Interstate Power & Light (IPLDP) 10-K 19 Feb 21 2020 FY Annual report Financial data

Document And Entity Information

Document And Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Jan. 31, 2021	Jun. 30, 2020
Entity Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
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
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 11,900
Entity Common Stock, Shares Outstanding		249,881,050
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Amendment Flag	false
Documents Incorporated by Reference	Portions of the Proxy Statement relating to Alliant Energy Corporation’s 2021 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, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues:
Electric utility	$ 2,920	$ 3,064	$ 3,000
Gas utility	373	455	447
Other utility	49	46	48
Non-utility	74	83	39
Total revenues	3,416	3,648	3,534
Operating expenses:
Electric production fuel and purchased power	652	777	855
Electric transmission service	449	481	496
Cost of gas sold	182	222	232
Other operation and maintenance	670	712	646
Depreciation and amortization	615	567	507
Taxes other than income taxes	108	111	104
Total operating expenses	2,676	2,870	2,840
Operating income (loss)	740	778	694
Other (income) and deductions:
Interest expense	275	273	247
Equity income from unconsolidated investments, net	(61)	(53)	(55)
Allowance for funds used during construction	(55)	(93)	(76)
Other	14	15	8
Total other (income) and deductions	173	142	124
Income before income taxes	567	636	570
Income tax expense (benefit)	(57)	69	48
Net income	624	567	522
Preferred dividend requirements of Interstate Power and Light Company	10	10	10
Net income attributable to common shareowners	$ 614	$ 557	$ 512
Weighted average number of common shares outstanding:
Basic (in shares)	248.4	238.5	233.6
Diluted (in shares)	248.7	239	233.6
Earnings per weighted average common share attributable to Alliant Energy common shareowners:
Basic (in dollars per share)	$ 2.47	$ 2.34	$ 2.19
Diluted (in dollars per share)	$ 2.47	$ 2.33	$ 2.19
IPL [Member]
Revenues:
Electric utility	$ 1,695	$ 1,781	$ 1,731
Gas utility	208	264	266
Other utility	44	44	45
Total revenues	1,947	2,089	2,042
Operating expenses:
Electric production fuel and purchased power	352	435	469
Electric transmission service	298	340	353
Cost of gas sold	99	120	129
Other operation and maintenance	375	404	403
Depreciation and amortization	356	327	283
Taxes other than income taxes	57	60	54
Total operating expenses	1,537	1,686	1,691
Operating income (loss)	410	403	351
Other (income) and deductions:
Interest expense	139	127	119
Allowance for funds used during construction	(24)	(50)	(42)
Other	8	8	3
Total other (income) and deductions	123	85	80
Income before income taxes	287	318	271
Income tax expense (benefit)	(47)	24	(3)
Net income	334	294	274
Preferred dividend requirements of Interstate Power and Light Company	10	10	10
Net income attributable to common shareowners	324	284	264
WPL [Member]
Revenues:
Electric utility	1,225	1,283	1,269
Gas utility	165	191	181
Other utility	5	2	3
Total revenues	1,395	1,476	1,453
Operating expenses:