service cost (credit) (b) (0.2 ) (0.1 ) 0.1 (0.2 ) (0.2 ) (0.2 ) Amortization of actuarial loss (c) 17.6 17.2 18.5 1.6 2.0 1.6 $16.2 $11.3 $16.8 $5.4 $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 2019 2018 2019 2018 Change in benefit obligation: Net benefit obligation at January 1 $1,175.0 $1,303.1 $206.1 $222.3 Service cost 9.7 12.1 3.3 4.2 Interest cost 49.8 46.8 8.5 7.7 Plan participants’ contributions — — 3.4 3.1 Actuarial (gain) loss 124.6 (96.2 ) 13.6 (8.2 ) Gross benefits paid (79.4 ) (90.8 ) (21.3 ) (23.0 ) Net benefit obligation at December 31 1,279.7 1,175.0 213.6 206.1 Change in plan assets: Fair value of plan assets at January 1 808.6 950.7 99.1 111.1 Actual return on plan assets 168.1 (57.8 ) 14.6 (2.6 ) Employer contributions 33.1 6.5 9.1 10.5 Plan participants’ contributions — — 3.4 3.1 Gross benefits paid (79.4 ) (90.8 ) (21.3 ) (23.0 ) Fair value of plan assets at December 31 930.4 808.6 104.9 99.1 Under funded status at December 31 ($349.3 ) ($366.4 ) ($108.7 ) ($107.0 ) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2019 2018 2019 2018 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $9.8 $7.5 Current liabilities (14.3 ) (2.3 ) (8.1 ) (9.6 ) Pension and other benefit obligations (335.0 ) (364.1 ) (110.4 ) (104.9 ) Net amounts recognized at December 31 ($349.3 ) ($366.4 ) ($108.7 ) ($107.0 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $485.3 $505.2 $45.2 $44.5 Prior service credit (5.1 ) (5.8 ) (0.9 ) (1.1 ) $480.2 $499.4 $44.3 $43.4 Defined Benefit Pension Plans OPEB Plans IPL 2019 2018 2019 2018 Change in benefit obligation: Net benefit obligation at January 1 $533.9 $592.9 $82.5 $89.4 Service cost 6.0 7.4 1.3 1.7 Interest cost 22.7 21.4 3.4 3.1 Plan participants’ contributions — — 1.3 1.0 Actuarial (gain) loss 55.9 (44.4 ) 5.7 (4.3 ) Gross benefits paid (37.3 ) (43.4 ) (8.5 ) (8.4 ) Net benefit obligation at December 31 581.2 533.9 85.7 82.5 Change in plan assets: Fair value of plan assets at January 1 377.8 443.7 66.7 72.9 Actual return on plan assets 78.9 (26.8 ) 10.1 (1.4 ) Employer contributions 16.6 4.3 2.3 2.6 Plan participants’ contributions — — 1.3 1.0 Gross benefits paid (37.3 ) (43.4 ) (8.5 ) (8.4 ) Fair value of plan assets at December 31 436.0 377.8 71.9 66.7 Under funded status at December 31 ($145.2 ) ($156.1 ) ($13.8 ) ($15.8 ) Defined Benefit Pension Plans OPEB Plans IPL 2019 2018 2019 2018 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $6.6 $4.7 Current liabilities (0.5 ) (0.6 ) (1.8 ) (1.9 ) Pension and other benefit obligations (144.7 ) (155.5 ) (18.6 ) (18.6 ) Net amounts recognized at December 31 ($145.2 ) ($156.1 ) ($13.8 ) ($15.8 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $208.2 $218.8 $16.6 $18.8 Prior service credit (1.7 ) (1.8 ) — — $206.5 $217.0 $16.6 $18.8 Defined Benefit Pension Plans OPEB Plans WPL 2019 2018 2019 2018 Change in benefit obligation: Net benefit obligation at January 1 $506.6 $559.8 $83.2 $90.4 Service cost 3.5 4.4 1.3 1.6 Interest cost 21.5 20.2 3.3 3.1 Plan participants’ contributions — — 1.5 1.4 Actuarial (gain) loss 53.8 (40.0 ) 5.0 (2.5 ) Gross benefits paid (35.3 ) (37.8 ) (9.0 ) (10.8 ) Net benefit obligation at December 31 550.1 506.6 85.3 83.2 Change in plan assets: Fair value of plan assets at January 1 352.2 415.0 16.7 18.7 Actual return on plan assets 73.4 (25.1 ) 1.7 (0.1 ) Employer contributions 15.7 0.1 6.2 7.5 Plan participants’ contributions — — 1.5 1.4 Gross benefits paid (35.3 ) (37.8 ) (9.0 ) (10.8 ) Fair value of plan assets at December 31 406.0 352.2 17.1 16.7 Under funded status at December 31 ($144.1 ) ($154.4 ) ($68.2 ) ($66.5 ) Defined Benefit Pension Plans OPEB Plans WPL 2019 2018 2019 2018 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $3.2 $2.8 Current liabilities (0.1 ) (0.1 ) (6.0 ) (7.4 ) Pension and other benefit obligations (144.0 ) (154.3 ) (65.4 ) (61.9 ) Net amounts recognized at December 31 ($144.1 ) ($154.4 ) ($68.2 ) ($66.5 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $212.0 $223.0 $22.2 $19.9 Prior service credit (1.2 ) (1.3 ) (0.9 ) (1.1 ) $210.8 $221.7 $21.3 $18.8 Actuarial losses related to benefit obligations in 2019 for defined benefit pension and OPEB plans were primarily due to decreases in the discount rates and experience losses, partially offset by the impact of the updated base mortality table. 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. 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 2019 2018 2019 2018 Accumulated benefit obligations $1,234.4 $1,139.9 $213.6 $206.1 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,234.4 1,139.9 213.6 206.1 Fair value of plan assets 930.4 808.6 104.9 99.1 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,279.7 1,175.0 N/A N/A Fair value of plan assets 930.4 808.6 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2019 2018 2019 2018 Accumulated benefit obligations $557.8 $514.3 $85.7 $82.5 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 557.8 514.3 85.7 82.5 Fair value of plan assets 436.0 377.8 71.9 66.7 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 581.2 533.9 N/A N/A Fair value of plan assets 436.0 377.8 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2019 2018 2019 2018 Accumulated benefit obligations $535.9 $494.8 $85.3 $83.2 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 535.9 494.8 85.3 83.2 Fair value of plan assets 406.0 352.2 17.1 16.7 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 550.1 506.6 N/A N/A Fair value of plan assets 406.0 352.2 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 2019 2018 2019 2018 Regulatory assets $39.4 $38.2 $29.0 $27.7 Estimated Future Employer Contributions and Benefit Payments - Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2020 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $59.8 $19.4 $21.8 OPEB plans 8.1 1.8 6.0 (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 2020 2021 2022 2023 2024 2025 - 2029 Defined benefit pension benefits $86.4 $74.5 $76.6 $77.5 $78.2 $390.2 OPEB 17.7 17.7 17.5 17.1 16.8 76.6 $104.1 $92.2 $94.1 $94.6 $95.0 $466.8 IPL 2020 2021 2022 2023 2024 2025 - 2029 Defined benefit pension benefits $35.1 $35.3 $36.6 $37.7 $36.6 $182.5 OPEB 7.2 7.1 7.1 7.0 6.9 30.6 $42.3 $42.4 $43.7 $44.7 $43.5 $213.1 WPL 2020 2021 2022 2023 2024 2025 - 2029 Defined benefit pension benefits $32.0 $32.0 $32.1 $32.4 $33.1 $165.2 OPEB 7.3 7.4 7.1 6.8 6.7 30.2 $39.3 $39.4 $39.2 $39.2 $39.8 $195.4 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 U.S. and international equity, fixed income, 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. 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, 2019 , 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% 5% Equity securities - U.S. 6 % - 56% 25% Equity securities - international 14 % - 34% 22% Global asset securities 5 % - 15% 9% Risk parity securities 5 % - 15% 10% Fixed income securities 20 % - 40% 29% 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, 2019 , 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% 3% Equity securities - U.S. 0 % - 46% 25% Equity securities - international 0 % - 34% 2% Fixed income securities 20 % - 100% 70% 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 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): 2019 2018 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $45.7 $3.7 $42.0 $— $36.4 $3.0 $33.4 $— Equity securities - U.S. 160.6 160.6 — — 130.2 130.2 — — Equity securities - international 135.9 135.9 — — 116.0 116.0 — — Global asset securities 47.2 47.2 — — 42.1 42.1 — — Fixed income securities 130.7 58.0 72.7 — 127.8 52.5 75.3 — Total assets in fair value hierarchy 520.1 $405.4 $114.7 $— 452.5 $343.8 $108.7 $— Assets measured at net asset value 410.1 355.4 Accrued investment income 0.9 1.2 Due to brokers, net (pending trades with brokers) (0.7 ) (0.5 ) Total pension plan assets $930.4 $808.6 2019 2018 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $21.4 $1.7 $19.7 $— $17.0 $1.4 $15.6 $— Equity securities - U.S. 75.2 75.2 — — 60.8 60.8 — — Equity securities - international 63.7 63.7 — — 54.2 54.2 — — Global asset securities 22.1 22.1 — — 19.7 19.7 — — Fixed income securities 61.3 27.2 34.1 — 59.7 24.5 35.2 — Total assets in fair value hierarchy 243.7 $189.9 $53.8 $— 211.4 $160.6 $50.8 $— Assets measured at net asset value 192.2 166.1 Accrued investment income 0.4 0.6 Due to brokers, net (pending trades with brokers) (0.3 ) (0.3 ) Total pension plan assets $436.0 $377.8 2019 2018 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $19.9 $1.6 $18.3 $— $15.9 $1.3 $14.6 $— Equity securities - U.S. 70.1 70.1 — — 56.7 56.7 — — Equity securities - international 59.3 59.3 — — 50.5 50.5 — — Global asset securities 20.6 20.6 — — 18.4 18.4 — — Fixed income securities 57.0 25.3 31.7 — 55.6 22.8 32.8 — Total assets in fair value hierarchy 226.9 $176.9 $50.0 $— 197.1 $149.7 $47.4 $— Assets measured at net asset value 179.0 154.8 Accrued investment income 0.4 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.2 ) Total pension plan assets $406.0 $352.2 At December 31, the fair values of OPEB plan assets were as follows (in millions): 2019 2018 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $3.4 $3.0 $0.4 $— $1.4 $1.1 $0.3 $— Equity securities - U.S. 3.7 3.7 — — 3.9 3.9 — — Equity securities - international 2.8 2.8 — — 2.9 2.9 — — Global asset securities 0.5 0.5 — — 0.4 0.4 — — Fixed income securities 68.5 67.8 0.7 — 68.2 67.5 0.7 — Total assets in fair value hierarchy 78.9 $77.8 $1.1 $— 76.8 $75.8 $1.0 $— Assets measured at net asset value 26.0 22.3 Total OPEB plan assets $104.9 $99.1 2019 2018 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $1.5 $1.5 $— $— $0.7 $0.7 $— $— Fixed income securities 48.3 48.3 — — 47.0 47.0 — — Total assets in fair value hierarchy 49.8 $49.8 $— $— 47.7 $47.7 $— $— Assets measured at net asset value 22.1 19.0 Total OPEB plan assets $71.9 $66.7 2019 2018 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.4 $0.4 $— $— $0.1 $0.1 $— $— Fixed income securities 16.7 16.7 — — 16.6 16.6 — — Total OPEB plan assets $17.1 $17.1 $— $— $16.7 $16.7 $— $— 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, 2019 and 2018 . 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, 2019 and 2018 , 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 2019 2018 2017 2019 2018 2017 2019 2018 2017 401(k) costs $25.5 $25.1 $24.8 $13.0 $13.0 $12.8 $11.5 $11.2 $11.1 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, 2019 , performance shares and restricted stock units (performance- and time-vesting) were outstanding under the Amended and Restated OIP, and 6.4 million shares of Alliant Energy 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, 2019 , performance units and restricted units (performance- and time-vesting) were outstanding under the DLIP, and 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 2019 2018 2017 2019 2018 2017 2019 2018 2017 Compensation expense $26.0 $17.0 $15.1 $14.5 $9.4 $8.3 $10.3 $6.9 $6.4 Income tax benefits 7.3 4.9 6.2 4.2 2.8 3.4 2.8 1.9 2.6 As of December 31, 2019 , Alliant Energy’s, IPL’s and WPL’s total unrecognized compensation cost related to share-based compensation awards was $5.4 million , $3.0 million and $2.2 million , respectively, which is expected to be recognized over a weighted average period of between one and two years . Share-based compensation expense is recognized on a straight-line basis over the requisite service periods and is recorded in “Other operation and maintenance” in the income statements. As of December 31, 2019 , 343,396 shares were included in the calculation of diluted EPS related to the nonvested equity awards. Performance Shares - Equity Awards - Payouts of 2019 performance shares under the Amended and Restated OIP are contingent upon achievement over a three-year period of specified performance criteria, which currently is total shareowner return relative to an investor-owned utility peer group. Performance shares granted in 2019 are to be paid out in shares of Alliant Energy common stock and are accounted for as equity awards. The fair value of each performance share granted in 2019 is based on the fair value of the underlying common stock on the grant date and the probability of satisfying the market condition contained in the agreement during a three-year performance period. The actual number of 2019 performance shares that will be paid out upon vesting is dependent upon actual performance and may range from zero to 200% of the target number of shares. Compensation expense is recorded ratably over the performance period based on the fair value of the awards at the grant date. A summary of the 2019 performance shares activity, with amounts representing the target number of awards, was as follows: 2019 Performance Shares - Equity Weighted Average Grant Date Fair Value Nonvested awards, January 1 — $— Granted 91,816 47.23 Forfeited (17,623 ) 46.35 Nonvested awards, December 31 74,193 47.44 Performance Shares and Performance Units - Liability Awards - Payouts of performance shares granted prior to 2019 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 is total shareowner return relative to an investor-owned utility peer group. Performance shares granted prior to 2019 can be paid out in shares of Alliant Energy 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 granted prior to 2019 and performance units in cash; therefore, these performance shares and performance units are accounted for as liability awards. Compensation expense for these performance shares and performance units is recorded ratably over the performance period based on the fair value of the awards at each reporting period. A summary of these performance shares and performance units activity, with amounts representing the target number of awards, was as follows: Performance Shares (granted prior to 2019) Performance Units (granted prior to 2019) 2019 2018 2017 2019 2018 2017 Nonvested awards, January 1 203,188 223,511 257,599 57,761 71,737 93,320 Granted — 74,163 65,350 — 19,840 21,558 Vested (66,322 ) (90,806 ) (99,438 ) (20,131 ) (31,910 ) (37,395 ) Forfeited (4,994 ) (3,680 ) — (3,056 ) (1,906 ) (5,746 ) Nonvested awards, December 31 131,872 203,188 223,511 34,574 57,761 71,737 Fair Value of Awards - The fair value of each performance share and performance unit is based on the closing market price of one share of Alliant Energy 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. At December 31, 2019 , the Alliant Energy common stock closing price was $54.72 . Additional information related to the fair value of nonvested performance shares and performance units at December 31, 2019 , by year of grant, was as follows: Performance Shares (granted prior to 2019) Performance Units (granted prior to 2019) 2018 Grant 2017 Grant 2018 Grant 2017 Grant Nonvested awards at target 68,307 63,565 17,622 16,952 Estimated payout percentage based on performance criteria 118 % 155 % 118 % 155 % Fair values of each nonvested award $64.57 $84.82 $64.57 $84.82 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 (granted prior to 2019) Performance Units (granted prior to 2019) 2019 2018 2017 2019 2018 2017 2016 Grant 2015 Grant 2014 Grant 2016 Grant 2015 Grant 2014 Grant Performance awards vested 66,322 90,806 99,438 20,131 31,910 37,395 Percentage of target number of performance awards 142.5% 137.5% 147.5% 142.5% 137.5% 147.5% Aggregate payout value (in millions) $4.3 $5.3 $5.6 $1.3 $1.4 $1.5 Payout - cash (in millions) $3.7 $4.9 $5.1 $1.3 $1.4 $1.5 Payout - common stock shares issued 6,447 5,078 5,185 N/A N/A N/A Restricted Stock Units - Equity Awards - Payouts of 2019 restricted stock units under the Amended and Restated OIP are based on the expiration of a three-year time-vesting period. Restricted stock units granted in 2019 are to be paid out in shares of Alliant Energy common stock and are accounted for as equity awards. The fair value of each restricted stock unit granted in 2019 is based on the closing market price of one share of Alliant Energy common stock on the grant date of the award. Compensation expense is recorded ratably over the performance period based on the fair value of the awards on the grant date. A summary of the 2019 restricted stock units activity was as follows: 2019 Restricted Stock Units - Equity Weighted Average Grant Date Fair Value Nonvested units, January 1 — $— Granted 105,348 45.98 Forfeited (16,067 ) 45.63 Nonvested units, December 31 89,281 46.04 Restricted Stock Units - Liability Awards - Payouts of restricted stock units granted prior to 2019 under the Amended and Restated OIP are based on the expiration of a three-year time-vesting period. Restricted stock units granted prior to 2019 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 granted prior to 2019 in cash; therefore, restricted stock units granted prior to 2019 are accounted for as liability awards. The fair value of each restricted stock unit granted prior to 2019 is based on the closing market price of one share of Alliant Energy 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. A summary of the restricted stock units granted prior to 2019 activity was as follows: 2019 2018 2017 Nonvested units, January 1 174,163 113,749 57,736 Granted — 63,568 56,013 Vested (56,849 ) — — Forfeited (4,281 ) (3,154 ) — Nonvested units, December 31 113,033 174,163 113,749 Performance Restricted Stock Units - Equity Awards - Payouts of performance restricted stock units under the Amended and Restated OIP are based upon achievement of certain performance targets during a three-year performance period, which currently includes specified growth of consolidated net income from continuing operations. 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. The fair value of each performance restricted stock unit is based on the closing market price of one share of Alliant Energy 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: 2019 2018 2017 Units Weighted Average Grant Date Fair Value Units Weighted Average Grant Date Fair Value Units Weighted Average Grant Date Fair Value Nonvested units, January 1 203,188 $37.23 132,705 $36.50 67,355 $33.96 Granted 91,816 46.10 74,163 38.60 65,350 39.12 Vested (66,322 ) 33.93 — — — — Forfeited (22,617 ) 44.00 (3,680 ) 38.60 — — Nonvested units, December 31 206,065 41.50 203,188 37
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. For IPL and WPL, amounts below represent the amounts for their plan participants covered under plans they sponsor, as well as amounts directly assigned to them related to certain participants in the Alliant Energy and Corporate Services sponsored plans. 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 2019 2018 2017 2019 2018 2017 Discount rate for benefit obligations 3.48% 4.34% 3.66% 3.40% 4.24% 3.53% Discount rate for net periodic cost 4.34% 3.66% 4.19% 4.24% 3.53% 3.98% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.44% 5.44% 5.80% Interest crediting rate for Alliant Energy Cash Balance Pension Plan 5.52% 5.04% 4.64% 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 Qualified Defined Benefit Pension Plan OPEB Plans IPL 2019 2018 2017 2019 2018 2017 Discount rate for benefit obligations 3.51% 4.35% 3.68% 3.39% 4.23% 3.51% Discount rate for net periodic cost 4.35% 3.68% 4.22% 4.23% 3.51% 3.95% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.60% 5.60% 6.20% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Qualified Defined Benefit Pension Plan OPEB Plans WPL 2019 2018 2017 2019 2018 2017 Discount rate for benefit obligations 3.50% 4.35% 3.69% 3.39% 4.23% 3.51% Discount rate for net periodic cost 4.35% 3.69% 4.23% 4.23% 3.51% 3.96% Expected rate of return on plan assets 7.60% 7.60% 7.60% 3.81% 3.84% 3.50% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A 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 2019 by the Society of Actuaries. Net Periodic Benefit Costs - The components of net periodic benefit costs for sponsored defined benefit pension and OPEB plans are included 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 2019 2018 2017 2019 2018 2017 Service cost $9.7 $12.1 $12.5 $3.3 $4.2 $5.0 Interest cost 49.8 46.8 51.0 8.5 7.7 8.6 Expected return on plan assets (a) (60.1 ) (69.7 ) (65.5 ) (5.0 ) (6.0 ) (6.1 ) Amortization of prior service credit (b) (0.7 ) (0.7 ) (0.4 ) (0.2 ) (0.2 ) (0.2 ) Amortization of actuarial loss (c) 36.4 35.2 37.6 3.3 3.4 3.8 Settlement losses (d) — — 0.9 — — — $35.1 $23.7 $36.1 $9.9 $9.1 $11.1 IPL Defined Benefit Pension Plans OPEB Plans 2019 2018 2017 2019 2018 2017 Service cost $6.0 $7.4 $7.3 $1.3 $1.7 $2.1 Interest cost 22.7 21.4 23.5 3.4 3.1 3.5 Expected return on plan assets (a) (28.1 ) (32.6 ) (30.8 ) (3.6 ) (4.4 ) (4.3 ) Amortization of prior service credit (b) (0.2 ) (0.2 ) (0.2 ) — — — Amortization of actuarial loss (c) 15.7 14.9 16.1 1.5 1.3 2.0 $16.1 $10.9 $15.9 $2.6 $1.7 $3.3 WPL Defined Benefit Pension Plans OPEB Plans 2019 2018 2017 2019 2018 2017 Service cost $3.5 $4.4 $4.9 $1.3 $1.6 $1.9 Interest cost 21.5 20.2 21.8 3.3 3.1 3.4 Expected return on plan assets (a) (26.2 ) (30.4 ) (28.5 ) (0.6 ) (0.6 ) (0.8 ) Amortization of prior service cost (credit) (b) (0.2 ) (0.1 ) 0.1 (0.2 ) (0.2 ) (0.2 ) Amortization of actuarial loss (c) 17.6 17.2 18.5 1.6 2.0 1.6 $16.2 $11.3 $16.8 $5.4 $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 2019 2018 2019 2018 Change in benefit obligation: Net benefit obligation at January 1 $1,175.0 $1,303.1 $206.1 $222.3 Service cost 9.7 12.1 3.3 4.2 Interest cost 49.8 46.8 8.5 7.7 Plan participants’ contributions — — 3.4 3.1 Actuarial (gain) loss 124.6 (96.2 ) 13.6 (8.2 ) Gross benefits paid (79.4 ) (90.8 ) (21.3 ) (23.0 ) Net benefit obligation at December 31 1,279.7 1,175.0 213.6 206.1 Change in plan assets: Fair value of plan assets at January 1 808.6 950.7 99.1 111.1 Actual return on plan assets 168.1 (57.8 ) 14.6 (2.6 ) Employer contributions 33.1 6.5 9.1 10.5 Plan participants’ contributions — — 3.4 3.1 Gross benefits paid (79.4 ) (90.8 ) (21.3 ) (23.0 ) Fair value of plan assets at December 31 930.4 808.6 104.9 99.1 Under funded status at December 31 ($349.3 ) ($366.4 ) ($108.7 ) ($107.0 ) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2019 2018 2019 2018 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $9.8 $7.5 Current liabilities (14.3 ) (2.3 ) (8.1 ) (9.6 ) Pension and other benefit obligations (335.0 ) (364.1 ) (110.4 ) (104.9 ) Net amounts recognized at December 31 ($349.3 ) ($366.4 ) ($108.7 ) ($107.0 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $485.3 $505.2 $45.2 $44.5 Prior service credit (5.1 ) (5.8 ) (0.9 ) (1.1 ) $480.2 $499.4 $44.3 $43.4 Defined Benefit Pension Plans OPEB Plans IPL 2019 2018 2019 2018 Change in benefit obligation: Net benefit obligation at January 1 $533.9 $592.9 $82.5 $89.4 Service cost 6.0 7.4 1.3 1.7 Interest cost 22.7 21.4 3.4 3.1 Plan participants’ contributions — — 1.3 1.0 Actuarial (gain) loss 55.9 (44.4 ) 5.7 (4.3 ) Gross benefits paid (37.3 ) (43.4 ) (8.5 ) (8.4 ) Net benefit obligation at December 31 581.2 533.9 85.7 82.5 Change in plan assets: Fair value of plan assets at January 1 377.8 443.7 66.7 72.9 Actual return on plan assets 78.9 (26.8 ) 10.1 (1.4 ) Employer contributions 16.6 4.3 2.3 2.6 Plan participants’ contributions — — 1.3 1.0 Gross benefits paid (37.3 ) (43.4 ) (8.5 ) (8.4 ) Fair value of plan assets at December 31 436.0 377.8 71.9 66.7 Under funded status at December 31 ($145.2 ) ($156.1 ) ($13.8 ) ($15.8 ) Defined Benefit Pension Plans OPEB Plans IPL 2019 2018 2019 2018 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $6.6 $4.7 Current liabilities (0.5 ) (0.6 ) (1.8 ) (1.9 ) Pension and other benefit obligations (144.7 ) (155.5 ) (18.6 ) (18.6 ) Net amounts recognized at December 31 ($145.2 ) ($156.1 ) ($13.8 ) ($15.8 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $208.2 $218.8 $16.6 $18.8 Prior service credit (1.7 ) (1.8 ) — — $206.5 $217.0 $16.6 $18.8 Defined Benefit Pension Plans OPEB Plans WPL 2019 2018 2019 2018 Change in benefit obligation: Net benefit obligation at January 1 $506.6 $559.8 $83.2 $90.4 Service cost 3.5 4.4 1.3 1.6 Interest cost 21.5 20.2 3.3 3.1 Plan participants’ contributions — — 1.5 1.4 Actuarial (gain) loss 53.8 (40.0 ) 5.0 (2.5 ) Gross benefits paid (35.3 ) (37.8 ) (9.0 ) (10.8 ) Net benefit obligation at December 31 550.1 506.6 85.3 83.2 Change in plan assets: Fair value of plan assets at January 1 352.2 415.0 16.7 18.7 Actual return on plan assets 73.4 (25.1 ) 1.7 (0.1 ) Employer contributions 15.7 0.1 6.2 7.5 Plan participants’ contributions — — 1.5 1.4 Gross benefits paid (35.3 ) (37.8 ) (9.0 ) (10.8 ) Fair value of plan assets at December 31 406.0 352.2 17.1 16.7 Under funded status at December 31 ($144.1 ) ($154.4 ) ($68.2 ) ($66.5 ) Defined Benefit Pension Plans OPEB Plans WPL 2019 2018 2019 2018 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $3.2 $2.8 Current liabilities (0.1 ) (0.1 ) (6.0 ) (7.4 ) Pension and other benefit obligations (144.0 ) (154.3 ) (65.4 ) (61.9 ) Net amounts recognized at December 31 ($144.1 ) ($154.4 ) ($68.2 ) ($66.5 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $212.0 $223.0 $22.2 $19.9 Prior service credit (1.2 ) (1.3 ) (0.9 ) (1.1 ) $210.8 $221.7 $21.3 $18.8 Actuarial losses related to benefit obligations in 2019 for defined benefit pension and OPEB plans were primarily due to decreases in the discount rates and experience losses, partially offset by the impact of the updated base mortality table. 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. 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 2019 2018 2019 2018 Accumulated benefit obligations $1,234.4 $1,139.9 $213.6 $206.1 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,234.4 1,139.9 213.6 206.1 Fair value of plan assets 930.4 808.6 104.9 99.1 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,279.7 1,175.0 N/A N/A Fair value of plan assets 930.4 808.6 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2019 2018 2019 2018 Accumulated benefit obligations $557.8 $514.3 $85.7 $82.5 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 557.8 514.3 85.7 82.5 Fair value of plan assets 436.0 377.8 71.9 66.7 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 581.2 533.9 N/A N/A Fair value of plan assets 436.0 377.8 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2019 2018 2019 2018 Accumulated benefit obligations $535.9 $494.8 $85.3 $83.2 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 535.9 494.8 85.3 83.2 Fair value of plan assets 406.0 352.2 17.1 16.7 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 550.1 506.6 N/A N/A Fair value of plan assets 406.0 352.2 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 2019 2018 2019 2018 Regulatory assets $39.4 $38.2 $29.0 $27.7 Estimated Future Employer Contributions and Benefit Payments - Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2020 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $59.8 $19.4 $21.8 OPEB plans 8.1 1.8 6.0 (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 2020 2021 2022 2023 2024 2025 - 2029 Defined benefit pension benefits $86.4 $74.5 $76.6 $77.5 $78.2 $390.2 OPEB 17.7 17.7 17.5 17.1 16.8 76.6 $104.1 $92.2 $94.1 $94.6 $95.0 $466.8 IPL 2020 2021 2022 2023 2024 2025 - 2029 Defined benefit pension benefits $35.1 $35.3 $36.6 $37.7 $36.6 $182.5 OPEB 7.2 7.1 7.1 7.0 6.9 30.6 $42.3 $42.4 $43.7 $44.7 $43.5 $213.1 WPL 2020 2021 2022 2023 2024 2025 - 2029 Defined benefit pension benefits $32.0 $32.0 $32.1 $32.4 $33.1 $165.2 OPEB 7.3 7.4 7.1 6.8 6.7 30.2 $39.3 $39.4 $39.2 $39.2 $39.8 $195.4 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 U.S. and international equity, fixed income, 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. 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, 2019 , 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% 5% Equity securities - U.S. 6 % - 56% 25% Equity securities - international 14 % - 34% 22% Global asset securities 5 % - 15% 9% Risk parity securities 5 % - 15% 10% Fixed income securities 20 % - 40% 29% 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, 2019 , 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% 3% Equity securities - U.S. 0 % - 46% 25% Equity securities - international 0 % - 34% 2% Fixed income securities 20 % - 100% 70% 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 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): 2019 2018 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $45.7 $3.7 $42.0 $— $36.4 $3.0 $33.4 $— Equity securities - U.S. 160.6 160.6 — — 130.2 130.2 — — Equity securities - international 135.9 135.9 — — 116.0 116.0 — — Global asset securities 47.2 47.2 — — 42.1 42.1 — — Fixed income securities 130.7 58.0 72.7 — 127.8 52.5 75.3 — Total assets in fair value hierarchy 520.1 $405.4 $114.7 $— 452.5 $343.8 $108.7 $— Assets measured at net asset value 410.1 355.4 Accrued investment income 0.9 1.2 Due to brokers, net (pending trades with brokers) (0.7 ) (0.5 ) Total pension plan assets $930.4 $808.6 2019 2018 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $21.4 $1.7 $19.7 $— $17.0 $1.4 $15.6 $— Equity securities - U.S. 75.2 75.2 — — 60.8 60.8 — — Equity securities - international 63.7 63.7 — — 54.2 54.2 — — Global asset securities 22.1 22.1 — — 19.7 19.7 — — Fixed income securities 61.3 27.2 34.1 — 59.7 24.5 35.2 — Total assets in fair value hierarchy 243.7 $189.9 $53.8 $— 211.4 $160.6 $50.8 $— Assets measured at net asset value 192.2 166.1 Accrued investment income 0.4 0.6 Due to brokers, net (pending trades with brokers) (0.3 ) (0.3 ) Total pension plan assets $436.0 $377.8 2019 2018 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $19.9 $1.6 $18.3 $— $15.9 $1.3 $14.6 $— Equity securities - U.S. 70.1 70.1 — — 56.7 56.7 — — Equity securities - international 59.3 59.3 — — 50.5 50.5 — — Global asset securities 20.6 20.6 — — 18.4 18.4 — — Fixed income securities 57.0 25.3 31.7 — 55.6 22.8 32.8 — Total assets in fair value hierarchy 226.9 $176.9 $50.0 $— 197.1 $149.7 $47.4 $— Assets measured at net asset value 179.0 154.8 Accrued investment income 0.4 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.2 ) Total pension plan assets $406.0 $352.2 At December 31, the fair values of OPEB plan assets were as follows (in millions): 2019 2018 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $3.4 $3.0 $0.4 $— $1.4 $1.1 $0.3 $— Equity securities - U.S. 3.7 3.7 — — 3.9 3.9 — — Equity securities - international 2.8 2.8 — — 2.9 2.9 — — Global asset securities 0.5 0.5 — — 0.4 0.4 — — Fixed income securities 68.5 67.8 0.7 — 68.2 67.5 0.7 — Total assets in fair value hierarchy 78.9 $77.8 $1.1 $— 76.8 $75.8 $1.0 $— Assets measured at net asset value 26.0 22.3 Total OPEB plan assets $104.9 $99.1 2019 2018 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $1.5 $1.5 $— $— $0.7 $0.7 $— $— Fixed income securities 48.3 48.3 — — 47.0 47.0 — — Total assets in fair value hierarchy 49.8 $49.8 $— $— 47.7 $47.7 $— $— Assets measured at net asset value 22.1 19.0 Total OPEB plan assets $71.9 $66.7 2019 2018 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.4 $0.4 $— $— $0.1 $0.1 $— $— Fixed income securities 16.7 16.7 — — 16.6 16.6 — — Total OPEB plan assets $17.1 $17.1 $— $— $16.7 $16.7 $— $— 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, 2019 and 2018 . 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, 2019 and 2018 , 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 2019 2018 2017 2019 2018 2017 2019 2018 2017 401(k) costs $25.5 $25.1 $24.8 $13.0 $13.0 $12.8 $11.5 $11.2 $11.1 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, 2019 , performance shares and restricted stock units (performance- and time-vesting) were outstanding under the Amended and Restated OIP, and 6.4 million shares of Alliant Energy 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, 2019 , performance units and restricted units (performance- and time-vesting) were outstanding under the DLIP, and 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 2019 2018 2017 2019 2018 2017 2019 2018 2017 Compensation expense $26.0 $17.0 $15.1 $14.5 $9.4 $8.3 $10.3 $6.9 $6.4 Income tax benefits 7.3 4.9 6.2 4.2 2.8 3.4 2.8 1.9 2.6 As of December 31, 2019 , Alliant Energy’s, IPL’s and WPL’s total unrecognized compensation cost related to share-based compensation awards was $5.4 million , $3.0 million and $2.2 million , respectively, which is expected to be recognized over a weighted average period of between one and two years . Share-based compensation expense is recognized on a straight-line basis over the requisite service periods and is recorded in “Other operation and maintenance” in the income statements. As of December 31, 2019 , 343,396 shares were included in the calculation of diluted EPS related to the nonvested equity awards. Performance Shares - Equity Awards - Payouts of 2019 performance shares under the Amended and Restated OIP are contingent upon achievement over a three-year period of specified performance criteria, which currently is total shareowner return relative to an investor-owned utility peer group. Performance shares granted in 2019 are to be paid out in shares of Alliant Energy common stock and are accounted for as equity awards. The fair value of each performance share granted in 2019 is based on the fair value of the underlying common stock on the grant date and the probability of satisfying the market condition contained in the agreement during a three-year performance period. The actual number of 2019 performance shares that will be paid out upon vesting is dependent upon actual performance and may range from zero to 200% of the target number of shares. Compensation expense is recorded ratably over the performance period based on the fair value of the awards at the grant date. A summary of the 2019 performance shares activity, with amounts representing the target number of awards, was as follows: 2019 Performance Shares - Equity Weighted Average Grant Date Fair Value Nonvested awards, January 1 — $— Granted 91,816 47.23 Forfeited (17,623 ) 46.35 Nonvested awards, December 31 74,193 47.44 Performance Shares and Performance Units - Liability Awards - Payouts of performance shares granted prior to 2019 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 is total shareowner return relative to an investor-owned utility peer group. Performance shares granted prior to 2019 can be paid out in shares of Alliant Energy 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 granted prior to 2019 and performance units in cash; therefore, these performance shares and performance units are accounted for as liability awards. Compensation expense for these performance shares and performance units is recorded ratably over the performance period based on the fair value of the awards at each reporting period. A summary of these performance shares and performance units activity, with amounts representing the target number of awards, was as follows: Performance Shares (granted prior to 2019) Performance Units (granted prior to 2019) 2019 2018 2017 2019 2018 2017 Nonvested awards, January 1 203,188 223,511 257,599 57,761 71,737 93,320 Granted — 74,163 65,350 — 19,840 21,558 Vested (66,322 ) (90,806 ) (99,438 ) (20,131 ) (31,910 ) (37,395 ) Forfeited (4,994 ) (3,680 ) — (3,056 ) (1,906 ) (5,746 ) Nonvested awards, December 31 131,872 203,188 223,511 34,574 57,761 71,737 Fair Value of Awards - The fair value of each performance share and performance unit is based on the closing market price of one share of Alliant Energy 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. At December 31, 2019 , the Alliant Energy common stock closing price was $54.72 . Additional information related to the fair value of nonvested performance shares and performance units at December 31, 2019 , by year of grant, was as follows: Performance Shares (granted prior to 2019) Performance Units (granted prior to 2019) 2018 Grant 2017 Grant 2018 Grant 2017 Grant Nonvested awards at target 68,307 63,565 17,622 16,952 Estimated payout percentage based on performance criteria 118 % 155 % 118 % 155 % Fair values of each nonvested award $64.57 $84.82 $64.57 $84.82 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 (granted prior to 2019) Performance Units (granted prior to 2019) 2019 2018 2017 2019 2018 2017 2016 Grant 2015 Grant 2014 Grant 2016 Grant 2015 Grant 2014 Grant Performance awards vested 66,322 90,806 99,438 20,131 31,910 37,395 Percentage of target number of performance awards 142.5% 137.5% 147.5% 142.5% 137.5% 147.5% Aggregate payout value (in millions) $4.3 $5.3 $5.6 $1.3 $1.4 $1.5 Payout - cash (in millions) $3.7 $4.9 $5.1 $1.3 $1.4 $1.5 Payout - common stock shares issued 6,447 5,078 5,185 N/A N/A N/A Restricted Stock Units - Equity Awards - Payouts of 2019 restricted stock units under the Amended and Restated OIP are based on the expiration of a three-year time-vesting period. Restricted stock units granted in 2019 are to be paid out in shares of Alliant Energy common stock and are accounted for as equity awards. The fair value of each restricted stock unit granted in 2019 is based on the closing market price of one share of Alliant Energy common stock on the grant date of the award. Compensation expense is recorded ratably over the performance period based on the fair value of the awards on the grant date. A summary of the 2019 restricted stock units activity was as follows: 2019 Restricted Stock Units - Equity Weighted Average Grant Date Fair Value Nonvested units, January 1 — $— Granted 105,348 45.98 Forfeited (16,067 ) 45.63 Nonvested units, December 31 89,281 46.04 Restricted Stock Units - Liability Awards - Payouts of restricted stock units granted prior to 2019 under the Amended and Restated OIP are based on the expiration of a three-year time-vesting period. Restricted stock units granted prior to 2019 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 granted prior to 2019 in cash; therefore, restricted stock units granted prior to 2019 are accounted for as liability awards. The fair value of each restricted stock unit granted prior to 2019 is based on the closing market price of one share of Alliant Energy 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. A summary of the restricted stock units granted prior to 2019 activity was as follows: 2019 2018 2017 Nonvested units, January 1 174,163 113,749 57,736 Granted — 63,568 56,013 Vested (56,849 ) — — Forfeited (4,281 ) (3,154 ) — Nonvested units, December 31 113,033 174,163 113,749 Performance Restricted Stock Units - Equity Awards - Payouts of performance restricted stock units under the Amended and Restated OIP are based upon achievement of certain performance targets during a three-year performance period, which currently includes specified growth of consolidated net income from continuing operations. 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. The fair value of each performance restricted stock unit is based on the closing market price of one share of Alliant Energy 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: 2019 2018 2017 Units Weighted Average Grant Date Fair Value Units Weighted Average Grant Date Fair Value Units Weighted Average Grant Date Fair Value Nonvested units, January 1 203,188 $37.23 132,705 $36.50 67,355 $33.96 Granted 91,816 46.10 74,163 38.60 65,350 39.12 Vested (66,322 ) 33.93 — — — — Forfeited (22,617 ) 44.00 (3,680 ) 38.60 — — Nonvested units, December 31 206,065 41.50 203,188 37

Asset Retirement Obligations

Asset Retirement Obligations	12 Months Ended
	Dec. 31, 2019
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, above ground storage tanks and solar generation. 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 2019 2018 2019 2018 2019 2018 Balance, January 1 $177.5 $184.5 $118.3 $134.1 $59.2 $50.4 Revisions in estimated cash flows (5.5 ) (10.1 ) (6.7 ) (10.1 ) 1.2 — Liabilities settled (8.8 ) (10.4 ) (8.4 ) (9.7 ) (0.4 ) (0.7 ) Liabilities incurred (a) 26.2 7.3 26.2 — — 7.3 Accretion expense 6.9 6.2 4.5 4.0 2.4 2.2 Balance, December 31 $196.3 $177.5 $133.9 $118.3 $62.4 $59.2 (a) In 2019, Alliant Energy and IPL recognized additional AROs related to IPL’s newly constructed Upland Prairie and English Farms wind sites. The increases in AROs resulted in corresponding increases in property, plant and equipment, net on the respective balance sheets.
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, above ground storage tanks and solar generation. 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 2019 2018 2019 2018 2019 2018 Balance, January 1 $177.5 $184.5 $118.3 $134.1 $59.2 $50.4 Revisions in estimated cash flows (5.5 ) (10.1 ) (6.7 ) (10.1 ) 1.2 — Liabilities settled (8.8 ) (10.4 ) (8.4 ) (9.7 ) (0.4 ) (0.7 ) Liabilities incurred (a) 26.2 7.3 26.2 — — 7.3 Accretion expense 6.9 6.2 4.5 4.0 2.4 2.2 Balance, December 31 $196.3 $177.5 $133.9 $118.3 $62.4 $59.2 (a) In 2019, Alliant Energy and IPL recognized additional AROs related to IPL’s newly constructed Upland Prairie and English Farms wind sites. The increases in AROs resulted in corresponding increases in property, plant and equipment, net on the respective balance sheets.
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, above ground storage tanks and solar generation. 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 2019 2018 2019 2018 2019 2018 Balance, January 1 $177.5 $184.5 $118.3 $134.1 $59.2 $50.4 Revisions in estimated cash flows (5.5 ) (10.1 ) (6.7 ) (10.1 ) 1.2 — Liabilities settled (8.8 ) (10.4 ) (8.4 ) (9.7 ) (0.4 ) (0.7 ) Liabilities incurred (a) 26.2 7.3 26.2 — — 7.3 Accretion expense 6.9 6.2 4.5 4.0 2.4 2.2 Balance, December 31 $196.3 $177.5 $133.9 $118.3 $62.4 $59.2 (a) In 2019, Alliant Energy and IPL recognized additional AROs related to IPL’s newly constructed Upland Prairie and English Farms wind sites. The increases in AROs resulted in corresponding increases in property, plant and equipment, net on the respective balance sheets.

Derivative Instruments

Derivative Instruments	12 Months Ended
	Dec. 31, 2019
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, 2019 , 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 11,671 2020 189,287 2020-2026 6,194 2020-2021 5,040 2020-2021 IPL 5,497 2020 102,758 2020-2026 2,700 2020-2021 — WPL 6,174 2020 86,529 2020-2026 3,494 2020-2021 5,040 2020-2021 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 2019 2018 2019 2018 2019 2018 Current derivative assets $15.8 $24.6 $12.1 $16.1 $3.7 $8.5 Non-current derivative assets 11.0 3.7 10.2 1.6 0.8 2.1 Current derivative liabilities 19.0 5.6 8.9 3.1 10.1 2.5 Non-current derivative liabilities 18.6 17.7 8.5 8.1 10.1 9.6 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, 2019 and 2018 , 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, 2019 and 2018 . 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, 2019 , 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 11,671 2020 189,287 2020-2026 6,194 2020-2021 5,040 2020-2021 IPL 5,497 2020 102,758 2020-2026 2,700 2020-2021 — WPL 6,174 2020 86,529 2020-2026 3,494 2020-2021 5,040 2020-2021 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 2019 2018 2019 2018 2019 2018 Current derivative assets $15.8 $24.6 $12.1 $16.1 $3.7 $8.5 Non-current derivative assets 11.0 3.7 10.2 1.6 0.8 2.1 Current derivative liabilities 19.0 5.6 8.9 3.1 10.1 2.5 Non-current derivative liabilities 18.6 17.7 8.5 8.1 10.1 9.6 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, 2019 and 2018 , 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, 2019 and 2018 . 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, 2019 , 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 11,671 2020 189,287 2020-2026 6,194 2020-2021 5,040 2020-2021 IPL 5,497 2020 102,758 2020-2026 2,700 2020-2021 — WPL 6,174 2020 86,529 2020-2026 3,494 2020-2021 5,040 2020-2021 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 2019 2018 2019 2018 2019 2018 Current derivative assets $15.8 $24.6 $12.1 $16.1 $3.7 $8.5 Non-current derivative assets 11.0 3.7 10.2 1.6 0.8 2.1 Current derivative liabilities 19.0 5.6 8.9 3.1 10.1 2.5 Non-current derivative liabilities 18.6 17.7 8.5 8.1 10.1 9.6 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, 2019 and 2018 , 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, 2019 and 2018 . 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, 2019
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 2019 2018 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $26.8 $— $4.8 $22.0 $26.8 $28.3 $— $8.9 $19.4 $28.3 Deferred proceeds 187.7 — — 187.7 187.7 119.4 — — 119.4 119.4 Liabilities and equity: Derivatives 37.6 — 36.8 0.8 37.6 23.3 — 16.1 7.2 23.3 Long-term debt (incl. current maturities) 6,190.2 — 6,917.9 2.0 6,919.9 5,502.8 — 5,858.4 2.4 5,860.8 IPL 2019 2018 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $22.3 $— $2.8 $19.5 $22.3 $17.7 $— $4.0 $13.7 $17.7 Deferred proceeds 187.7 — — 187.7 187.7 119.4 — — 119.4 119.4 Liabilities and equity: Derivatives 17.4 — 16.6 0.8 17.4 11.2 — 6.5 4.7 11.2 Long-term debt (incl. current maturities) 3,147.3 — 3,489.1 — 3,489.1 2,552.3 — 2,691.2 — 2,691.2 WPL 2019 2018 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $4.5 $— $2.0 $2.5 $4.5 $10.6 $— $4.9 $5.7 $10.6 Liabilities and equity: Derivatives 20.2 — 20.2 — 20.2 12.1 — 9.6 2.5 12.1 Long-term debt (incl. current maturities) 1,932.7 — 2,268.2 — 2,268.2 1,834.9 — 2,043.7 — 2,043.7 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 2019 2018 2019 2018 Beginning balance, January 1 $12.2 ($12.2 ) $119.4 $222.1 Total net gains included in changes in net assets (realized/unrealized) 8.2 9.1 — — Transfers out of Level 3 (a) 4.0 16.1 — — Purchases 13.8 26.7 — — Sales (0.2 ) (0.5 ) — — Settlements (b) (16.8 ) (27.0 ) 68.3 (102.7 ) Ending balance, December 31 $21.2 $12.2 $187.7 $119.4 The amount of total net gains for the period included in changes in net assets attributable to the change in unrealized gains relating to assets and liabilities held at December 31 $11.4 $10.7 $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2019 2018 2019 2018 Beginning balance, January 1 $9.0 ($1.4 ) $119.4 $222.1 Total net gains (losses) included in changes in net assets (realized/unrealized) 10.7 (0.5 ) — — Transfers out of Level 3 (a) 2.6 11.0 — — Purchases 9.5 22.5 — — Sales (0.1 ) (0.4 ) — — Settlements (b) (13.0 ) (22.2 ) 68.3 (102.7 ) Ending balance, December 31 $18.7 $9.0 $187.7 $119.4 The amount of total net gains for the period included in changes in net assets attributable to the change in unrealized gains relating to assets and liabilities held at December 31 $12.6 $0.2 $— $— WPL Commodity Contract Derivative Assets and (Liabilities), net 2019 2018 Beginning balance, January 1 $3.2 ($10.8 ) Total net gains (losses) included in changes in net assets (realized/unrealized) (2.5 ) 9.6 Transfers out of Level 3 (a) 1.4 5.1 Purchases 4.3 4.2 Sales (0.1 ) (0.1 ) Settlements (3.8 ) (4.8 ) Ending balance, December 31 $2.5 $3.2 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 ($1.2 ) $10.5 (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 FTR and natural gas commodity contracts categorized as Level 3 was recognized as net derivative assets at December 31 as follows (in millions): Alliant Energy IPL WPL Excluding FTRs FTRs Excluding FTRs FTRs Excluding FTRs FTRs 2019 $14.6 $6.6 $13.6 $5.1 $1.0 $1.5 2018 3.2 9.0 1.8 7.2 1.4 1.8
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 2019 2018 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $26.8 $— $4.8 $22.0 $26.8 $28.3 $— $8.9 $19.4 $28.3 Deferred proceeds 187.7 — — 187.7 187.7 119.4 — — 119.4 119.4 Liabilities and equity: Derivatives 37.6 — 36.8 0.8 37.6 23.3 — 16.1 7.2 23.3 Long-term debt (incl. current maturities) 6,190.2 — 6,917.9 2.0 6,919.9 5,502.8 — 5,858.4 2.4 5,860.8 IPL 2019 2018 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $22.3 $— $2.8 $19.5 $22.3 $17.7 $— $4.0 $13.7 $17.7 Deferred proceeds 187.7 — — 187.7 187.7 119.4 — — 119.4 119.4 Liabilities and equity: Derivatives 17.4 — 16.6 0.8 17.4 11.2 — 6.5 4.7 11.2 Long-term debt (incl. current maturities) 3,147.3 — 3,489.1 — 3,489.1 2,552.3 — 2,691.2 — 2,691.2 WPL 2019 2018 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $4.5 $— $2.0 $2.5 $4.5 $10.6 $— $4.9 $5.7 $10.6 Liabilities and equity: Derivatives 20.2 — 20.2 — 20.2 12.1 — 9.6 2.5 12.1 Long-term debt (incl. current maturities) 1,932.7 — 2,268.2 — 2,268.2 1,834.9 — 2,043.7 — 2,043.7 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 2019 2018 2019 2018 Beginning balance, January 1 $12.2 ($12.2 ) $119.4 $222.1 Total net gains included in changes in net assets (realized/unrealized) 8.2 9.1 — — Transfers out of Level 3 (a) 4.0 16.1 — — Purchases 13.8 26.7 — — Sales (0.2 ) (0.5 ) — — Settlements (b) (16.8 ) (27.0 ) 68.3 (102.7 ) Ending balance, December 31 $21.2 $12.2 $187.7 $119.4 The amount of total net gains for the period included in changes in net assets attributable to the change in unrealized gains relating to assets and liabilities held at December 31 $11.4 $10.7 $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2019 2018 2019 2018 Beginning balance, January 1 $9.0 ($1.4 ) $119.4 $222.1 Total net gains (losses) included in changes in net assets (realized/unrealized) 10.7 (0.5 ) — — Transfers out of Level 3 (a) 2.6 11.0 — — Purchases 9.5 22.5 — — Sales (0.1 ) (0.4 ) — — Settlements (b) (13.0 ) (22.2 ) 68.3 (102.7 ) Ending balance, December 31 $18.7 $9.0 $187.7 $119.4 The amount of total net gains for the period included in changes in net assets attributable to the change in unrealized gains relating to assets and liabilities held at December 31 $12.6 $0.2 $— $— WPL Commodity Contract Derivative Assets and (Liabilities), net 2019 2018 Beginning balance, January 1 $3.2 ($10.8 ) Total net gains (losses) included in changes in net assets (realized/unrealized) (2.5 ) 9.6 Transfers out of Level 3 (a) 1.4 5.1 Purchases 4.3 4.2 Sales (0.1 ) (0.1 ) Settlements (3.8 ) (4.8 ) Ending balance, December 31 $2.5 $3.2 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 ($1.2 ) $10.5 (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 FTR and natural gas commodity contracts categorized as Level 3 was recognized as net derivative assets at December 31 as follows (in millions): Alliant Energy IPL WPL Excluding FTRs FTRs Excluding FTRs FTRs Excluding FTRs FTRs 2019 $14.6 $6.6 $13.6 $5.1 $1.0 $1.5 2018 3.2 9.0 1.8 7.2 1.4 1.8
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 2019 2018 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $26.8 $— $4.8 $22.0 $26.8 $28.3 $— $8.9 $19.4 $28.3 Deferred proceeds 187.7 — — 187.7 187.7 119.4 — — 119.4 119.4 Liabilities and equity: Derivatives 37.6 — 36.8 0.8 37.6 23.3 — 16.1 7.2 23.3 Long-term debt (incl. current maturities) 6,190.2 — 6,917.9 2.0 6,919.9 5,502.8 — 5,858.4 2.4 5,860.8 IPL 2019 2018 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $22.3 $— $2.8 $19.5 $22.3 $17.7 $— $4.0 $13.7 $17.7 Deferred proceeds 187.7 — — 187.7 187.7 119.4 — — 119.4 119.4 Liabilities and equity: Derivatives 17.4 — 16.6 0.8 17.4 11.2 — 6.5 4.7 11.2 Long-term debt (incl. current maturities) 3,147.3 — 3,489.1 — 3,489.1 2,552.3 — 2,691.2 — 2,691.2 WPL 2019 2018 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $4.5 $— $2.0 $2.5 $4.5 $10.6 $— $4.9 $5.7 $10.6 Liabilities and equity: Derivatives 20.2 — 20.2 — 20.2 12.1 — 9.6 2.5 12.1 Long-term debt (incl. current maturities) 1,932.7 — 2,268.2 — 2,268.2 1,834.9 — 2,043.7 — 2,043.7 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 2019 2018 2019 2018 Beginning balance, January 1 $12.2 ($12.2 ) $119.4 $222.1 Total net gains included in changes in net assets (realized/unrealized) 8.2 9.1 — — Transfers out of Level 3 (a) 4.0 16.1 — — Purchases 13.8 26.7 — — Sales (0.2 ) (0.5 ) — — Settlements (b) (16.8 ) (27.0 ) 68.3 (102.7 ) Ending balance, December 31 $21.2 $12.2 $187.7 $119.4 The amount of total net gains for the period included in changes in net assets attributable to the change in unrealized gains relating to assets and liabilities held at December 31 $11.4 $10.7 $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2019 2018 2019 2018 Beginning balance, January 1 $9.0 ($1.4 ) $119.4 $222.1 Total net gains (losses) included in changes in net assets (realized/unrealized) 10.7 (0.5 ) — — Transfers out of Level 3 (a) 2.6 11.0 — — Purchases 9.5 22.5 — — Sales (0.1 ) (0.4 ) — — Settlements (b) (13.0 ) (22.2 ) 68.3 (102.7 ) Ending balance, December 31 $18.7 $9.0 $187.7 $119.4 The amount of total net gains for the period included in changes in net assets attributable to the change in unrealized gains relating to assets and liabilities held at December 31 $12.6 $0.2 $— $— WPL Commodity Contract Derivative Assets and (Liabilities), net 2019 2018 Beginning balance, January 1 $3.2 ($10.8 ) Total net gains (losses) included in changes in net assets (realized/unrealized) (2.5 ) 9.6 Transfers out of Level 3 (a) 1.4 5.1 Purchases 4.3 4.2 Sales (0.1 ) (0.1 ) Settlements (3.8 ) (4.8 ) Ending balance, December 31 $2.5 $3.2 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 ($1.2 ) $10.5 (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 FTR and natural gas commodity contracts categorized as Level 3 was recognized as net derivative assets at December 31 as follows (in millions): Alliant Energy IPL WPL Excluding FTRs FTRs Excluding FTRs FTRs Excluding FTRs FTRs 2019 $14.6 $6.6 $13.6 $5.1 $1.0 $1.5 2018 3.2 9.0 1.8 7.2 1.4 1.8

Commitments And Contingencies

Commitments And Contingencies	12 Months Ended
	Dec. 31, 2019
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 and WPL’s projects include the expansion of wind generation. At December 31, 2019 , Alliant Energy’s, IPL’s and WPL’s minimum future commitments for these projects were $77 million , $60 million and $17 million , respectively. 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, 2019 , the related minimum future commitments were as follows (in millions): Alliant Energy 2020 2021 2022 2023 2024 Thereafter Total Purchased power (a) $111 $— $— $— $— $— $111 Natural gas 257 183 139 109 77 187 952 Coal (b) 75 27 10 10 — — 122 Other (c) 56 13 11 9 7 12 108 $499 $223 $160 $128 $84 $199 $1,293 IPL 2020 2021 2022 2023 2024 Thereafter Total Purchased power (a) $110 $— $— $— $— $— $110 Natural gas 135 101 77 67 55 89 524 Coal (b) 46 21 10 10 — — 87 Other (c) 24 4 4 3 1 10 46 $315 $126 $91 $80 $56 $99 $767 WPL 2020 2021 2022 2023 2024 Thereafter Total Purchased power $1 $— $— $— $— $— $1 Natural gas 122 82 62 42 22 98 428 Coal (b) 29 6 — — — — 35 Other (c) 24 1 1 1 1 1 29 $176 $89 $63 $43 $23 $99 $493 (a) Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. As a result of an amendment to shorten the term of the DAEC PPA, Alliant Energy’s and IPL’s amounts include minimum future commitments related to IPL’s purchase of capacity and the resulting energy from DAEC through September 2020, and do not include the September 2020 buyout payment of $110 million . (b) Corporate Services has historically entered into system-wide coal contracts on behalf of IPL and WPL that include minimum future commitments. Such commitments were assigned to IPL and WPL based on information available as of December 31, 2019 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, 2019 . 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. 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, 2019 , the present value of the abandonment obligations is estimated at $39 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, 2019 and 2018 . Non-utility Wind Farm in Oklahoma - In 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 $82 million as of December 31, 2019 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, 2019 and 2018 . IPL’s Minnesota Electric Distribution Assets - IPL provided indemnifications associated with the 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, 2019 and 2018 . The general terms of the indemnifications provided by IPL included a maximum limit of $17 million and expire in October 2020 . 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, 2019 , 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, 2019 , such amounts for WPL were not material. Alliant Energy IPL Range of estimated future costs $14 - $30 $12 - $25 Current and non-current environmental liabilities 18 15 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 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. 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. At December 31, 2019 , employees covered by collective bargaining agreements represented 54% , 62% and 83% of total employees of Alliant Energy, IPL and WPL, respectively. In August 2020, IPL’s collective bargaining agreement with International Brotherhood of Electrical Workers Local 204 (Cedar Rapids) expires, representing 18% and 46% of total employees of Alliant Energy and IPL, respectively. A group of MISO cooperative and municipal utilities previously filed two complaints with FERC requesting a reduction to the base return on equity used by MISO transmission owners, including ITC and ATC. The first complaint covered the period from November 12, 2013 through February 11, 2015. The second complaint covered the period from February 12, 2015 through May 11, 2016. In 2016, FERC issued an order on the first complaint and established a base return on equity of 10.32% , excluding any incentive adders granted by FERC, effective September 28, 2016. In 2017, IPL and WPL received the refunds for this first complaint period, which were subsequently refunded to their retail and wholesale customers. In November 2019, FERC issued an order on the previously filed two complaints. The order adjusted the base return on equity to 9.88% , excluding any incentive adders granted by FERC, from the 10.32% previously established in the 2016 FERC order. The 9.88% base return on equity established in the November 2019 order is effective for the first complaint period and the period subsequent to September 28, 2016. Additional refunds for the first complaint period and the period subsequent to September 28, 2016 are currently expected to be issued later in 2020. The November 2019 FERC order also dismissed the second complaint, therefore FERC did not direct refunds to be made for that complaint. For the period ending December 31, 2019, Alliant Energy’s share of earnings in ATC Holdings reflects the recent FERC Order including, (1) estimates of the additional reserves for refunds expected for the first complaint period and the period subsequent to September 28, 2016 and (2) reversals of the reserves that were previously established for the second complaint period. As of December 31, 2019, Alliant Energy’s reserves related to both complaints was $5.8 million . Subsequent to the November 2019 FERC order, various rehearing requests were filed, and in January 2020, FERC issued an order providing an open-ended amount of time for FERC to consider these requests. Any changes in FERC’s decision may have an impact on Alliant Energy’s share of ATC’s future earnings.
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 and WPL’s projects include the expansion of wind generation. At December 31, 2019 , Alliant Energy’s, IPL’s and WPL’s minimum future commitments for these projects were $77 million , $60 million and $17 million , respectively. 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, 2019 , the related minimum future commitments were as follows (in millions): Alliant Energy 2020 2021 2022 2023 2024 Thereafter Total Purchased power (a) $111 $— $— $— $— $— $111 Natural gas 257 183 139 109 77 187 952 Coal (b) 75 27 10 10 — — 122 Other (c) 56 13 11 9 7 12 108 $499 $223 $160 $128 $84 $199 $1,293 IPL 2020 2021 2022 2023 2024 Thereafter Total Purchased power (a) $110 $— $— $— $— $— $110 Natural gas 135 101 77 67 55 89 524 Coal (b) 46 21 10 10 — — 87 Other (c) 24 4 4 3 1 10 46 $315 $126 $91 $80 $56 $99 $767 WPL 2020 2021 2022 2023 2024 Thereafter Total Purchased power $1 $— $— $— $— $— $1 Natural gas 122 82 62 42 22 98 428 Coal (b) 29 6 — — — — 35 Other (c) 24 1 1 1 1 1 29 $176 $89 $63 $43 $23 $99 $493 (a) Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. As a result of an amendment to shorten the term of the DAEC PPA, Alliant Energy’s and IPL’s amounts include minimum future commitments related to IPL’s purchase of capacity and the resulting energy from DAEC through September 2020, and do not include the September 2020 buyout payment of $110 million . (b) Corporate Services has historically entered into system-wide coal contracts on behalf of IPL and WPL that include minimum future commitments. Such commitments were assigned to IPL and WPL based on information available as of December 31, 2019 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, 2019 . 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. 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, 2019 , the present value of the abandonment obligations is estimated at $39 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, 2019 and 2018 . Non-utility Wind Farm in Oklahoma - In 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 $82 million as of December 31, 2019 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, 2019 and 2018 . IPL’s Minnesota Electric Distribution Assets - IPL provided indemnifications associated with the 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, 2019 and 2018 . The general terms of the indemnifications provided by IPL included a maximum limit of $17 million and expire in October 2020 . 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, 2019 , 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, 2019 , such amounts for WPL were not material. Alliant Energy IPL Range of estimated future costs $14 - $30 $12 - $25 Current and non-current environmental liabilities 18 15 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 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. 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. At December 31, 2019 , employees covered by collective bargaining agreements represented 54% , 62% and 83% of total employees of Alliant Energy, IPL and WPL, respectively. In August 2020, IPL’s collective bargaining agreement with International Brotherhood of Electrical Workers Local 204 (Cedar Rapids) expires, representing 18% and 46% of total employees of Alliant Energy and IPL, respectively. A group of MISO cooperative and municipal utilities previously filed two complaints with FERC requesting a reduction to the base return on equity used by MISO transmission owners, including ITC and ATC. The first complaint covered the period from November 12, 2013 through February 11, 2015. The second complaint covered the period from February 12, 2015 through May 11, 2016. In 2016, FERC issued an order on the first complaint and established a base return on equity of 10.32% , excluding any incentive adders granted by FERC, effective September 28, 2016. In 2017, IPL and WPL received the refunds for this first complaint period, which were subsequently refunded to their retail and wholesale customers. In November 2019, FERC issued an order on the previously filed two complaints. The order adjusted the base return on equity to 9.88% , excluding any incentive adders granted by FERC, from the 10.32% previously established in the 2016 FERC order. The 9.88% base return on equity established in the November 2019 order is effective for the first complaint period and the period subsequent to September 28, 2016. Additional refunds for the first complaint period and the period subsequent to September 28, 2016 are currently expected to be issued later in 2020. The November 2019 FERC order also dismissed the second complaint, therefore FERC did not direct refunds to be made for that complaint. For the period ending December 31, 2019, Alliant Energy’s share of earnings in ATC Holdings reflects the recent FERC Order including, (1) estimates of the additional reserves for refunds expected for the first complaint period and the period subsequent to September 28, 2016 and (2) reversals of the reserves that were previously established for the second complaint period. As of December 31, 2019, Alliant Energy’s reserves related to both complaints was $5.8 million . Subsequent to the November 2019 FERC order, various rehearing requests were filed, and in January 2020, FERC issued an order providing an open-ended amount of time for FERC to consider these requests. Any changes in FERC’s decision may have an impact on Alliant Energy’s share of ATC’s future earnings.
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 and WPL’s projects include the expansion of wind generation. At December 31, 2019 , Alliant Energy’s, IPL’s and WPL’s minimum future commitments for these projects were $77 million , $60 million and $17 million , respectively. 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, 2019 , the related minimum future commitments were as follows (in millions): Alliant Energy 2020 2021 2022 2023 2024 Thereafter Total Purchased power (a) $111 $— $— $— $— $— $111 Natural gas 257 183 139 109 77 187 952 Coal (b) 75 27 10 10 — — 122 Other (c) 56 13 11 9 7 12 108 $499 $223 $160 $128 $84 $199 $1,293 IPL 2020 2021 2022 2023 2024 Thereafter Total Purchased power (a) $110 $— $— $— $— $— $110 Natural gas 135 101 77 67 55 89 524 Coal (b) 46 21 10 10 — — 87 Other (c) 24 4 4 3 1 10 46 $315 $126 $91 $80 $56 $99 $767 WPL 2020 2021 2022 2023 2024 Thereafter Total Purchased power $1 $— $— $— $— $— $1 Natural gas 122 82 62 42 22 98 428 Coal (b) 29 6 — — — — 35 Other (c) 24 1 1 1 1 1 29 $176 $89 $63 $43 $23 $99 $493 (a) Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. As a result of an amendment to shorten the term of the DAEC PPA, Alliant Energy’s and IPL’s amounts include minimum future commitments related to IPL’s purchase of capacity and the resulting energy from DAEC through September 2020, and do not include the September 2020 buyout payment of $110 million . (b) Corporate Services has historically entered into system-wide coal contracts on behalf of IPL and WPL that include minimum future commitments. Such commitments were assigned to IPL and WPL based on information available as of December 31, 2019 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, 2019 . 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. 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, 2019 , 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, 2019 , such amounts for WPL were not material. Alliant Energy IPL Range of estimated future costs $14 - $30 $12 - $25 Current and non-current environmental liabilities 18 15 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 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. 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. At December 31, 2019 , employees covered by collective bargaining agreements represented 54% , 62% and 83% of total employees of Alliant Energy, IPL and WPL, respectively. In August 2020, IPL’s collective bargaining agreement with International Brotherhood of Electrical Workers Local 204 (Cedar Rapids) expires, representing 18% and 46% of total employees of Alliant Energy and IPL, respectively. A group of MISO cooperative and municipal utilities previously filed two complaints with FERC requesting a reduction to the base return on equity used by MISO transmission owners, including ITC and ATC. The first complaint covered the period from November 12, 2013 through February 11, 2015. The second complaint covered the period from February 12, 2015 through May 11, 2016. In 2016, FERC issued an order on the first complaint and established a base return on equity of 10.32% , excluding any incentive adders granted by FERC, effective September 28, 2016. In 2017, IPL and WPL received the refunds for this first complaint period, which were subsequently refunded to their retail and wholesale customers. In November 2019, FERC issued an order on the previously filed two complaints. The order adjusted the base return on equity to 9.88% , excluding any incentive adders granted by FERC, from the 10.32% previously established in the 2016 FERC order. The 9.88% base return on equity established in the November 2019 order is effective for the first complaint period and the period subsequent to September 28, 2016. Additional refunds for the first complaint period and the period subsequent to September 28, 2016 are currently expected to be issued later in 2020. The November 2019 FERC order also dismissed the second complaint, therefore FERC did not direct refunds to be made for that complaint. For the period ending December 31, 2019, Alliant Energy’s share of earnings in ATC Holdings reflects the recent FERC Order including, (1) estimates of the additional reserves for refunds expected for the first complaint period and the period subsequent to September 28, 2016 and (2) reversals of the reserves that were previously established for the second complaint period. As of December 31, 2019, Alliant Energy’s reserves related to both complaints was $5.8 million . Subsequent to the November 2019 FERC order, various rehearing requests were filed, and in January 2020, FERC issued an order providing an open-ended amount of time for FERC to consider these requests. Any changes in FERC’s decision may have an impact on Alliant Energy’s share of ATC’s future earnings.

Segments Of Business

Segments Of Business	12 Months Ended
	Dec. 31, 2019
Segment Reporting Information [Line Items]
Segments Of Business	SEGMENTS OF BUSINESS Alliant Energy - Alliant Energy’s principal businesses as of December 31, 2019 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 2019 Electric Gas Other Total Parent and Other Consolidated Revenues $3,063.6 $455.2 $46.5 $3,565.3 $82.4 $3,647.7 Depreciation and amortization 512.7 46.4 3.2 562.3 4.9 567.2 Operating income 678.9 69.8 1.3 750.0 27.7 777.7 Interest expense 229.1 43.8 272.9 Equity income from unconsolidated investments, net (1.4 ) — — (1.4 ) (51.6 ) (53.0 ) Income tax expense (benefit) 73.4 (4.7 ) 68.7 Net income attributable to Alliant Energy common shareowners 517.1 40.1 557.2 Total assets 13,659.0 1,268.5 856.5 15,784.0 916.7 16,700.7 Investments in equity method subsidiaries 9.5 — — 9.5 448.8 458.3 Construction and acquisition expenditures 1,438.4 99.7 0.3 1,538.4 101.7 1,640.1 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 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): 2019 Electric Gas Other Total Revenues $1,781.2 $264.2 $44.2 $2,089.6 Depreciation and amortization 294.9 28.6 3.2 326.7 Operating income 359.7 38.9 4.2 402.8 Interest expense 126.9 Income taxes 24.1 Net income available for common stock 284.1 Total assets 8,074.6 733.8 469.1 9,277.5 Construction and acquisition expenditures 963.4 55.9 0.3 1,019.6 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 ) Net income 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 ) Net income 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 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 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): 2019 Electric Gas Other Total Revenues $1,282.4 $191.0 $2.3 $1,475.7 Depreciation and amortization 217.8 17.8 — 235.6 Operating income (loss) 319.2 30.9 (2.9 ) 347.2 Interest expense 102.2 Income taxes 49.3 Net income 233.0 Total assets 5,584.4 534.7 387.4 6,506.5 Construction and acquisition expenditures 475.0 43.8 — 518.8 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 Net income 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 Net income 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
IPL [Member]
Segment Reporting Information [Line Items]
Segments Of Business	SEGMENTS OF BUSINESS Alliant Energy - Alliant Energy’s principal businesses as of December 31, 2019 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 2019 Electric Gas Other Total Parent and Other Consolidated Revenues $3,063.6 $455.2 $46.5 $3,565.3 $82.4 $3,647.7 Depreciation and amortization 512.7 46.4 3.2 562.3 4.9 567.2 Operating income 678.9 69.8 1.3 750.0 27.7 777.7 Interest expense 229.1 43.8 272.9 Equity income from unconsolidated investments, net (1.4 ) — — (1.4 ) (51.6 ) (53.0 ) Income tax expense (benefit) 73.4 (4.7 ) 68.7 Net income attributable to Alliant Energy common shareowners 517.1 40.1 557.2 Total assets 13,659.0 1,268.5 856.5 15,784.0 916.7 16,700.7 Investments in equity method subsidiaries 9.5 — — 9.5 448.8 458.3 Construction and acquisition expenditures 1,438.4 99.7 0.3 1,538.4 101.7 1,640.1 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 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): 2019 Electric Gas Other Total Revenues $1,781.2 $264.2 $44.2 $2,089.6 Depreciation and amortization 294.9 28.6 3.2 326.7 Operating income 359.7 38.9 4.2 402.8 Interest expense 126.9 Income taxes 24.1 Net income available for common stock 284.1 Total assets 8,074.6 733.8 469.1 9,277.5 Construction and acquisition expenditures 963.4 55.9 0.3 1,019.6 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 ) Net income 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 ) Net income 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 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 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): 2019 Electric Gas Other Total Revenues $1,282.4 $191.0 $2.3 $1,475.7 Depreciation and amortization 217.8 17.8 — 235.6 Operating income (loss) 319.2 30.9 (2.9 ) 347.2 Interest expense 102.2 Income taxes 49.3 Net income 233.0 Total assets 5,584.4 534.7 387.4 6,506.5 Construction and acquisition expenditures 475.0 43.8 — 518.8 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 Net income 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 Net income 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
WPL [Member]
Segment Reporting Information [Line Items]
Segments Of Business	SEGMENTS OF BUSINESS Alliant Energy - Alliant Energy’s principal businesses as of December 31, 2019 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 2019 Electric Gas Other Total Parent and Other Consolidated Revenues $3,063.6 $455.2 $46.5 $3,565.3 $82.4 $3,647.7 Depreciation and amortization 512.7 46.4 3.2 562.3 4.9 567.2 Operating income 678.9 69.8 1.3 750.0 27.7 777.7 Interest expense 229.1 43.8 272.9 Equity income from unconsolidated investments, net (1.4 ) — — (1.4 ) (51.6 ) (53.0 ) Income tax expense (benefit) 73.4 (4.7 ) 68.7 Net income attributable to Alliant Energy common shareowners 517.1 40.1 557.2 Total assets 13,659.0 1,268.5 856.5 15,784.0 916.7 16,700.7 Investments in equity method subsidiaries 9.5 — — 9.5 448.8 458.3 Construction and acquisition expenditures 1,438.4 99.7 0.3 1,538.4 101.7 1,640.1 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 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): 2019 Electric Gas Other Total Revenues $1,781.2 $264.2 $44.2 $2,089.6 Depreciation and amortization 294.9 28.6 3.2 326.7 Operating income 359.7 38.9 4.2 402.8 Interest expense 126.9 Income taxes 24.1 Net income available for common stock 284.1 Total assets 8,074.6 733.8 469.1 9,277.5 Construction and acquisition expenditures 963.4 55.9 0.3 1,019.6 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 ) Net income 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 ) Net income 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 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 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): 2019 Electric Gas Other Total Revenues $1,282.4 $191.0 $2.3 $1,475.7 Depreciation and amortization 217.8 17.8 — 235.6 Operating income (loss) 319.2 30.9 (2.9 ) 347.2 Interest expense 102.2 Income taxes 49.3 Net income 233.0 Total assets 5,584.4 534.7 387.4 6,506.5 Construction and acquisition expenditures 475.0 43.8 — 518.8 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 Net income 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 Net income 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

Related Parties

Related Parties	12 Months Ended
	Dec. 31, 2019
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 2019 2018 2017 2019 2018 2017 Corporate Services billings $185 $170 $177 $142 $132 $135 Sales credited 68 48 23 7 28 13 Purchases billed 331 358 364 120 81 115 As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2019 2018 IPL $112 $95 WPL 85 71 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): 2019 2018 2017 ATC billings to WPL $109 $106 $105 WPL billings to ATC 13 11 10 As of December 31, 2019 and 2018 , WPL owed ATC net amounts of $9 million and $8 million , respectively. WPL’s Sheboygan Falls Energy Facility Lease - Refer to Note 10 for discussion of WPL’s Sheboygan Falls Energy Facility lease.
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 2019 2018 2017 2019 2018 2017 Corporate Services billings $185 $170 $177 $142 $132 $135 Sales credited 68 48 23 7 28 13 Purchases billed 331 358 364 120 81 115 As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2019 2018 IPL $112 $95 WPL 85 71
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 2019 2018 2017 2019 2018 2017 Corporate Services billings $185 $170 $177 $142 $132 $135 Sales credited 68 48 23 7 28 13 Purchases billed 331 358 364 120 81 115 As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2019 2018 IPL $112 $95 WPL 85 71 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): 2019 2018 2017 ATC billings to WPL $109 $106 $105 WPL billings to ATC 13 11 10 As of December 31, 2019 and 2018 , WPL owed ATC net amounts of $9 million and $8 million , respectively. WPL’s Sheboygan Falls Energy Facility Lease - Refer to Note 10 for discussion of WPL’s Sheboygan Falls Energy Facility lease.

Selected Consolidated Quarterly

Selected Consolidated Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2019
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. 2019 2018 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions, except per share data) Revenues $987.2 $790.2 $990.2 $880.1 $916.3 $816.1 $928.6 $873.5 Operating income 176.8 149.8 290.2 160.9 165.7 151.2 256.1 121.4 Net income attributable to Alliant Energy common shareowners 125.1 94.6 226.0 111.5 120.9 100.4 205.5 85.3 Earnings per weighted average common share attributable to Alliant Energy common shareowners 0.53 0.40 0.94 0.46 0.52 0.43 0.87 0.36 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. 2019 2018 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Revenues $555.1 $441.2 $596.5 $496.8 $525.8 $474.8 $547.6 $494.1 Operating income 73.5 74.5 173.5 81.3 75.6 77.7 143.1 54.4 Net income 55.9 47.5 143.7 47.2 49.3 54.2 129.1 41.6 Net income available for common stock 53.3 45.0 141.1 44.7 46.7 51.7 126.5 39.1 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. 2019 2018 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Revenues $415.2 $326.1 $372.1 $362.3 $381.7 $330.8 $370.7 $369.4 Operating income 98.1 67.0 108.3 73.8 84.0 63.4 104.2 61.3 Net income 65.7 42.0 75.5 49.8 54.0 39.8 76.3 38.0
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. 2019 2018 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions, except per share data) Revenues $987.2 $790.2 $990.2 $880.1 $916.3 $816.1 $928.6 $873.5 Operating income 176.8 149.8 290.2 160.9 165.7 151.2 256.1 121.4 Net income attributable to Alliant Energy common shareowners 125.1 94.6 226.0 111.5 120.9 100.4 205.5 85.3 Earnings per weighted average common share attributable to Alliant Energy common shareowners 0.53 0.40 0.94 0.46 0.52 0.43 0.87 0.36 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. 2019 2018 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Revenues $555.1 $441.2 $596.5 $496.8 $525.8 $474.8 $547.6 $494.1 Operating income 73.5 74.5 173.5 81.3 75.6 77.7 143.1 54.4 Net income 55.9 47.5 143.7 47.2 49.3 54.2 129.1 41.6 Net income available for common stock 53.3 45.0 141.1 44.7 46.7 51.7 126.5 39.1 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. 2019 2018 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Revenues $415.2 $326.1 $372.1 $362.3 $381.7 $330.8 $370.7 $369.4 Operating income 98.1 67.0 108.3 73.8 84.0 63.4 104.2 61.3 Net income 65.7 42.0 75.5 49.8 54.0 39.8 76.3 38.0
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. 2019 2018 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions, except per share data) Revenues $987.2 $790.2 $990.2 $880.1 $916.3 $816.1 $928.6 $873.5 Operating income 176.8 149.8 290.2 160.9 165.7 151.2 256.1 121.4 Net income attributable to Alliant Energy common shareowners 125.1 94.6 226.0 111.5 120.9 100.4 205.5 85.3 Earnings per weighted average common share attributable to Alliant Energy common shareowners 0.53 0.40 0.94 0.46 0.52 0.43 0.87 0.36 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. 2019 2018 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Revenues $555.1 $441.2 $596.5 $496.8 $525.8 $474.8 $547.6 $494.1 Operating income 73.5 74.5 173.5 81.3 75.6 77.7 143.1 54.4 Net income 55.9 47.5 143.7 47.2 49.3 54.2 129.1 41.6 Net income available for common stock 53.3 45.0 141.1 44.7 46.7 51.7 126.5 39.1 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. 2019 2018 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Revenues $415.2 $326.1 $372.1 $362.3 $381.7 $330.8 $370.7 $369.4 Operating income 98.1 67.0 108.3 73.8 84.0 63.4 104.2 61.3 Net income 65.7 42.0 75.5 49.8 54.0 39.8 76.3 38.0

Condensed Parent Company Financ

Condensed Parent Company Financial Statements	12 Months Ended
	Dec. 31, 2019
Condensed Financial Information 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 2019 2018 2017 (in millions) Operating expenses $2 $5 $2 Operating loss (2 ) (5 ) (2 ) Other (income) and deductions: Equity earnings from consolidated subsidiaries (562 ) (523 ) (457 ) Interest expense 9 4 3 Other 7 2 — Total other (income) and deductions (546 ) (517 ) (454 ) Income before income taxes 544 512 452 Income tax benefit (15 ) (1 ) (6 ) Net income $559 $513 $458 Refer to accompanying Notes to Condensed Financial Statements. ALLIANT ENERGY CORPORATION (Parent Company Only) December 31, CONDENSED BALANCE SHEETS 2019 2018 (in millions) ASSETS Current assets: Notes receivable from affiliated companies $27 $23 Other 6 4 Total current assets 33 27 Investments: Investments in consolidated subsidiaries 6,017 5,518 Other 1 1 Total investments 6,018 5,519 Other assets 89 81 Total assets $6,140 $5,627 LIABILITIES AND EQUITY Current liabilities: Commercial paper $169 $285 Notes payable to affiliated companies 731 719 Other 19 21 Total current liabilities 919 1,025 Other liabilities 15 17 Common equity: Common stock and additional paid-in capital 2,448 2,048 Retained earnings 2,766 2,545 Accumulated other comprehensive income 2 2 Shares in deferred compensation trust (10 ) (10 ) Total common equity 5,206 4,585 Total liabilities and equity $6,140 $5,627 Refer to accompanying Notes to Condensed Financial Statements. ALLIANT ENERGY CORPORATION (Parent Company Only) Year Ended December 31, CONDENSED STATEMENTS OF CASH FLOWS 2019 2018 2017 (in millions) Net cash flows from operating activities $305 $311 $273 Cash flows used for investing activities: Capital contributions to consolidated subsidiaries (250 ) (625 ) (290 ) Net change in notes receivable from and payable to affiliates 8 441 54 Net cash flows used for investing activities (242 ) (184 ) (236 ) Cash flows used for financing activities: Common stock dividends (338 ) (312 ) (288 ) Proceeds from issuance of common stock, net 390 197 150 Net change in commercial paper (116 ) (10 ) 103 Other 1 (2 ) (2 ) Net cash flows used for financing activities (63 ) (127 ) (37 ) 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 ($9 ) ($4 ) ($3 ) Income taxes, net 14 5 — 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 2019 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, 2019
SEC Schedule, 12-09, 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, 2019 $10.5 $17.3 $2.4 $22.9 $7.3 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 IPL (c) Year ended December 31, 2019 $3.1 $16.5 $— $18.4 $1.2 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 WPL Year ended December 31, 2019 $7.4 $0.8 $2.4 $4.5 $6.1 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 Note: The above provisions relate to various customer, notes and other receivable balances included in various line items on the respective balance sheets. (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.
IPL [Member]
SEC Schedule, 12-09, 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, 2019 $10.5 $17.3 $2.4 $22.9 $7.3 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 IPL (c) Year ended December 31, 2019 $3.1 $16.5 $— $18.4 $1.2 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 WPL Year ended December 31, 2019 $7.4 $0.8 $2.4 $4.5 $6.1 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 Note: The above provisions relate to various customer, notes and other receivable balances included in various line items on the respective balance sheets. (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.
WPL [Member]
SEC Schedule, 12-09, 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, 2019 $10.5 $17.3 $2.4 $22.9 $7.3 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 IPL (c) Year ended December 31, 2019 $3.1 $16.5 $— $18.4 $1.2 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 WPL Year ended December 31, 2019 $7.4 $0.8 $2.4 $4.5 $6.1 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 Note: The above provisions relate to various customer, notes and other receivable balances included in various line items on the respective balance sheets. (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.

Summary Of Significant Accoun_2

Summary Of Significant Accounting Policies (Policy)	12 Months Ended
	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
General, Basis of Presentation	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 income subsequent to the sale. 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 that Alliant Energy and WPL do not control 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.
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.
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 alternative minimum tax and allows unutilized alternative minimum tax 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 2019 , 2018 and 2017 .
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, 2019 and 2018 , restricted cash primarily related to requirements in Sheboygan Power, LLC’s debt agreement.
Property, Plant and Equipment	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. 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 2019 2018 2017 2019 2018 2017 Electric - generation 3.8% 3.6% 3.5% 3.6% 3.6% 3.5% Electric - distribution 2.9% 2.8% 2.4% 2.6% 2.6% 2.6% Electric - other 5.3% 4.7% 4.5% 5.8% 5.7% 6.9% Gas 3.3% 3.2% 3.4% 2.5% 2.5% 2.5% Other 5.9% 5.2% 4.0% 5.6% 5.8% 6.0%
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: 2019 2018 2017 IPL (Marshalltown CWIP) N/A N/A 7.8% IPL (Wind generation CWIP) 7.4% 7.5% 7.6% IPL (other CWIP) 7.5% 7.5% 7.6% WPL (retail jurisdiction) 6.8% 7.7% 7.6% WPL (wholesale jurisdiction) 6.9% 7.2% 6.0% 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, 2019 , 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 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. Renewable Energy Rider - Effective with the implementation of final rates covering the 2020 forward-looking Test Period, IPL will recover a return of, as well as earn a return on, its 1,000 MW of new wind generation placed in service in 2019 and 2020 from its retail electric customers through a renewable energy rider. Other applicable costs and tax benefits associated with the new wind generation, excluding operation and maintenance expenses, will also be included in the rider. This cost recovery mechanism provides for annual adjustments to electric rates charged to IPL’s retail electric customers for actual renewable energy costs and tax benefits. Changes in the under-/over-collection of these costs will be recognized in “Electric utility revenue” in Alliant Energy’s and IPL’s income statements. The cumulative effects of the under-/over-collection of these costs for IPL will be 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.
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 or when sufficient information becomes available to determine a reasonable estimate of the fair value of future retirement costs. When an ARO is recorded as a liability, an equivalent amount is added to the asset cost. The fair value of AROs at inception is determined using discounted cash flows analyses. The liability is accreted to its present value and the capitalized cost is depreciated over the useful life of the related asset. Accretion and depreciation expenses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory assets on the balance sheets. Upon regulatory approval to recover IPL’s AROs expenditures, its regulatory assets are amortized to depreciation and amortization expenses in Alliant Energy’s and IPL’s income statements over the same time period the ARO expenditures are recovered from IPL’s customers. WPL’s regulatory assets related to AROs are being recovered as a component of depreciation rates pursuant to PSCW and FERC orders. Accretion and depreciation expenses related to AROs for Alliant Energy’s non-utility operations are recorded to depreciation and amortization expenses in Alliant Energy’s income statements. Upon settlement of the ARO liability, an entity settles the obligation for its recorded amount or incurs a gain or loss. Any gains or losses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory liabilities or regulatory assets on the balance sheets.
Debt Issuance and Retirement Costs	Debt issuance costs and debt premiums or discounts are presented on the balance sheet as a direct 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.
Leases	The determination of whether an arrangement qualifies as a lease occurs at the inception of the arrangement. Arrangements that qualify as leases are classified as either operating or finance. Operating and finance lease liabilities represent obligations to make payments arising from the lease. Operating and finance lease assets represent the right to use an underlying asset for the lease term and are recognized at the lease commencement date based on the present value of the lease payments over the lease term. Leases with initial terms less than 12 months are not recognized as leases. For operating leases, an incremental borrowing rate, as determined at the lease commencement date, is used to determine the present value of the lease payments. For finance leases, the rate implicit in the lease, if known, is used to determine the present value of the lease payments. Lease terms include options to extend or terminate the lease when it is reasonably certain that the option will be exercised. Operating lease expense is recognized on a straight-line basis over the expected lease term. Finance lease expense is comprised of depreciation and interest expenses. Finance lease assets are depreciated on a straight-line basis over the shorter of the useful life of the underlying asset or the lease term.
New Accounting Standards	Credit Losses - In June 2016, the FASB issued an accounting standard requiring use of a current expected credit loss model rather than an incurred loss method, which is intended to result in more timely recognition of credit losses on trade receivables and certain other assets. Alliant Energy, IPL and WPL adopted this standard on January 1, 2020 using a modified retrospective method of adoption, which requires cumulative effect adjustments to retained earnings on January 1, 2020. IPL and WPL did not record a cumulative effect adjustment to retained earnings and do not expect a material change to their financial condition or results of operations as a result of adopting this standard. Alliant Energy’s non-regulated entities continue to evaluate the final adoption impact for credit loss exposures related to their existing guarantees (described in Note 17(d) ); however, Alliant Energy does not expect a material change in its financial condition or results of operations as a result of adopting this standard. Cloud Computing Arrangements - In August 2018, the FASB issued an accounting standard that clarifies capitalization and presentation requirements of implementation costs incurred in cloud computing arrangements. Alliant Energy, IPL and WPL adopted this standard on January 1, 2020 and do not expect a material change to their financial condition or results of operations as a result of adopting 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. The accounting for capital leases, referred to as finance leases, remains unchanged with the adoption of this standard. 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 the balance sheets as of January 1, 2019. 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. Upon transition to the new standard, Alliant Energy, IPL and WPL elected the land easement transition practical expedient, which does not require existing land easements that were not previously accounted for as leases under the original accounting standards to be reassessed under the new 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, which under the new standard, do not meet the criteria for leases. Therefore, these land easement arrangements are no longer reflected as operating leases effective January 1, 2019. Refer to Note 10 for further discussion of leases.
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 that Alliant Energy and WPL do not control 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.
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.
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 alternative minimum tax and allows unutilized alternative minimum tax 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 2019 , 2018 and 2017 .
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, 2019 and 2018 , restricted cash primarily related to 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 2019 2018 2017 2019 2018 2017 Electric - generation 3.8% 3.6% 3.5% 3.6% 3.6% 3.5% Electric - distribution 2.9% 2.8% 2.4% 2.6% 2.6% 2.6% Electric - other 5.3% 4.7% 4.5% 5.8% 5.7% 6.9% Gas 3.3% 3.2% 3.4% 2.5% 2.5% 2.5% Other 5.9% 5.2% 4.0% 5.6% 5.8% 6.0%
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: 2019 2018 2017 IPL (Marshalltown CWIP) N/A N/A 7.8% IPL (Wind generation CWIP) 7.4% 7.5% 7.6% IPL (other CWIP) 7.5% 7.5% 7.6% WPL (retail jurisdiction) 6.8% 7.7% 7.6% WPL (wholesale jurisdiction) 6.9% 7.2% 6.0% 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, 2019 , 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 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. Renewable Energy Rider - Effective with the implementation of final rates covering the 2020 forward-looking Test Period, IPL will recover a return of, as well as earn a return on, its 1,000 MW of new wind generation placed in service in 2019 and 2020 from its retail electric customers through a renewable energy rider. Other applicable costs and tax benefits associated with the new wind generation, excluding operation and maintenance expenses, will also be included in the rider. This cost recovery mechanism provides for annual adjustments to electric rates charged to IPL’s retail electric customers for actual renewable energy costs and tax benefits. Changes in the under-/over-collection of these costs will be recognized in “Electric utility revenue” in Alliant Energy’s and IPL’s income statements. The cumulative effects of the under-/over-collection of these costs for IPL will be 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.
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 or when sufficient information becomes available to determine a reasonable estimate of the fair value of future retirement costs. When an ARO is recorded as a liability, an equivalent amount is added to the asset cost. The fair value of AROs at inception is determined using discounted cash flows analyses. The liability is accreted to its present value and the capitalized cost is depreciated over the useful life of the related asset. Accretion and depreciation expenses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory assets on the balance sheets. Upon regulatory approval to recover IPL’s AROs expenditures, its regulatory assets are amortized to depreciation and amortization expenses in Alliant Energy’s and IPL’s income statements over the same time period the ARO expenditures are recovered from IPL’s customers. WPL’s regulatory assets related to AROs are being recovered as a component of depreciation rates pursuant to PSCW and FERC orders. Accretion and depreciation expenses related to AROs for Alliant Energy’s non-utility operations are recorded to depreciation and amortization expenses in Alliant Energy’s income statements. Upon settlement of the ARO liability, an entity settles the obligation for its recorded amount or incurs a gain or loss. Any gains or losses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory liabilities or regulatory assets on the balance sheets.
Debt Issuance and Retirement Costs	Debt issuance costs and debt premiums or discounts are presented on the balance sheet as a direct 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.
Leases	The determination of whether an arrangement qualifies as a lease occurs at the inception of the arrangement. Arrangements that qualify as leases are classified as either operating or finance. Operating and finance lease liabilities represent obligations to make payments arising from the lease. Operating and finance lease assets represent the right to use an underlying asset for the lease term and are recognized at the lease commencement date based on the present value of the lease payments over the lease term. Leases with initial terms less than 12 months are not recognized as leases. For operating leases, an incremental borrowing rate, as determined at the lease commencement date, is used to determine the present value of the lease payments. For finance leases, the rate implicit in the lease, if known, is used to determine the present value of the lease payments. Lease terms include options to extend or terminate the lease when it is reasonably certain that the option will be exercised. Operating lease expense is recognized on a straight-line basis over the expected lease term. Finance lease expense is comprised of depreciation and interest expenses. Finance lease assets are depreciated on a straight-line basis over the shorter of the useful life of the underlying asset or the lease term.
New Accounting Standards	Credit Losses - In June 2016, the FASB issued an accounting standard requiring use of a current expected credit loss model rather than an incurred loss method, which is intended to result in more timely recognition of credit losses on trade receivables and certain other assets. Alliant Energy, IPL and WPL adopted this standard on January 1, 2020 using a modified retrospective method of adoption, which requires cumulative effect adjustments to retained earnings on January 1, 2020. IPL and WPL did not record a cumulative effect adjustment to retained earnings and do not expect a material change to their financial condition or results of operations as a result of adopting this standard. Alliant Energy’s non-regulated entities continue to evaluate the final adoption impact for credit loss exposures related to their existing guarantees (described in Note 17(d) ); however, Alliant Energy does not expect a material change in its financial condition or results of operations as a result of adopting this standard. Cloud Computing Arrangements - In August 2018, the FASB issued an accounting standard that clarifies capitalization and presentation requirements of implementation costs incurred in cloud computing arrangements. Alliant Energy, IPL and WPL adopted this standard on January 1, 2020 and do not expect a material change to their financial condition or results of operations as a result of adopting 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. The accounting for capital leases, referred to as finance leases, remains unchanged with the adoption of this standard. 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 the balance sheets as of January 1, 2019. 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. Upon transition to the new standard, Alliant Energy, IPL and WPL elected the land easement transition practical expedient, which does not require existing land easements that were not previously accounted for as leases under the original accounting standards to be reassessed under the new 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, which under the new standard, do not meet the criteria for leases. Therefore, these land easement arrangements are no longer reflected as operating leases effective January 1, 2019. Refer to Note 10 for further discussion of leases.
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 that Alliant Energy and WPL do not control 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.
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.
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 alternative minimum tax and allows unutilized alternative minimum tax 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 2019 , 2018 and 2017 .
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, 2019 and 2018 , restricted cash primarily related to 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 2019 2018 2017 2019 2018 2017 Electric - generation 3.8% 3.6% 3.5% 3.6% 3.6% 3.5% Electric - distribution 2.9% 2.8% 2.4% 2.6% 2.6% 2.6% Electric - other 5.3% 4.7% 4.5% 5.8% 5.7% 6.9% Gas 3.3% 3.2% 3.4% 2.5% 2.5% 2.5% Other 5.9% 5.2% 4.0% 5.6% 5.8% 6.0%
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: 2019 2018 2017 IPL (Marshalltown CWIP) N/A N/A 7.8% IPL (Wind generation CWIP) 7.4% 7.5% 7.6% IPL (other CWIP) 7.5% 7.5% 7.6% WPL (retail jurisdiction) 6.8% 7.7% 7.6% WPL (wholesale jurisdiction) 6.9% 7.2% 6.0% 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, 2019 , 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 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. Renewable Energy Rider - Effective with the implementation of final rates covering the 2020 forward-looking Test Period, IPL will recover a return of, as well as earn a return on, its 1,000 MW of new wind generation placed in service in 2019 and 2020 from its retail electric customers through a renewable energy rider. Other applicable costs and tax benefits associated with the new wind generation, excluding operation and maintenance expenses, will also be included in the rider. This cost recovery mechanism provides for annual adjustments to electric rates charged to IPL’s retail electric customers for actual renewable energy costs and tax benefits. Changes in the under-/over-collection of these costs will be recognized in “Electric utility revenue” in Alliant Energy’s and IPL’s income statements. The cumulative effects of the under-/over-collection of these costs for IPL will be 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.
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 or when sufficient information becomes available to determine a reasonable estimate of the fair value of future retirement costs. When an ARO is recorded as a liability, an equivalent amount is added to the asset cost. The fair value of AROs at inception is determined using discounted cash flows analyses. The liability is accreted to its present value and the capitalized cost is depreciated over the useful life of the related asset. Accretion and depreciation expenses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory assets on the balance sheets. Upon regulatory approval to recover IPL’s AROs expenditures, its regulatory assets are amortized to depreciation and amortization expenses in Alliant Energy’s and IPL’s income statements over the same time period the ARO expenditures are recovered from IPL’s customers. WPL’s regulatory assets related to AROs are being recovered as a component of depreciation rates pursuant to PSCW and FERC orders. Accretion and depreciation expenses related to AROs for Alliant Energy’s non-utility operations are recorded to depreciation and amortization expenses in Alliant Energy’s income statements. Upon settlement of the ARO liability, an entity settles the obligation for its recorded amount or incurs a gain or loss. Any gains or losses related to AROs for IPL’s and WPL’s regulated operations are recorded to regulatory liabilities or regulatory assets on the balance sheets.
Debt Issuance and Retirement Costs	Debt issuance costs and debt premiums or discounts are presented on the balance sheet as a direct 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.
Leases	The determination of whether an arrangement qualifies as a lease occurs at the inception of the arrangement. Arrangements that qualify as leases are classified as either operating or finance. Operating and finance lease liabilities represent obligations to make payments arising from the lease. Operating and finance lease assets represent the right to use an underlying asset for the lease term and are recognized at the lease commencement date based on the present value of the lease payments over the lease term. Leases with initial terms less than 12 months are not recognized as leases. For operating leases, an incremental borrowing rate, as determined at the lease commencement date, is used to determine the present value of the lease payments. For finance leases, the rate implicit in the lease, if known, is used to determine the present value of the lease payments. Lease terms include options to extend or terminate the lease when it is reasonably certain that the option will be exercised. Operating lease expense is recognized on a straight-line basis over the expected lease term. Finance lease expense is comprised of depreciation and interest expenses. Finance lease assets are depreciated on a straight-line basis over the shorter of the useful life of the underlying asset or the lease term.
New Accounting Standards	Credit Losses - In June 2016, the FASB issued an accounting standard requiring use of a current expected credit loss model rather than an incurred loss method, which is intended to result in more timely recognition of credit losses on trade receivables and certain other assets. Alliant Energy, IPL and WPL adopted this standard on January 1, 2020 using a modified retrospective method of adoption, which requires cumulative effect adjustments to retained earnings on January 1, 2020. IPL and WPL did not record a cumulative effect adjustment to retained earnings and do not expect a material change to their financial condition or results of operations as a result of adopting this standard. Alliant Energy’s non-regulated entities continue to evaluate the final adoption impact for credit loss exposures related to their existing guarantees (described in Note 17(d) ); however, Alliant Energy does not expect a material change in its financial condition or results of operations as a result of adopting this standard. Cloud Computing Arrangements - In August 2018, the FASB issued an accounting standard that clarifies capitalization and presentation requirements of implementation costs incurred in cloud computing arrangements. Alliant Energy, IPL and WPL adopted this standard on January 1, 2020 and do not expect a material change to their financial condition or results of operations as a result of adopting 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. The accounting for capital leases, referred to as finance leases, remains unchanged with the adoption of this standard. 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 the balance sheets as of January 1, 2019. 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. Upon transition to the new standard, Alliant Energy, IPL and WPL elected the land easement transition practical expedient, which does not require existing land easements that were not previously accounted for as leases under the original accounting standards to be reassessed under the new 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, which under the new standard, do not meet the criteria for leases. Therefore, these land easement arrangements are no longer reflected as operating leases effective January 1, 2019. Refer to Note 10 for further discussion of leases.

Summary Of Significant Accoun_3

Summary Of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2019
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 2019 2018 2017 2019 2018 2017 Electric - generation 3.8% 3.6% 3.5% 3.6% 3.6% 3.5% Electric - distribution 2.9% 2.8% 2.4% 2.6% 2.6% 2.6% Electric - other 5.3% 4.7% 4.5% 5.8% 5.7% 6.9% Gas 3.3% 3.2% 3.4% 2.5% 2.5% 2.5% Other 5.9% 5.2% 4.0% 5.6% 5.8% 6.0%
Schedule of Allowance for Funds Used During Construction Rate	The AFUDC rates, computed in accordance with the prescribed regulatory formula, were as follows: 2019 2018 2017 IPL (Marshalltown CWIP) N/A N/A 7.8% IPL (Wind generation CWIP) 7.4% 7.5% 7.6% IPL (other CWIP) 7.5% 7.5% 7.6% WPL (retail jurisdiction) 6.8% 7.7% 7.6% WPL (wholesale jurisdiction) 6.9% 7.2% 6.0%
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 2019 2018 2017 2019 2018 2017 Electric - generation 3.8% 3.6% 3.5% 3.6% 3.6% 3.5% Electric - distribution 2.9% 2.8% 2.4% 2.6% 2.6% 2.6% Electric - other 5.3% 4.7% 4.5% 5.8% 5.7% 6.9% Gas 3.3% 3.2% 3.4% 2.5% 2.5% 2.5% Other 5.9% 5.2% 4.0% 5.6% 5.8% 6.0%
Schedule of Allowance for Funds Used During Construction Rate	The AFUDC rates, computed in accordance with the prescribed regulatory formula, were as follows: 2019 2018 2017 IPL (Marshalltown CWIP) N/A N/A 7.8% IPL (Wind generation CWIP) 7.4% 7.5% 7.6% IPL (other CWIP) 7.5% 7.5% 7.6% WPL (retail jurisdiction) 6.8% 7.7% 7.6% WPL (wholesale jurisdiction) 6.9% 7.2% 6.0%
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 2019 2018 2017 2019 2018 2017 Electric - generation 3.8% 3.6% 3.5% 3.6% 3.6% 3.5% Electric - distribution 2.9% 2.8% 2.4% 2.6% 2.6% 2.6% Electric - other 5.3% 4.7% 4.5% 5.8% 5.7% 6.9% Gas 3.3% 3.2% 3.4% 2.5% 2.5% 2.5% Other 5.9% 5.2% 4.0% 5.6% 5.8% 6.0%
Schedule of Allowance for Funds Used During Construction Rate	The AFUDC rates, computed in accordance with the prescribed regulatory formula, were as follows: 2019 2018 2017 IPL (Marshalltown CWIP) N/A N/A 7.8% IPL (Wind generation CWIP) 7.4% 7.5% 7.6% IPL (other CWIP) 7.5% 7.5% 7.6% WPL (retail jurisdiction) 6.8% 7.7% 7.6% WPL (wholesale jurisdiction) 6.9% 7.2% 6.0%

Regulatory Matters (Tables)

Regulatory Matters (Tables)	12 Months Ended
	Dec. 31, 2019
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 2019 2018 2019 2018 2019 2018 Tax-related $817.6 $820.6 $776.8 $783.1 $40.8 $37.5 Pension and OPEB costs 524.0 542.3 262.5 274.0 261.5 268.3 Assets retired early 134.0 111.6 87.9 55.4 46.1 56.2 AROs 111.8 110.8 76.2 76.3 35.6 34.5 IPL’s DAEC PPA amendment 108.2 — 108.2 — — — Derivatives 39.5 28.0 18.3 15.1 21.2 12.9 Emission allowances 21.1 23.6 21.1 23.6 — — Other 88.5 100.4 48.3 51.5 40.2 48.9 $1,844.7 $1,737.3 $1,399.3 $1,279.0 $445.4 $458.3
Electric Generating Units Retired Early	Details regarding the recovery of the remaining net book value of these assets from IPL’s and WPL’s customers are as follows: Entity Asset Retirement Date Regulatory Asset Balance as of Dec. 31, 2019 Recovery Regulatory Approval IPL Sutherland Units 1 and 3 2017 $28.5 Return of and return on remaining net book value over 10 years (return on effective with new rates expected to be implemented by the end of the first quarter of 2020) IUB and FERC IPL M.L. Kapp Unit 2 2018 23.6 Return of and return on remaining net book value over 10 years IUB and FERC IPL Analog electric meters 2019 35.8 Return of remaining net book value over 10 years (effective with new rates expected to be implemented by the end of the first quarter of 2020) IUB WPL Nelson Dewey Units 1 and 2 and Edgewater Unit 3 2015 20.2 Return of and return on remaining net book value over 10 years PSCW and FERC WPL Edgewater Unit 4 2018 25.9 Return of and return on remaining net book value over 10 years PSCW and FERC
Schedule of Regulatory Liabilities	At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2019 2018 2019 2018 2019 2018 Tax-related $835.6 $890.6 $350.9 $390.1 $484.7 $500.5 Cost of removal obligations 387.7 401.2 257.0 273.3 130.7 127.9 Electric transmission cost recovery 88.6 104.0 51.3 47.7 37.3 56.3 Commodity cost recovery 24.2 16.8 8.8 11.9 15.4 4.9 WPL’s earnings sharing mechanism 21.9 25.4 — — 21.9 25.4 Derivatives 19.9 18.5 17.4 10.2 2.5 8.3 Other 45.7 36.7 29.3 21.7 16.4 15.0 $1,423.6 $1,493.2 $714.7 $754.9 $708.9 $738.3
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 2019 2018 2019 2018 2019 2018 Tax-related $817.6 $820.6 $776.8 $783.1 $40.8 $37.5 Pension and OPEB costs 524.0 542.3 262.5 274.0 261.5 268.3 Assets retired early 134.0 111.6 87.9 55.4 46.1 56.2 AROs 111.8 110.8 76.2 76.3 35.6 34.5 IPL’s DAEC PPA amendment 108.2 — 108.2 — — — Derivatives 39.5 28.0 18.3 15.1 21.2 12.9 Emission allowances 21.1 23.6 21.1 23.6 — — Other 88.5 100.4 48.3 51.5 40.2 48.9 $1,844.7 $1,737.3 $1,399.3 $1,279.0 $445.4 $458.3
Electric Generating Units Retired Early	Details regarding the recovery of the remaining net book value of these assets from IPL’s and WPL’s customers are as follows: Entity Asset Retirement Date Regulatory Asset Balance as of Dec. 31, 2019 Recovery Regulatory Approval IPL Sutherland Units 1 and 3 2017 $28.5 Return of and return on remaining net book value over 10 years (return on effective with new rates expected to be implemented by the end of the first quarter of 2020) IUB and FERC IPL M.L. Kapp Unit 2 2018 23.6 Return of and return on remaining net book value over 10 years IUB and FERC IPL Analog electric meters 2019 35.8 Return of remaining net book value over 10 years (effective with new rates expected to be implemented by the end of the first quarter of 2020) IUB WPL Nelson Dewey Units 1 and 2 and Edgewater Unit 3 2015 20.2 Return of and return on remaining net book value over 10 years PSCW and FERC WPL Edgewater Unit 4 2018 25.9 Return of and return on remaining net book value over 10 years PSCW and FERC
Schedule of Regulatory Liabilities	At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2019 2018 2019 2018 2019 2018 Tax-related $835.6 $890.6 $350.9 $390.1 $484.7 $500.5 Cost of removal obligations 387.7 401.2 257.0 273.3 130.7 127.9 Electric transmission cost recovery 88.6 104.0 51.3 47.7 37.3 56.3 Commodity cost recovery 24.2 16.8 8.8 11.9 15.4 4.9 WPL’s earnings sharing mechanism 21.9 25.4 — — 21.9 25.4 Derivatives 19.9 18.5 17.4 10.2 2.5 8.3 Other 45.7 36.7 29.3 21.7 16.4 15.0 $1,423.6 $1,493.2 $714.7 $754.9 $708.9 $738.3
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 2019 2018 2019 2018 2019 2018 Tax-related $817.6 $820.6 $776.8 $783.1 $40.8 $37.5 Pension and OPEB costs 524.0 542.3 262.5 274.0 261.5 268.3 Assets retired early 134.0 111.6 87.9 55.4 46.1 56.2 AROs 111.8 110.8 76.2 76.3 35.6 34.5 IPL’s DAEC PPA amendment 108.2 — 108.2 — — — Derivatives 39.5 28.0 18.3 15.1 21.2 12.9 Emission allowances 21.1 23.6 21.1 23.6 — — Other 88.5 100.4 48.3 51.5 40.2 48.9 $1,844.7 $1,737.3 $1,399.3 $1,279.0 $445.4 $458.3
Electric Generating Units Retired Early	Details regarding the recovery of the remaining net book value of these assets from IPL’s and WPL’s customers are as follows: Entity Asset Retirement Date Regulatory Asset Balance as of Dec. 31, 2019 Recovery Regulatory Approval IPL Sutherland Units 1 and 3 2017 $28.5 Return of and return on remaining net book value over 10 years (return on effective with new rates expected to be implemented by the end of the first quarter of 2020) IUB and FERC IPL M.L. Kapp Unit 2 2018 23.6 Return of and return on remaining net book value over 10 years IUB and FERC IPL Analog electric meters 2019 35.8 Return of remaining net book value over 10 years (effective with new rates expected to be implemented by the end of the first quarter of 2020) IUB WPL Nelson Dewey Units 1 and 2 and Edgewater Unit 3 2015 20.2 Return of and return on remaining net book value over 10 years PSCW and FERC WPL Edgewater Unit 4 2018 25.9 Return of and return on remaining net book value over 10 years PSCW and FERC
Schedule of Regulatory Liabilities	At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2019 2018 2019 2018 2019 2018 Tax-related $835.6 $890.6 $350.9 $390.1 $484.7 $500.5 Cost of removal obligations 387.7 401.2 257.0 273.3 130.7 127.9 Electric transmission cost recovery 88.6 104.0 51.3 47.7 37.3 56.3 Commodity cost recovery 24.2 16.8 8.8 11.9 15.4 4.9 WPL’s earnings sharing mechanism 21.9 25.4 — — 21.9 25.4 Derivatives 19.9 18.5 17.4 10.2 2.5 8.3 Other 45.7 36.7 29.3 21.7 16.4 15.0 $1,423.6 $1,493.2 $714.7 $754.9 $708.9 $738.3

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	12 Months Ended
	Dec. 31, 2019
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 2019 2018 2019 2018 2019 2018 Utility: Electric plant: Generation in service $7,625.3 $6,800.6 $4,432.4 $3,610.4 $3,192.9 $3,190.2 Distribution in service 5,783.3 5,452.2 3,190.0 3,023.7 2,593.3 2,428.5 Other in service 456.0 410.8 298.6 260.4 157.4 150.4 Total electric plant 13,864.6 12,663.6 7,921.0 6,894.5 5,943.6 5,769.1 Gas plant in service 1,462.7 1,387.6 801.5 763.1 661.2 624.5 Other plant in service 519.7 513.2 344.7 322.4 175.0 190.8 Accumulated depreciation (4,601.4 ) (4,314.6 ) (2,498.3 ) (2,294.7 ) (2,103.1 ) (2,019.9 ) Net plant 11,245.6 10,249.8 6,568.9 5,685.3 4,676.7 4,564.5 Leased Sheboygan Falls Energy Facility, net (a) — — — — 32.3 38.1 Construction work in progress 1,835.0 1,774.8 906.8 1,091.2 928.2 683.6 Other, net 6.1 6.1 5.0 5.0 1.1 1.1 Total utility 13,086.7 12,030.7 7,480.7 6,781.5 5,638.3 5,287.3 Non-utility and other: Non-utility Generation, net (b) 82.9 86.9 — — — — Corporate Services and other, net (c) 357.5 344.8 — — — — Total non-utility and other 440.4 431.7 — — — — Total property, plant and equipment $13,527.1 $12,462.4 $7,480.7 $6,781.5 $5,638.3 $5,287.3 (a) Less accumulated amortization of $88.7 million and $82.8 million for WPL as of December 31, 2019 and 2018 , 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. (b) Less accumulated depreciation of $58.5 million and $54.5 million for Alliant Energy as of December 31, 2019 and 2018 , respectively. (c) Less accumulated depreciation of $172.4 million and $167.5 million for Alliant Energy as of December 31, 2019 and 2018 , 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 2019 2018 2017 2019 2018 2017 2019 2018 2017 Equity $65.5 $51.4 $33.6 $35.2 $28.6 $21.1 $30.3 $22.8 $12.5 Debt 27.2 24.2 16.1 14.2 13.6 10.3 13.0 10.6 5.8 $92.7 $75.6 $49.7 $49.4 $42.2 $31.4 $43.3 $33.4 $18.3
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 2019 2018 2019 2018 2019 2018 Utility: Electric plant: Generation in service $7,625.3 $6,800.6 $4,432.4 $3,610.4 $3,192.9 $3,190.2 Distribution in service 5,783.3 5,452.2 3,190.0 3,023.7 2,593.3 2,428.5 Other in service 456.0 410.8 298.6 260.4 157.4 150.4 Total electric plant 13,864.6 12,663.6 7,921.0 6,894.5 5,943.6 5,769.1 Gas plant in service 1,462.7 1,387.6 801.5 763.1 661.2 624.5 Other plant in service 519.7 513.2 344.7 322.4 175.0 190.8 Accumulated depreciation (4,601.4 ) (4,314.6 ) (2,498.3 ) (2,294.7 ) (2,103.1 ) (2,019.9 ) Net plant 11,245.6 10,249.8 6,568.9 5,685.3 4,676.7 4,564.5 Leased Sheboygan Falls Energy Facility, net (a) — — — — 32.3 38.1 Construction work in progress 1,835.0 1,774.8 906.8 1,091.2 928.2 683.6 Other, net 6.1 6.1 5.0 5.0 1.1 1.1 Total utility 13,086.7 12,030.7 7,480.7 6,781.5 5,638.3 5,287.3 Non-utility and other: Non-utility Generation, net (b) 82.9 86.9 — — — — Corporate Services and other, net (c) 357.5 344.8 — — — — Total non-utility and other 440.4 431.7 — — — — Total property, plant and equipment $13,527.1 $12,462.4 $7,480.7 $6,781.5 $5,638.3 $5,287.3 (a) Less accumulated amortization of $88.7 million and $82.8 million for WPL as of December 31, 2019 and 2018 , 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. (b) Less accumulated depreciation of $58.5 million and $54.5 million for Alliant Energy as of December 31, 2019 and 2018 , respectively. (c) Less accumulated depreciation of $172.4 million and $167.5 million for Alliant Energy as of December 31, 2019 and 2018 , 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 2019 2018 2017 2019 2018 2017 2019 2018 2017 Equity $65.5 $51.4 $33.6 $35.2 $28.6 $21.1 $30.3 $22.8 $12.5 Debt 27.2 24.2 16.1 14.2 13.6 10.3 13.0 10.6 5.8 $92.7 $75.6 $49.7 $49.4 $42.2 $31.4 $43.3 $33.4 $18.3
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 2019 2018 2019 2018 2019 2018 Utility: Electric plant: Generation in service $7,625.3 $6,800.6 $4,432.4 $3,610.4 $3,192.9 $3,190.2 Distribution in service 5,783.3 5,452.2 3,190.0 3,023.7 2,593.3 2,428.5 Other in service 456.0 410.8 298.6 260.4 157.4 150.4 Total electric plant 13,864.6 12,663.6 7,921.0 6,894.5 5,943.6 5,769.1 Gas plant in service 1,462.7 1,387.6 801.5 763.1 661.2 624.5 Other plant in service 519.7 513.2 344.7 322.4 175.0 190.8 Accumulated depreciation (4,601.4 ) (4,314.6 ) (2,498.3 ) (2,294.7 ) (2,103.1 ) (2,019.9 ) Net plant 11,245.6 10,249.8 6,568.9 5,685.3 4,676.7 4,564.5 Leased Sheboygan Falls Energy Facility, net (a) — — — — 32.3 38.1 Construction work in progress 1,835.0 1,774.8 906.8 1,091.2 928.2 683.6 Other, net 6.1 6.1 5.0 5.0 1.1 1.1 Total utility 13,086.7 12,030.7 7,480.7 6,781.5 5,638.3 5,287.3 Non-utility and other: Non-utility Generation, net (b) 82.9 86.9 — — — — Corporate Services and other, net (c) 357.5 344.8 — — — — Total non-utility and other 440.4 431.7 — — — — Total property, plant and equipment $13,527.1 $12,462.4 $7,480.7 $6,781.5 $5,638.3 $5,287.3 (a) Less accumulated amortization of $88.7 million and $82.8 million for WPL as of December 31, 2019 and 2018 , 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. (b) Less accumulated depreciation of $58.5 million and $54.5 million for Alliant Energy as of December 31, 2019 and 2018 , respectively. (c) Less accumulated depreciation of $172.4 million and $167.5 million for Alliant Energy as of December 31, 2019 and 2018 , 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 2019 2018 2017 2019 2018 2017 2019 2018 2017 Equity $65.5 $51.4 $33.6 $35.2 $28.6 $21.1 $30.3 $22.8 $12.5 Debt 27.2 24.2 16.1 14.2 13.6 10.3 13.0 10.6 5.8 $92.7 $75.6 $49.7 $49.4 $42.2 $31.4 $43.3 $33.4 $18.3

Jointly-Owned Electric Utilit_2

Jointly-Owned Electric Utility Plant (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0 % $571.5 $180.8 $18.8 George Neal Unit 4 25.7 % 191.8 93.5 0.8 George Neal Unit 3 28.0 % 163.7 63.2 0.9 Louisa Unit 1 4.0 % 39.9 24.9 0.2 966.9 362.4 20.7 WPL Columbia Units 1-2 53.3 % 777.0 264.7 5.5 FWEC 42.6 % 120.2 43.6 — West Riverside 91.8 % — — 647.5 897.2 308.3 653.0 Alliant Energy $1,864.1 $670.7 $673.7
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, 2019 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0 % $571.5 $180.8 $18.8 George Neal Unit 4 25.7 % 191.8 93.5 0.8 George Neal Unit 3 28.0 % 163.7 63.2 0.9 Louisa Unit 1 4.0 % 39.9 24.9 0.2 966.9 362.4 20.7 WPL Columbia Units 1-2 53.3 % 777.0 264.7 5.5 FWEC 42.6 % 120.2 43.6 — West Riverside 91.8 % — — 647.5 897.2 308.3 653.0 Alliant Energy $1,864.1 $670.7 $673.7
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, 2019 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0 % $571.5 $180.8 $18.8 George Neal Unit 4 25.7 % 191.8 93.5 0.8 George Neal Unit 3 28.0 % 163.7 63.2 0.9 Louisa Unit 1 4.0 % 39.9 24.9 0.2 966.9 362.4 20.7 WPL Columbia Units 1-2 53.3 % 777.0 264.7 5.5 FWEC 42.6 % 120.2 43.6 — West Riverside 91.8 % — — 647.5 897.2 308.3 653.0 Alliant Energy $1,864.1 $670.7 $673.7

Receivables (Tables)

Receivables (Tables)	12 Months Ended
	Dec. 31, 2019
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 2019 2018 2019 2018 2019 2018 Customer $91.6 $91.0 $— $— $83.5 $84.8 Unbilled utility revenues 82.1 74.2 — — 82.1 74.2 Deferred proceeds 187.7 119.4 187.7 119.4 — — Other 48.0 76.3 16.3 37.2 31.4 38.5 Allowance for doubtful accounts (7.3 ) (10.5 ) (1.2 ) (3.1 ) (6.1 ) (7.4 ) $402.1 $350.4 $202.8 $153.5 $190.9 $190.1
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 2019 2018 2017 2019 2018 2017 Outstanding aggregate cash proceeds $108.0 $116.0 $112.0 $35.9 $53.4 $62.2
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): 2019 2018 Customer accounts receivable $124.7 $140.1 Unbilled utility revenues 95.5 97.1 Other receivables 0.9 0.1 Receivables sold to third party 221.1 237.3 Less: cash proceeds 27.0 108.0 Deferred proceeds 194.1 129.3 Less: allowance for doubtful accounts 6.4 9.9 Fair value of deferred proceeds $187.7 $119.4 Outstanding receivables past due $26.0 $35.5
Additional Attributes Of Receivables Sold Under The Agreement	Additional attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2019 2018 2017 Collections $2,192.8 $2,076.7 $1,647.1 Write-offs, net of recoveries 19.2 21.3 17.7
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 2019 2018 2019 2018 2019 2018 Customer $91.6 $91.0 $— $— $83.5 $84.8 Unbilled utility revenues 82.1 74.2 — — 82.1 74.2 Deferred proceeds 187.7 119.4 187.7 119.4 — — Other 48.0 76.3 16.3 37.2 31.4 38.5 Allowance for doubtful accounts (7.3 ) (10.5 ) (1.2 ) (3.1 ) (6.1 ) (7.4 ) $402.1 $350.4 $202.8 $153.5 $190.9 $190.1
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 2019 2018 2017 2019 2018 2017 Outstanding aggregate cash proceeds $108.0 $116.0 $112.0 $35.9 $53.4 $62.2
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): 2019 2018 Customer accounts receivable $124.7 $140.1 Unbilled utility revenues 95.5 97.1 Other receivables 0.9 0.1 Receivables sold to third party 221.1 237.3 Less: cash proceeds 27.0 108.0 Deferred proceeds 194.1 129.3 Less: allowance for doubtful accounts 6.4 9.9 Fair value of deferred proceeds $187.7 $119.4 Outstanding receivables past due $26.0 $35.5
Additional Attributes Of Receivables Sold Under The Agreement	Additional attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2019 2018 2017 Collections $2,192.8 $2,076.7 $1,647.1 Write-offs, net of recoveries 19.2 21.3 17.7
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 2019 2018 2019 2018 2019 2018 Customer $91.6 $91.0 $— $— $83.5 $84.8 Unbilled utility revenues 82.1 74.2 — — 82.1 74.2 Deferred proceeds 187.7 119.4 187.7 119.4 — — Other 48.0 76.3 16.3 37.2 31.4 38.5 Allowance for doubtful accounts (7.3 ) (10.5 ) (1.2 ) (3.1 ) (6.1 ) (7.4 ) $402.1 $350.4 $202.8 $153.5 $190.9 $190.1

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2019
Investment [Line Items]
Unconsolidated Equity Investments	Alliant Energy’s 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, 2019 2019 2018 2019 2018 2017 ATC Holdings 16%, 20% $320.1 $293.6 ($45.3 ) ($38.1 ) ($42.4 ) Non-utility wind farm in Oklahoma 50% 105.3 105.1 (5.2 ) (15.6 ) (1.8 ) Other Various 32.9 22.6 (2.5 ) (0.9 ) (0.6 ) $458.3 $421.3 ($53.0 ) ($54.6 ) ($44.8 )

Common Equity (Tables)

Common Equity (Tables)	12 Months Ended
	Dec. 31, 2019
Schedule of Common Equity [Line Items]
Common Stock Activity	A summary of Alliant Energy’s common stock activity was as follows: 2019 2018 2017 Shares outstanding, January 1 236,063,279 231,348,646 227,673,654 Equity forward agreements 8,358,973 — — At-the-market offering programs — 4,171,013 3,074,931 Shareowner Direct Plan 501,808 576,965 640,723 Equity-based compensation plans 101,478 5,078 5,185 Other (2,738 ) (38,423 ) (45,847 ) Shares outstanding, December 31 245,022,800 236,063,279 231,348,646

Preferred Stock (Tables)

Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2019
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 2019 2018 (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 2019 2018 (in millions) 5.1% $25 8,000,000 8,000,000 $200.0 $200.0

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2019
Debt Instrument [Line Items]
Schedule of 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 2019 2018 2019 2018 2019 2018 Commercial paper outstanding $337.4 $441.2 $— $50.4 $168.2 $105.5 Commercial paper weighted average interest rates 1.9% 2.8% N/A 2.8% 1.8% 2.5% Available credit facility capacity $662.6 $558.8 $250.0 $199.6 $131.8 $244.5 Alliant Energy IPL WPL For the year ended 2019 2018 2019 2018 2019 2018 Maximum amount outstanding (based on daily outstanding balances) $600.6 $446.5 $50.4 $50.4 $195.1 $126.0 Average amount outstanding (based on daily outstanding balances) $453.5 $221.4 $0.1 $1.5 $92.6 $36.6 Weighted average interest rates 2.5% 2.2% 2.8% 2.3% 2.4% 2.1%
Schedule of Debt-to-Capital Ratios	The required debt-to-capital ratios compared to the actual debt-to-capital ratios at December 31, 2019 were as follows: Alliant Energy IPL WPL Requirement, not to exceed 65% 65% 65% Actual 55% 48% 48%
Schedule of Long-term Debt	Long-term debt, net as of December 31 was as follows (dollars in millions): 2019 2018 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 500.0 500.0 — 500.0 500.0 — 3.6%, due 2029 (b) 300.0 300.0 — — — — 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 — 3.5%, due 2049 (b) 300.0 300.0 — — — — 3,175.0 3,175.0 — 2,575.0 2,575.0 — Debentures (a): 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 3%, due 2029 (c) 350.0 — 350.0 — — — 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 5% (Retired in 2019) — — — 250.0 — 250.0 1,950.0 — 1,950.0 1,850.0 — 1,850.0 Other: AEF term loan credit agreement through April 2020, 2% at December 31, 2019 (with Alliant Energy as guarantor) 300.0 — — 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) 400.0 — — 400.0 — — AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a) 300.0 — — 300.0 — — Sheboygan Power, LLC 5.06% senior secured notes, due 2020 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 38.2 — — 44.3 — — Other, 1% at December 31, 2019, due 2020 to 2025 2.1 — — 2.4 — — 1,115.3 — — 1,121.7 — — Subtotal 6,240.3 3,175.0 1,950.0 5,546.7 2,575.0 1,850.0 Current maturities (657.2 ) (200.0 ) (150.0 ) (256.5 ) — (250.0 ) Unamortized debt issuance costs (36.9 ) (21.0 ) (11.3 ) (32.1 ) (17.2 ) (9.6 ) Unamortized debt (discount) and premium, net (13.2 ) (6.7 ) (6.0 ) (11.8 ) (5.5 ) (5.5 ) Long-term debt, net (d) $5,533.0 $2,947.3 $1,782.7 $5,246.3 $2,552.3 $1,584.9 (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 April 2019, IPL issued $300 million of 3.6% senior debentures due 2029. In September 2019, IPL issued $300 million of 3.5% senior debentures due 2049. The senior debentures were issued as green bonds, and all of the net proceeds were allocated for the construction and development of IPL’s wind projects. (c) In June 2019, WPL issued $350 million of 3% debentures due 2029. The net proceeds from the issuance were used by WPL to reduce its outstanding commercial paper and retire its $250 million 5% debentures that matured in July 2019. (d) There were no significant sinking fund requirements related to the outstanding long-term debt.
Schedule of Maturities of Long-term Debt	At December 31, 2019 , long-term debt maturities for 2020 through 2024 were as follows (in millions): 2020 2021 2022 2023 2024 IPL $200 $— $— $— $500 WPL 150 — 250 — — Corporate Services — — 75 — — AEF 307 8 8 408 9 Alliant Energy $657 $8 $333 $408 $509
IPL [Member]
Debt Instrument [Line Items]
Schedule of 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 2019 2018 2019 2018 2019 2018 Commercial paper outstanding $337.4 $441.2 $— $50.4 $168.2 $105.5 Commercial paper weighted average interest rates 1.9% 2.8% N/A 2.8% 1.8% 2.5% Available credit facility capacity $662.6 $558.8 $250.0 $199.6 $131.8 $244.5 Alliant Energy IPL WPL For the year ended 2019 2018 2019 2018 2019 2018 Maximum amount outstanding (based on daily outstanding balances) $600.6 $446.5 $50.4 $50.4 $195.1 $126.0 Average amount outstanding (based on daily outstanding balances) $453.5 $221.4 $0.1 $1.5 $92.6 $36.6 Weighted average interest rates 2.5% 2.2% 2.8% 2.3% 2.4% 2.1%
Schedule of Debt-to-Capital Ratios	The required debt-to-capital ratios compared to the actual debt-to-capital ratios at December 31, 2019 were as follows: Alliant Energy IPL WPL Requirement, not to exceed 65% 65% 65% Actual 55% 48% 48%
Schedule of Long-term Debt	Long-term debt, net as of December 31 was as follows (dollars in millions): 2019 2018 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 500.0 500.0 — 500.0 500.0 — 3.6%, due 2029 (b) 300.0 300.0 — — — — 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 — 3.5%, due 2049 (b) 300.0 300.0 — — — — 3,175.0 3,175.0 — 2,575.0 2,575.0 — Debentures (a): 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 3%, due 2029 (c) 350.0 — 350.0 — — — 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 5% (Retired in 2019) — — — 250.0 — 250.0 1,950.0 — 1,950.0 1,850.0 — 1,850.0 Other: AEF term loan credit agreement through April 2020, 2% at December 31, 2019 (with Alliant Energy as guarantor) 300.0 — — 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) 400.0 — — 400.0 — — AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a) 300.0 — — 300.0 — — Sheboygan Power, LLC 5.06% senior secured notes, due 2020 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 38.2 — — 44.3 — — Other, 1% at December 31, 2019, due 2020 to 2025 2.1 — — 2.4 — — 1,115.3 — — 1,121.7 — — Subtotal 6,240.3 3,175.0 1,950.0 5,546.7 2,575.0 1,850.0 Current maturities (657.2 ) (200.0 ) (150.0 ) (256.5 ) — (250.0 ) Unamortized debt issuance costs (36.9 ) (21.0 ) (11.3 ) (32.1 ) (17.2 ) (9.6 ) Unamortized debt (discount) and premium, net (13.2 ) (6.7 ) (6.0 ) (11.8 ) (5.5 ) (5.5 ) Long-term debt, net (d) $5,533.0 $2,947.3 $1,782.7 $5,246.3 $2,552.3 $1,584.9 (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 April 2019, IPL issued $300 million of 3.6% senior debentures due 2029. In September 2019, IPL issued $300 million of 3.5% senior debentures due 2049. The senior debentures were issued as green bonds, and all of the net proceeds were allocated for the construction and development of IPL’s wind projects. (c) In June 2019, WPL issued $350 million of 3% debentures due 2029. The net proceeds from the issuance were used by WPL to reduce its outstanding commercial paper and retire its $250 million 5% debentures that matured in July 2019. (d) There were no significant sinking fund requirements related to the outstanding long-term debt.
Schedule of Maturities of Long-term Debt	At December 31, 2019 , long-term debt maturities for 2020 through 2024 were as follows (in millions): 2020 2021 2022 2023 2024 IPL $200 $— $— $— $500 WPL 150 — 250 — — Corporate Services — — 75 — — AEF 307 8 8 408 9 Alliant Energy $657 $8 $333 $408 $509
WPL [Member]
Debt Instrument [Line Items]
Schedule of 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 2019 2018 2019 2018 2019 2018 Commercial paper outstanding $337.4 $441.2 $— $50.4 $168.2 $105.5 Commercial paper weighted average interest rates 1.9% 2.8% N/A 2.8% 1.8% 2.5% Available credit facility capacity $662.6 $558.8 $250.0 $199.6 $131.8 $244.5 Alliant Energy IPL WPL For the year ended 2019 2018 2019 2018 2019 2018 Maximum amount outstanding (based on daily outstanding balances) $600.6 $446.5 $50.4 $50.4 $195.1 $126.0 Average amount outstanding (based on daily outstanding balances) $453.5 $221.4 $0.1 $1.5 $92.6 $36.6 Weighted average interest rates 2.5% 2.2% 2.8% 2.3% 2.4% 2.1%
Schedule of Debt-to-Capital Ratios	The required debt-to-capital ratios compared to the actual debt-to-capital ratios at December 31, 2019 were as follows: Alliant Energy IPL WPL Requirement, not to exceed 65% 65% 65% Actual 55% 48% 48%
Schedule of Long-term Debt	Long-term debt, net as of December 31 was as follows (dollars in millions): 2019 2018 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 500.0 500.0 — 500.0 500.0 — 3.6%, due 2029 (b) 300.0 300.0 — — — — 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 — 3.5%, due 2049 (b) 300.0 300.0 — — — — 3,175.0 3,175.0 — 2,575.0 2,575.0 — Debentures (a): 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 3%, due 2029 (c) 350.0 — 350.0 — — — 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 5% (Retired in 2019) — — — 250.0 — 250.0 1,950.0 — 1,950.0 1,850.0 — 1,850.0 Other: AEF term loan credit agreement through April 2020, 2% at December 31, 2019 (with Alliant Energy as guarantor) 300.0 — — 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) 400.0 — — 400.0 — — AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a) 300.0 — — 300.0 — — Sheboygan Power, LLC 5.06% senior secured notes, due 2020 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 38.2 — — 44.3 — — Other, 1% at December 31, 2019, due 2020 to 2025 2.1 — — 2.4 — — 1,115.3 — — 1,121.7 — — Subtotal 6,240.3 3,175.0 1,950.0 5,546.7 2,575.0 1,850.0 Current maturities (657.2 ) (200.0 ) (150.0 ) (256.5 ) — (250.0 ) Unamortized debt issuance costs (36.9 ) (21.0 ) (11.3 ) (32.1 ) (17.2 ) (9.6 ) Unamortized debt (discount) and premium, net (13.2 ) (6.7 ) (6.0 ) (11.8 ) (5.5 ) (5.5 ) Long-term debt, net (d) $5,533.0 $2,947.3 $1,782.7 $5,246.3 $2,552.3 $1,584.9 (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 April 2019, IPL issued $300 million of 3.6% senior debentures due 2029. In September 2019, IPL issued $300 million of 3.5% senior debentures due 2049. The senior debentures were issued as green bonds, and all of the net proceeds were allocated for the construction and development of IPL’s wind projects. (c) In June 2019, WPL issued $350 million of 3% debentures due 2029. The net proceeds from the issuance were used by WPL to reduce its outstanding commercial paper and retire its $250 million 5% debentures that matured in July 2019. (d) There were no significant sinking fund requirements related to the outstanding long-term debt.
Schedule of Maturities of Long-term Debt	At December 31, 2019 , long-term debt maturities for 2020 through 2024 were as follows (in millions): 2020 2021 2022 2023 2024 IPL $200 $— $— $— $500 WPL 150 — 250 — — Corporate Services — — 75 — — AEF 307 8 8 408 9 Alliant Energy $657 $8 $333 $408 $509

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2019
Leases [Line Items]
Schedule of Lease Details	Operating lease details are as follows (dollars in millions): December 31, 2019 Alliant Energy IPL WPL Property, plant and equipment, net $16 $10 $6 Other current liabilities $2 $1 $1 Other liabilities 14 9 5 Total operating lease liabilities $16 $10 $6 Weighted average remaining lease term 11 years 12 years 10 years Weighted average discount rate 4 % 4 % 4 % 2019 Alliant Energy IPL WPL Operating lease cost $2 $1 $1
Schedule of Operating Lease Liability Maturities	As of December 31, 2019 , expected maturities of lease liabilities were as follows (in millions): 2020 2021 2022 2023 2024 Thereafter Total Less: amount representing interest Present value of minimum lease payments Operating Leases: Alliant Energy $2 $2 $2 $2 $2 $10 $20 $4 $16 IPL 1 1 1 1 1 7 12 2 10 WPL 1 1 1 1 1 3 8 2 6 WPL’s Finance Lease: Sheboygan Falls Energy Facility 15 15 15 15 15 5 80 20 60
Schedule of Future Minimum Payments for Operating Leases	As of December 31, 2018 , future minimum operating (excluding contingent rentals) and capital lease payments were as follows (in millions): 2019 2020 2021 2022 2023 Thereafter Total Less: amount representing interest Present value of minimum capital lease payments Operating Leases: Alliant Energy $5 $5 $3 $3 $2 $12 $30 IPL 3 2 2 2 2 12 23 WPL 2 3 1 — — — 6 WPL’s Capital Lease: Sheboygan Falls Energy Facility $15 $15 $15 $15 $15 $19 $94 $26 $68
IPL [Member]
Leases [Line Items]
Schedule of Lease Details	Operating lease details are as follows (dollars in millions): December 31, 2019 Alliant Energy IPL WPL Property, plant and equipment, net $16 $10 $6 Other current liabilities $2 $1 $1 Other liabilities 14 9 5 Total operating lease liabilities $16 $10 $6 Weighted average remaining lease term 11 years 12 years 10 years Weighted average discount rate 4 % 4 % 4 % 2019 Alliant Energy IPL WPL Operating lease cost $2 $1 $1
Schedule of Operating Lease Liability Maturities	As of December 31, 2019 , expected maturities of lease liabilities were as follows (in millions): 2020 2021 2022 2023 2024 Thereafter Total Less: amount representing interest Present value of minimum lease payments Operating Leases: Alliant Energy $2 $2 $2 $2 $2 $10 $20 $4 $16 IPL 1 1 1 1 1 7 12 2 10 WPL 1 1 1 1 1 3 8 2 6 WPL’s Finance Lease: Sheboygan Falls Energy Facility 15 15 15 15 15 5 80 20 60
Schedule of Future Minimum Payments for Operating Leases	As of December 31, 2018 , future minimum operating (excluding contingent rentals) and capital lease payments were as follows (in millions): 2019 2020 2021 2022 2023 Thereafter Total Less: amount representing interest Present value of minimum capital lease payments Operating Leases: Alliant Energy $5 $5 $3 $3 $2 $12 $30 IPL 3 2 2 2 2 12 23 WPL 2 3 1 — — — 6 WPL’s Capital Lease: Sheboygan Falls Energy Facility $15 $15 $15 $15 $15 $19 $94 $26 $68
WPL [Member]
Leases [Line Items]
Schedule of Lease Details	Operating lease details are as follows (dollars in millions): December 31, 2019 Alliant Energy IPL WPL Property, plant and equipment, net $16 $10 $6 Other current liabilities $2 $1 $1 Other liabilities 14 9 5 Total operating lease liabilities $16 $10 $6 Weighted average remaining lease term 11 years 12 years 10 years Weighted average discount rate 4 % 4 % 4 % 2019 Alliant Energy IPL WPL Operating lease cost $2 $1 $1 December 31, 2019 Property, plant and equipment, net $32 Other current liabilities $9 Finance lease obligations - Sheboygan Falls Energy Facility 51 Total finance lease liabilities $60 Remaining lease term 5 years Discount rate 11 % 2019 Depreciation expense $6 Interest expense 7 Total finance lease expense $13
Schedule of Operating Lease Liability Maturities	As of December 31, 2019 , expected maturities of lease liabilities were as follows (in millions): 2020 2021 2022 2023 2024 Thereafter Total Less: amount representing interest Present value of minimum lease payments Operating Leases: Alliant Energy $2 $2 $2 $2 $2 $10 $20 $4 $16 IPL 1 1 1 1 1 7 12 2 10 WPL 1 1 1 1 1 3 8 2 6 WPL’s Finance Lease: Sheboygan Falls Energy Facility 15 15 15 15 15 5 80 20 60
Schedule of Finance Lease Liability Maturities	As of December 31, 2019 , expected maturities of lease liabilities were as follows (in millions): 2020 2021 2022 2023 2024 Thereafter Total Less: amount representing interest Present value of minimum lease payments Operating Leases: Alliant Energy $2 $2 $2 $2 $2 $10 $20 $4 $16 IPL 1 1 1 1 1 7 12 2 10 WPL 1 1 1 1 1 3 8 2 6 WPL’s Finance Lease: Sheboygan Falls Energy Facility 15 15 15 15 15 5 80 20 60
Schedule of Future Minimum Payments for Operating Leases	As of December 31, 2018 , future minimum operating (excluding contingent rentals) and capital lease payments were as follows (in millions): 2019 2020 2021 2022 2023 Thereafter Total Less: amount representing interest Present value of minimum capital lease payments Operating Leases: Alliant Energy $5 $5 $3 $3 $2 $12 $30 IPL 3 2 2 2 2 12 23 WPL 2 3 1 — — — 6 WPL’s Capital Lease: Sheboygan Falls Energy Facility $15 $15 $15 $15 $15 $19 $94 $26 $68
Schedule of Future Minimum Payments for Capital Leases	As of December 31, 2018 , future minimum operating (excluding contingent rentals) and capital lease payments were as follows (in millions): 2019 2020 2021 2022 2023 Thereafter Total Less: amount representing interest Present value of minimum capital lease payments Operating Leases: Alliant Energy $5 $5 $3 $3 $2 $12 $30 IPL 3 2 2 2 2 12 23 WPL 2 3 1 — — — 6 WPL’s Capital Lease: Sheboygan Falls Energy Facility $15 $15 $15 $15 $15 $19 $94 $26 $68

Revenues (Tables)

Revenues (Tables)	12 Months Ended
	Dec. 31, 2019
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 2019 2018 2017 2019 2018 2017 2019 2018 2017 Electric Utility: Retail - residential $1,092.4 $1,063.4 $1,006.2 $601.5 $590.6 $535.6 $490.9 $472.8 $470.6 Retail - commercial 769.7 735.6 710.3 510.3 486.8 452.7 259.4 248.8 257.6 Retail - industrial 889.1 888.8 853.1 503.9 500.8 459.7 385.2 388.0 393.4 Wholesale 176.4 188.4 238.4 63.5 71.2 95.5 112.9 117.2 142.9 Bulk power and other 136.0 124.1 86.7 102.0 81.7 55.4 34.0 42.4 31.3 Total Electric Utility 3,063.6 3,000.3 2,894.7 1,781.2 1,731.1 1,598.9 1,282.4 1,269.2 1,295.8 Gas Utility: Retail - residential 259.4 254.4 224.7 149.3 152.3 123.2 110.1 102.1 101.5 Retail - commercial 133.0 133.0 123.2 74.9 75.9 67.9 58.1 57.1 55.3 Retail - industrial 16.0 14.9 16.7 11.7 10.2 11.1 4.3 4.7 5.6 Transportation/other 46.8 44.3 36.3 28.3 27.8 23.8 18.5 16.5 12.5 Total Gas Utility 455.2 446.6 400.9 264.2 266.2 226.0 191.0 180.4 174.9 Other Utility: Steam 37.2 35.2 34.6 37.2 35.2 34.6 — — — Other utility 9.3 12.8 12.9 7.0 9.8 10.8 2.3 3.0 2.1 Total Other Utility 46.5 48.0 47.5 44.2 45.0 45.4 2.3 3.0 2.1 Non-Utility and Other: Transportation and other 82.4 39.6 39.1 — — — — — — Total Non-Utility and Other 82.4 39.6 39.1 — — — — — — Total revenues $3,647.7 $3,534.5 $3,382.2 $2,089.6 $2,042.3 $1,870.3 $1,475.7 $1,452.6 $1,472.8
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 2019 2018 2017 2019 2018 2017 2019 2018 2017 Electric Utility: Retail - residential $1,092.4 $1,063.4 $1,006.2 $601.5 $590.6 $535.6 $490.9 $472.8 $470.6 Retail - commercial 769.7 735.6 710.3 510.3 486.8 452.7 259.4 248.8 257.6 Retail - industrial 889.1 888.8 853.1 503.9 500.8 459.7 385.2 388.0 393.4 Wholesale 176.4 188.4 238.4 63.5 71.2 95.5 112.9 117.2 142.9 Bulk power and other 136.0 124.1 86.7 102.0 81.7 55.4 34.0 42.4 31.3 Total Electric Utility 3,063.6 3,000.3 2,894.7 1,781.2 1,731.1 1,598.9 1,282.4 1,269.2 1,295.8 Gas Utility: Retail - residential 259.4 254.4 224.7 149.3 152.3 123.2 110.1 102.1 101.5 Retail - commercial 133.0 133.0 123.2 74.9 75.9 67.9 58.1 57.1 55.3 Retail - industrial 16.0 14.9 16.7 11.7 10.2 11.1 4.3 4.7 5.6 Transportation/other 46.8 44.3 36.3 28.3 27.8 23.8 18.5 16.5 12.5 Total Gas Utility 455.2 446.6 400.9 264.2 266.2 226.0 191.0 180.4 174.9 Other Utility: Steam 37.2 35.2 34.6 37.2 35.2 34.6 — — — Other utility 9.3 12.8 12.9 7.0 9.8 10.8 2.3 3.0 2.1 Total Other Utility 46.5 48.0 47.5 44.2 45.0 45.4 2.3 3.0 2.1 Non-Utility and Other: Transportation and other 82.4 39.6 39.1 — — — — — — Total Non-Utility and Other 82.4 39.6 39.1 — — — — — — Total revenues $3,647.7 $3,534.5 $3,382.2 $2,089.6 $2,042.3 $1,870.3 $1,475.7 $1,452.6 $1,472.8
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 2019 2018 2017 2019 2018 2017 2019 2018 2017 Electric Utility: Retail - residential $1,092.4 $1,063.4 $1,006.2 $601.5 $590.6 $535.6 $490.9 $472.8 $470.6 Retail - commercial 769.7 735.6 710.3 510.3 486.8 452.7 259.4 248.8 257.6 Retail - industrial 889.1 888.8 853.1 503.9 500.8 459.7 385.2 388.0 393.4 Wholesale 176.4 188.4 238.4 63.5 71.2 95.5 112.9 117.2 142.9 Bulk power and other 136.0 124.1 86.7 102.0 81.7 55.4 34.0 42.4 31.3 Total Electric Utility 3,063.6 3,000.3 2,894.7 1,781.2 1,731.1 1,598.9 1,282.4 1,269.2 1,295.8 Gas Utility: Retail - residential 259.4 254.4 224.7 149.3 152.3 123.2 110.1 102.1 101.5 Retail - commercial 133.0 133.0 123.2 74.9 75.9 67.9 58.1 57.1 55.3 Retail - industrial 16.0 14.9 16.7 11.7 10.2 11.1 4.3 4.7 5.6 Transportation/other 46.8 44.3 36.3 28.3 27.8 23.8 18.5 16.5 12.5 Total Gas Utility 455.2 446.6 400.9 264.2 266.2 226.0 191.0 180.4 174.9 Other Utility: Steam 37.2 35.2 34.6 37.2 35.2 34.6 — — — Other utility 9.3 12.8 12.9 7.0 9.8 10.8 2.3 3.0 2.1 Total Other Utility 46.5 48.0 47.5 44.2 45.0 45.4 2.3 3.0 2.1 Non-Utility and Other: Transportation and other 82.4 39.6 39.1 — — — — — — Total Non-Utility and Other 82.4 39.6 39.1 — — — — — — Total revenues $3,647.7 $3,534.5 $3,382.2 $2,089.6 $2,042.3 $1,870.3 $1,475.7 $1,452.6 $1,472.8

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2019
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 2019 2018 2017 2019 2018 2017 2019 2018 2017 Current tax expense (benefit): Federal ($6.6 ) ($1.0 ) ($41.0 ) ($11.0 ) $14.9 ($27.9 ) $12.0 ($9.2 ) $5.5 State 24.2 (5.1 ) 8.5 24.3 (7.1 ) 1.6 13.7 (4.4 ) 2.5 IPL’s tax benefit riders (4.5 ) (13.2 ) (40.4 ) (4.5 ) (13.2 ) (40.4 ) — — — Deferred tax expense (benefit): Federal 69.7 67.9 159.5 26.4 9.5 72.5 30.7 43.8 55.0 State 41.4 29.8 12.3 30.9 7.3 (2.2 ) 6.3 22.1 16.6 Production tax credits (54.7 ) (29.5 ) (31.1 ) (41.8 ) (14.0 ) (14.1 ) (12.8 ) (15.5 ) (17.0 ) Investment tax credits (0.8 ) (1.2 ) (1.1 ) (0.2 ) (0.6 ) (0.4 ) (0.6 ) (0.6 ) (0.7 ) $68.7 $47.7 $66.7 $24.1 ($3.2 ) ($10.9 ) $49.3 $36.2 $61.9
Schedule Of Effective Income Tax Rates	The overall income tax rates shown in the following table were computed by dividing income tax expense (benefit) by income from continuing operations before income taxes. Alliant Energy IPL WPL 2019 2018 2017 2019 2018 2017 2019 2018 2017 Statutory federal income tax rate 21.0 % 21.0 % 35.0 % 21.0 % 21.0 % 35.0 % 21.0 % 21.0 % 35.0 % State income taxes, net of federal benefits 6.8 7.0 5.5 8.5 7.7 6.5 6.2 6.2 5.1 Production tax credits (8.6 ) (5.2 ) (6.1 ) (13.1 ) (5.2 ) (6.7 ) (4.5 ) (6.4 ) (7.1 ) Effect of rate-making on property-related differences (6.1 ) (7.6 ) (8.5 ) (9.8 ) (14.0 ) (19.1 ) (2.7 ) (2.3 ) (1.7 ) Amortization of excess deferred taxes (1.0 ) (0.3 ) (0.1 ) (0.3 ) — (0.1 ) (1.8 ) (0.2 ) (0.1 ) IPL’s tax benefit riders (0.7 ) (2.3 ) (7.6 ) (1.4 ) (4.9 ) (18.7 ) — — — Adjustment for prior period taxes 0.6 (2.3 ) (1.5 ) 3.1 (4.8 ) (3.4 ) (0.3 ) (0.2 ) — Federal Tax Reform adjustments — (1.0 ) (3.4 ) — (0.4 ) 1.7 — (2.3 ) (5.8 ) Other items, net (1.2 ) (0.9 ) (0.8 ) (0.4 ) (0.6 ) (0.2 ) (0.4 ) (1.0 ) (0.5 ) Overall income tax rate 10.8 % 8.4 % 12.5 % 7.6 % (1.2 %) (5.0 %) 17.5 % 14.8 % 24.9 %
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 2019 2018 2019 2018 2019 2018 Deferred tax liabilities: Property $2,021.5 $1,975.5 $1,183.5 $1,158.4 $770.0 $735.2 ATC Holdings 110.8 102.4 — — — — Other 85.1 80.5 76.5 69.4 32.1 34.4 Total deferred tax liabilities 2,217.4 2,158.4 1,260.0 1,227.8 802.1 769.6 Deferred tax assets: Federal credit carryforwards 355.4 299.1 175.0 133.2 160.3 147.4 Net operating losses carryforwards - federal 60.5 158.6 56.3 114.1 0.5 26.4 Net operating losses carryforwards - state 36.6 47.0 0.6 0.9 0.1 0.5 Other 60.5 59.8 20.6 22.8 15.8 14.1 Subtotal deferred tax assets 513.0 564.5 252.5 271.0 176.7 188.4 Valuation allowances (9.6 ) (9.2 ) (0.5 ) (0.5 ) (0.8 ) (0.8 ) Total deferred tax assets 503.4 555.3 252.0 270.5 175.9 187.6 Total deferred tax liabilities, net $1,714.0 $1,603.1 $1,008.0 $957.3 $626.2 $582.0
Summary Of Tax Credit Carryforwards	At December 31, 2019 , carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2037 $297 $276 $2 State net operating losses 2020-2039 615 9 2 Federal tax credits 2022-2039 355 175 160
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) 2016 - 2018 Consolidated Iowa income tax returns (b) 2016 - 2018 Wisconsin combined tax returns (c) 2015 - 2018 (a) The 2016 and 2017 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 2019 2018 2017 2019 2018 2017 2019 2018 2017 Current tax expense (benefit): Federal ($6.6 ) ($1.0 ) ($41.0 ) ($11.0 ) $14.9 ($27.9 ) $12.0 ($9.2 ) $5.5 State 24.2 (5.1 ) 8.5 24.3 (7.1 ) 1.6 13.7 (4.4 ) 2.5 IPL’s tax benefit riders (4.5 ) (13.2 ) (40.4 ) (4.5 ) (13.2 ) (40.4 ) — — — Deferred tax expense (benefit): Federal 69.7 67.9 159.5 26.4 9.5 72.5 30.7 43.8 55.0 State 41.4 29.8 12.3 30.9 7.3 (2.2 ) 6.3 22.1 16.6 Production tax credits (54.7 ) (29.5 ) (31.1 ) (41.8 ) (14.0 ) (14.1 ) (12.8 ) (15.5 ) (17.0 ) Investment tax credits (0.8 ) (1.2 ) (1.1 ) (0.2 ) (0.6 ) (0.4 ) (0.6 ) (0.6 ) (0.7 ) $68.7 $47.7 $66.7 $24.1 ($3.2 ) ($10.9 ) $49.3 $36.2 $61.9
Schedule Of Effective Income Tax Rates	The overall income tax rates shown in the following table were computed by dividing income tax expense (benefit) by income from continuing operations before income taxes. Alliant Energy IPL WPL 2019 2018 2017 2019 2018 2017 2019 2018 2017 Statutory federal income tax rate 21.0 % 21.0 % 35.0 % 21.0 % 21.0 % 35.0 % 21.0 % 21.0 % 35.0 % State income taxes, net of federal benefits 6.8 7.0 5.5 8.5 7.7 6.5 6.2 6.2 5.1 Production tax credits (8.6 ) (5.2 ) (6.1 ) (13.1 ) (5.2 ) (6.7 ) (4.5 ) (6.4 ) (7.1 ) Effect of rate-making on property-related differences (6.1 ) (7.6 ) (8.5 ) (9.8 ) (14.0 ) (19.1 ) (2.7 ) (2.3 ) (1.7 ) Amortization of excess deferred taxes (1.0 ) (0.3 ) (0.1 ) (0.3 ) — (0.1 ) (1.8 ) (0.2 ) (0.1 ) IPL’s tax benefit riders (0.7 ) (2.3 ) (7.6 ) (1.4 ) (4.9 ) (18.7 ) — — — Adjustment for prior period taxes 0.6 (2.3 ) (1.5 ) 3.1 (4.8 ) (3.4 ) (0.3 ) (0.2 ) — Federal Tax Reform adjustments — (1.0 ) (3.4 ) — (0.4 ) 1.7 — (2.3 ) (5.8 ) Other items, net (1.2 ) (0.9 ) (0.8 ) (0.4 ) (0.6 ) (0.2 ) (0.4 ) (1.0 ) (0.5 ) Overall income tax rate 10.8 % 8.4 % 12.5 % 7.6 % (1.2 %) (5.0 %) 17.5 % 14.8 % 24.9 %
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 2019 2018 2019 2018 2019 2018 Deferred tax liabilities: Property $2,021.5 $1,975.5 $1,183.5 $1,158.4 $770.0 $735.2 ATC Holdings 110.8 102.4 — — — — Other 85.1 80.5 76.5 69.4 32.1 34.4 Total deferred tax liabilities 2,217.4 2,158.4 1,260.0 1,227.8 802.1 769.6 Deferred tax assets: Federal credit carryforwards 355.4 299.1 175.0 133.2 160.3 147.4 Net operating losses carryforwards - federal 60.5 158.6 56.3 114.1 0.5 26.4 Net operating losses carryforwards - state 36.6 47.0 0.6 0.9 0.1 0.5 Other 60.5 59.8 20.6 22.8 15.8 14.1 Subtotal deferred tax assets 513.0 564.5 252.5 271.0 176.7 188.4 Valuation allowances (9.6 ) (9.2 ) (0.5 ) (0.5 ) (0.8 ) (0.8 ) Total deferred tax assets 503.4 555.3 252.0 270.5 175.9 187.6 Total deferred tax liabilities, net $1,714.0 $1,603.1 $1,008.0 $957.3 $626.2 $582.0
Summary Of Tax Credit Carryforwards	At December 31, 2019 , carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2037 $297 $276 $2 State net operating losses 2020-2039 615 9 2 Federal tax credits 2022-2039 355 175 160
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) 2016 - 2018 Consolidated Iowa income tax returns (b) 2016 - 2018 Wisconsin combined tax returns (c) 2015 - 2018 (a) The 2016 and 2017 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 2019 2018 2017 2019 2018 2017 2019 2018 2017 Current tax expense (benefit): Federal ($6.6 ) ($1.0 ) ($41.0 ) ($11.0 ) $14.9 ($27.9 ) $12.0 ($9.2 ) $5.5 State 24.2 (5.1 ) 8.5 24.3 (7.1 ) 1.6 13.7 (4.4 ) 2.5 IPL’s tax benefit riders (4.5 ) (13.2 ) (40.4 ) (4.5 ) (13.2 ) (40.4 ) — — — Deferred tax expense (benefit): Federal 69.7 67.9 159.5 26.4 9.5 72.5 30.7 43.8 55.0 State 41.4 29.8 12.3 30.9 7.3 (2.2 ) 6.3 22.1 16.6 Production tax credits (54.7 ) (29.5 ) (31.1 ) (41.8 ) (14.0 ) (14.1 ) (12.8 ) (15.5 ) (17.0 ) Investment tax credits (0.8 ) (1.2 ) (1.1 ) (0.2 ) (0.6 ) (0.4 ) (0.6 ) (0.6 ) (0.7 ) $68.7 $47.7 $66.7 $24.1 ($3.2 ) ($10.9 ) $49.3 $36.2 $61.9
Schedule Of Effective Income Tax Rates	The overall income tax rates shown in the following table were computed by dividing income tax expense (benefit) by income from continuing operations before income taxes. Alliant Energy IPL WPL 2019 2018 2017 2019 2018 2017 2019 2018 2017 Statutory federal income tax rate 21.0 % 21.0 % 35.0 % 21.0 % 21.0 % 35.0 % 21.0 % 21.0 % 35.0 % State income taxes, net of federal benefits 6.8 7.0 5.5 8.5 7.7 6.5 6.2 6.2 5.1 Production tax credits (8.6 ) (5.2 ) (6.1 ) (13.1 ) (5.2 ) (6.7 ) (4.5 ) (6.4 ) (7.1 ) Effect of rate-making on property-related differences (6.1 ) (7.6 ) (8.5 ) (9.8 ) (14.0 ) (19.1 ) (2.7 ) (2.3 ) (1.7 ) Amortization of excess deferred taxes (1.0 ) (0.3 ) (0.1 ) (0.3 ) — (0.1 ) (1.8 ) (0.2 ) (0.1 ) IPL’s tax benefit riders (0.7 ) (2.3 ) (7.6 ) (1.4 ) (4.9 ) (18.7 ) — — — Adjustment for prior period taxes 0.6 (2.3 ) (1.5 ) 3.1 (4.8 ) (3.4 ) (0.3 ) (0.2 ) — Federal Tax Reform adjustments — (1.0 ) (3.4 ) — (0.4 ) 1.7 — (2.3 ) (5.8 ) Other items, net (1.2 ) (0.9 ) (0.8 ) (0.4 ) (0.6 ) (0.2 ) (0.4 ) (1.0 ) (0.5 ) Overall income tax rate 10.8 % 8.4 % 12.5 % 7.6 % (1.2 %) (5.0 %) 17.5 % 14.8 % 24.9 %
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 2019 2018 2019 2018 2019 2018 Deferred tax liabilities: Property $2,021.5 $1,975.5 $1,183.5 $1,158.4 $770.0 $735.2 ATC Holdings 110.8 102.4 — — — — Other 85.1 80.5 76.5 69.4 32.1 34.4 Total deferred tax liabilities 2,217.4 2,158.4 1,260.0 1,227.8 802.1 769.6 Deferred tax assets: Federal credit carryforwards 355.4 299.1 175.0 133.2 160.3 147.4 Net operating losses carryforwards - federal 60.5 158.6 56.3 114.1 0.5 26.4 Net operating losses carryforwards - state 36.6 47.0 0.6 0.9 0.1 0.5 Other 60.5 59.8 20.6 22.8 15.8 14.1 Subtotal deferred tax assets 513.0 564.5 252.5 271.0 176.7 188.4 Valuation allowances (9.6 ) (9.2 ) (0.5 ) (0.5 ) (0.8 ) (0.8 ) Total deferred tax assets 503.4 555.3 252.0 270.5 175.9 187.6 Total deferred tax liabilities, net $1,714.0 $1,603.1 $1,008.0 $957.3 $626.2 $582.0
Summary Of Tax Credit Carryforwards	At December 31, 2019 , carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2037 $297 $276 $2 State net operating losses 2020-2039 615 9 2 Federal tax credits 2022-2039 355 175 160
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) 2016 - 2018 Consolidated Iowa income tax returns (b) 2016 - 2018 Wisconsin combined tax returns (c) 2015 - 2018 (a) The 2016 and 2017 federal tax returns are effectively settled as a result of participation in the IRS Compliance Assurance Program, which allows Alliant Energy and the IRS to work together to resolve issues related to Alliant Energy’s current tax year before filing its federal income tax return. The statute of limitations for these federal tax returns expires three years from each filing date. (b) The statute of limitations for these Iowa tax returns expires three years from each filing date. (c) The statute of limitations for these Wisconsin combined tax returns expires four years from each filing date.

Benefit Plans (Tables)

Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2019
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 2019 2018 2017 2019 2018 2017 Discount rate for benefit obligations 3.48% 4.34% 3.66% 3.40% 4.24% 3.53% Discount rate for net periodic cost 4.34% 3.66% 4.19% 4.24% 3.53% 3.98% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.44% 5.44% 5.80% Interest crediting rate for Alliant Energy Cash Balance Pension Plan 5.52% 5.04% 4.64% 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 Qualified Defined Benefit Pension Plan OPEB Plans IPL 2019 2018 2017 2019 2018 2017 Discount rate for benefit obligations 3.51% 4.35% 3.68% 3.39% 4.23% 3.51% Discount rate for net periodic cost 4.35% 3.68% 4.22% 4.23% 3.51% 3.95% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.60% 5.60% 6.20% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Qualified Defined Benefit Pension Plan OPEB Plans WPL 2019 2018 2017 2019 2018 2017 Discount rate for benefit obligations 3.50% 4.35% 3.69% 3.39% 4.23% 3.51% Discount rate for net periodic cost 4.35% 3.69% 4.23% 4.23% 3.51% 3.96% Expected rate of return on plan assets 7.60% 7.60% 7.60% 3.81% 3.84% 3.50% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A
Net Periodic Benefit Costs	The components of net periodic benefit costs for sponsored defined benefit pension and OPEB plans are included 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 2019 2018 2017 2019 2018 2017 Service cost $9.7 $12.1 $12.5 $3.3 $4.2 $5.0 Interest cost 49.8 46.8 51.0 8.5 7.7 8.6 Expected return on plan assets (a) (60.1 ) (69.7 ) (65.5 ) (5.0 ) (6.0 ) (6.1 ) Amortization of prior service credit (b) (0.7 ) (0.7 ) (0.4 ) (0.2 ) (0.2 ) (0.2 ) Amortization of actuarial loss (c) 36.4 35.2 37.6 3.3 3.4 3.8 Settlement losses (d) — — 0.9 — — — $35.1 $23.7 $36.1 $9.9 $9.1 $11.1 IPL Defined Benefit Pension Plans OPEB Plans 2019 2018 2017 2019 2018 2017 Service cost $6.0 $7.4 $7.3 $1.3 $1.7 $2.1 Interest cost 22.7 21.4 23.5 3.4 3.1 3.5 Expected return on plan assets (a) (28.1 ) (32.6 ) (30.8 ) (3.6 ) (4.4 ) (4.3 ) Amortization of prior service credit (b) (0.2 ) (0.2 ) (0.2 ) — — — Amortization of actuarial loss (c) 15.7 14.9 16.1 1.5 1.3 2.0 $16.1 $10.9 $15.9 $2.6 $1.7 $3.3 WPL Defined Benefit Pension Plans OPEB Plans 2019 2018 2017 2019 2018 2017 Service cost $3.5 $4.4 $4.9 $1.3 $1.6 $1.9 Interest cost 21.5 20.2 21.8 3.3 3.1 3.4 Expected return on plan assets (a) (26.2 ) (30.4 ) (28.5 ) (0.6 ) (0.6 ) (0.8 ) Amortization of prior service cost (credit) (b) (0.2 ) (0.1 ) 0.1 (0.2 ) (0.2 ) (0.2 ) Amortization of actuarial loss (c) 17.6 17.2 18.5 1.6 2.0 1.6 $16.2 $11.3 $16.8 $5.4 $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 2019 2018 2019 2018 Change in benefit obligation: Net benefit obligation at January 1 $1,175.0 $1,303.1 $206.1 $222.3 Service cost 9.7 12.1 3.3 4.2 Interest cost 49.8 46.8 8.5 7.7 Plan participants’ contributions — — 3.4 3.1 Actuarial (gain) loss 124.6 (96.2 ) 13.6 (8.2 ) Gross benefits paid (79.4 ) (90.8 ) (21.3 ) (23.0 ) Net benefit obligation at December 31 1,279.7 1,175.0 213.6 206.1 Change in plan assets: Fair value of plan assets at January 1 808.6 950.7 99.1 111.1 Actual return on plan assets 168.1 (57.8 ) 14.6 (2.6 ) Employer contributions 33.1 6.5 9.1 10.5 Plan participants’ contributions — — 3.4 3.1 Gross benefits paid (79.4 ) (90.8 ) (21.3 ) (23.0 ) Fair value of plan assets at December 31 930.4 808.6 104.9 99.1 Under funded status at December 31 ($349.3 ) ($366.4 ) ($108.7 ) ($107.0 ) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2019 2018 2019 2018 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $9.8 $7.5 Current liabilities (14.3 ) (2.3 ) (8.1 ) (9.6 ) Pension and other benefit obligations (335.0 ) (364.1 ) (110.4 ) (104.9 ) Net amounts recognized at December 31 ($349.3 ) ($366.4 ) ($108.7 ) ($107.0 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $485.3 $505.2 $45.2 $44.5 Prior service credit (5.1 ) (5.8 ) (0.9 ) (1.1 ) $480.2 $499.4 $44.3 $43.4 Defined Benefit Pension Plans OPEB Plans IPL 2019 2018 2019 2018 Change in benefit obligation: Net benefit obligation at January 1 $533.9 $592.9 $82.5 $89.4 Service cost 6.0 7.4 1.3 1.7 Interest cost 22.7 21.4 3.4 3.1 Plan participants’ contributions — — 1.3 1.0 Actuarial (gain) loss 55.9 (44.4 ) 5.7 (4.3 ) Gross benefits paid (37.3 ) (43.4 ) (8.5 ) (8.4 ) Net benefit obligation at December 31 581.2 533.9 85.7 82.5 Change in plan assets: Fair value of plan assets at January 1 377.8 443.7 66.7 72.9 Actual return on plan assets 78.9 (26.8 ) 10.1 (1.4 ) Employer contributions 16.6 4.3 2.3 2.6 Plan participants’ contributions — — 1.3 1.0 Gross benefits paid (37.3 ) (43.4 ) (8.5 ) (8.4 ) Fair value of plan assets at December 31 436.0 377.8 71.9 66.7 Under funded status at December 31 ($145.2 ) ($156.1 ) ($13.8 ) ($15.8 ) Defined Benefit Pension Plans OPEB Plans IPL 2019 2018 2019 2018 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $6.6 $4.7 Current liabilities (0.5 ) (0.6 ) (1.8 ) (1.9 ) Pension and other benefit obligations (144.7 ) (155.5 ) (18.6 ) (18.6 ) Net amounts recognized at December 31 ($145.2 ) ($156.1 ) ($13.8 ) ($15.8 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $208.2 $218.8 $16.6 $18.8 Prior service credit (1.7 ) (1.8 ) — — $206.5 $217.0 $16.6 $18.8 Defined Benefit Pension Plans OPEB Plans WPL 2019 2018 2019 2018 Change in benefit obligation: Net benefit obligation at January 1 $506.6 $559.8 $83.2 $90.4 Service cost 3.5 4.4 1.3 1.6 Interest cost 21.5 20.2 3.3 3.1 Plan participants’ contributions — — 1.5 1.4 Actuarial (gain) loss 53.8 (40.0 ) 5.0 (2.5 ) Gross benefits paid (35.3 ) (37.8 ) (9.0 ) (10.8 ) Net benefit obligation at December 31 550.1 506.6 85.3 83.2 Change in plan assets: Fair value of plan assets at January 1 352.2 415.0 16.7 18.7 Actual return on plan assets 73.4 (25.1 ) 1.7 (0.1 ) Employer contributions 15.7 0.1 6.2 7.5 Plan participants’ contributions — — 1.5 1.4 Gross benefits paid (35.3 ) (37.8 ) (9.0 ) (10.8 ) Fair value of plan assets at December 31 406.0 352.2 17.1 16.7 Under funded status at December 31 ($144.1 ) ($154.4 ) ($68.2 ) ($66.5 ) Defined Benefit Pension Plans OPEB Plans WPL 2019 2018 2019 2018 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $3.2 $2.8 Current liabilities (0.1 ) (0.1 ) (6.0 ) (7.4 ) Pension and other benefit obligations (144.0 ) (154.3 ) (65.4 ) (61.9 ) Net amounts recognized at December 31 ($144.1 ) ($154.4 ) ($68.2 ) ($66.5 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $212.0 $223.0 $22.2 $19.9 Prior service credit (1.2 ) (1.3 ) (0.9 ) (1.1 ) $210.8 $221.7 $21.3 $18.8
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 2019 2018 2019 2018 Accumulated benefit obligations $1,234.4 $1,139.9 $213.6 $206.1 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,234.4 1,139.9 213.6 206.1 Fair value of plan assets 930.4 808.6 104.9 99.1 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,279.7 1,175.0 N/A N/A Fair value of plan assets 930.4 808.6 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2019 2018 2019 2018 Accumulated benefit obligations $557.8 $514.3 $85.7 $82.5 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 557.8 514.3 85.7 82.5 Fair value of plan assets 436.0 377.8 71.9 66.7 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 581.2 533.9 N/A N/A Fair value of plan assets 436.0 377.8 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2019 2018 2019 2018 Accumulated benefit obligations $535.9 $494.8 $85.3 $83.2 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 535.9 494.8 85.3 83.2 Fair value of plan assets 406.0 352.2 17.1 16.7 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 550.1 506.6 N/A N/A Fair value of plan assets 406.0 352.2 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 2019 2018 2019 2018 Regulatory assets $39.4 $38.2 $29.0 $27.7
Estimated Future Employer Contributions	Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2020 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $59.8 $19.4 $21.8 OPEB plans 8.1 1.8 6.0 (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 2020 2021 2022 2023 2024 2025 - 2029 Defined benefit pension benefits $86.4 $74.5 $76.6 $77.5 $78.2 $390.2 OPEB 17.7 17.7 17.5 17.1 16.8 76.6 $104.1 $92.2 $94.1 $94.6 $95.0 $466.8 IPL 2020 2021 2022 2023 2024 2025 - 2029 Defined benefit pension benefits $35.1 $35.3 $36.6 $37.7 $36.6 $182.5 OPEB 7.2 7.1 7.1 7.0 6.9 30.6 $42.3 $42.4 $43.7 $44.7 $43.5 $213.1 WPL 2020 2021 2022 2023 2024 2025 - 2029 Defined benefit pension benefits $32.0 $32.0 $32.1 $32.4 $33.1 $165.2 OPEB 7.3 7.4 7.1 6.8 6.7 30.2 $39.3 $39.4 $39.2 $39.2 $39.8 $195.4
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 2019 2018 2017 2019 2018 2017 2019 2018 2017 Compensation expense $26.0 $17.0 $15.1 $14.5 $9.4 $8.3 $10.3 $6.9 $6.4 Income tax benefits 7.3 4.9 6.2 4.2 2.8 3.4 2.8 1.9 2.6
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): 2019 2018 Carrying value $10.0 $9.8 Fair market value 20.9 16.2
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 2019 2018 2017 2019 2018 2017 Discount rate for benefit obligations 3.48% 4.34% 3.66% 3.40% 4.24% 3.53% Discount rate for net periodic cost 4.34% 3.66% 4.19% 4.24% 3.53% 3.98% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.44% 5.44% 5.80% Interest crediting rate for Alliant Energy Cash Balance Pension Plan 5.52% 5.04% 4.64% 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 Qualified Defined Benefit Pension Plan OPEB Plans IPL 2019 2018 2017 2019 2018 2017 Discount rate for benefit obligations 3.51% 4.35% 3.68% 3.39% 4.23% 3.51% Discount rate for net periodic cost 4.35% 3.68% 4.22% 4.23% 3.51% 3.95% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.60% 5.60% 6.20% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Qualified Defined Benefit Pension Plan OPEB Plans WPL 2019 2018 2017 2019 2018 2017 Discount rate for benefit obligations 3.50% 4.35% 3.69% 3.39% 4.23% 3.51% Discount rate for net periodic cost 4.35% 3.69% 4.23% 4.23% 3.51% 3.96% Expected rate of return on plan assets 7.60% 7.60% 7.60% 3.81% 3.84% 3.50% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A
Net Periodic Benefit Costs	The components of net periodic benefit costs for sponsored defined benefit pension and OPEB plans are included 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 2019 2018 2017 2019 2018 2017 Service cost $9.7 $12.1 $12.5 $3.3 $4.2 $5.0 Interest cost 49.8 46.8 51.0 8.5 7.7 8.6 Expected return on plan assets (a) (60.1 ) (69.7 ) (65.5 ) (5.0 ) (6.0 ) (6.1 ) Amortization of prior service credit (b) (0.7 ) (0.7 ) (0.4 ) (0.2 ) (0.2 ) (0.2 ) Amortization of actuarial loss (c) 36.4 35.2 37.6 3.3 3.4 3.8 Settlement losses (d) — — 0.9 — — — $35.1 $23.7 $36.1 $9.9 $9.1 $11.1 IPL Defined Benefit Pension Plans OPEB Plans 2019 2018 2017 2019 2018 2017 Service cost $6.0 $7.4 $7.3 $1.3 $1.7 $2.1 Interest cost 22.7 21.4 23.5 3.4 3.1 3.5 Expected return on plan assets (a) (28.1 ) (32.6 ) (30.8 ) (3.6 ) (4.4 ) (4.3 ) Amortization of prior service credit (b) (0.2 ) (0.2 ) (0.2 ) — — — Amortization of actuarial loss (c) 15.7 14.9 16.1 1.5 1.3 2.0 $16.1 $10.9 $15.9 $2.6 $1.7 $3.3 WPL Defined Benefit Pension Plans OPEB Plans 2019 2018 2017 2019 2018 2017 Service cost $3.5 $4.4 $4.9 $1.3 $1.6 $1.9 Interest cost 21.5 20.2 21.8 3.3 3.1 3.4 Expected return on plan assets (a) (26.2 ) (30.4 ) (28.5 ) (0.6 ) (0.6 ) (0.8 ) Amortization of prior service cost (credit) (b) (0.2 ) (0.1 ) 0.1 (0.2 ) (0.2 ) (0.2 ) Amortization of actuarial loss (c) 17.6 17.2 18.5 1.6 2.0 1.6 $16.2 $11.3 $16.8 $5.4 $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 2019 2018 2019 2018 Change in benefit obligation: Net benefit obligation at January 1 $1,175.0 $1,303.1 $206.1 $222.3 Service cost 9.7 12.1 3.3 4.2 Interest cost 49.8 46.8 8.5 7.7 Plan participants’ contributions — — 3.4 3.1 Actuarial (gain) loss 124.6 (96.2 ) 13.6 (8.2 ) Gross benefits paid (79.4 ) (90.8 ) (21.3 ) (23.0 ) Net benefit obligation at December 31 1,279.7 1,175.0 213.6 206.1 Change in plan assets: Fair value of plan assets at January 1 808.6 950.7 99.1 111.1 Actual return on plan assets 168.1 (57.8 ) 14.6 (2.6 ) Employer contributions 33.1 6.5 9.1 10.5 Plan participants’ contributions — — 3.4 3.1 Gross benefits paid (79.4 ) (90.8 ) (21.3 ) (23.0 ) Fair value of plan assets at December 31 930.4 808.6 104.9 99.1 Under funded status at December 31 ($349.3 ) ($366.4 ) ($108.7 ) ($107.0 ) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2019 2018 2019 2018 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $9.8 $7.5 Current liabilities (14.3 ) (2.3 ) (8.1 ) (9.6 ) Pension and other benefit obligations (335.0 ) (364.1 ) (110.4 ) (104.9 ) Net amounts recognized at December 31 ($349.3 ) ($366.4 ) ($108.7 ) ($107.0 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $485.3 $505.2 $45.2 $44.5 Prior service credit (5.1 ) (5.8 ) (0.9 ) (1.1 ) $480.2 $499.4 $44.3 $43.4 Defined Benefit Pension Plans OPEB Plans IPL 2019 2018 2019 2018 Change in benefit obligation: Net benefit obligation at January 1 $533.9 $592.9 $82.5 $89.4 Service cost 6.0 7.4 1.3 1.7 Interest cost 22.7 21.4 3.4 3.1 Plan participants’ contributions — — 1.3 1.0 Actuarial (gain) loss 55.9 (44.4 ) 5.7 (4.3 ) Gross benefits paid (37.3 ) (43.4 ) (8.5 ) (8.4 ) Net benefit obligation at December 31 581.2 533.9 85.7 82.5 Change in plan assets: Fair value of plan assets at January 1 377.8 443.7 66.7 72.9 Actual return on plan assets 78.9 (26.8 ) 10.1 (1.4 ) Employer contributions 16.6 4.3 2.3 2.6 Plan participants’ contributions — — 1.3 1.0 Gross benefits paid (37.3 ) (43.4 ) (8.5 ) (8.4 ) Fair value of plan assets at December 31 436.0 377.8 71.9 66.7 Under funded status at December 31 ($145.2 ) ($156.1 ) ($13.8 ) ($15.8 ) Defined Benefit Pension Plans OPEB Plans IPL 2019 2018 2019 2018 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $6.6 $4.7 Current liabilities (0.5 ) (0.6 ) (1.8 ) (1.9 ) Pension and other benefit obligations (144.7 ) (155.5 ) (18.6 ) (18.6 ) Net amounts recognized at December 31 ($145.2 ) ($156.1 ) ($13.8 ) ($15.8 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $208.2 $218.8 $16.6 $18.8 Prior service credit (1.7 ) (1.8 ) — — $206.5 $217.0 $16.6 $18.8 Defined Benefit Pension Plans OPEB Plans WPL 2019 2018 2019 2018 Change in benefit obligation: Net benefit obligation at January 1 $506.6 $559.8 $83.2 $90.4 Service cost 3.5 4.4 1.3 1.6 Interest cost 21.5 20.2 3.3 3.1 Plan participants’ contributions — — 1.5 1.4 Actuarial (gain) loss 53.8 (40.0 ) 5.0 (2.5 ) Gross benefits paid (35.3 ) (37.8 ) (9.0 ) (10.8 ) Net benefit obligation at December 31 550.1 506.6 85.3 83.2 Change in plan assets: Fair value of plan assets at January 1 352.2 415.0 16.7 18.7 Actual return on plan assets 73.4 (25.1 ) 1.7 (0.1 ) Employer contributions 15.7 0.1 6.2 7.5 Plan participants’ contributions — — 1.5 1.4 Gross benefits paid (35.3 ) (37.8 ) (9.0 ) (10.8 ) Fair value of plan assets at December 31 406.0 352.2 17.1 16.7 Under funded status at December 31 ($144.1 ) ($154.4 ) ($68.2 ) ($66.5 ) Defined Benefit Pension Plans OPEB Plans WPL 2019 2018 2019 2018 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $3.2 $2.8 Current liabilities (0.1 ) (0.1 ) (6.0 ) (7.4 ) Pension and other benefit obligations (144.0 ) (154.3 ) (65.4 ) (61.9 ) Net amounts recognized at December 31 ($144.1 ) ($154.4 ) ($68.2 ) ($66.5 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $212.0 $223.0 $22.2 $19.9 Prior service credit (1.2 ) (1.3 ) (0.9 ) (1.1 ) $210.8 $221.7 $21.3 $18.8
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 2019 2018 2019 2018 Accumulated benefit obligations $1,234.4 $1,139.9 $213.6 $206.1 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,234.4 1,139.9 213.6 206.1 Fair value of plan assets 930.4 808.6 104.9 99.1 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,279.7 1,175.0 N/A N/A Fair value of plan assets 930.4 808.6 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2019 2018 2019 2018 Accumulated benefit obligations $557.8 $514.3 $85.7 $82.5 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 557.8 514.3 85.7 82.5 Fair value of plan assets 436.0 377.8 71.9 66.7 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 581.2 533.9 N/A N/A Fair value of plan assets 436.0 377.8 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2019 2018 2019 2018 Accumulated benefit obligations $535.9 $494.8 $85.3 $83.2 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 535.9 494.8 85.3 83.2 Fair value of plan assets 406.0 352.2 17.1 16.7 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 550.1 506.6 N/A N/A Fair value of plan assets 406.0 352.2 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 2019 2018 2019 2018 Regulatory assets $39.4 $38.2 $29.0 $27.7
Estimated Future Employer Contributions	Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2020 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $59.8 $19.4 $21.8 OPEB plans 8.1 1.8 6.0 (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 2020 2021 2022 2023 2024 2025 - 2029 Defined benefit pension benefits $86.4 $74.5 $76.6 $77.5 $78.2 $390.2 OPEB 17.7 17.7 17.5 17.1 16.8 76.6 $104.1 $92.2 $94.1 $94.6 $95.0 $466.8 IPL 2020 2021 2022 2023 2024 2025 - 2029 Defined benefit pension benefits $35.1 $35.3 $36.6 $37.7 $36.6 $182.5 OPEB 7.2 7.1 7.1 7.0 6.9 30.6 $42.3 $42.4 $43.7 $44.7 $43.5 $213.1 WPL 2020 2021 2022 2023 2024 2025 - 2029 Defined benefit pension benefits $32.0 $32.0 $32.1 $32.4 $33.1 $165.2 OPEB 7.3 7.4 7.1 6.8 6.7 30.2 $39.3 $39.4 $39.2 $39.2 $39.8 $195.4
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 2019 2018 2017 2019 2018 2017 2019 2018 2017 Compensation expense $26.0 $17.0 $15.1 $14.5 $9.4 $8.3 $10.3 $6.9 $6.4 Income tax benefits 7.3 4.9 6.2 4.2 2.8 3.4 2.8 1.9 2.6
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 2019 2018 2017 2019 2018 2017 Discount rate for benefit obligations 3.48% 4.34% 3.66% 3.40% 4.24% 3.53% Discount rate for net periodic cost 4.34% 3.66% 4.19% 4.24% 3.53% 3.98% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.44% 5.44% 5.80% Interest crediting rate for Alliant Energy Cash Balance Pension Plan 5.52% 5.04% 4.64% 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 Qualified Defined Benefit Pension Plan OPEB Plans IPL 2019 2018 2017 2019 2018 2017 Discount rate for benefit obligations 3.51% 4.35% 3.68% 3.39% 4.23% 3.51% Discount rate for net periodic cost 4.35% 3.68% 4.22% 4.23% 3.51% 3.95% Expected rate of return on plan assets 7.60% 7.60% 7.60% 5.60% 5.60% 6.20% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Qualified Defined Benefit Pension Plan OPEB Plans WPL 2019 2018 2017 2019 2018 2017 Discount rate for benefit obligations 3.50% 4.35% 3.69% 3.39% 4.23% 3.51% Discount rate for net periodic cost 4.35% 3.69% 4.23% 4.23% 3.51% 3.96% Expected rate of return on plan assets 7.60% 7.60% 7.60% 3.81% 3.84% 3.50% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A
Net Periodic Benefit Costs	The components of net periodic benefit costs for sponsored defined benefit pension and OPEB plans are included 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 2019 2018 2017 2019 2018 2017 Service cost $9.7 $12.1 $12.5 $3.3 $4.2 $5.0 Interest cost 49.8 46.8 51.0 8.5 7.7 8.6 Expected return on plan assets (a) (60.1 ) (69.7 ) (65.5 ) (5.0 ) (6.0 ) (6.1 ) Amortization of prior service credit (b) (0.7 ) (0.7 ) (0.4 ) (0.2 ) (0.2 ) (0.2 ) Amortization of actuarial loss (c) 36.4 35.2 37.6 3.3 3.4 3.8 Settlement losses (d) — — 0.9 — — — $35.1 $23.7 $36.1 $9.9 $9.1 $11.1 IPL Defined Benefit Pension Plans OPEB Plans 2019 2018 2017 2019 2018 2017 Service cost $6.0 $7.4 $7.3 $1.3 $1.7 $2.1 Interest cost 22.7 21.4 23.5 3.4 3.1 3.5 Expected return on plan assets (a) (28.1 ) (32.6 ) (30.8 ) (3.6 ) (4.4 ) (4.3 ) Amortization of prior service credit (b) (0.2 ) (0.2 ) (0.2 ) — — — Amortization of actuarial loss (c) 15.7 14.9 16.1 1.5 1.3 2.0 $16.1 $10.9 $15.9 $2.6 $1.7 $3.3 WPL Defined Benefit Pension Plans OPEB Plans 2019 2018 2017 2019 2018 2017 Service cost $3.5 $4.4 $4.9 $1.3 $1.6 $1.9 Interest cost 21.5 20.2 21.8 3.3 3.1 3.4 Expected return on plan assets (a) (26.2 ) (30.4 ) (28.5 ) (0.6 ) (0.6 ) (0.8 ) Amortization of prior service cost (credit) (b) (0.2 ) (0.1 ) 0.1 (0.2 ) (0.2 ) (0.2 ) Amortization of actuarial loss (c) 17.6 17.2 18.5 1.6 2.0 1.6 $16.2 $11.3 $16.8 $5.4 $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 2019 2018 2019 2018 Change in benefit obligation: Net benefit obligation at January 1 $1,175.0 $1,303.1 $206.1 $222.3 Service cost 9.7 12.1 3.3 4.2 Interest cost 49.8 46.8 8.5 7.7 Plan participants’ contributions — — 3.4 3.1 Actuarial (gain) loss 124.6 (96.2 ) 13.6 (8.2 ) Gross benefits paid (79.4 ) (90.8 ) (21.3 ) (23.0 ) Net benefit obligation at December 31 1,279.7 1,175.0 213.6 206.1 Change in plan assets: Fair value of plan assets at January 1 808.6 950.7 99.1 111.1 Actual return on plan assets 168.1 (57.8 ) 14.6 (2.6 ) Employer contributions 33.1 6.5 9.1 10.5 Plan participants’ contributions — — 3.4 3.1 Gross benefits paid (79.4 ) (90.8 ) (21.3 ) (23.0 ) Fair value of plan assets at December 31 930.4 808.6 104.9 99.1 Under funded status at December 31 ($349.3 ) ($366.4 ) ($108.7 ) ($107.0 ) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2019 2018 2019 2018 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $9.8 $7.5 Current liabilities (14.3 ) (2.3 ) (8.1 ) (9.6 ) Pension and other benefit obligations (335.0 ) (364.1 ) (110.4 ) (104.9 ) Net amounts recognized at December 31 ($349.3 ) ($366.4 ) ($108.7 ) ($107.0 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $485.3 $505.2 $45.2 $44.5 Prior service credit (5.1 ) (5.8 ) (0.9 ) (1.1 ) $480.2 $499.4 $44.3 $43.4 Defined Benefit Pension Plans OPEB Plans IPL 2019 2018 2019 2018 Change in benefit obligation: Net benefit obligation at January 1 $533.9 $592.9 $82.5 $89.4 Service cost 6.0 7.4 1.3 1.7 Interest cost 22.7 21.4 3.4 3.1 Plan participants’ contributions — — 1.3 1.0 Actuarial (gain) loss 55.9 (44.4 ) 5.7 (4.3 ) Gross benefits paid (37.3 ) (43.4 ) (8.5 ) (8.4 ) Net benefit obligation at December 31 581.2 533.9 85.7 82.5 Change in plan assets: Fair value of plan assets at January 1 377.8 443.7 66.7 72.9 Actual return on plan assets 78.9 (26.8 ) 10.1 (1.4 ) Employer contributions 16.6 4.3 2.3 2.6 Plan participants’ contributions — — 1.3 1.0 Gross benefits paid (37.3 ) (43.4 ) (8.5 ) (8.4 ) Fair value of plan assets at December 31 436.0 377.8 71.9 66.7 Under funded status at December 31 ($145.2 ) ($156.1 ) ($13.8 ) ($15.8 ) Defined Benefit Pension Plans OPEB Plans IPL 2019 2018 2019 2018 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $6.6 $4.7 Current liabilities (0.5 ) (0.6 ) (1.8 ) (1.9 ) Pension and other benefit obligations (144.7 ) (155.5 ) (18.6 ) (18.6 ) Net amounts recognized at December 31 ($145.2 ) ($156.1 ) ($13.8 ) ($15.8 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $208.2 $218.8 $16.6 $18.8 Prior service credit (1.7 ) (1.8 ) — — $206.5 $217.0 $16.6 $18.8 Defined Benefit Pension Plans OPEB Plans WPL 2019 2018 2019 2018 Change in benefit obligation: Net benefit obligation at January 1 $506.6 $559.8 $83.2 $90.4 Service cost 3.5 4.4 1.3 1.6 Interest cost 21.5 20.2 3.3 3.1 Plan participants’ contributions — — 1.5 1.4 Actuarial (gain) loss 53.8 (40.0 ) 5.0 (2.5 ) Gross benefits paid (35.3 ) (37.8 ) (9.0 ) (10.8 ) Net benefit obligation at December 31 550.1 506.6 85.3 83.2 Change in plan assets: Fair value of plan assets at January 1 352.2 415.0 16.7 18.7 Actual return on plan assets 73.4 (25.1 ) 1.7 (0.1 ) Employer contributions 15.7 0.1 6.2 7.5 Plan participants’ contributions — — 1.5 1.4 Gross benefits paid (35.3 ) (37.8 ) (9.0 ) (10.8 ) Fair value of plan assets at December 31 406.0 352.2 17.1 16.7 Under funded status at December 31 ($144.1 ) ($154.4 ) ($68.2 ) ($66.5 ) Defined Benefit Pension Plans OPEB Plans WPL 2019 2018 2019 2018 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $3.2 $2.8 Current liabilities (0.1 ) (0.1 ) (6.0 ) (7.4 ) Pension and other benefit obligations (144.0 ) (154.3 ) (65.4 ) (61.9 ) Net amounts recognized at December 31 ($144.1 ) ($154.4 ) ($68.2 ) ($66.5 ) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $212.0 $223.0 $22.2 $19.9 Prior service credit (1.2 ) (1.3 ) (0.9 ) (1.1 ) $210.8 $221.7 $21.3 $18.8
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 2019 2018 2019 2018 Accumulated benefit obligations $1,234.4 $1,139.9 $213.6 $206.1 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,234.4 1,139.9 213.6 206.1 Fair value of plan assets 930.4 808.6 104.9 99.1 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,279.7 1,175.0 N/A N/A Fair value of plan assets 930.4 808.6 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2019 2018 2019 2018 Accumulated benefit obligations $557.8 $514.3 $85.7 $82.5 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 557.8 514.3 85.7 82.5 Fair value of plan assets 436.0 377.8 71.9 66.7 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 581.2 533.9 N/A N/A Fair value of plan assets 436.0 377.8 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2019 2018 2019 2018 Accumulated benefit obligations $535.9 $494.8 $85.3 $83.2 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 535.9 494.8 85.3 83.2 Fair value of plan assets 406.0 352.2 17.1 16.7 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 550.1 506.6 N/A N/A Fair value of plan assets 406.0 352.2 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 2019 2018 2019 2018 Regulatory assets $39.4 $38.2 $29.0 $27.7
Estimated Future Employer Contributions	Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2020 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $59.8 $19.4 $21.8 OPEB plans 8.1 1.8 6.0 (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 2020 2021 2022 2023 2024 2025 - 2029 Defined benefit pension benefits $86.4 $74.5 $76.6 $77.5 $78.2 $390.2 OPEB 17.7 17.7 17.5 17.1 16.8 76.6 $104.1 $92.2 $94.1 $94.6 $95.0 $466.8 IPL 2020 2021 2022 2023 2024 2025 - 2029 Defined benefit pension benefits $35.1 $35.3 $36.6 $37.7 $36.6 $182.5 OPEB 7.2 7.1 7.1 7.0 6.9 30.6 $42.3 $42.4 $43.7 $44.7 $43.5 $213.1 WPL 2020 2021 2022 2023 2024 2025 - 2029 Defined benefit pension benefits $32.0 $32.0 $32.1 $32.4 $33.1 $165.2 OPEB 7.3 7.4 7.1 6.8 6.7 30.2 $39.3 $39.4 $39.2 $39.2 $39.8 $195.4
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 2019 2018 2017 2019 2018 2017 2019 2018 2017 Compensation expense $26.0 $17.0 $15.1 $14.5 $9.4 $8.3 $10.3 $6.9 $6.4 Income tax benefits 7.3 4.9 6.2 4.2 2.8 3.4 2.8 1.9 2.6
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, 2019 , 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% 5% Equity securities - U.S. 6 % - 56% 25% Equity securities - international 14 % - 34% 22% Global asset securities 5 % - 15% 9% Risk parity securities 5 % - 15% 10% Fixed income securities 20 % - 40% 29% At December 31, the fair values of qualified and non-qualified defined benefit pension plan assets were as follows (in millions): 2019 2018 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $45.7 $3.7 $42.0 $— $36.4 $3.0 $33.4 $— Equity securities - U.S. 160.6 160.6 — — 130.2 130.2 — — Equity securities - international 135.9 135.9 — — 116.0 116.0 — — Global asset securities 47.2 47.2 — — 42.1 42.1 — — Fixed income securities 130.7 58.0 72.7 — 127.8 52.5 75.3 — Total assets in fair value hierarchy 520.1 $405.4 $114.7 $— 452.5 $343.8 $108.7 $— Assets measured at net asset value 410.1 355.4 Accrued investment income 0.9 1.2 Due to brokers, net (pending trades with brokers) (0.7 ) (0.5 ) Total pension plan assets $930.4 $808.6 2019 2018 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $21.4 $1.7 $19.7 $— $17.0 $1.4 $15.6 $— Equity securities - U.S. 75.2 75.2 — — 60.8 60.8 — — Equity securities - international 63.7 63.7 — — 54.2 54.2 — — Global asset securities 22.1 22.1 — — 19.7 19.7 — — Fixed income securities 61.3 27.2 34.1 — 59.7 24.5 35.2 — Total assets in fair value hierarchy 243.7 $189.9 $53.8 $— 211.4 $160.6 $50.8 $— Assets measured at net asset value 192.2 166.1 Accrued investment income 0.4 0.6 Due to brokers, net (pending trades with brokers) (0.3 ) (0.3 ) Total pension plan assets $436.0 $377.8 2019 2018 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $19.9 $1.6 $18.3 $— $15.9 $1.3 $14.6 $— Equity securities - U.S. 70.1 70.1 — — 56.7 56.7 — — Equity securities - international 59.3 59.3 — — 50.5 50.5 — — Global asset securities 20.6 20.6 — — 18.4 18.4 — — Fixed income securities 57.0 25.3 31.7 — 55.6 22.8 32.8 — Total assets in fair value hierarchy 226.9 $176.9 $50.0 $— 197.1 $149.7 $47.4 $— Assets measured at net asset value 179.0 154.8 Accrued investment income 0.4 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.2 ) Total pension plan assets $406.0 $352.2
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): 2019 2018 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $45.7 $3.7 $42.0 $— $36.4 $3.0 $33.4 $— Equity securities - U.S. 160.6 160.6 — — 130.2 130.2 — — Equity securities - international 135.9 135.9 — — 116.0 116.0 — — Global asset securities 47.2 47.2 — — 42.1 42.1 — — Fixed income securities 130.7 58.0 72.7 — 127.8 52.5 75.3 — Total assets in fair value hierarchy 520.1 $405.4 $114.7 $— 452.5 $343.8 $108.7 $— Assets measured at net asset value 410.1 355.4 Accrued investment income 0.9 1.2 Due to brokers, net (pending trades with brokers) (0.7 ) (0.5 ) Total pension plan assets $930.4 $808.6 2019 2018 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $21.4 $1.7 $19.7 $— $17.0 $1.4 $15.6 $— Equity securities - U.S. 75.2 75.2 — — 60.8 60.8 — — Equity securities - international 63.7 63.7 — — 54.2 54.2 — — Global asset securities 22.1 22.1 — — 19.7 19.7 — — Fixed income securities 61.3 27.2 34.1 — 59.7 24.5 35.2 — Total assets in fair value hierarchy 243.7 $189.9 $53.8 $— 211.4 $160.6 $50.8 $— Assets measured at net asset value 192.2 166.1 Accrued investment income 0.4 0.6 Due to brokers, net (pending trades with brokers) (0.3 ) (0.3 ) Total pension plan assets $436.0 $377.8 2019 2018 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $19.9 $1.6 $18.3 $— $15.9 $1.3 $14.6 $— Equity securities - U.S. 70.1 70.1 — — 56.7 56.7 — — Equity securities - international 59.3 59.3 — — 50.5 50.5 — — Global asset securities 20.6 20.6 — — 18.4 18.4 — — Fixed income securities 57.0 25.3 31.7 — 55.6 22.8 32.8 — Total assets in fair value hierarchy 226.9 $176.9 $50.0 $— 197.1 $149.7 $47.4 $— Assets measured at net asset value 179.0 154.8 Accrued investment income 0.4 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.2 ) Total pension plan assets $406.0 $352.2 At December 31, 2019 , 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% 5% Equity securities - U.S. 6 % - 56% 25% Equity securities - international 14 % - 34% 22% Global asset securities 5 % - 15% 9% Risk parity securities 5 % - 15% 10% Fixed income securities 20 % - 40% 29%
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): 2019 2018 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $45.7 $3.7 $42.0 $— $36.4 $3.0 $33.4 $— Equity securities - U.S. 160.6 160.6 — — 130.2 130.2 — — Equity securities - international 135.9 135.9 — — 116.0 116.0 — — Global asset securities 47.2 47.2 — — 42.1 42.1 — — Fixed income securities 130.7 58.0 72.7 — 127.8 52.5 75.3 — Total assets in fair value hierarchy 520.1 $405.4 $114.7 $— 452.5 $343.8 $108.7 $— Assets measured at net asset value 410.1 355.4 Accrued investment income 0.9 1.2 Due to brokers, net (pending trades with brokers) (0.7 ) (0.5 ) Total pension plan assets $930.4 $808.6 2019 2018 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $21.4 $1.7 $19.7 $— $17.0 $1.4 $15.6 $— Equity securities - U.S. 75.2 75.2 — — 60.8 60.8 — — Equity securities - international 63.7 63.7 — — 54.2 54.2 — — Global asset securities 22.1 22.1 — — 19.7 19.7 — — Fixed income securities 61.3 27.2 34.1 — 59.7 24.5 35.2 — Total assets in fair value hierarchy 243.7 $189.9 $53.8 $— 211.4 $160.6 $50.8 $— Assets measured at net asset value 192.2 166.1 Accrued investment income 0.4 0.6 Due to brokers, net (pending trades with brokers) (0.3 ) (0.3 ) Total pension plan assets $436.0 $377.8 2019 2018 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $19.9 $1.6 $18.3 $— $15.9 $1.3 $14.6 $— Equity securities - U.S. 70.1 70.1 — — 56.7 56.7 — — Equity securities - international 59.3 59.3 — — 50.5 50.5 — — Global asset securities 20.6 20.6 — — 18.4 18.4 — — Fixed income securities 57.0 25.3 31.7 — 55.6 22.8 32.8 — Total assets in fair value hierarchy 226.9 $176.9 $50.0 $— 197.1 $149.7 $47.4 $— Assets measured at net asset value 179.0 154.8 Accrued investment income 0.4 0.5 Due to brokers, net (pending trades with brokers) (0.3 ) (0.2 ) Total pension plan assets $406.0 $352.2 At December 31, 2019 , 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% 5% Equity securities - U.S. 6 % - 56% 25% Equity securities - international 14 % - 34% 22% Global asset securities 5 % - 15% 9% Risk parity securities 5 % - 15% 10% Fixed income securities 20 % - 40% 29%
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, 2019 , 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% 3% Equity securities - U.S. 0 % - 46% 25% Equity securities - international 0 % - 34% 2% Fixed income securities 20 % - 100% 70% At December 31, the fair values of OPEB plan assets were as follows (in millions): 2019 2018 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $3.4 $3.0 $0.4 $— $1.4 $1.1 $0.3 $— Equity securities - U.S. 3.7 3.7 — — 3.9 3.9 — — Equity securities - international 2.8 2.8 — — 2.9 2.9 — — Global asset securities 0.5 0.5 — — 0.4 0.4 — — Fixed income securities 68.5 67.8 0.7 — 68.2 67.5 0.7 — Total assets in fair value hierarchy 78.9 $77.8 $1.1 $— 76.8 $75.8 $1.0 $— Assets measured at net asset value 26.0 22.3 Total OPEB plan assets $104.9 $99.1 2019 2018 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $1.5 $1.5 $— $— $0.7 $0.7 $— $— Fixed income securities 48.3 48.3 — — 47.0 47.0 — — Total assets in fair value hierarchy 49.8 $49.8 $— $— 47.7 $47.7 $— $— Assets measured at net asset value 22.1 19.0 Total OPEB plan assets $71.9 $66.7 2019 2018 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.4 $0.4 $— $— $0.1 $0.1 $— $— Fixed income securities 16.7 16.7 — — 16.6 16.6 — — Total OPEB plan assets $17.1 $17.1 $— $— $16.7 $16.7 $— $—
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, the fair values of OPEB plan assets were as follows (in millions): 2019 2018 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $3.4 $3.0 $0.4 $— $1.4 $1.1 $0.3 $— Equity securities - U.S. 3.7 3.7 — — 3.9 3.9 — — Equity securities - international 2.8 2.8 — — 2.9 2.9 — — Global asset securities 0.5 0.5 — — 0.4 0.4 — — Fixed income securities 68.5 67.8 0.7 — 68.2 67.5 0.7 — Total assets in fair value hierarchy 78.9 $77.8 $1.1 $— 76.8 $75.8 $1.0 $— Assets measured at net asset value 26.0 22.3 Total OPEB plan assets $104.9 $99.1 2019 2018 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $1.5 $1.5 $— $— $0.7 $0.7 $— $— Fixed income securities 48.3 48.3 — — 47.0 47.0 — — Total assets in fair value hierarchy 49.8 $49.8 $— $— 47.7 $47.7 $— $— Assets measured at net asset value 22.1 19.0 Total OPEB plan assets $71.9 $66.7 2019 2018 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.4 $0.4 $— $— $0.1 $0.1 $— $— Fixed income securities 16.7 16.7 — — 16.6 16.6 — — Total OPEB plan assets $17.1 $17.1 $— $— $16.7 $16.7 $— $— December 31, 2019 , 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% 3% Equity securities - U.S. 0 % - 46% 25% Equity securities - international 0 % - 34% 2% Fixed income securities 20 % - 100% 70%
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, 2019 , 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% 3% Equity securities - U.S. 0 % - 46% 25% Equity securities - international 0 % - 34% 2% Fixed income securities 20 % - 100% 70% At December 31, the fair values of OPEB plan assets were as follows (in millions): 2019 2018 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $3.4 $3.0 $0.4 $— $1.4 $1.1 $0.3 $— Equity securities - U.S. 3.7 3.7 — — 3.9 3.9 — — Equity securities - international 2.8 2.8 — — 2.9 2.9 — — Global asset securities 0.5 0.5 — — 0.4 0.4 — — Fixed income securities 68.5 67.8 0.7 — 68.2 67.5 0.7 — Total assets in fair value hierarchy 78.9 $77.8 $1.1 $— 76.8 $75.8 $1.0 $— Assets measured at net asset value 26.0 22.3 Total OPEB plan assets $104.9 $99.1 2019 2018 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $1.5 $1.5 $— $— $0.7 $0.7 $— $— Fixed income securities 48.3 48.3 — — 47.0 47.0 — — Total assets in fair value hierarchy 49.8 $49.8 $— $— 47.7 $47.7 $— $— Assets measured at net asset value 22.1 19.0 Total OPEB plan assets $71.9 $66.7 2019 2018 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $0.4 $0.4 $— $— $0.1 $0.1 $— $— Fixed income securities 16.7 16.7 — — 16.6 16.6 — — Total OPEB plan assets $17.1 $17.1 $— $— $16.7 $16.7 $— $—
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 2019 2018 2017 2019 2018 2017 2019 2018 2017 401(k) costs $25.5 $25.1 $24.8 $13.0 $13.0 $12.8 $11.5 $11.2 $11.1
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 2019 2018 2017 2019 2018 2017 2019 2018 2017 401(k) costs $25.5 $25.1 $24.8 $13.0 $13.0 $12.8 $11.5 $11.2 $11.1
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 2019 2018 2017 2019 2018 2017 2019 2018 2017 401(k) costs $25.5 $25.1 $24.8 $13.0 $13.0 $12.8 $11.5 $11.2 $11.1
Performance Shares and Performance Units Liability Awards [Member]
Defined Benefit Plan Disclosure [Line Items]
Schedule Of Equity-based Compensation Plans Activity	Additional information related to the fair value of nonvested performance shares and performance units at December 31, 2019 , by year of grant, was as follows: Performance Shares (granted prior to 2019) Performance Units (granted prior to 2019) 2018 Grant 2017 Grant 2018 Grant 2017 Grant Nonvested awards at target 68,307 63,565 17,622 16,952 Estimated payout percentage based on performance criteria 118 % 155 % 118 % 155 % Fair values of each nonvested award $64.57 $84.82 $64.57 $84.82 Performance Shares (granted prior to 2019) Performance Units (granted prior to 2019) 2019 2018 2017 2019 2018 2017 Nonvested awards, January 1 203,188 223,511 257,599 57,761 71,737 93,320 Granted — 74,163 65,350 — 19,840 21,558 Vested (66,322 ) (90,806 ) (99,438 ) (20,131 ) (31,910 ) (37,395 ) Forfeited (4,994 ) (3,680 ) — (3,056 ) (1,906 ) (5,746 ) Nonvested awards, December 31 131,872 203,188 223,511 34,574 57,761 71,737 Performance Shares (granted prior to 2019) Performance Units (granted prior to 2019) 2019 2018 2017 2019 2018 2017 2016 Grant 2015 Grant 2014 Grant 2016 Grant 2015 Grant 2014 Grant Performance awards vested 66,322 90,806 99,438 20,131 31,910 37,395 Percentage of target number of performance awards 142.5% 137.5% 147.5% 142.5% 137.5% 147.5% Aggregate payout value (in millions) $4.3 $5.3 $5.6 $1.3 $1.4 $1.5 Payout - cash (in millions) $3.7 $4.9 $5.1 $1.3 $1.4 $1.5 Payout - common stock shares issued 6,447 5,078 5,185 N/A N/A N/A
Performance Shares Equity Awards [Member]
Defined Benefit Plan Disclosure [Line Items]
Schedule Of Equity-based Compensation Plans Activity	A summary of the 2019 performance shares activity, with amounts representing the target number of awards, was as follows: 2019 Performance Shares - Equity Weighted Average Grant Date Fair Value Nonvested awards, January 1 — $— Granted 91,816 47.23 Forfeited (17,623 ) 46.35 Nonvested awards, December 31 74,193 47.44
Performance Restricted Stock Units Equity Awards [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: 2019 2018 2017 Units Weighted Average Grant Date Fair Value Units Weighted Average Grant Date Fair Value Units Weighted Average Grant Date Fair Value Nonvested units, January 1 203,188 $37.23 132,705 $36.50 67,355 $33.96 Granted 91,816 46.10 74,163 38.60 65,350 39.12 Vested (66,322 ) 33.93 — — — — Forfeited (22,617 ) 44.00 (3,680 ) 38.60 — — Nonvested units, December 31 206,065 41.50 203,188 37.23 132,705 36.50
Restricted Stock Units Equity Awards [Member]
Defined Benefit Plan Disclosure [Line Items]
Schedule Of Equity-based Compensation Plans Activity	A summary of the 2019 restricted stock units activity was as follows: 2019 Restricted Stock Units - Equity Weighted Average Grant Date Fair Value Nonvested units, January 1 — $— Granted 105,348 45.98 Forfeited (16,067 ) 45.63 Nonvested units, December 31 89,281 46.04
Restricted Stock Unit Liability Awards [Member]
Defined Benefit Plan Disclosure [Line Items]
Schedule Of Equity-based Compensation Plans Activity	A summary of the restricted stock units granted prior to 2019 activity was as follows: 2019 2018 2017 Nonvested units, January 1 174,163 113,749 57,736 Granted — 63,568 56,013 Vested (56,849 ) — — Forfeited (4,281 ) (3,154 ) — Nonvested units, December 31 113,033 174,163 113,749

Asset Retirement Obligations (T

Asset Retirement Obligations (Tables)	12 Months Ended
	Dec. 31, 2019
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 2019 2018 2019 2018 2019 2018 Balance, January 1 $177.5 $184.5 $118.3 $134.1 $59.2 $50.4 Revisions in estimated cash flows (5.5 ) (10.1 ) (6.7 ) (10.1 ) 1.2 — Liabilities settled (8.8 ) (10.4 ) (8.4 ) (9.7 ) (0.4 ) (0.7 ) Liabilities incurred (a) 26.2 7.3 26.2 — — 7.3 Accretion expense 6.9 6.2 4.5 4.0 2.4 2.2 Balance, December 31 $196.3 $177.5 $133.9 $118.3 $62.4 $59.2 (a) In 2019, Alliant Energy and IPL recognized additional AROs related to IPL’s newly constructed Upland Prairie and English Farms wind sites. The increases in AROs resulted in corresponding increases in property, plant and equipment, net on the respective balance sheets.
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 2019 2018 2019 2018 2019 2018 Balance, January 1 $177.5 $184.5 $118.3 $134.1 $59.2 $50.4 Revisions in estimated cash flows (5.5 ) (10.1 ) (6.7 ) (10.1 ) 1.2 — Liabilities settled (8.8 ) (10.4 ) (8.4 ) (9.7 ) (0.4 ) (0.7 ) Liabilities incurred (a) 26.2 7.3 26.2 — — 7.3 Accretion expense 6.9 6.2 4.5 4.0 2.4 2.2 Balance, December 31 $196.3 $177.5 $133.9 $118.3 $62.4 $59.2 (a) In 2019, Alliant Energy and IPL recognized additional AROs related to IPL’s newly constructed Upland Prairie and English Farms wind sites. The increases in AROs resulted in corresponding increases in property, plant and equipment, net on the respective balance sheets.
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 2019 2018 2019 2018 2019 2018 Balance, January 1 $177.5 $184.5 $118.3 $134.1 $59.2 $50.4 Revisions in estimated cash flows (5.5 ) (10.1 ) (6.7 ) (10.1 ) 1.2 — Liabilities settled (8.8 ) (10.4 ) (8.4 ) (9.7 ) (0.4 ) (0.7 ) Liabilities incurred (a) 26.2 7.3 26.2 — — 7.3 Accretion expense 6.9 6.2 4.5 4.0 2.4 2.2 Balance, December 31 $196.3 $177.5 $133.9 $118.3 $62.4 $59.2 (a) In 2019, Alliant Energy and IPL recognized additional AROs related to IPL’s newly constructed Upland Prairie and English Farms wind sites. The increases in AROs resulted in corresponding increases in property, plant and equipment, net on the respective balance sheets.

Derivative Instruments (Tables)

Derivative Instruments (Tables)	12 Months Ended
	Dec. 31, 2019
Derivative [Line Items]
Notional Amounts of Derivative Instruments	As of December 31, 2019 , 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 11,671 2020 189,287 2020-2026 6,194 2020-2021 5,040 2020-2021 IPL 5,497 2020 102,758 2020-2026 2,700 2020-2021 — WPL 6,174 2020 86,529 2020-2026 3,494 2020-2021 5,040 2020-2021
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 2019 2018 2019 2018 2019 2018 Current derivative assets $15.8 $24.6 $12.1 $16.1 $3.7 $8.5 Non-current derivative assets 11.0 3.7 10.2 1.6 0.8 2.1 Current derivative liabilities 19.0 5.6 8.9 3.1 10.1 2.5 Non-current derivative liabilities 18.6 17.7 8.5 8.1 10.1 9.6
IPL [Member]
Derivative [Line Items]
Notional Amounts of Derivative Instruments	As of December 31, 2019 , 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 11,671 2020 189,287 2020-2026 6,194 2020-2021 5,040 2020-2021 IPL 5,497 2020 102,758 2020-2026 2,700 2020-2021 — WPL 6,174 2020 86,529 2020-2026 3,494 2020-2021 5,040 2020-2021
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 2019 2018 2019 2018 2019 2018 Current derivative assets $15.8 $24.6 $12.1 $16.1 $3.7 $8.5 Non-current derivative assets 11.0 3.7 10.2 1.6 0.8 2.1 Current derivative liabilities 19.0 5.6 8.9 3.1 10.1 2.5 Non-current derivative liabilities 18.6 17.7 8.5 8.1 10.1 9.6
WPL [Member]
Derivative [Line Items]
Notional Amounts of Derivative Instruments	As of December 31, 2019 , 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 11,671 2020 189,287 2020-2026 6,194 2020-2021 5,040 2020-2021 IPL 5,497 2020 102,758 2020-2026 2,700 2020-2021 — WPL 6,174 2020 86,529 2020-2026 3,494 2020-2021 5,040 2020-2021
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 2019 2018 2019 2018 2019 2018 Current derivative assets $15.8 $24.6 $12.1 $16.1 $3.7 $8.5 Non-current derivative assets 11.0 3.7 10.2 1.6 0.8 2.1 Current derivative liabilities 19.0 5.6 8.9 3.1 10.1 2.5 Non-current derivative liabilities 18.6 17.7 8.5 8.1 10.1 9.6

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2019
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 2019 2018 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $26.8 $— $4.8 $22.0 $26.8 $28.3 $— $8.9 $19.4 $28.3 Deferred proceeds 187.7 — — 187.7 187.7 119.4 — — 119.4 119.4 Liabilities and equity: Derivatives 37.6 — 36.8 0.8 37.6 23.3 — 16.1 7.2 23.3 Long-term debt (incl. current maturities) 6,190.2 — 6,917.9 2.0 6,919.9 5,502.8 — 5,858.4 2.4 5,860.8 IPL 2019 2018 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $22.3 $— $2.8 $19.5 $22.3 $17.7 $— $4.0 $13.7 $17.7 Deferred proceeds 187.7 — — 187.7 187.7 119.4 — — 119.4 119.4 Liabilities and equity: Derivatives 17.4 — 16.6 0.8 17.4 11.2 — 6.5 4.7 11.2 Long-term debt (incl. current maturities) 3,147.3 — 3,489.1 — 3,489.1 2,552.3 — 2,691.2 — 2,691.2 WPL 2019 2018 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $4.5 $— $2.0 $2.5 $4.5 $10.6 $— $4.9 $5.7 $10.6 Liabilities and equity: Derivatives 20.2 — 20.2 — 20.2 12.1 — 9.6 2.5 12.1 Long-term debt (incl. current maturities) 1,932.7 — 2,268.2 — 2,268.2 1,834.9 — 2,043.7 — 2,043.7
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 2019 2018 2019 2018 Beginning balance, January 1 $12.2 ($12.2 ) $119.4 $222.1 Total net gains included in changes in net assets (realized/unrealized) 8.2 9.1 — — Transfers out of Level 3 (a) 4.0 16.1 — — Purchases 13.8 26.7 — — Sales (0.2 ) (0.5 ) — — Settlements (b) (16.8 ) (27.0 ) 68.3 (102.7 ) Ending balance, December 31 $21.2 $12.2 $187.7 $119.4 The amount of total net gains for the period included in changes in net assets attributable to the change in unrealized gains relating to assets and liabilities held at December 31 $11.4 $10.7 $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2019 2018 2019 2018 Beginning balance, January 1 $9.0 ($1.4 ) $119.4 $222.1 Total net gains (losses) included in changes in net assets (realized/unrealized) 10.7 (0.5 ) — — Transfers out of Level 3 (a) 2.6 11.0 — — Purchases 9.5 22.5 — — Sales (0.1 ) (0.4 ) — — Settlements (b) (13.0 ) (22.2 ) 68.3 (102.7 ) Ending balance, December 31 $18.7 $9.0 $187.7 $119.4 The amount of total net gains for the period included in changes in net assets attributable to the change in unrealized gains relating to assets and liabilities held at December 31 $12.6 $0.2 $— $— WPL Commodity Contract Derivative Assets and (Liabilities), net 2019 2018 Beginning balance, January 1 $3.2 ($10.8 ) Total net gains (losses) included in changes in net assets (realized/unrealized) (2.5 ) 9.6 Transfers out of Level 3 (a) 1.4 5.1 Purchases 4.3 4.2 Sales (0.1 ) (0.1 ) Settlements (3.8 ) (4.8 ) Ending balance, December 31 $2.5 $3.2 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 ($1.2 ) $10.5 (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 FTR and natural gas commodity contracts categorized as Level 3 was recognized as net derivative assets at December 31 as follows (in millions): Alliant Energy IPL WPL Excluding FTRs FTRs Excluding FTRs FTRs Excluding FTRs FTRs 2019 $14.6 $6.6 $13.6 $5.1 $1.0 $1.5 2018 3.2 9.0 1.8 7.2 1.4 1.8
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 2019 2018 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $26.8 $— $4.8 $22.0 $26.8 $28.3 $— $8.9 $19.4 $28.3 Deferred proceeds 187.7 — — 187.7 187.7 119.4 — — 119.4 119.4 Liabilities and equity: Derivatives 37.6 — 36.8 0.8 37.6 23.3 — 16.1 7.2 23.3 Long-term debt (incl. current maturities) 6,190.2 — 6,917.9 2.0 6,919.9 5,502.8 — 5,858.4 2.4 5,860.8 IPL 2019 2018 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $22.3 $— $2.8 $19.5 $22.3 $17.7 $— $4.0 $13.7 $17.7 Deferred proceeds 187.7 — — 187.7 187.7 119.4 — — 119.4 119.4 Liabilities and equity: Derivatives 17.4 — 16.6 0.8 17.4 11.2 — 6.5 4.7 11.2 Long-term debt (incl. current maturities) 3,147.3 — 3,489.1 — 3,489.1 2,552.3 — 2,691.2 — 2,691.2 WPL 2019 2018 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $4.5 $— $2.0 $2.5 $4.5 $10.6 $— $4.9 $5.7 $10.6 Liabilities and equity: Derivatives 20.2 — 20.2 — 20.2 12.1 — 9.6 2.5 12.1 Long-term debt (incl. current maturities) 1,932.7 — 2,268.2 — 2,268.2 1,834.9 — 2,043.7 — 2,043.7
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 2019 2018 2019 2018 Beginning balance, January 1 $12.2 ($12.2 ) $119.4 $222.1 Total net gains included in changes in net assets (realized/unrealized) 8.2 9.1 — — Transfers out of Level 3 (a) 4.0 16.1 — — Purchases 13.8 26.7 — — Sales (0.2 ) (0.5 ) — — Settlements (b) (16.8 ) (27.0 ) 68.3 (102.7 ) Ending balance, December 31 $21.2 $12.2 $187.7 $119.4 The amount of total net gains for the period included in changes in net assets attributable to the change in unrealized gains relating to assets and liabilities held at December 31 $11.4 $10.7 $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2019 2018 2019 2018 Beginning balance, January 1 $9.0 ($1.4 ) $119.4 $222.1 Total net gains (losses) included in changes in net assets (realized/unrealized) 10.7 (0.5 ) — — Transfers out of Level 3 (a) 2.6 11.0 — — Purchases 9.5 22.5 — — Sales (0.1 ) (0.4 ) — — Settlements (b) (13.0 ) (22.2 ) 68.3 (102.7 ) Ending balance, December 31 $18.7 $9.0 $187.7 $119.4 The amount of total net gains for the period included in changes in net assets attributable to the change in unrealized gains relating to assets and liabilities held at December 31 $12.6 $0.2 $— $— WPL Commodity Contract Derivative Assets and (Liabilities), net 2019 2018 Beginning balance, January 1 $3.2 ($10.8 ) Total net gains (losses) included in changes in net assets (realized/unrealized) (2.5 ) 9.6 Transfers out of Level 3 (a) 1.4 5.1 Purchases 4.3 4.2 Sales (0.1 ) (0.1 ) Settlements (3.8 ) (4.8 ) Ending balance, December 31 $2.5 $3.2 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 ($1.2 ) $10.5 (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 FTR and natural gas commodity contracts categorized as Level 3 was recognized as net derivative assets at December 31 as follows (in millions): Alliant Energy IPL WPL Excluding FTRs FTRs Excluding FTRs FTRs Excluding FTRs FTRs 2019 $14.6 $6.6 $13.6 $5.1 $1.0 $1.5 2018 3.2 9.0 1.8 7.2 1.4 1.8
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 2019 2018 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $26.8 $— $4.8 $22.0 $26.8 $28.3 $— $8.9 $19.4 $28.3 Deferred proceeds 187.7 — — 187.7 187.7 119.4 — — 119.4 119.4 Liabilities and equity: Derivatives 37.6 — 36.8 0.8 37.6 23.3 — 16.1 7.2 23.3 Long-term debt (incl. current maturities) 6,190.2 — 6,917.9 2.0 6,919.9 5,502.8 — 5,858.4 2.4 5,860.8 IPL 2019 2018 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $22.3 $— $2.8 $19.5 $22.3 $17.7 $— $4.0 $13.7 $17.7 Deferred proceeds 187.7 — — 187.7 187.7 119.4 — — 119.4 119.4 Liabilities and equity: Derivatives 17.4 — 16.6 0.8 17.4 11.2 — 6.5 4.7 11.2 Long-term debt (incl. current maturities) 3,147.3 — 3,489.1 — 3,489.1 2,552.3 — 2,691.2 — 2,691.2 WPL 2019 2018 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $4.5 $— $2.0 $2.5 $4.5 $10.6 $— $4.9 $5.7 $10.6 Liabilities and equity: Derivatives 20.2 — 20.2 — 20.2 12.1 — 9.6 2.5 12.1 Long-term debt (incl. current maturities) 1,932.7 — 2,268.2 — 2,268.2 1,834.9 — 2,043.7 — 2,043.7
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 2019 2018 2019 2018 Beginning balance, January 1 $12.2 ($12.2 ) $119.4 $222.1 Total net gains included in changes in net assets (realized/unrealized) 8.2 9.1 — — Transfers out of Level 3 (a) 4.0 16.1 — — Purchases 13.8 26.7 — — Sales (0.2 ) (0.5 ) — — Settlements (b) (16.8 ) (27.0 ) 68.3 (102.7 ) Ending balance, December 31 $21.2 $12.2 $187.7 $119.4 The amount of total net gains for the period included in changes in net assets attributable to the change in unrealized gains relating to assets and liabilities held at December 31 $11.4 $10.7 $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2019 2018 2019 2018 Beginning balance, January 1 $9.0 ($1.4 ) $119.4 $222.1 Total net gains (losses) included in changes in net assets (realized/unrealized) 10.7 (0.5 ) — — Transfers out of Level 3 (a) 2.6 11.0 — — Purchases 9.5 22.5 — — Sales (0.1 ) (0.4 ) — — Settlements (b) (13.0 ) (22.2 ) 68.3 (102.7 ) Ending balance, December 31 $18.7 $9.0 $187.7 $119.4 The amount of total net gains for the period included in changes in net assets attributable to the change in unrealized gains relating to assets and liabilities held at December 31 $12.6 $0.2 $— $— WPL Commodity Contract Derivative Assets and (Liabilities), net 2019 2018 Beginning balance, January 1 $3.2 ($10.8 ) Total net gains (losses) included in changes in net assets (realized/unrealized) (2.5 ) 9.6 Transfers out of Level 3 (a) 1.4 5.1 Purchases 4.3 4.2 Sales (0.1 ) (0.1 ) Settlements (3.8 ) (4.8 ) Ending balance, December 31 $2.5 $3.2 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 ($1.2 ) $10.5 (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 FTR and natural gas commodity contracts categorized as Level 3 was recognized as net derivative assets at December 31 as follows (in millions): Alliant Energy IPL WPL Excluding FTRs FTRs Excluding FTRs FTRs Excluding FTRs FTRs 2019 $14.6 $6.6 $13.6 $5.1 $1.0 $1.5 2018 3.2 9.0 1.8 7.2 1.4 1.8

Commitments And Contingencies (

Commitments And Contingencies (Tables)	12 Months Ended
	Dec. 31, 2019
Long-term Purchase Commitment [Line Items]
Other Purchase Commitments	At December 31, 2019 , the related minimum future commitments were as follows (in millions): Alliant Energy 2020 2021 2022 2023 2024 Thereafter Total Purchased power (a) $111 $— $— $— $— $— $111 Natural gas 257 183 139 109 77 187 952 Coal (b) 75 27 10 10 — — 122 Other (c) 56 13 11 9 7 12 108 $499 $223 $160 $128 $84 $199 $1,293 IPL 2020 2021 2022 2023 2024 Thereafter Total Purchased power (a) $110 $— $— $— $— $— $110 Natural gas 135 101 77 67 55 89 524 Coal (b) 46 21 10 10 — — 87 Other (c) 24 4 4 3 1 10 46 $315 $126 $91 $80 $56 $99 $767 WPL 2020 2021 2022 2023 2024 Thereafter Total Purchased power $1 $— $— $— $— $— $1 Natural gas 122 82 62 42 22 98 428 Coal (b) 29 6 — — — — 35 Other (c) 24 1 1 1 1 1 29 $176 $89 $63 $43 $23 $99 $493 (a) Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. As a result of an amendment to shorten the term of the DAEC PPA, Alliant Energy’s and IPL’s amounts include minimum future commitments related to IPL’s purchase of capacity and the resulting energy from DAEC through September 2020, and do not include the September 2020 buyout payment of $110 million . (b) Corporate Services has historically entered into system-wide coal contracts on behalf of IPL and WPL that include minimum future commitments. Such commitments were assigned to IPL and WPL based on information available as of December 31, 2019 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, 2019 .
Schedule of Environmental Liabilities	At December 31, 2019 , 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, 2019 , such amounts for WPL were not material. Alliant Energy IPL Range of estimated future costs $14 - $30 $12 - $25 Current and non-current environmental liabilities 18 15
IPL [Member]
Long-term Purchase Commitment [Line Items]
Other Purchase Commitments	At December 31, 2019 , the related minimum future commitments were as follows (in millions): Alliant Energy 2020 2021 2022 2023 2024 Thereafter Total Purchased power (a) $111 $— $— $— $— $— $111 Natural gas 257 183 139 109 77 187 952 Coal (b) 75 27 10 10 — — 122 Other (c) 56 13 11 9 7 12 108 $499 $223 $160 $128 $84 $199 $1,293 IPL 2020 2021 2022 2023 2024 Thereafter Total Purchased power (a) $110 $— $— $— $— $— $110 Natural gas 135 101 77 67 55 89 524 Coal (b) 46 21 10 10 — — 87 Other (c) 24 4 4 3 1 10 46 $315 $126 $91 $80 $56 $99 $767 WPL 2020 2021 2022 2023 2024 Thereafter Total Purchased power $1 $— $— $— $— $— $1 Natural gas 122 82 62 42 22 98 428 Coal (b) 29 6 — — — — 35 Other (c) 24 1 1 1 1 1 29 $176 $89 $63 $43 $23 $99 $493 (a) Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. As a result of an amendment to shorten the term of the DAEC PPA, Alliant Energy’s and IPL’s amounts include minimum future commitments related to IPL’s purchase of capacity and the resulting energy from DAEC through September 2020, and do not include the September 2020 buyout payment of $110 million . (b) Corporate Services has historically entered into system-wide coal contracts on behalf of IPL and WPL that include minimum future commitments. Such commitments were assigned to IPL and WPL based on information available as of December 31, 2019 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, 2019 .
Schedule of Environmental Liabilities	At December 31, 2019 , 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, 2019 , such amounts for WPL were not material. Alliant Energy IPL Range of estimated future costs $14 - $30 $12 - $25 Current and non-current environmental liabilities 18 15
WPL [Member]
Long-term Purchase Commitment [Line Items]
Other Purchase Commitments	At December 31, 2019 , the related minimum future commitments were as follows (in millions): Alliant Energy 2020 2021 2022 2023 2024 Thereafter Total Purchased power (a) $111 $— $— $— $— $— $111 Natural gas 257 183 139 109 77 187 952 Coal (b) 75 27 10 10 — — 122 Other (c) 56 13 11 9 7 12 108 $499 $223 $160 $128 $84 $199 $1,293 IPL 2020 2021 2022 2023 2024 Thereafter Total Purchased power (a) $110 $— $— $— $— $— $110 Natural gas 135 101 77 67 55 89 524 Coal (b) 46 21 10 10 — — 87 Other (c) 24 4 4 3 1 10 46 $315 $126 $91 $80 $56 $99 $767 WPL 2020 2021 2022 2023 2024 Thereafter Total Purchased power $1 $— $— $— $— $— $1 Natural gas 122 82 62 42 22 98 428 Coal (b) 29 6 — — — — 35 Other (c) 24 1 1 1 1 1 29 $176 $89 $63 $43 $23 $99 $493 (a) Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. As a result of an amendment to shorten the term of the DAEC PPA, Alliant Energy’s and IPL’s amounts include minimum future commitments related to IPL’s purchase of capacity and the resulting energy from DAEC through September 2020, and do not include the September 2020 buyout payment of $110 million . (b) Corporate Services has historically entered into system-wide coal contracts on behalf of IPL and WPL that include minimum future commitments. Such commitments were assigned to IPL and WPL based on information available as of December 31, 2019 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, 2019 .

Segments Of Business (Tables)

Segments Of Business (Tables)	12 Months Ended
	Dec. 31, 2019
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 2019 Electric Gas Other Total Parent and Other Consolidated Revenues $3,063.6 $455.2 $46.5 $3,565.3 $82.4 $3,647.7 Depreciation and amortization 512.7 46.4 3.2 562.3 4.9 567.2 Operating income 678.9 69.8 1.3 750.0 27.7 777.7 Interest expense 229.1 43.8 272.9 Equity income from unconsolidated investments, net (1.4 ) — — (1.4 ) (51.6 ) (53.0 ) Income tax expense (benefit) 73.4 (4.7 ) 68.7 Net income attributable to Alliant Energy common shareowners 517.1 40.1 557.2 Total assets 13,659.0 1,268.5 856.5 15,784.0 916.7 16,700.7 Investments in equity method subsidiaries 9.5 — — 9.5 448.8 458.3 Construction and acquisition expenditures 1,438.4 99.7 0.3 1,538.4 101.7 1,640.1 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
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): 2019 Electric Gas Other Total Revenues $1,781.2 $264.2 $44.2 $2,089.6 Depreciation and amortization 294.9 28.6 3.2 326.7 Operating income 359.7 38.9 4.2 402.8 Interest expense 126.9 Income taxes 24.1 Net income available for common stock 284.1 Total assets 8,074.6 733.8 469.1 9,277.5 Construction and acquisition expenditures 963.4 55.9 0.3 1,019.6 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 ) Net income 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 ) Net income 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
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): 2019 Electric Gas Other Total Revenues $1,282.4 $191.0 $2.3 $1,475.7 Depreciation and amortization 217.8 17.8 — 235.6 Operating income (loss) 319.2 30.9 (2.9 ) 347.2 Interest expense 102.2 Income taxes 49.3 Net income 233.0 Total assets 5,584.4 534.7 387.4 6,506.5 Construction and acquisition expenditures 475.0 43.8 — 518.8 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 Net income 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 Net income 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

Related Parties (Tables)

Related Parties (Tables)	12 Months Ended
	Dec. 31, 2019
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 2019 2018 2017 2019 2018 2017 Corporate Services billings $185 $170 $177 $142 $132 $135 Sales credited 68 48 23 7 28 13 Purchases billed 331 358 364 120 81 115
Net Intercompany Payables	As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2019 2018 IPL $112 $95 WPL 85 71
Related Amounts Billed Between Parties	The related amounts billed between the parties were as follows (in millions): 2019 2018 2017 ATC billings to WPL $109 $106 $105 WPL billings to ATC 13 11 10
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 2019 2018 2017 2019 2018 2017 Corporate Services billings $185 $170 $177 $142 $132 $135 Sales credited 68 48 23 7 28 13 Purchases billed 331 358 364 120 81 115
Net Intercompany Payables	As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2019 2018 IPL $112 $95 WPL 85 71
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 2019 2018 2017 2019 2018 2017 Corporate Services billings $185 $170 $177 $142 $132 $135 Sales credited 68 48 23 7 28 13 Purchases billed 331 358 364 120 81 115
Net Intercompany Payables	As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2019 2018 IPL $112 $95 WPL 85 71
Related Amounts Billed Between Parties	The related amounts billed between the parties were as follows (in millions): 2019 2018 2017 ATC billings to WPL $109 $106 $105 WPL billings to ATC 13 11 10

Selected Consolidated Quarter_2

Selected Consolidated Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2019
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. 2019 2018 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions, except per share data) Revenues $987.2 $790.2 $990.2 $880.1 $916.3 $816.1 $928.6 $873.5 Operating income 176.8 149.8 290.2 160.9 165.7 151.2 256.1 121.4 Net income attributable to Alliant Energy common shareowners 125.1 94.6 226.0 111.5 120.9 100.4 205.5 85.3 Earnings per weighted average common share attributable to Alliant Energy common shareowners 0.53 0.40 0.94 0.46 0.52 0.43 0.87 0.36
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. 2019 2018 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Revenues $555.1 $441.2 $596.5 $496.8 $525.8 $474.8 $547.6 $494.1 Operating income 73.5 74.5 173.5 81.3 75.6 77.7 143.1 54.4 Net income 55.9 47.5 143.7 47.2 49.3 54.2 129.1 41.6 Net income available for common stock 53.3 45.0 141.1 44.7 46.7 51.7 126.5 39.1
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. 2019 2018 March 31 June 30 Sep. 30 Dec. 31 March 31 June 30 Sep. 30 Dec. 31 (in millions) Revenues $415.2 $326.1 $372.1 $362.3 $381.7 $330.8 $370.7 $369.4 Operating income 98.1 67.0 108.3 73.8 84.0 63.4 104.2 61.3 Net income 65.7 42.0 75.5 49.8 54.0 39.8 76.3 38.0

Condensed Parent Company Fina_2

Condensed Parent Company Financial Statements (Tables)	12 Months Ended
	Dec. 31, 2019
Condensed Financial Information Disclosure [Abstract]
Condensed Statements of Income	ALLIANT ENERGY CORPORATION (Parent Company Only) Year Ended December 31, CONDENSED STATEMENTS OF INCOME 2019 2018 2017 (in millions) Operating expenses $2 $5 $2 Operating loss (2 ) (5 ) (2 ) Other (income) and deductions: Equity earnings from consolidated subsidiaries (562 ) (523 ) (457 ) Interest expense 9 4 3 Other 7 2 — Total other (income) and deductions (546 ) (517 ) (454 ) Income before income taxes 544 512 452 Income tax benefit (15 ) (1 ) (6 ) Net income $559 $513 $458 Refer to accompanying Notes to Condensed Financial Statements.
Condensed Balance Sheets	ALLIANT ENERGY CORPORATION (Parent Company Only) December 31, CONDENSED BALANCE SHEETS 2019 2018 (in millions) ASSETS Current assets: Notes receivable from affiliated companies $27 $23 Other 6 4 Total current assets 33 27 Investments: Investments in consolidated subsidiaries 6,017 5,518 Other 1 1 Total investments 6,018 5,519 Other assets 89 81 Total assets $6,140 $5,627 LIABILITIES AND EQUITY Current liabilities: Commercial paper $169 $285 Notes payable to affiliated companies 731 719 Other 19 21 Total current liabilities 919 1,025 Other liabilities 15 17 Common equity: Common stock and additional paid-in capital 2,448 2,048 Retained earnings 2,766 2,545 Accumulated other comprehensive income 2 2 Shares in deferred compensation trust (10 ) (10 ) Total common equity 5,206 4,585 Total liabilities and equity $6,140 $5,627 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 2019 2018 2017 (in millions) Net cash flows from operating activities $305 $311 $273 Cash flows used for investing activities: Capital contributions to consolidated subsidiaries (250 ) (625 ) (290 ) Net change in notes receivable from and payable to affiliates 8 441 54 Net cash flows used for investing activities (242 ) (184 ) (236 ) Cash flows used for financing activities: Common stock dividends (338 ) (312 ) (288 ) Proceeds from issuance of common stock, net 390 197 150 Net change in commercial paper (116 ) (10 ) 103 Other 1 (2 ) (2 ) Net cash flows used for financing activities (63 ) (127 ) (37 ) 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 ($9 ) ($4 ) ($3 ) Income taxes, net 14 5 — Refer to accompanying Notes to Condensed Financial Statements.

Summary Of Significant Accoun_4

Summary Of Significant Accounting Policies (Narrative) (Details)	12 Months Ended
	Dec. 31, 2019customerMW
General:
Electric capacity of wind farm (in megawatts)	1,000
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%
WPL [Member]
Property, Plant and Equipment:
AFUDC accrual recorded, percentage of estimated CWIP	50.00%
AFUDC rates, projects with approval	100.00%
Alliant Energy Finance, LLC [Member]
General:
Ownership interest	50.00%
Electric capacity of wind farm (in megawatts)	225
Sheboygan Falls Energy Facility [Member]
General:
Fossil-fueled EGU capacity (in megawatts)	347
Finance lease, lease term	20 years

Summary Of Significant Accoun_5

Summary Of Significant Accounting Policies (Schedule Of Average Rates Of Depreciation) (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Electric - generation [Member] | IPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	3.80%	3.60%	3.50%
Electric - generation [Member] | WPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	3.60%	3.60%	3.50%
Electric - distribution [Member] | IPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	2.90%	2.80%	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	5.30%	4.70%	4.50%
Electric - other [Member] | WPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	5.80%	5.70%	6.90%
Gas [Member] | IPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	3.30%	3.20%	3.40%
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	5.90%	5.20%	4.00%
Other [Member] | WPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	5.60%	5.80%	6.00%

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, 2019	Dec. 31, 2018	Dec. 31, 2017
IPL [Member] | IPL (Marshalltown CWIP) [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Recovery rates			7.80%
IPL [Member] | IPL (Wind generation CWIP) [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Recovery rates	7.40%	7.50%	7.60%
IPL [Member] | IPL (other CWIP) [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Recovery rates	7.50%	7.50%	7.60%
WPL [Member] | WPL (retail jurisdiction) [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Recovery rates	6.80%	7.70%	7.60%
WPL [Member] | WPL (wholesale jurisdiction) [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Recovery rates	6.90%	7.20%	6.00%

Regulatory Matters (Narrative)

Regulatory Matters (Narrative) (Details) $ in Millions	1 Months Ended	12 Months Ended
	Sep. 30, 2020USD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)MW	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Regulatory Matters [Line Items]
Electric capacity of wind farm (in megawatts) | MW			1,000
Regulatory liabilities			$ 1,423.6	$ 1,493.2
IPL [Member]
Regulatory Matters [Line Items]
Regulatory assets not earning a return			78
Regulatory liabilities			714.7	754.9
WPL [Member]
Regulatory Matters [Line Items]
Regulatory assets not earning a return			6
Regulatory liabilities			708.9	738.3
2020 Test Period Retail Electric [Member] | IPL [Member]
Regulatory Matters [Line Items]
Interim rate increase (decrease), amount			90
2020 Test Period Retail Electric [Member] | IPL [Member] | Tax-related [Member]
Regulatory Matters [Line Items]
Regulatory liabilities			27
2020 Test Period Retail Electric [Member] | IPL [Member] | Revenue Subject to Refund [Member]
Regulatory Matters [Line Items]
Regulatory liabilities			$ 8
2016 Test Year Retail Electric [Member] | IPL [Member]
Regulatory Matters [Line Items]
Interim rate increase (decrease), amount					$ 102
Authorized increase (decrease) in final rates, amount				$ 130
Forward Contracts [Member] | IUB [Member] | IPL [Member]
Regulatory Matters [Line Items]
Recovery period, authorized			10 years
DAEC PPA [Member]
Regulatory Matters [Line Items]
PPA term amendment			5 years
Recovery period, authorized			5 years
Forecast [Member] | 2020 Test Period Retail Electric [Member] | IPL [Member]
Regulatory Matters [Line Items]
Authorized increase (decrease) in final rates, amount		$ 127
Billing credit, period		12 months
Forecast [Member] | 2020 Test Period Retail Gas [Member] | IPL [Member]
Regulatory Matters [Line Items]
Authorized increase (decrease) in final rates, amount		$ 12
Forecast [Member] | DAEC PPA [Member]
Regulatory Matters [Line Items]
Buyout payment	$ 110

Regulatory Matters (Regulatory

Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Regulatory Assets [Line Items]
Regulatory assets	$ 1,844.7	$ 1,737.3
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	817.6	820.6
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	524	542.3
Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	134	111.6
AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	111.8	110.8
IPL's DAEC PPA amendment [Member]
Regulatory Assets [Line Items]
Regulatory assets	108.2	0
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	39.5	28
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	21.1	23.6
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	88.5	100.4
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,399.3	1,279
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	776.8	783.1
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	262.5	274
IPL [Member] | Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	87.9	55.4
IPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	76.2	76.3
IPL [Member] | IPL's DAEC PPA amendment [Member]
Regulatory Assets [Line Items]
Regulatory assets	108.2	0
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	18.3	15.1
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	21.1	23.6
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	48.3	51.5
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	445.4	458.3
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	40.8	37.5
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	261.5	268.3
WPL [Member] | Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	46.1	56.2
WPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	35.6	34.5
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	21.2	12.9
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 40.2	$ 48.9

Regulatory Matters (Assets Reti

Regulatory Matters (Assets Retired Early (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Regulatory Assets [Line Items]
Regulatory assets	$ 1,844.7	$ 1,737.3
Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	134	111.6
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,399.3	1,279
IPL [Member] | Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	87.9	55.4
IPL [Member] | Assets retired early [Member] | Sutherland Units 1 and 3 [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 28.5
Recovery period, authorized	10 years
IPL [Member] | Assets retired early [Member] | M.L. Kapp Unit 2 [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 23.6
Recovery period, authorized	10 years
IPL [Member] | Assets retired early [Member] | Analog Electric Meters [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 35.8
Recovery period, authorized	10 years
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 445.4	458.3
WPL [Member] | Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	46.1	$ 56.2
WPL [Member] | Assets retired early [Member] | Nelson Dewey Units 1 and 2 and Edgewater Unit 3 [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 20.2
Recovery period, authorized	10 years
WPL [Member] | Assets retired early [Member] | Edgewater Unit 4 [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 25.9
Recovery period, authorized	10 years

Regulatory Matters (Regulator_2

Regulatory Matters (Regulatory Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 1,423.6	$ 1,493.2
Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	835.6	890.6
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	387.7	401.2
Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	88.6	104
Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	24.2	16.8
WPL's earnings sharing mechanism [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	21.9	25.4
Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	19.9	18.5
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	45.7	36.7
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	714.7	754.9
IPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	350.9	390.1
IPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	257	273.3
IPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	51.3	47.7
IPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	8.8	11.9
IPL [Member] | Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	17.4	10.2
IPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	29.3	21.7
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	708.9	738.3
WPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	484.7	500.5
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	130.7	127.9
WPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	37.3	56.3
WPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	15.4	4.9
WPL [Member] | WPL's earnings sharing mechanism [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	21.9	25.4
WPL [Member] | Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	2.5	8.3
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 16.4	$ 15

Property, Plant and Equipment_2

Property, Plant and Equipment (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Mar. 31, 2019
Property, Plant and Equipment [Line Items]
Acquisitions of freight management companies		$ 21
Contingent consideration		$ 8
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
Sheboygan Falls Energy Facility [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	35 years
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

Property, Plant and Equipment_3

Property, Plant and Equipment (Property, Plant and Equipment) (Details) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Total electric plant		$ 13,864.6	$ 12,663.6
Gas plant in service		1,462.7	1,387.6
Other plant in service		519.7	513.2
Accumulated depreciation		(4,601.4)	(4,314.6)
Net plant		11,245.6	10,249.8
Construction work in progress		1,835	1,774.8
Other, net		6.1	6.1
Total utility		13,086.7	12,030.7
Non-utility Generation, net	[1]	82.9	86.9
Corporate Services and other, net	[2]	357.5	344.8
Total non-utility and other		440.4	431.7
Total property, plant and equipment		13,527.1	12,462.4
Non-utility Generation, accumulated depreciation		58.5	54.5
Corporate Services and other, accumulated depreciation		172.4	167.5
Generation [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		7,625.3	6,800.6
Distribution [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		5,783.3	5,452.2
Other [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		456	410.8
IPL [Member]
Property, Plant and Equipment [Line Items]
Total electric plant		7,921	6,894.5
Gas plant in service		801.5	763.1
Other plant in service		344.7	322.4
Accumulated depreciation		(2,498.3)	(2,294.7)
Net plant		6,568.9	5,685.3
Construction work in progress		906.8	1,091.2
Other, net		5	5
Total utility		7,480.7	6,781.5
Total property, plant and equipment		7,480.7	6,781.5
IPL [Member] | Generation [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		4,432.4	3,610.4
IPL [Member] | Distribution [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		3,190	3,023.7
IPL [Member] | Other [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		298.6	260.4
WPL [Member]
Property, Plant and Equipment [Line Items]
Total electric plant		5,943.6	5,769.1
Gas plant in service		661.2	624.5
Other plant in service		175	190.8
Accumulated depreciation		(2,103.1)	(2,019.9)
Net plant		4,676.7	4,564.5
Leased Sheboygan Falls Energy Facility, net	[3]	32.3	38.1
Construction work in progress		928.2	683.6
Other, net		1.1	1.1
Total utility		5,638.3	5,287.3
Total property, plant and equipment		5,638.3	5,287.3
Leased Sheboygan Falls Energy Facility, accumulated depreciation		88.7	82.8
WPL [Member] | Generation [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		3,192.9	3,190.2
WPL [Member] | Distribution [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		2,593.3	2,428.5
WPL [Member] | Other [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		$ 157.4	$ 150.4
[1]	Less accumulated depreciation of $58.5 million and $54.5 million for Alliant Energy as of December 31, 2019 and 2018 , respectively.
[2]	Less accumulated depreciation of $172.4 million and $167.5 million for Alliant Energy as of December 31, 2019 and 2018 , respectively.
[3]	Less accumulated amortization of $88.7 million and $82.8 million for WPL as of December 31, 2019 and 2018 , 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 (Equity and Debt AFUDC) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 92.7	$ 75.6	$ 49.7
Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	65.5	51.4	33.6
Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	27.2	24.2	16.1
IPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	49.4	42.2	31.4
IPL [Member] | Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	35.2	28.6	21.1
IPL [Member] | Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	14.2	13.6	10.3
WPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	43.3	33.4	18.3
WPL [Member] | Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	30.3	22.8	12.5
WPL [Member] | Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 13	$ 10.6	$ 5.8

Jointly-Owned Electric Utilit_3

Jointly-Owned Electric Utility Plant (Details) $ in Millions	Dec. 31, 2019USD ($)
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	$ 1,864.1
Accumulated Provision for Depreciation	670.7
Construction Work in Progress	673.7
IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	966.9
Accumulated Provision for Depreciation	362.4
Construction Work in Progress	20.7
WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	897.2
Accumulated Provision for Depreciation	308.3
Construction Work in Progress	$ 653
Ottumwa Unit 1 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	48.00%
Electric Plant	$ 571.5
Accumulated Provision for Depreciation	180.8
Construction Work in Progress	$ 18.8
George Neal Unit 4 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	25.70%
Electric Plant	$ 191.8
Accumulated Provision for Depreciation	93.5
Construction Work in Progress	$ 0.8
George Neal Unit 3 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	28.00%
Electric Plant	$ 163.7
Accumulated Provision for Depreciation	63.2
Construction Work in Progress	$ 0.9
Louisa Unit 1 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	4.00%
Electric Plant	$ 39.9
Accumulated Provision for Depreciation	24.9
Construction Work in Progress	$ 0.2
Columbia Units 1-2 [Member] | WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	53.30%
Electric Plant	$ 777
Accumulated Provision for Depreciation	264.7
Construction Work in Progress	$ 5.5
Forward Wind Energy Center (FWEC) [Member] | WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	42.60%
Electric Plant	$ 120.2
Accumulated Provision for Depreciation	43.6
Construction Work in Progress	$ 0
West Riverside [Member] | WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	91.80%
Electric Plant	$ 0
Accumulated Provision for Depreciation	0
Construction Work in Progress	$ 647.5

Receivables (Narrative) (Detail

Receivables (Narrative) (Details) $ in Millions	Dec. 31, 2019USD ($)
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
Financing Receivable [Member] | IPL [Member]
Accounts Receivable [Line Items]
Available capacity	$ 83

Receivables (Details of Account

Receivables (Details of Accounts Receivable) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Accounts Receivable [Line Items]
Customer	$ 91.6	$ 91
Unbilled utility revenues	82.1	74.2
Deferred proceeds	187.7	119.4
Other	48	76.3
Allowance for doubtful accounts	(7.3)	(10.5)
Accounts receivable, less allowance for doubtful accounts	402.1	350.4
IPL [Member]
Accounts Receivable [Line Items]
Customer	0	0
Unbilled utility revenues	0	0
Deferred proceeds	187.7	119.4
Other	16.3	37.2
Allowance for doubtful accounts	(1.2)	(3.1)
Accounts receivable, less allowance for doubtful accounts	202.8	153.5
WPL [Member]
Accounts Receivable [Line Items]
Customer	83.5	84.8
Unbilled utility revenues	82.1	74.2
Other	31.4	38.5
Allowance for doubtful accounts	(6.1)	(7.4)
Accounts receivable, less allowance for doubtful accounts	$ 190.9	$ 190.1

Receivables (Maximum And Averag

Receivables (Maximum And Average Outstanding Cash Proceeds) (Details) - IPL [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Maximum [Member]
Accounts Receivable [Line Items]
Outstanding aggregate cash proceeds (based on daily outstanding balances)	$ 108	$ 116	$ 112
Average [Member]
Accounts Receivable [Line Items]
Outstanding aggregate cash proceeds (based on daily outstanding balances)	$ 35.9	$ 53.4	$ 62.2

Receivables (Receivables Sold U

Receivables (Receivables Sold Under The Agreement) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Accounts Receivable [Line Items]
Fair value of deferred proceeds	$ 187.7	$ 119.4
IPL [Member]
Accounts Receivable [Line Items]
Fair value of deferred proceeds	187.7	119.4
Financing Receivable [Member] | IPL [Member]
Accounts Receivable [Line Items]
Customer accounts receivable	124.7	140.1
Unbilled utility revenues	95.5	97.1
Other receivables	0.9	0.1
Receivables sold to third party	221.1	237.3
Less: cash proceeds	27	108
Deferred proceeds	194.1	129.3
Less: allowance for doubtful accounts	6.4	9.9
Fair value of deferred proceeds	187.7	119.4
Outstanding receivables, past due	$ 26	$ 35.5

Receivables (Additional Attribu

Receivables (Additional Attributes Of Receivables Sold Under The Agreement) (Details) - IPL [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts Receivable [Line Items]
Collections	$ 2,192.8	$ 2,076.7	$ 1,647.1
Write-offs, net of recoveries	$ 19.2	$ 21.3	$ 17.7

Investments (Narrative) (Detail

Investments (Narrative) (Details)	Dec. 31, 2019
ATC LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	16.00%
ATC Holdco LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	20.00%

Investments (Unconsolidated Equ

Investments (Unconsolidated Equity Investments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Carrying value	$ 458.3	$ 421.3	$ 381.4
Equity (income) / loss	(53)	(54.6)	(44.8)
ATC Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value	320.1	293.6
Equity (income) / loss	$ (45.3)	(38.1)	(42.4)
ATC LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	16.00%
ATC Holdco LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	20.00%
Non-utility wind farm in Oklahoma [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	50.00%
Carrying value	$ 105.3	105.1
Equity (income) / loss	(5.2)	(15.6)	(1.8)
Other [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value	32.9	22.6
Equity (income) / loss	(2.5)	(0.9)	(0.6)
Total [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value	458.3	421.3
Equity (income) / loss	$ (53)	$ (54.6)	$ (44.8)

Common Equity (Narrative) (Deta

Common Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Common Equity [Line Items]
Shares available for issuance under the Amended and Restated OIP, Shareowner Direct Plan and 401(k) Savings Plan (in shares)		12,100,000
Proceeds from issuance of common stock, net		$ 390.3	$ 196.6	$ 149.6
Other comprehensive income (loss)		(0.4)	2.2	(0.1)
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
Equity Forward Agreements [Member]
Schedule of Common Equity [Line Items]
Common stock issued during the period (in shares)		8,358,973	0	0
Proceeds from issuance of common stock, net		$ 366
Initial forward sale price (in dollars per share)		$ 43.75
Shares included in diluted earnings per share (in shares)		67,689
At-the-market Offering Program [Member]
Schedule of Common Equity [Line Items]
Common stock issued during the period (in shares)		0	4,171,013	3,074,931
Proceeds from issuance of common stock, net			$ 173	$ 124
Fees and commissions from issuance of common stock			2	1
Maximum aggregate gross sales price of common stock that can be offered and sold			$ 175	$ 125
Forecast [Member] | Equity Forward Agreements [Member]
Schedule of Common Equity [Line Items]
Common stock issued during the period (in shares)	4,275,127
Initial forward sale price (in dollars per share)	$ 52.235

Preferred Stock (Narrative) (De

Preferred Stock (Narrative) (Details) - IPL [Member]	12 Months Ended
	Dec. 31, 2019Qboard_member$ / sharesshares	Dec. 31, 2018shares
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
Cumulative Preferred Stock [Member]
Temporary Equity [Line Items]
Shares authorized (in shares)	16,000,000
Cumulative 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

Common Equity (Common Share Act

Common Equity (Common Share Activity) (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Common Stock Oustanding [Roll Forward]
Shares outstanding, January 1 (in shares)	236,063,279	231,348,646	227,673,654
Common stock issued during the period, Shareowner Direct Plan (in shares)	501,808	576,965	640,723
Common stock issued during the period, Equity-based compensation plans (in shares)	101,478	5,078	5,185
Common stock issued during the period, Other (in shares)	(2,738)	(38,423)	(45,847)
Shares outstanding, December 31 (in shares)	245,022,800	236,063,279	231,348,646
Equity Forward Agreements [Member]
Common Stock Oustanding [Roll Forward]
Common stock issued during the period (in shares)	8,358,973	0	0
At-the-market Offering Program [Member]
Common Stock Oustanding [Roll Forward]
Common stock issued during the period (in shares)	0	4,171,013	3,074,931

Preferred Stock (Carrying Value

Preferred Stock (Carrying Value Of Cumulative Preferred Stock) (Details) - IPL [Member] - Cumulative Preferred Stock [Member] - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
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 ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity		$ 1,000
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		$ 450
IPL [Member]
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity		250
WPL [Member]
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity		$ 300
Forecast [Member] | Parent Company [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 500
Forecast [Member] | IPL [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	400
Forecast [Member] | WPL [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 500

Debt (Credit Facilities) (Detai

Debt (Credit Facilities) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Commercial paper:
Amount outstanding	$ 337.4	$ 441.2
Weighted average interest rates	1.90%	2.80%
Available credit facility capacity	$ 662.6	$ 558.8
IPL [Member]
Commercial paper:
Amount outstanding	0	$ 50.4
Weighted average interest rates		2.80%
Available credit facility capacity	250	$ 199.6
WPL [Member]
Commercial paper:
Amount outstanding	$ 168.2	$ 105.5
Weighted average interest rates	1.80%	2.50%
Available credit facility capacity	$ 131.8	$ 244.5

Debt (Other Short-Term Borrowin

Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 600.6	$ 446.5
Average amount outstanding (based on daily outstanding balances)	$ 453.5	$ 221.4
Weighted average interest rates	2.50%	2.20%
IPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 50.4	$ 50.4
Average amount outstanding (based on daily outstanding balances)	$ 0.1	$ 1.5
Weighted average interest rates	2.80%	2.30%
WPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 195.1	$ 126
Average amount outstanding (based on daily outstanding balances)	$ 92.6	$ 36.6
Weighted average interest rates	2.40%	2.10%

Debt (Schedule Of Debt-To-Capit

Debt (Schedule Of Debt-To-Capital Ratios) (Details)	Dec. 31, 2019
Actual debt-to-capital ratio	55.00%
IPL [Member]
Actual debt-to-capital ratio	48.00%
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, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term debt		$ 6,240.3	$ 5,546.7
Current maturities		(657.2)	(256.5)
Unamortized debt issuance costs		(36.9)	(32.1)
Unamortized debt (discount) and premium, net		(13.2)	(11.8)
Long-term debt, net	[1]	5,533	5,246.3
Senior Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	3,175	2,575
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]	$ 500	$ 500
Interest rate		4.10%	4.10%
Senior Debentures [Member] | 3.6% senior debenture, due 2029 [Member]
Debt Instrument [Line Items]
Long-term debt	[2],[3]	$ 300	$ 0
Interest rate		3.60%
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] | 3.5% senior debenture, due 2049 [Member]
Debt Instrument [Line Items]
Long-term debt	[2],[3]	$ 300	$ 0
Interest rate		3.50%
Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 1,950	1,850
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] | 3% debenture, due 2029 [Member]
Debt Instrument [Line Items]
Long-term debt	[2],[4]	$ 350	$ 0
Interest rate		3.00%
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%
Debentures [Member] | 5% debenture, retired in 2019 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 0	$ 250
Interest rate			5.00%
Other Long Term Debt [Member]
Debt Instrument [Line Items]
Long-term debt		1,115.3	$ 1,121.7
Other [Member] | Other, 1% at December 31, 2019, due 2020 to 2025 [Member]
Debt Instrument [Line Items]
Long-term debt		$ 2.1	2.4
Interest rate		1.00%
IPL [Member]
Debt Instrument [Line Items]
Long-term debt		$ 3,175	2,575
Current maturities		(200)	0
Unamortized debt issuance costs		(21)	(17.2)
Unamortized debt (discount) and premium, net		(6.7)	(5.5)
Long-term debt, net	[1]	2,947.3	2,552.3
IPL [Member] | Senior Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	3,175	2,575
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]	$ 500	$ 500
Interest rate		4.10%	4.10%
IPL [Member] | Senior Debentures [Member] | 3.6% senior debenture, due 2029 [Member]
Debt Instrument [Line Items]
Long-term debt	[2],[3]	$ 300	$ 0
Interest rate		3.60%
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] | 3.5% senior debenture, due 2049 [Member]
Debt Instrument [Line Items]
Long-term debt	[2],[3]	$ 300	$ 0
Interest rate		3.50%
WPL [Member]
Debt Instrument [Line Items]
Long-term debt		$ 1,950	1,850
Current maturities		(150)	(250)
Unamortized debt issuance costs		(11.3)	(9.6)
Unamortized debt (discount) and premium, net		(6)	(5.5)
Long-term debt, net	[1]	1,782.7	1,584.9
WPL [Member] | Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	1,950	1,850
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] | 3% debenture, due 2029 [Member]
Debt Instrument [Line Items]
Long-term debt	[2],[4]	$ 350	$ 0
Interest rate		3.00%
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%
WPL [Member] | Debentures [Member] | 5% debenture, retired in 2019 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 0	$ 250
Interest rate			5.00%
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		$ 300	$ 300
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]	$ 400	$ 400
Interest rate		3.75%	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]	$ 300	$ 300
Interest rate		4.25%	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 2020 to 2024 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 38.2	$ 44.3
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 April 2019, IPL issued $300 million of 3.6% senior debentures due 2029. In September 2019, IPL issued $300 million of 3.5% senior debentures due 2049. The senior debentures were issued as green bonds, and all of the net proceeds were allocated for the construction and development of IPL’s wind projects.
[4]	In June 2019, WPL issued $350 million of 3% debentures due 2029. The net proceeds from the issuance were used by WPL to reduce its outstanding commercial paper and retire its $250 million 5% debentures that matured in July 2019.

Debt (Schedule Of Debt Maturiti

Debt (Schedule Of Debt Maturities) (Details) $ in Millions	Dec. 31, 2019USD ($)
2020	$ 657
2021	8
2022	333
2023	408
2024	509
IPL [Member]
2020	200
2021	0
2022	0
2023	0
2024	500
WPL [Member]
2020	150
2021	0
2022	250
2023	0
2024	0
Corporate Services [Member]
2020	0
2021	0
2022	75
2023	0
2024	0
Alliant Energy Finance, LLC [Member]
2020	307
2021	8
2022	8
2023	408
2024	$ 9

Leases (Narrative) (Details)

Leases (Narrative) (Details)	12 Months Ended
	Dec. 31, 2019lease_renewal_period_option
Sheboygan Falls Energy Facility [Member] | WPL [Member]
Leases [Line Items]
Finance lease, renewal options (in number of renewal periods)	2

Leases (Operating Leases) (Deta

Leases (Operating Leases) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)
Leases [Line Items]
Operating leases assets, Property, plant and equipment, net	$ 16
Operating leases liabilities, Other current liabilities	2
Operating leases liabilities, Other liabilities	14
Operating leases liabilities, Total	$ 16
Operating leases, Weighted average remaining lease term	11 years
Operating leases, Weighted average discount rate, percent	4.00%
Operating leases, Cost	$ 2
IPL [Member]
Leases [Line Items]
Operating leases assets, Property, plant and equipment, net	10
Operating leases liabilities, Other current liabilities	1
Operating leases liabilities, Other liabilities	9
Operating leases liabilities, Total	$ 10
Operating leases, Weighted average remaining lease term	12 years
Operating leases, Weighted average discount rate, percent	4.00%
Operating leases, Cost	$ 1
WPL [Member]
Leases [Line Items]
Operating leases assets, Property, plant and equipment, net	6
Operating leases liabilities, Other current liabilities	1
Operating leases liabilities, Other liabilities	5
Operating leases liabilities, Total	$ 6
Operating leases, Weighted average remaining lease term	10 years
Operating leases, Weighted average discount rate, percent	4.00%
Operating leases, Cost	$ 1

Leases (Finance Lease) (Details

Leases (Finance Lease) (Details) - WPL [Member] - Sheboygan Falls Energy Facility [Member] $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)
Leases [Line Items]
Finance lease assets, Property, plant and equipment, net	$ 32
Finance lease liabilities, Other current liabilities	9
Finance lease liabilities, Sheboygan Falls Energy Facility	51
Finance lease liabilities, Total	$ 60
Finance lease, Remaining lease term	5 years
Finance lease, Discount rate, percent	11.00%
Finance lease, Depreciation expense	$ 6
Finance lease, Interest expense	7
Finance lease, Total expense	$ 13

Leases (Expected Maturities of

Leases (Expected Maturities of Operating and Finance Lease Liabilities (Details) $ in Millions	Dec. 31, 2019USD ($)
Leases [Line Items]
Operating Leases Liability, 2020	$ 2
Operating Leases Liability, 2021	2
Operating Leases Liability, 2022	2
Operating Leases Liability, 2023	2
Operating Leases Liability, 2024	2
Operating Leases Liability, Thereafter	10
Operating Leases Liability, Total	20
Operating Leases Liability, Less: amount representing interest	4
Operating Leases Liability, Present value of minimum lease payments	16
IPL [Member]
Leases [Line Items]
Operating Leases Liability, 2020	1
Operating Leases Liability, 2021	1
Operating Leases Liability, 2022	1
Operating Leases Liability, 2023	1
Operating Leases Liability, 2024	1
Operating Leases Liability, Thereafter	7
Operating Leases Liability, Total	12
Operating Leases Liability, Less: amount representing interest	2
Operating Leases Liability, Present value of minimum lease payments	10
WPL [Member]
Leases [Line Items]
Operating Leases Liability, 2020	1
Operating Leases Liability, 2021	1
Operating Leases Liability, 2022	1
Operating Leases Liability, 2023	1
Operating Leases Liability, 2024	1
Operating Leases Liability, Thereafter	3
Operating Leases Liability, Total	8
Operating Leases Liability, Less: amount representing interest	2
Operating Leases Liability, Present value of minimum lease payments	6
Sheboygan Falls Energy Facility [Member] | WPL [Member]
Leases [Line Items]
Finance Lease Liability, 2020	15
Finance Lease Liability, 2021	15
Finance Lease Liability, 2022	15
Finance Lease Liability, 2023	15
Finance Lease Liability, 2024	15
Finance Lease Liability, Thereafter	5
Finance Lease Liability, Total	80
Finance Lease Liability, Less: amount representing interest	20
Finance Lease Liability, Present value of minimum lease payments	$ 60

Leases (Future Minimum Operatin

Leases (Future Minimum Operating and Capital Lease Payments) (Details) $ in Millions	Dec. 31, 2019USD ($)
Leases [Line Items]
Operating Leases, 2019	$ 5
Operating Leases, 2020	5
Operating Leases, 2021	3
Operating Leases, 2022	3
Operating Leases, 2023	2
Operating Leases, Thereafter	12
Operating Leases, Total	30
IPL [Member]
Leases [Line Items]
Operating Leases, 2019	3
Operating Leases, 2020	2
Operating Leases, 2021	2
Operating Leases, 2022	2
Operating Leases, 2023	2
Operating Leases, Thereafter	12
Operating Leases, Total	23
WPL [Member]
Leases [Line Items]
Operating Leases, 2019	2
Operating Leases, 2020	3
Operating Leases, 2021	1
Operating Leases, 2022	0
Operating Leases, 2023	0
Operating Leases, Thereafter	0
Operating Leases, Total	6
WPL [Member] | Sheboygan Falls Energy Facility [Member]
Leases [Line Items]
Capital Leases, 2019	15
Capital Leases, 2020	15
Capital Leases, 2021	15
Capital Leases, 2022	15
Capital Leases, 2023	15
Capital Leases, Thereafter	19
Capital Leases, Total	94
Capital Leases, Less: amount representing interest	26
Capital Leases, Present value of minimum capital lease payments	$ 68

Disaggregation of Revenues (Det

Disaggregation of Revenues (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 3,647.7	$ 3,534.5	$ 3,382.2
IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,089.6	2,042.3	1,870.3
WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,475.7	1,452.6	1,472.8
Electric [Member]
Disaggregation of Revenue [Line Items]
Revenues	3,063.6	3,000.3	2,894.7
Electric [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,781.2	1,731.1	1,598.9
Electric [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,282.4	1,269.2	1,295.8
Electric [Member] | Retail - Residential [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,092.4	1,063.4	1,006.2
Electric [Member] | Retail - Residential [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	601.5	590.6	535.6
Electric [Member] | Retail - Residential [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	490.9	472.8	470.6
Electric [Member] | Retail - Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenues	769.7	735.6	710.3
Electric [Member] | Retail - Commercial [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	510.3	486.8	452.7
Electric [Member] | Retail - Commercial [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	259.4	248.8	257.6
Electric [Member] | Retail - Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenues	889.1	888.8	853.1
Electric [Member] | Retail - Industrial [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	503.9	500.8	459.7
Electric [Member] | Retail - Industrial [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	385.2	388	393.4
Electric [Member] | Wholesale [Member]
Disaggregation of Revenue [Line Items]
Revenues	176.4	188.4	238.4
Electric [Member] | Wholesale [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	63.5	71.2	95.5
Electric [Member] | Wholesale [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	112.9	117.2	142.9
Electric [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	136	124.1	86.7
Electric [Member] | Other [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	102	81.7	55.4
Electric [Member] | Other [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	34	42.4	31.3
Gas [Member]
Disaggregation of Revenue [Line Items]
Revenues	455.2	446.6	400.9
Gas [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	264.2	266.2	226
Gas [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	191	180.4	174.9
Gas [Member] | Retail - Residential [Member]
Disaggregation of Revenue [Line Items]
Revenues	259.4	254.4	224.7
Gas [Member] | Retail - Residential [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	149.3	152.3	123.2
Gas [Member] | Retail - Residential [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	110.1	102.1	101.5
Gas [Member] | Retail - Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenues	133	133	123.2
Gas [Member] | Retail - Commercial [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	74.9	75.9	67.9
Gas [Member] | Retail - Commercial [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	58.1	57.1	55.3
Gas [Member] | Retail - Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenues	16	14.9	16.7
Gas [Member] | Retail - Industrial [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	11.7	10.2	11.1
Gas [Member] | Retail - Industrial [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	4.3	4.7	5.6
Gas [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	46.8	44.3	36.3
Gas [Member] | Other [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	28.3	27.8	23.8
Gas [Member] | Other [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	18.5	16.5	12.5
Other Utility [Member]
Disaggregation of Revenue [Line Items]
Revenues	46.5	48	47.5
Other Utility [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	44.2	45	45.4
Other Utility [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	2.3	3	2.1
Other Utility [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	9.3	12.8	12.9
Other Utility [Member] | Other [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	7	9.8	10.8
Other Utility [Member] | Other [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	2.3	3	2.1
Other Utility [Member] | Steam [Member]
Disaggregation of Revenue [Line Items]
Revenues	37.2	35.2	34.6
Other Utility [Member] | Steam [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	37.2	35.2	34.6
Non-Utility and Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	82.4	39.6	39.1
Non-Utility and Other [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 82.4	$ 39.6	$ 39.1

Income Taxes (Narrative) (Detai

Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax [Line Items]
Statutory federal income tax rate		21.00%	21.00%	35.00%
Income tax expense (benefits) Federal Tax Reform			$ (5.6)	$ (18.1)
Overall income tax rate		10.80%	8.40%	12.50%
IPL [Member]
Income Tax [Line Items]
Statutory federal income tax rate		21.00%	21.00%	35.00%
Income tax expense (benefits) Federal Tax Reform			$ (1.1)	$ 3.8
Overall income tax rate		7.60%	(1.20%)	(5.00%)
WPL [Member]
Income Tax [Line Items]
Statutory federal income tax rate		21.00%	21.00%	35.00%
Income tax expense (benefits) Federal Tax Reform			$ (5.5)	$ (14.5)
Overall income tax rate		17.50%	14.80%	24.90%
State [Member]
Income Tax [Line Items]
Overall income tax rate		12.00%
State [Member] | Forecast [Member]
Income Tax [Line Items]
Overall income tax rate	9.80%
State [Member] | IPL [Member]
Income Tax [Line Items]
Overall income tax rate		12.00%
State [Member] | IPL [Member] | Forecast [Member]
Income Tax [Line Items]
Overall income tax rate	9.80%
State [Member] | WPL [Member]
Income Tax [Line Items]
Overall income tax rate		12.00%
State [Member] | WPL [Member] | 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, 2019	Dec. 31, 2018	Dec. 31, 2017
Current tax expense (benefit):
Federal	$ (6.6)	$ (1)	$ (41)
State	24.2	(5.1)	8.5
Deferred tax expense (benefit):
Federal	69.7	67.9	159.5
State	41.4	29.8	12.3
Production tax credits	(54.7)	(29.5)	(31.1)
Investment tax credits	(0.8)	(1.2)	(1.1)
Income tax expense (benefit)	68.7	47.7	66.7
IPL's tax benefit riders [Member]
Current tax expense (benefit):
IPL's tax benefit riders	(4.5)	(13.2)	(40.4)
IPL [Member]
Current tax expense (benefit):
Federal	(11)	14.9	(27.9)
State	24.3	(7.1)	1.6
Deferred tax expense (benefit):
Federal	26.4	9.5	72.5
State	30.9	7.3	(2.2)
Production tax credits	(41.8)	(14)	(14.1)
Investment tax credits	(0.2)	(0.6)	(0.4)
Income tax expense (benefit)	24.1	(3.2)	(10.9)
IPL [Member] | IPL's tax benefit riders [Member]
Current tax expense (benefit):
IPL's tax benefit riders	(4.5)	(13.2)	(40.4)
WPL [Member]
Current tax expense (benefit):
Federal	12	(9.2)	5.5
State	13.7	(4.4)	2.5
Deferred tax expense (benefit):
Federal	30.7	43.8	55
State	6.3	22.1	16.6
Production tax credits	(12.8)	(15.5)	(17)
Investment tax credits	(0.6)	(0.6)	(0.7)
Income tax expense (benefit)	$ 49.3	$ 36.2	$ 61.9

Income Taxes (Schedule Of Effec

Income Taxes (Schedule Of Effective Income Tax Rates) (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective Tax Rate [Line Items]
Statutory federal income tax rate	21.00%	21.00%	35.00%
State income taxes, net of federal benefits	6.80%	7.00%	5.50%
Production tax credits	(8.60%)	(5.20%)	(6.10%)
Effect of rate-making on property-related differences	(6.10%)	(7.60%)	(8.50%)
Amortization of excess deferred taxes	(1.00%)	(0.30%)	(0.10%)
Adjustment for prior period taxes	0.60%	(2.30%)	(1.50%)
Federal Tax Reform adjustments	0.00%	(1.00%)	(3.40%)
Other items, net	(1.20%)	(0.90%)	(0.80%)
Overall income tax rate	10.80%	8.40%	12.50%
IPL [Member]
Effective Tax Rate [Line Items]
Statutory federal income tax rate	21.00%	21.00%	35.00%
State income taxes, net of federal benefits	8.50%	7.70%	6.50%
Production tax credits	(13.10%)	(5.20%)	(6.70%)
Effect of rate-making on property-related differences	(9.80%)	(14.00%)	(19.10%)
Amortization of excess deferred taxes	(0.30%)	0.00%	(0.10%)
Adjustment for prior period taxes	3.10%	(4.80%)	(3.40%)
Federal Tax Reform adjustments	0.00%	(0.40%)	1.70%
Other items, net	(0.40%)	(0.60%)	(0.20%)
Overall income tax rate	7.60%	(1.20%)	(5.00%)
WPL [Member]
Effective Tax Rate [Line Items]
Statutory federal income tax rate	21.00%	21.00%	35.00%
State income taxes, net of federal benefits	6.20%	6.20%	5.10%
Production tax credits	(4.50%)	(6.40%)	(7.10%)
Effect of rate-making on property-related differences	(2.70%)	(2.30%)	(1.70%)
Amortization of excess deferred taxes	(1.80%)	(0.20%)	(0.10%)
Adjustment for prior period taxes	(0.30%)	(0.20%)	0.00%
Federal Tax Reform adjustments	0.00%	(2.30%)	(5.80%)
Other items, net	(0.40%)	(1.00%)	(0.50%)
Overall income tax rate	17.50%	14.80%	24.90%
IPL's tax benefit riders [Member]
Effective Tax Rate [Line Items]
IPL’s tax benefit riders	(0.70%)	(2.30%)	(7.60%)
IPL's tax benefit riders [Member] | IPL [Member]
Effective Tax Rate [Line Items]
IPL’s tax benefit riders	(1.40%)	(4.90%)	(18.70%)

Income Taxes (Schedule Of Defer

Income Taxes (Schedule Of Deferred Tax Assets And Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Deferred tax liabilities, property	$ 2,021.5	$ 1,975.5
Deferred tax liabilities, other	85.1	80.5
Total deferred tax liabilities	2,217.4	2,158.4
Deferred tax assets, federal credit carryforwards	355.4	299.1
Deferred tax assets, net operating losses carryforwards - federal	60.5	158.6
Deferred tax assets, net operating losses carryforwards - state	36.6	47
Deferred tax assets, other	60.5	59.8
Subtotal deferred tax assets	513	564.5
Deferred tax assets, valuation allowances	(9.6)	(9.2)
Total deferred tax assets	503.4	555.3
Total deferred tax liabilities, net	1,714	1,603.1
IPL [Member]
Deferred tax liabilities, property	1,183.5	1,158.4
Deferred tax liabilities, other	76.5	69.4
Total deferred tax liabilities	1,260	1,227.8
Deferred tax assets, federal credit carryforwards	175	133.2
Deferred tax assets, net operating losses carryforwards - federal	56.3	114.1
Deferred tax assets, net operating losses carryforwards - state	0.6	0.9
Deferred tax assets, other	20.6	22.8
Subtotal deferred tax assets	252.5	271
Deferred tax assets, valuation allowances	(0.5)	(0.5)
Total deferred tax assets	252	270.5
Total deferred tax liabilities, net	1,008	957.3
WPL [Member]
Deferred tax liabilities, property	770	735.2
Deferred tax liabilities, other	32.1	34.4
Total deferred tax liabilities	802.1	769.6
Deferred tax assets, federal credit carryforwards	160.3	147.4
Deferred tax assets, net operating losses carryforwards - federal	0.5	26.4
Deferred tax assets, net operating losses carryforwards - state	0.1	0.5
Deferred tax assets, other	15.8	14.1
Subtotal deferred tax assets	176.7	188.4
Deferred tax assets, valuation allowances	(0.8)	(0.8)
Total deferred tax assets	175.9	187.6
Total deferred tax liabilities, net	626.2	582
ATC Holdings [Member]
Deferred tax liabilities, ATC Holdings	$ 110.8	$ 102.4

Income Taxes (Summary Of Tax Cr

Income Taxes (Summary Of Tax Credit Carryforwards) (Details) $ in Millions	Dec. 31, 2019USD ($)
Federal [Member]
Tax carryforwards, net operating losses	$ 297
Tax carryforwards, tax credits	355
Federal [Member] | IPL [Member]
Tax carryforwards, net operating losses	276
Tax carryforwards, tax credits	175
Federal [Member] | WPL [Member]
Tax carryforwards, net operating losses	2
Tax carryforwards, tax credits	160
State [Member]
Tax carryforwards, net operating losses	615
State [Member] | IPL [Member]
Tax carryforwards, net operating losses	9
State [Member] | WPL [Member]
Tax carryforwards, net operating losses	$ 2

Income Taxes (Schedule Of Uncer

Income Taxes (Schedule Of Uncertain Tax Positions) (Details)	12 Months Ended
	Dec. 31, 2019
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, 2019	Dec. 31, 2018
Benefit Plans [Line Items]
Total plan assets (less than 1%), percentage of common stock	1.00%	1.00%
Common stock percentage in assets held in 401(k) saving plans	11.50%	11.50%
Unrecognized compensation cost	$ 5.4
Alliant Energy common stock closing price on December 31, 2019 (in dollars per share)	$ 54.72
Percentage of base salary and short-term cash incentive compensation	100.00%
Carrying value of deferred compensation obligations	$ 21.2	$ 21
Performance Shares Equity Awards [Member]
Benefit Plans [Line Items]
Performance period	3 years
Performance Shares and Performance Units Liability Awards [Member]
Benefit Plans [Line Items]
Performance period	3 years
Instrument valuation based on shares of common stock, number of shares	1
Restricted Stock Units Equity Awards [Member]
Benefit Plans [Line Items]
Performance period	3 years
Instrument valuation based on shares of common stock, number of shares	1
Restricted Stock Units Liability Awards [Member]
Benefit Plans [Line Items]
Performance period	3 years
Instrument valuation based on shares of common stock, number of shares	1
Performance Restricted Stock Units Equity Awards [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
Minimum [Member] | Performance Shares Equity Awards [Member]
Benefit Plans [Line Items]
Actual number of shares paid out upon vesting, percentage of target shares	0.00%
Minimum [Member] | Performance Shares and Performance Units Liability Awards [Member]
Benefit Plans [Line Items]
Actual number of shares paid out upon vesting, percentage of target shares	0.00%
Minimum [Member] | Performance Restricted Stock Units Equity Awards [Member]
Benefit Plans [Line Items]
Actual number of shares paid out upon vesting, percentage of target shares	0.00%
Maximum [Member]
Benefit Plans [Line Items]
Unrecognized compensation cost recognized over a weighted average period	2 years
Maximum [Member] | Performance Shares Equity Awards [Member]
Benefit Plans [Line Items]
Actual number of shares paid out upon vesting, percentage of target shares	200.00%
Maximum [Member] | Performance Shares and Performance Units Liability Awards [Member]
Benefit Plans [Line Items]
Actual number of shares paid out upon vesting, percentage of target shares	200.00%
Maximum [Member] | Performance Restricted Stock Units Equity Awards [Member]
Benefit Plans [Line Items]
Actual number of shares paid out upon vesting, percentage of target shares	200.00%
IPL [Member]
Benefit Plans [Line Items]
Unrecognized compensation cost	$ 3
WPL [Member]
Benefit Plans [Line Items]
Unrecognized compensation cost	2.2
Other Postretirement Benefits Plans [Member]
Benefit Plans [Line Items]
Plan asset threshold for long-term allocation targets	5
Other Postretirement Benefits Plans [Member] | IPL [Member]
Benefit Plans [Line Items]
Plan asset threshold for long-term allocation targets	5
Other Postretirement Benefits Plans [Member] | WPL [Member]
Benefit Plans [Line Items]
Plan asset threshold for long-term allocation targets	$ 5
Omnibus Incentive Plan [Member]
Benefit Plans [Line Items]
Shares available for issuance under the Amended and Restated OIP	6,400,000
Shares included in diluted earnings per share (in shares)	343,396

Benefit Plans (Assumptions Used

Benefit Plans (Assumptions Used To Measure Benefit Plans) (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate for benefit obligations	3.48%	4.34%	3.66%
Discount rate for net periodic cost	4.34%	3.66%	4.19%
Expected rate of return on plan assets	7.60%	7.60%	7.60%
Interest crediting rate for Alliant Energy Cash Balance Pension Plan	5.52%	5.04%	4.64%
Pension Plans, Defined Benefit [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate for benefit obligations	3.51%	4.35%	3.68%
Discount rate for net periodic cost	4.35%	3.68%	4.22%
Expected rate of return on plan assets	7.60%	7.60%	7.60%
Rate of compensation increase	3.65%	3.65%	3.65%
Pension Plans, Defined Benefit [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate for benefit obligations	3.50%	4.35%	3.69%
Discount rate for net periodic cost	4.35%	3.69%	4.23%
Expected rate of return on plan assets	7.60%	7.60%	7.60%
Rate of compensation increase	3.65%	3.65%	3.65%
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate for benefit obligations	3.40%	4.24%	3.53%
Discount rate for net periodic cost	4.24%	3.53%	3.98%
Expected rate of return on plan assets	5.44%	5.44%	5.80%
Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate for benefit obligations	3.39%	4.23%	3.51%
Discount rate for net periodic cost	4.23%	3.51%	3.95%
Expected rate of return on plan assets	5.60%	5.60%	6.20%
Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate for benefit obligations	3.39%	4.23%	3.51%
Discount rate for net periodic cost	4.23%	3.51%	3.96%
Expected rate of return on plan assets	3.81%	3.84%	3.50%
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, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 9.7	$ 12.1	$ 12.5
Interest cost		49.8	46.8	51
Expected return on plan assets	[1]	(60.1)	(69.7)	(65.5)
Amortization of prior service cost (credit)		(0.7)	(0.7)	(0.4)
Amortization of actuarial loss	[2]	36.4	35.2	37.6
Settlement losses	[3]	0	0	0.9
Total		35.1	23.7	36.1
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		3.3	4.2	5
Interest cost		8.5	7.7	8.6
Expected return on plan assets	[1]	(5)	(6)	(6.1)
Amortization of prior service cost (credit)	[4]	(0.2)	(0.2)	(0.2)
Amortization of actuarial loss	[2]	3.3	3.4	3.8
Settlement losses		0	0	0
Total		9.9	9.1	11.1
IPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		6	7.4	7.3
Interest cost		22.7	21.4	23.5
Expected return on plan assets	[1]	(28.1)	(32.6)	(30.8)
Amortization of prior service cost (credit)		(0.2)	(0.2)	(0.2)
Amortization of actuarial loss	[2]	15.7	14.9	16.1
Total		16.1	10.9	15.9
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		1.3	1.7	2.1
Interest cost		3.4	3.1	3.5
Expected return on plan assets	[1]	(3.6)	(4.4)	(4.3)
Amortization of prior service cost (credit)	[4]	0	0	0
Amortization of actuarial loss	[2]	1.5	1.3	2
Total		2.6	1.7	3.3
WPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		3.5	4.4	4.9
Interest cost		21.5	20.2	21.8
Expected return on plan assets	[1]	(26.2)	(30.4)	(28.5)
Amortization of prior service cost (credit)		(0.2)	(0.1)	0.1
Amortization of actuarial loss	[2]	17.6	17.2	18.5
Total		16.2	11.3	16.8
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		1.3	1.6	1.9
Interest cost		3.3	3.1	3.4
Expected return on plan assets	[1]	(0.6)	(0.6)	(0.8)
Amortization of prior service cost (credit)	[4]	(0.2)	(0.2)	(0.2)
Amortization of actuarial loss	[2]	1.6	2	1.6
Total		$ 5.4	$ 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, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations	$ 1,279.7	$ 1,175
Change in benefit obligation:
Net benefit obligation at January 1	1,175	1,303.1
Service cost	9.7	12.1	$ 12.5
Interest cost	49.8	46.8	51
Plan participants' contributions	0	0
Actuarial (gain) loss	124.6	(96.2)
Gross benefits paid	(79.4)	(90.8)
Net benefit obligation at December 31	1,279.7	1,175	1,303.1
Change in plan assets:
Fair value of plan assets at January 1	808.6	950.7
Actual return on plan assets	168.1	(57.8)
Employer contributions	33.1	6.5
Plan participants' contributions	0	0
Gross benefits paid	(79.4)	(90.8)
Fair value of plan assets at December 31	930.4	808.6	950.7
Under funded status at December 31	(349.3)	(366.4)
Fair value of plan assets	930.4	808.6
Pension Plans, Defined Benefit [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations	581.2	533.9
Change in benefit obligation:
Net benefit obligation at January 1	533.9	592.9
Service cost	6	7.4	7.3
Interest cost	22.7	21.4	23.5
Plan participants' contributions	0	0
Actuarial (gain) loss	55.9	(44.4)
Gross benefits paid	(37.3)	(43.4)
Net benefit obligation at December 31	581.2	533.9	592.9
Change in plan assets:
Fair value of plan assets at January 1	377.8	443.7
Actual return on plan assets	78.9	(26.8)
Employer contributions	16.6	4.3
Plan participants' contributions	0	0
Gross benefits paid	(37.3)	(43.4)
Fair value of plan assets at December 31	436	377.8	443.7
Under funded status at December 31	(145.2)	(156.1)
Fair value of plan assets	436	377.8
Pension Plans, Defined Benefit [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations	550.1	506.6
Change in benefit obligation:
Net benefit obligation at January 1	506.6	559.8
Service cost	3.5	4.4	4.9
Interest cost	21.5	20.2	21.8
Plan participants' contributions	0	0
Actuarial (gain) loss	53.8	(40)
Gross benefits paid	(35.3)	(37.8)
Net benefit obligation at December 31	550.1	506.6	559.8
Change in plan assets:
Fair value of plan assets at January 1	352.2	415
Actual return on plan assets	73.4	(25.1)
Employer contributions	15.7	0.1
Plan participants' contributions	0	0
Gross benefits paid	(35.3)	(37.8)
Fair value of plan assets at December 31	406	352.2	415
Under funded status at December 31	(144.1)	(154.4)
Fair value of plan assets	406	352.2
Other Postretirement Benefits Plans [Member]
Change in benefit obligation:
Net benefit obligation at January 1	206.1	222.3
Service cost	3.3	4.2	5
Interest cost	8.5	7.7	8.6
Plan participants' contributions	3.4	3.1
Actuarial (gain) loss	13.6	(8.2)
Gross benefits paid	(21.3)	(23)
Net benefit obligation at December 31	213.6	206.1	222.3
Change in plan assets:
Fair value of plan assets at January 1	99.1	111.1
Actual return on plan assets	14.6	(2.6)
Employer contributions	9.1	10.5
Plan participants' contributions	3.4	3.1
Gross benefits paid	(21.3)	(23)
Fair value of plan assets at December 31	104.9	99.1	111.1
Under funded status at December 31	(108.7)	(107)
Other Postretirement Benefits Plans [Member] | IPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	82.5	89.4
Service cost	1.3	1.7	2.1
Interest cost	3.4	3.1	3.5
Plan participants' contributions	1.3	1
Actuarial (gain) loss	5.7	(4.3)
Gross benefits paid	(8.5)	(8.4)
Net benefit obligation at December 31	85.7	82.5	89.4
Change in plan assets:
Fair value of plan assets at January 1	66.7	72.9
Actual return on plan assets	10.1	(1.4)
Employer contributions	2.3	2.6
Plan participants' contributions	1.3	1
Gross benefits paid	(8.5)	(8.4)
Fair value of plan assets at December 31	71.9	66.7	72.9
Under funded status at December 31	(13.8)	(15.8)
Other Postretirement Benefits Plans [Member] | WPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	83.2	90.4
Service cost	1.3	1.6	1.9
Interest cost	3.3	3.1	3.4
Plan participants' contributions	1.5	1.4
Actuarial (gain) loss	5	(2.5)
Gross benefits paid	(9)	(10.8)
Net benefit obligation at December 31	85.3	83.2	90.4
Change in plan assets:
Fair value of plan assets at January 1	16.7	18.7
Actual return on plan assets	1.7	(0.1)
Employer contributions	6.2	7.5
Plan participants' contributions	1.5	1.4
Gross benefits paid	(9)	(10.8)
Fair value of plan assets at December 31	17.1	16.7	$ 18.7
Under funded status at December 31	$ (68.2)	$ (66.5)

Benefit Plans (Amounts Recogniz

Benefit Plans (Amounts Recognized On The Consolidated Balance Sheets) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Amounts recognized on the balance sheets consist of:
Pension and other benefit obligations	$ (484)	$ (509.1)
Pension Plans, Defined Benefit [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	0	0
Current liabilities	(14.3)	(2.3)
Pension and other benefit obligations	(335)	(364.1)
Net amounts recognized at December 31	(349.3)	(366.4)
Amounts recognized in Regulatory Assets consist of:
Net actuarial loss	485.3	505.2
Prior service credit	(5.1)	(5.8)
Net amount recognized at December 31	480.2	499.4
Other Postretirement Benefits Plans [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	9.8	7.5
Current liabilities	(8.1)	(9.6)
Pension and other benefit obligations	(110.4)	(104.9)
Net amounts recognized at December 31	(108.7)	(107)
Amounts recognized in Regulatory Assets consist of:
Net actuarial loss	45.2	44.5
Prior service credit	(0.9)	(1.1)
Net amount recognized at December 31	44.3	43.4
IPL [Member]
Amounts recognized on the balance sheets consist of:
Pension and other benefit obligations	(167.7)	(178.4)
IPL [Member] | Pension Plans, Defined Benefit [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	0	0
Current liabilities	(0.5)	(0.6)
Pension and other benefit obligations	(144.7)	(155.5)
Net amounts recognized at December 31	(145.2)	(156.1)
Amounts recognized in Regulatory Assets consist of:
Net actuarial loss	208.2	218.8
Prior service credit	(1.7)	(1.8)
Net amount recognized at December 31	206.5	217
IPL [Member] | Other Postretirement Benefits Plans [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	6.6	4.7
Current liabilities	(1.8)	(1.9)
Pension and other benefit obligations	(18.6)	(18.6)
Net amounts recognized at December 31	(13.8)	(15.8)
Amounts recognized in Regulatory Assets consist of:
Net actuarial loss	16.6	18.8
Prior service credit	0	0
Net amount recognized at December 31	16.6	18.8
WPL [Member]
Amounts recognized on the balance sheets consist of:
Pension and other benefit obligations	(210.8)	(217.7)
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	(144)	(154.3)
Net amounts recognized at December 31	(144.1)	(154.4)
Amounts recognized in Regulatory Assets consist of:
Net actuarial loss	212	223
Prior service credit	(1.2)	(1.3)
Net amount recognized at December 31	210.8	221.7
WPL [Member] | Other Postretirement Benefits Plans [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	3.2	2.8
Current liabilities	(6)	(7.4)
Pension and other benefit obligations	(65.4)	(61.9)
Net amounts recognized at December 31	(68.2)	(66.5)
Amounts recognized in Regulatory Assets consist of:
Net actuarial loss	22.2	19.9
Prior service credit	(0.9)	(1.1)
Net amount recognized at December 31	$ 21.3	$ 18.8

Benefit Plans (Accumulated Bene

Benefit Plans (Accumulated Benefit Obligations) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	$ 1,234.4	$ 1,139.9
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	1,234.4	1,139.9
Fair value of plan assets	930.4	808.6
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	1,279.7	1,175
Fair value of plan assets	930.4	808.6
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	213.6	206.1
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	213.6	206.1
Fair value of plan assets	104.9	99.1
IPL [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	557.8	514.3
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	557.8	514.3
Fair value of plan assets	436	377.8
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	581.2	533.9
Fair value of plan assets	436	377.8
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	85.7	82.5
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	85.7	82.5
Fair value of plan assets	71.9	66.7
WPL [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	535.9	494.8
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	535.9	494.8
Fair value of plan assets	406	352.2
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	550.1	506.6
Fair value of plan assets	406	352.2
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	85.3	83.2
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	85.3	83.2
Fair value of plan assets	$ 17.1	$ 16.7

Benefit Plans (Regulatory Asset

Benefit Plans (Regulatory Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	$ 39.4	$ 38.2
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	$ 29	$ 27.7

Benefit Plans (Estimated Future

Benefit Plans (Estimated Future Funding) (Details) - Forecast [Member] $ in Millions	Dec. 31, 2020USD ($)
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2020	$ 59.8	[1]
Defined Benefit Pension Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2020	19.4	[1]
Defined Benefit Pension Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2020	21.8	[1]
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2020	8.1
Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2020	1.8
Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2020	$ 6
[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, 2019USD ($)
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2020	$ 104.1
Expected benefit payments, 2021	92.2
Expected benefit payments, 2022	94.1
Expected benefit payments, 2023	94.6
Expected benefit payments, 2024	95
Expected benefit payments, 2025 - 2029	466.8
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2020	86.4
Expected benefit payments, 2021	74.5
Expected benefit payments, 2022	76.6
Expected benefit payments, 2023	77.5
Expected benefit payments, 2024	78.2
Expected benefit payments, 2025 - 2029	390.2
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2020	17.7
Expected benefit payments, 2021	17.7
Expected benefit payments, 2022	17.5
Expected benefit payments, 2023	17.1
Expected benefit payments, 2024	16.8
Expected benefit payments, 2025 - 2029	76.6
IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2020	42.3
Expected benefit payments, 2021	42.4
Expected benefit payments, 2022	43.7
Expected benefit payments, 2023	44.7
Expected benefit payments, 2024	43.5
Expected benefit payments, 2025 - 2029	213.1
IPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2020	35.1
Expected benefit payments, 2021	35.3
Expected benefit payments, 2022	36.6
Expected benefit payments, 2023	37.7
Expected benefit payments, 2024	36.6
Expected benefit payments, 2025 - 2029	182.5
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2020	7.2
Expected benefit payments, 2021	7.1
Expected benefit payments, 2022	7.1
Expected benefit payments, 2023	7
Expected benefit payments, 2024	6.9
Expected benefit payments, 2025 - 2029	30.6
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2020	39.3
Expected benefit payments, 2021	39.4
Expected benefit payments, 2022	39.2
Expected benefit payments, 2023	39.2
Expected benefit payments, 2024	39.8
Expected benefit payments, 2025 - 2029	195.4
WPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2020	32
Expected benefit payments, 2021	32
Expected benefit payments, 2022	32.1
Expected benefit payments, 2023	32.4
Expected benefit payments, 2024	33.1
Expected benefit payments, 2025 - 2029	165.2
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2020	7.3
Expected benefit payments, 2021	7.4
Expected benefit payments, 2022	7.1
Expected benefit payments, 2023	6.8
Expected benefit payments, 2024	6.7
Expected benefit payments, 2025 - 2029	$ 30.2

Benefit Plans (Allocation Of Pl

Benefit Plans (Allocation Of Plan Assets) (Details)	Dec. 31, 2019
Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation, percentage	5.00%
Pension Plans, Defined Benefit [Member] | Equity securites - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation, percentage	25.00%
Pension Plans, Defined Benefit [Member] | Equity securities - international [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation, percentage	22.00%
Pension Plans, Defined Benefit [Member] | Global asset securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation, percentage	9.00%
Pension Plans, Defined Benefit [Member] | Risk parity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation, percentage	10.00%
Pension Plans, Defined Benefit [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation, percentage	29.00%
Other Postretirement Benefits Plans [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation, percentage	3.00%
Other Postretirement Benefits Plans [Member] | Equity securites - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation, percentage	25.00%
Other Postretirement Benefits Plans [Member] | Equity securities - international [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation, percentage	2.00%
Other Postretirement Benefits Plans [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation, percentage	70.00%
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	6.00%
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	56.00%
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, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 930.4	$ 808.6	$ 950.7
Assets measured at net asset value	410.1	355.4
Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	45.7	36.4
Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3.7	3
Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	42	33.4
Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | Equity securites - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	160.6	130.2
Pension Plans, Defined Benefit [Member] | Equity securites - U.S. [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	160.6	130.2
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	135.9	116
Pension Plans, Defined Benefit [Member] | Equity securities - international [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	135.9	116
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	47.2	42.1
Pension Plans, Defined Benefit [Member] | Global asset securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	47.2	42.1
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	130.7	127.8
Pension Plans, Defined Benefit [Member] | Fixed income securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	58	52.5
Pension Plans, Defined Benefit [Member] | Fixed income securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	72.7	75.3
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	520.1	452.5
Pension Plans, Defined Benefit [Member] | Subtotal [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	405.4	343.8
Pension Plans, Defined Benefit [Member] | Subtotal [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	114.7	108.7
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	0.9	1.2
Pension Plans, Defined Benefit [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	436	377.8	443.7
Assets measured at net asset value	192.2	166.1
Pension Plans, Defined Benefit [Member] | IPL [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	21.4	17
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	1.7	1.4
Pension Plans, Defined Benefit [Member] | IPL [Member] | Cash and equivalents [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	19.7	15.6
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	75.2	60.8
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	75.2	60.8
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	63.7	54.2
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	63.7	54.2
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	22.1	19.7
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	22.1	19.7
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	61.3	59.7
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	27.2	24.5
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	34.1	35.2
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	243.7	211.4
Pension Plans, Defined Benefit [Member] | IPL [Member] | Subtotal [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	189.9	160.6
Pension Plans, Defined Benefit [Member] | IPL [Member] | Subtotal [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	53.8	50.8
Pension Plans, Defined Benefit [Member] | IPL [Member] | Subtotal [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | IPL [Member] | Accrued investment income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.4	0.6
Pension Plans, Defined Benefit [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	406	352.2	415
Assets measured at net asset value	179	154.8
Pension Plans, Defined Benefit [Member] | WPL [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	19.9	15.9
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	1.6	1.3
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	18.3	14.6
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	70.1	56.7
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	70.1	56.7
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	59.3	50.5
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	59.3	50.5
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	20.6	18.4
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	20.6	18.4
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	57	55.6
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	25.3	22.8
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	31.7	32.8
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	226.9	197.1
Pension Plans, Defined Benefit [Member] | WPL [Member] | Subtotal [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	176.9	149.7
Pension Plans, Defined Benefit [Member] | WPL [Member] | Subtotal [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	50	47.4
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	0.4	0.5
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	104.9	99.1	111.1
Assets measured at net asset value	26	22.3
Total assets in fair value hierarchy	78.9	76.8
Other Postretirement Benefit Plans, Defined Benefit [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	77.8	75.8
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	3.4	1.4
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	3	1.1
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	0.4	0.3
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	3.7	3.9
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	3.7	3.9
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	2.8	2.9
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	2.8	2.9
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	0.5	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	0.5	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	68.5	68.2
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	67.8	67.5
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	0.7
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	71.9	66.7	72.9
Assets measured at net asset value	22.1	19
Total assets in fair value hierarchy	49.8	47.7
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	49.8	47.7
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	1.5	0.7
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	1.5	0.7
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] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	48.3	47
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	48.3	47
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	17.1	16.7	$ 18.7
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	17.1	16.7
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
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	0.4	0.1
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	0.4	0.1
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
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	16.7	16.6
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	16.7	16.6
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Fixed income securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Fixed income securities [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Defined Benefit Plan Due To Brokers Net [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(0.7)	(0.5)
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	$ (0.3)	$ (0.2)

Benefit Plans (Employees Partic

Benefit Plans (Employees Participate In Defined Contribution Retirement Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
401(k) costs	$ 25.5	$ 25.1	$ 24.8
IPL [Member]
401(k) costs	13	13	12.8
WPL [Member]
401(k) costs	$ 11.5	$ 11.2	$ 11.1

Benefit Plans (Recognized Compe

Benefit Plans (Recognized Compensation Expense And Income Tax Benefits) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Compensation expense	$ 26	$ 17	$ 15.1
Income tax benefits	7.3	4.9	6.2
IPL [Member]
Compensation expense	14.5	9.4	8.3
Income tax benefits	4.2	2.8	3.4
WPL [Member]
Compensation expense	10.3	6.9	6.4
Income tax benefits	$ 2.8	$ 1.9	$ 2.6

Benefit Plans (Summary Of Perfo

Benefit Plans (Summary Of Performance Shares and Performance Units Activity) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Performance Shares Equity Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Nonvested, January 1 (in shares/awards)	0
Nonvested, January 1, weighted average grant date fair value (in dollars per share)	$ 0
Granted (in shares/awards)	91,816
Granted, weighted average grant date fair value (in dollars per share)	$ 47.23
Forfeited (in shares/awards)	(17,623)
Forfeited, weighted average grant date fair value (in dollars per share)	$ 46.35
Nonvested, December 31 (in shares/awards)	74,193	0
Nonvested, December 31, weighted average grant date fair value (in dollars per share)	$ 47.44	$ 0
Performance Shares Liability Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Nonvested, January 1 (in shares/awards)	203,188	223,511	257,599
Granted (in shares/awards)	0	74,163	65,350
Vested (in shares/awards)	(66,322)	(90,806)	(99,438)
Forfeited (in shares/awards)	(4,994)	(3,680)	0
Nonvested, December 31 (in shares/awards)	131,872	203,188	223,511
Performance Units Liability Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Nonvested, January 1 (in shares/awards)	57,761	71,737	93,320
Granted (in shares/awards)	0	19,840	21,558
Vested (in shares/awards)	(20,131)	(31,910)	(37,395)
Forfeited (in shares/awards)	(3,056)	(1,906)	(5,746)
Nonvested, December 31 (in shares/awards)	34,574	57,761	71,737

Benefit Plans (Fair Values Of N

Benefit Plans (Fair Values Of Nonvested Performance Shares And Performance Units) (Details)	12 Months Ended
	Dec. 31, 2019$ / sharesshares
Performance Shares Liability Awards [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards)	131,872
Performance Units Liability Awards [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards)	34,574
2018 Grant [Member] | Performance Shares Liability Awards [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards)	68,307
Estimated payout percentage based on performance criteria	118.00%
Fair values of each nonvested award (in dollars per share) | $ / shares	$ 64.57
2018 Grant [Member] | Performance Units Liability Awards [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards)	17,622
Estimated payout percentage based on performance criteria	118.00%
Fair values of each nonvested award (in dollars per share) | $ / shares	$ 64.57
2017 Grant [Member] | Performance Shares Liability Awards [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards)	63,565
Estimated payout percentage based on performance criteria	155.00%
Fair values of each nonvested award (in dollars per share) | $ / shares	$ 84.82
2017 Grant [Member] | Performance Units Liability Awards [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards)	16,952
Estimated payout percentage based on performance criteria	155.00%
Fair values of each nonvested award (in dollars per share) | $ / shares	$ 84.82

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, 2019	Dec. 31, 2018	Dec. 31, 2017
Performance Shares Liability Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares/awards)	66,322	90,806	99,438
Performance Units Liability Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares/awards)	20,131	31,910	37,395
2016 Grant [Member] | Performance Shares Liability Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares/awards)	66,322
Vested percentage of the target	142.50%
Cash and stock payout value	$ 4.3
Cash payout value	$ 3.7
Common stock shares from vested performance shares (in shares)	6,447
2016 Grant [Member] | Performance Units Liability Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares/awards)	20,131
Vested percentage of the target	142.50%
Cash and stock payout value	$ 1.3
Cash payout value	$ 1.3
2015 Grant [Member] | Performance Shares Liability Awards [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 Liability Awards [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 Liability Awards [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 Liability Awards [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

Benefit Plans (Summary of Restr

Benefit Plans (Summary of Restricted Stock Units) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Restricted Stock Units Equity Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Nonvested, January 1 (in shares/awards)	0
Nonvested, January 1, weighted average grant date fair value (in dollars per share)	$ 0
Granted (in shares/awards)	105,348
Granted, weighted average grant date fair value (in dollars per share)	$ 45.98
Forfeited (in shares/awards)	(16,067)
Forfeited, weighted average grant date fair value (in dollars per share)	$ 45.63
Nonvested, December 31 (in shares/awards)	89,281	0
Nonvested, December 31, weighted average grant date fair value (in dollars per share)	$ 46.04	$ 0
Restricted Stock Units Liability Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Nonvested, January 1 (in shares/awards)	174,163	113,749	57,736
Granted (in shares/awards)	0	63,568	56,013
Vested (in shares/awards)	(56,849)	0	0
Forfeited (in shares/awards)	(4,281)	(3,154)	0
Nonvested, December 31 (in shares/awards)	113,033	174,163	113,749

Benefit Plans (Summary of Per_3

Benefit Plans (Summary of Performance Restricted Stock Units) (Details) - Performance Restricted Stock Units Equity Awards [Member] - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Nonvested, January 1 (in shares/awards)	203,188	132,705	67,355
Nonvested, January 1, weighted average grant date fair value (in dollars per share)	$ 37.23	$ 36.50	$ 33.96
Granted (in shares/awards)	91,816	74,163	65,350
Granted, weighted average grant date fair value (in dollars per share)	$ 46.10	$ 38.60	$ 39.12
Vested (in shares/awards)	(66,322)	0	0
Vested, weighted average grant date fair value (in dollars per share)	$ 33.93	$ 0	$ 0
Forfeited (in shares/awards)	(22,617)	(3,680)	0
Forfeited, weighted average grant date fair value (in dollars per share)	$ 44	$ 38.60	$ 0
Nonvested, December 31 (in shares/awards)	206,065	203,188	132,705
Nonvested, December 31, weighted average grant date fair value (in dollars per share)	$ 41.50	$ 37.23	$ 36.50

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, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Carrying value of deferred compensation obligation	$ 10	$ 9.8
Fair market value of deferred compensation obligation	$ 20.9	$ 16.2

Asset Retirement Obligations (R

Asset Retirement Obligations (Reconciliation Of Changes In Asset Retirement Obligations) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1		$ 177.5	$ 184.5
Revisions in estimated cash flows		(5.5)	(10.1)
Liabilities settled		(8.8)	(10.4)
Liabilities incurred	[1]	26.2	7.3
Accretion expense		6.9	6.2
Balance, December 31		196.3	177.5
IPL [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1		118.3	134.1
Revisions in estimated cash flows		(6.7)	(10.1)
Liabilities settled		(8.4)	(9.7)
Liabilities incurred	[1]	26.2	0
Accretion expense		4.5	4
Balance, December 31		133.9	118.3
WPL [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1		59.2	50.4
Revisions in estimated cash flows		1.2	0
Liabilities settled		(0.4)	(0.7)
Liabilities incurred		0	7.3
Accretion expense		2.4	2.2
Balance, December 31		$ 62.4	$ 59.2
[1]	In 2019, Alliant Energy and IPL recognized additional AROs related to IPL’s newly constructed Upland Prairie and English Farms wind sites. The increases in AROs resulted in corresponding increases in property, plant and equipment, net on the respective balance sheets.

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, 2019DekathermsMWhTgal
FTRs (MWhs) [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs) | MWh	11,671
FTRs (MWhs) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs) | MWh	5,497
FTRs (MWhs) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs) | MWh	6,174
Gas (Dths) [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in Dths) | Dekatherms	189,287
Gas (Dths) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in Dths) | Dekatherms	102,758
Gas (Dths) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in Dths) | Dekatherms	86,529
Coal (Tons) [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in Tons) | T	6,194
Coal (Tons) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in Tons) | T	2,700
Coal (Tons) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in Tons) | T	3,494
Diesel Fuel (gallons) [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in Gallons) | gal	5,040
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	5,040

Derivative Instruments (Fair Va

Derivative Instruments (Fair Value Of Financial Instruments) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Current derivative assets	$ 15.8	$ 24.6
Non-current derivative assets	11	3.7
Current derivative liabilities	19	5.6
Non-current derivative liabilities	18.6	17.7
IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	12.1	16.1
Non-current derivative assets	10.2	1.6
Current derivative liabilities	8.9	3.1
Non-current derivative liabilities	8.5	8.1
WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	3.7	8.5
Non-current derivative assets	0.8	2.1
Current derivative liabilities	10.1	2.5
Non-current derivative liabilities	$ 10.1	$ 9.6

Fair Value Measurements (Recurr

Fair Value Measurements (Recurring Fair Value Measurements) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Assets:
Deferred proceeds	$ 187.7	$ 119.4
Liabilities and equity:
Long-term debt (including current maturities)	6,919.9	5,860.8
Commodity Contracts [Member]
Assets:
Derivatives	26.8	28.3
Liabilities and equity:
Derivatives	37.6	23.3
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)	6,917.9	5,858.4
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	4.8	8.9
Liabilities and equity:
Derivatives	36.8	16.1
Level 3 [Member]
Assets:
Deferred proceeds	187.7	119.4
Liabilities and equity:
Long-term debt (including current maturities)	2	2.4
Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	22	19.4
Liabilities and equity:
Derivatives	0.8	7.2
Carrying Amount [Member]
Assets:
Deferred proceeds	187.7	119.4
Liabilities and equity:
Long-term debt (including current maturities)	6,190.2	5,502.8
Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	26.8	28.3
Liabilities and equity:
Derivatives	37.6	23.3
IPL [Member]
Assets:
Deferred proceeds	187.7	119.4
Liabilities and equity:
Long-term debt (including current maturities)	3,489.1	2,691.2
IPL [Member] | Commodity Contracts [Member]
Assets:
Derivatives	22.3	17.7
Liabilities and equity:
Derivatives	17.4	11.2
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)	3,489.1	2,691.2
IPL [Member] | Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	2.8	4
Liabilities and equity:
Derivatives	16.6	6.5
IPL [Member] | Level 3 [Member]
Assets:
Deferred proceeds	187.7	119.4
Liabilities and equity:
Long-term debt (including current maturities)	0	0
IPL [Member] | Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	19.5	13.7
Liabilities and equity:
Derivatives	0.8	4.7
IPL [Member] | Carrying Amount [Member]
Assets:
Deferred proceeds	187.7	119.4
Liabilities and equity:
Long-term debt (including current maturities)	3,147.3	2,552.3
IPL [Member] | Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	22.3	17.7
Liabilities and equity:
Derivatives	17.4	11.2
WPL [Member]
Liabilities and equity:
Long-term debt (including current maturities)	2,268.2	2,043.7
WPL [Member] | Commodity Contracts [Member]
Assets:
Derivatives	4.5	10.6
Liabilities and equity:
Derivatives	20.2	12.1
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)	2,268.2	2,043.7
WPL [Member] | Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	2	4.9
Liabilities and equity:
Derivatives	20.2	9.6
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	2.5	5.7
Liabilities and equity:
Derivatives	0	2.5
WPL [Member] | Carrying Amount [Member]
Liabilities and equity:
Long-term debt (including current maturities)	1,932.7	1,834.9
WPL [Member] | Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	4.5	10.6
Liabilities and equity:
Derivatives	$ 20.2	$ 12.1

Fair Value Measurements (Fair V

Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018
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	$ (12.2)
Total net gains (losses) included in changes in net assets (realized/unrealized)		8.2	9.1
Transfers out of Level 3	[1]	4	16.1
Purchases		13.8	26.7
Sales		(0.2)	(0.5)
Settlements		(16.8)	(27)
Ending balance, December 31		21.2	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		11.4	10.7
Commodity Contracts Derivative Assets and (Liabilities), net [Member] | IPL [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance, January 1		9	(1.4)
Total net gains (losses) included in changes in net assets (realized/unrealized)		10.7	(0.5)
Transfers out of Level 3	[1]	2.6	11
Purchases		9.5	22.5
Sales		(0.1)	(0.4)
Settlements		(13)	(22.2)
Ending balance, December 31		18.7	9
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		12.6	0.2
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		3.2	(10.8)
Total net gains (losses) included in changes in net assets (realized/unrealized)		(2.5)	9.6
Transfers out of Level 3	[1]	1.4	5.1
Purchases		4.3	4.2
Sales		(0.1)	(0.1)
Settlements		(3.8)	(4.8)
Ending balance, December 31		2.5	3.2
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		(1.2)	10.5
Deferred Proceeds [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance, January 1		119.4	222.1
Total net gains (losses) included in changes in net assets (realized/unrealized)		0	0
Transfers out of Level 3	[1]	0	0
Purchases		0	0
Sales		0	0
Settlements	[2]	68.3	(102.7)
Ending balance, December 31		187.7	119.4
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	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		119.4	222.1
Total net gains (losses) included in changes in net assets (realized/unrealized)		0	0
Transfers out of Level 3	[1]	0	0
Purchases		0	0
Sales		0	0
Settlements	[2]	68.3	(102.7)
Ending balance, December 31		187.7	119.4
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	$ 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, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	$ 21.2	$ 12.2	$ (12.2)
Excluding FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	14.6	3.2
FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	6.6	9
IPL [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	18.7	9	(1.4)
IPL [Member] | Excluding FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	13.6	1.8
IPL [Member] | FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	5.1	7.2
WPL [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	2.5	3.2	$ (10.8)
WPL [Member] | Excluding FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	1	1.4
WPL [Member] | FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	$ 1.5	$ 1.8

Commitments And Contingencies_2

Commitments And Contingencies (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)	Dec. 31, 2016	Dec. 31, 2018USD ($)
Long-term Purchase Commitment [Line Items]
Present value abandonment obligation	$ 39
Other	$ 298.9		$ 287.2
Workforce Subject to Collective Bargaining Arrangements [Member]
Long-term Purchase Commitment [Line Items]
Employees covered by collective bargaining agreement	54.00%
Workforce Subject to Collective Bargaining Arrangements Expiring within One Year [Member]
Long-term Purchase Commitment [Line Items]
Employees covered by collective bargaining agreement	18.00%
Capital Purchase Commitment [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	$ 77
IPL [Member]
Long-term Purchase Commitment [Line Items]
Maximum indemnification limit	17
Other	$ 253.4		220.7
IPL [Member] | Workforce Subject to Collective Bargaining Arrangements [Member]
Long-term Purchase Commitment [Line Items]
Employees covered by collective bargaining agreement	62.00%
IPL [Member] | Workforce Subject to Collective Bargaining Arrangements Expiring within One Year [Member]
Long-term Purchase Commitment [Line Items]
Employees covered by collective bargaining agreement	46.00%
IPL [Member] | Capital Purchase Commitment [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	$ 60
WPL [Member]
Long-term Purchase Commitment [Line Items]
Other	$ 168.7		$ 178.2
WPL [Member] | Workforce Subject to Collective Bargaining Arrangements [Member]
Long-term Purchase Commitment [Line Items]
Employees covered by collective bargaining agreement	83.00%
WPL [Member] | Capital Purchase Commitment [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	$ 17
Indemnification Agreement [Member]
Long-term Purchase Commitment [Line Items]
Maximum indemnification limit	82
Indemnification Agreement [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Maximum indemnification limit	$ 17
FERC [Member]
Long-term Purchase Commitment [Line Items]
Number of FERC complaints (in number of complaints)	2
Base return on equity, percentage	9.88%	10.32%
Other	$ 5.8

Commitments And Contingencies_3

Commitments And Contingencies (Other Purchase Commitments) (Details) - USD ($) $ in Millions		1 Months Ended
		Sep. 30, 2020	Dec. 31, 2019
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2020			$ 499
Minimum future commitments, 2021			223
Minimum future commitments, 2022			160
Minimum future commitments, 2023			128
Minimum future commitments, 2024			84
Minimum future commitments, Thereafter			199
Minimum future commitments, Total			1,293
Individual commitments incurred			1
Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2020	[1]		111
Minimum future commitments, 2021	[1]		0
Minimum future commitments, 2022	[1]		0
Minimum future commitments, 2023	[1]		0
Minimum future commitments, 2024	[1]		0
Minimum future commitments, Thereafter	[1]		0
Minimum future commitments, Total	[1]		111
Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2020			257
Minimum future commitments, 2021			183
Minimum future commitments, 2022			139
Minimum future commitments, 2023			109
Minimum future commitments, 2024			77
Minimum future commitments, Thereafter			187
Minimum future commitments, Total			952
Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2020	[2]		75
Minimum future commitments, 2021	[2]		27
Minimum future commitments, 2022	[2]		10
Minimum future commitments, 2023	[2]		10
Minimum future commitments, 2024	[2]		0
Minimum future commitments, Thereafter	[2]		0
Minimum future commitments, Total	[2]		122
Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2020	[3]		56
Minimum future commitments, 2021	[3]		13
Minimum future commitments, 2022	[3]		11
Minimum future commitments, 2023	[3]		9
Minimum future commitments, 2024	[3]		7
Minimum future commitments, Thereafter	[3]		12
Minimum future commitments, Total	[3]		108
IPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2020			315
Minimum future commitments, 2021			126
Minimum future commitments, 2022			91
Minimum future commitments, 2023			80
Minimum future commitments, 2024			56
Minimum future commitments, Thereafter			99
Minimum future commitments, Total			767
IPL [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2020	[1]		110
Minimum future commitments, 2021	[1]		0
Minimum future commitments, 2022	[1]		0
Minimum future commitments, 2023	[1]		0
Minimum future commitments, 2024	[1]		0
Minimum future commitments, Thereafter	[1]		0
Minimum future commitments, Total	[1]		110
IPL [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2020			135
Minimum future commitments, 2021			101
Minimum future commitments, 2022			77
Minimum future commitments, 2023			67
Minimum future commitments, 2024			55
Minimum future commitments, Thereafter			89
Minimum future commitments, Total			524
IPL [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2020	[2]		46
Minimum future commitments, 2021	[2]		21
Minimum future commitments, 2022	[2]		10
Minimum future commitments, 2023	[2]		10
Minimum future commitments, 2024	[2]		0
Minimum future commitments, Thereafter	[2]		0
Minimum future commitments, Total	[2]		87
IPL [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2020	[3]		24
Minimum future commitments, 2021	[3]		4
Minimum future commitments, 2022	[3]		4
Minimum future commitments, 2023	[3]		3
Minimum future commitments, 2024	[3]		1
Minimum future commitments, Thereafter	[3]		10
Minimum future commitments, Total	[3]		46
WPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2020			176
Minimum future commitments, 2021			89
Minimum future commitments, 2022			63
Minimum future commitments, 2023			43
Minimum future commitments, 2024			23
Minimum future commitments, Thereafter			99
Minimum future commitments, Total			493
WPL [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2020			1
Minimum future commitments, 2021			0
Minimum future commitments, 2022			0
Minimum future commitments, 2023			0
Minimum future commitments, 2024			0
Minimum future commitments, Thereafter			0
Minimum future commitments, Total			1
WPL [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2020			122
Minimum future commitments, 2021			82
Minimum future commitments, 2022			62
Minimum future commitments, 2023			42
Minimum future commitments, 2024			22
Minimum future commitments, Thereafter			98
Minimum future commitments, Total			428
WPL [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2020	[2]		29
Minimum future commitments, 2021	[2]		6
Minimum future commitments, 2022	[2]		0
Minimum future commitments, 2023	[2]		0
Minimum future commitments, 2024	[2]		0
Minimum future commitments, Thereafter	[2]		0
Minimum future commitments, Total	[2]		35
WPL [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2020	[3]		24
Minimum future commitments, 2021	[3]		1
Minimum future commitments, 2022	[3]		1
Minimum future commitments, 2023	[3]		1
Minimum future commitments, 2024	[3]		1
Minimum future commitments, Thereafter	[3]		1
Minimum future commitments, Total	[3]		$ 29
Forecast [Member] | DAEC PPA [Member]
Long-term Purchase Commitment [Line Items]
Buyout payment		$ 110
[1]	Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. As a result of an amendment to shorten the term of the DAEC PPA, Alliant Energy’s and IPL’s amounts include minimum future commitments related to IPL’s purchase of capacity and the resulting energy from DAEC through September 2020, and do not include the September 2020 buyout payment of $110 million .
[2]	Corporate Services has historically entered into system-wide coal contracts on behalf of IPL and WPL that include minimum future commitments. Such commitments were assigned to IPL and WPL based on information available as of December 31, 2019 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, 2019 .

Commitments And Contingencies_4

Commitments And Contingencies (Schedule Of Environmental Liabilities) (Details) - Natural Gas Processing Plant [Member] $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)
Current and non-current environmental liabilities	$ 18
IPL [Member]
Current and non-current environmental liabilities	15
Minimum [Member]
Range of estimated future costs	14
Minimum [Member] | IPL [Member]
Range of estimated future costs	12
Maximum [Member]
Range of estimated future costs	30
Maximum [Member] | IPL [Member]
Range of estimated future costs	$ 25

Segments Of Business (Narrative

Segments Of Business (Narrative) (Details)	12 Months Ended
	Dec. 31, 2019segment
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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ 880.1	$ 990.2	$ 790.2	$ 987.2	$ 873.5	$ 928.6	$ 816.1	$ 916.3	$ 3,647.7	$ 3,534.5	$ 3,382.2
Depreciation and amortization									567.2	506.9	461.8
Operating income (loss)	160.9	290.2	149.8	176.8	121.4	256.1	151.2	165.7	777.7	694.4	671.2
Interest expense									272.9	247	215.6
Equity income from unconsolidated investments, net									(53)	(54.6)	(44.8)
Income tax expense (benefit)									68.7	47.7	66.7
Net income attributable to common shareowners	111.5	226	94.6	125.1	85.3	205.5	100.4	120.9	557.2	512.1	457.3
Total assets	16,700.7				15,426				16,700.7	15,426	14,187.8
Investments in equity method subsidiaries	458.3				421.3				458.3	421.3	381.4
Construction and acquisition expenditures									1,640.1	1,633.9	1,466.9
IPL [Member]
Segment Reporting Information [Line Items]
Revenues	496.8	596.5	441.2	555.1	494.1	547.6	474.8	525.8	2,089.6	2,042.3	1,870.3
Operating income (loss)	81.3	173.5	74.5	73.5	54.4	143.1	77.7	75.6	402.8	350.8	304.1
Interest expense									126.9	119.4	112.4
Income tax expense (benefit)									24.1	(3.2)	(10.9)
Net income attributable to common shareowners	44.7	141.1	45	53.3	39.1	126.5	51.7	46.7	284.1	264	216.8
Total assets	9,277.5				8,411.4				9,277.5	8,411.4
WPL [Member]
Segment Reporting Information [Line Items]
Revenues	362.3	372.1	326.1	415.2	369.4	370.7	330.8	381.7	1,475.7	1,452.6	1,472.8
Operating income (loss)	73.8	$ 108.3	$ 67	$ 98.1	61.3	$ 104.2	$ 63.4	$ 84	347.2	312.9	333.7
Interest expense									102.2	97.8	93.8
Income tax expense (benefit)									49.3	36.2	61.9
Total assets	6,506.5				6,152.5				6,506.5	6,152.5
Electric [Member]
Segment Reporting Information [Line Items]
Revenues									3,063.6	3,000.3	2,894.7
Depreciation and amortization									512.7	457.3	412
Operating income (loss)									678.9	610.2	601.7
Equity income from unconsolidated investments, net									(1.4)	(0.9)	(0.7)
Total assets	13,659				12,486.3				13,659	12,486.3	11,396.2
Investments in equity method subsidiaries	9.5				8.1				9.5	8.1	8.3
Construction and acquisition expenditures									1,438.4	1,421.1	1,154.9
Electric [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Revenues									1,781.2	1,731.1	1,598.9
Depreciation and amortization									294.9	254.7	215.1
Operating income (loss)									359.7	318.2	287.3
Total assets	8,074.6				7,219.9				8,074.6	7,219.9	6,524.4
Construction and acquisition expenditures									963.4	890.6	594.1
Electric [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Revenues									1,282.4	1,269.2	1,295.8
Depreciation and amortization									217.8	202.6	196.9
Operating income (loss)									319.2	292	314.4
Total assets	5,584.4				5,266.4				5,584.4	5,266.4	4,871.8
Construction and acquisition expenditures									475	530.5	592.4
Gas [Member]
Segment Reporting Information [Line Items]
Revenues									455.2	446.6	400.9
Depreciation and amortization									46.4	42	38.2
Operating income (loss)									69.8	53.2	47.7
Equity income from unconsolidated investments, net									0	0	0
Total assets	1,268.5				1,184.4				1,268.5	1,184.4	1,199.8
Investments in equity method subsidiaries	0				0				0	0	0
Construction and acquisition expenditures									99.7	146.8	125.2
Gas [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Revenues									264.2	266.2	226
Depreciation and amortization									28.6	25.2	22.2
Operating income (loss)									38.9	28.3	21.7
Total assets	733.8				687.5				733.8	687.5	727.9
Construction and acquisition expenditures									55.9	99.7	80.7
Gas [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Revenues									191	180.4	174.9
Depreciation and amortization									17.8	16.8	16
Operating income (loss)									30.9	24.9	26
Total assets	534.7				496.9				534.7	496.9	471.9
Construction and acquisition expenditures									43.8	47.1	44.5
Other Utility [Member]
Segment Reporting Information [Line Items]
Revenues									46.5	48	47.5
Depreciation and amortization									3.2	3.6	7.7
Operating income (loss)									1.3	0.3	(11.6)
Equity income from unconsolidated investments, net									0	0	0
Total assets	856.5				893.2				856.5	893.2	766.5
Investments in equity method subsidiaries	0				0				0	0	0
Construction and acquisition expenditures									0.3	0.4	1.7
Other Utility [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Revenues									44.2	45	45.4
Depreciation and amortization									3.2	3.6	7.7
Operating income (loss)									4.2	4.3	(4.9)
Total assets	469.1				504				469.1	504	353.7
Construction and acquisition expenditures									0.3	0.4	1.2
Other Utility [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Revenues									2.3	3	2.1
Depreciation and amortization									0	0	0
Operating income (loss)									(2.9)	(4)	(6.7)
Total assets	387.4				389.2				387.4	389.2	412.8
Construction and acquisition expenditures									0	0	0.5
Utility Business [Member]
Segment Reporting Information [Line Items]
Revenues									3,565.3	3,494.9	3,343.1
Depreciation and amortization									562.3	502.9	457.9
Operating income (loss)									750	663.7	637.8
Interest expense									229.1	217.2	206.2
Equity income from unconsolidated investments, net									(1.4)	(0.9)	(0.7)
Income tax expense (benefit)									73.4	33	51
Net income attributable to common shareowners									517.1	472.1	403.4
Total assets	15,784				14,563.9				15,784	14,563.9	13,362.5
Investments in equity method subsidiaries	9.5				8.1				9.5	8.1	8.3
Construction and acquisition expenditures									1,538.4	1,568.3	1,281.8
Utility Business [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Revenues									2,089.6	2,042.3	1,870.3
Depreciation and amortization									326.7	283.5	245
Operating income (loss)									402.8	350.8	304.1
Interest expense									126.9	119.4	112.4
Income tax expense (benefit)									24.1	(3.2)	(10.9)
Net income attributable to common shareowners									284.1	264	216.8
Total assets	9,277.5				8,411.4				9,277.5	8,411.4	7,606
Construction and acquisition expenditures									1,019.6	990.7	676
Utility Business [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Revenues									1,475.7	1,452.6	1,472.8
Depreciation and amortization									235.6	219.4	212.9
Operating income (loss)									347.2	312.9	333.7
Interest expense									102.2	97.8	93.8
Income tax expense (benefit)									49.3	36.2	61.9
Net income attributable to common shareowners									233	208.1	186.6
Total assets	6,506.5				6,152.5				6,506.5	6,152.5	5,756.5
Construction and acquisition expenditures									518.8	577.6	637.4
Non-Utility [Member]
Segment Reporting Information [Line Items]
Revenues									82.4	39.6	39.1
Depreciation and amortization									4.9	4	3.9
Operating income (loss)									27.7	30.7	33.4
Interest expense									43.8	29.8	9.4
Equity income from unconsolidated investments, net									(51.6)	(53.7)	(44.1)
Income tax expense (benefit)									(4.7)	14.7	15.7
Net income attributable to common shareowners									40.1	40	53.9
Total assets	916.7				862.1				916.7	862.1	825.3
Investments in equity method subsidiaries	$ 448.8				$ 413.2				448.8	413.2	373.1
Construction and acquisition expenditures									$ 101.7	$ 65.6	$ 185.1

Related Parties (Narrative) (De

Related Parties (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
WPL [Member] | WPL Owed ATC [Member]
Related Party Transaction [Line Items]
Net amounts owed	$ 9	$ 8

Related Parties (Service Agreem

Related Parties (Service Agreements) (Details) - Corporate Services [Member] - Subsidiary of Common Parent [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Administrative and General Services Billings [Member] | IPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	$ 185	$ 170	$ 177
Administrative and General Services Billings [Member] | WPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	142	132	135
Transmission Sales Credited [Member] | IPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	68	48	23
Transmission Sales Credited [Member] | WPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	7	28	13
Transmission Purchases Billed [Member] | IPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	331	358	364
Transmission Purchases Billed [Member] | WPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	$ 120	$ 81	$ 115

Related Parties (Net Intercompa

Related Parties (Net Intercompany Payables) (Details) - Corporate Services [Member] - Subsidiary of Common Parent [Member] - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
IPL [Member]
Related Party Transaction [Line Items]
Intercompany payables to Corporate Services	$ 112	$ 95
WPL [Member]
Related Party Transaction [Line Items]
Intercompany payables to Corporate Services	$ 85	$ 71

Related Parties (Related Amount

Related Parties (Related Amounts Billed Between Parties) (Details) - WPL [Member] - ATC LLC [Member] - Equity Method Investment [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
ATC Billings To WPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	$ 109	$ 106	$ 105
WPL Billings To ATC [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	$ 13	$ 11	$ 10

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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues	$ 880.1	$ 990.2	$ 790.2	$ 987.2	$ 873.5	$ 928.6	$ 816.1	$ 916.3	$ 3,647.7	$ 3,534.5	$ 3,382.2
Operating income	160.9	290.2	149.8	176.8	121.4	256.1	151.2	165.7	777.7	694.4	671.2
Net income									567.4	522.3	467.5
Net income attributable to common shareowners	$ 111.5	$ 226	$ 94.6	$ 125.1	$ 85.3	$ 205.5	$ 100.4	$ 120.9	$ 557.2	$ 512.1	$ 457.3
Earnings per weighted average common share attributable to Alliant Energy common shareowners (in dollars per share)	$ 0.46	$ 0.94	$ 0.40	$ 0.53	$ 0.36	$ 0.87	$ 0.43	$ 0.52	$ 2.33	$ 2.19	$ 1.99
IPL [Member]
Revenues	$ 496.8	$ 596.5	$ 441.2	$ 555.1	$ 494.1	$ 547.6	$ 474.8	$ 525.8	$ 2,089.6	$ 2,042.3	$ 1,870.3
Operating income	81.3	173.5	74.5	73.5	54.4	143.1	77.7	75.6	402.8	350.8	304.1
Net income	47.2	143.7	47.5	55.9	41.6	129.1	54.2	49.3	294.3	274.2	227
Net income attributable to common shareowners	44.7	141.1	45	53.3	39.1	126.5	51.7	46.7	284.1	264	216.8
WPL [Member]
Revenues	362.3	372.1	326.1	415.2	369.4	370.7	330.8	381.7	1,475.7	1,452.6	1,472.8
Operating income	73.8	108.3	67	98.1	61.3	104.2	63.4	84	347.2	312.9	333.7
Net income	$ 49.8	$ 75.5	$ 42	$ 65.7	$ 38	$ 76.3	$ 39.8	$ 54	$ 233	$ 208.1	$ 186.6

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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating expenses									$ 2,870	$ 2,840.1	$ 2,711
Operating income (loss)	$ 160.9	$ 290.2	$ 149.8	$ 176.8	$ 121.4	$ 256.1	$ 151.2	$ 165.7	777.7	694.4	671.2
Other (income) and deductions:
Equity earnings from consolidated subsidiaries									(53)	(54.6)	(44.8)
Interest expense									272.9	247	215.6
Total other (income) and deductions									141.6	124.4	138.4
Income from continuing operations before income taxes									636.1	570	532.8
Income tax benefit									68.7	47.7	66.7
Net income									567.4	522.3	467.5
Parent Company [Member]
Operating expenses									2	5	2
Operating income (loss)									(2)	(5)	(2)
Other (income) and deductions:
Equity earnings from consolidated subsidiaries									(562)	(523)	(457)
Interest expense									9	4	3
Other									7	2	0
Total other (income) and deductions									(546)	(517)	(454)
Income from continuing operations before income taxes									544	512	452
Income tax benefit									(15)	(1)	(6)
Net income									$ 559	$ 513	$ 458

Condensed Parent Company Fina_4

Condensed Parent Company Financial Statements (Condensed Balance Sheets) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Other	$ 101.7	$ 80
Total current assets	875.5	785.1
Investments:
Other	147.7	137.7
Total investments	467.8	431.3
Other assets	72	89.7
Total assets	16,700.7	15,426	$ 14,187.8
Current liabilities:
Commercial paper	337.4	441.2
Other	425.2	260.4
Total current liabilities	2,054.1	1,644.1
Other liabilities	298.9	287.2
Common Equity:
Retained earnings	2,765.4	2,545.9
Accumulated other comprehensive income	1.3	1.7
Shares in deferred compensation trust	(10)	(9.8)
Total common equity	5,205.1	4,585.7
Total liabilities and equity	16,700.7	15,426
Parent Company [Member]
Current assets:
Notes receivable from affiliated companies	27	23
Other	6	4
Total current assets	33	27
Investments:
Investments in consolidated subsidiaries	6,017	5,518
Other	1	1
Total investments	6,018	5,519
Other assets	89	81
Total assets	6,140	5,627
Current liabilities:
Commercial paper	169	285
Notes payable to affiliated companies	731	719
Other	19	21
Total current liabilities	919	1,025
Other liabilities	15	17
Common Equity:
Common stock and additional paid-in capital	2,448	2,048
Retained earnings	2,766	2,545
Accumulated other comprehensive income	2	2
Shares in deferred compensation trust	(10)	(10)
Total common equity	5,206	4,585
Total liabilities and equity	$ 6,140	$ 5,627

Condensed Parent Company Fina_5

Condensed Parent Company Financial Statements (Condensed Statements of Cash Flows) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net cash flows from operating activities	$ 660.4	$ 527.7	$ 521.6
Cash flows used for investing activities:
Other	(60.4)	(38.2)	(28.3)
Net cash flows used for investing activities	(1,287.3)	(1,066.8)	(1,033.4)
Cash flows used for financing activities:
Common stock dividends	(337.7)	(312.2)	(288.3)
Proceeds from issuance of common stock, net	390.3	196.6	149.6
Net change in commercial paper	(103.8)	26	171.1
Other	(23.2)	(24)	(45.2)
Net cash flows from (used for) financing activities	619.1	530.7	532.6
Net increase (decrease) in cash, cash equivalents and restricted cash	(7.8)	(8.4)	20.8
Cash, cash equivalents and restricted cash at beginning of period	25.5	33.9	13.1
Cash, cash equivalents and restricted cash at end of period	17.7	25.5	33.9
Supplemental cash flows information:
Interest	(267.9)	(247.5)	(212.6)
Income taxes, net	20.5	(5)	(11.3)
Parent Company [Member]
Net cash flows from operating activities	305	311	273
Cash flows used for investing activities:
Capital contributions to consolidated subsidiaries	(250)	(625)	(290)
Net change in notes receivable from and payable to affiliates	8	441	54
Net cash flows used for investing activities	(242)	(184)	(236)
Cash flows used for financing activities:
Common stock dividends	(338)	(312)	(288)
Proceeds from issuance of common stock, net	390	197	150
Net change in commercial paper	(116)	(10)	103
Other	1	(2)	(2)
Net cash flows from (used for) financing activities	(63)	(127)	(37)
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	(9)	(4)	(3)
Income taxes, net	$ 14	$ 5	$ 0

Valuation And Qualifying Acco_2

Valuation And Qualifying Accounts and Reserves (Details) - Accumulated Provision for Uncollectible Accounts [Member] - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	[1]	$ 10.5	$ 12	$ 8.7
Charged to Expense	[1]	17.3	21.2	15.1
Charged to Other Accounts	[1],[2]	2.4	1	5.4
Deductions	[1],[3]	22.9	23.7	17.2
Ending Balance	[1]	7.3	10.5	12
IPL [Member]
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	[1]	3.1	1.3	1.1
Charged to Expense	[1]	16.5	20.9	14.9
Charged to Other Accounts	[1],[2]	0	0	0
Deductions	[1],[3]	18.4	19.1	14.7
Ending Balance	[1]	1.2	3.1	1.3
WPL [Member]
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance		7.4	10.7	7.1
Charged to Expense		0.8	0.3	0.2
Charged to Other Accounts	[2]	2.4	1	5.4
Deductions	[3]	4.5	4.6	2
Ending Balance		$ 6.1	$ 7.4	$ 10.7
[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.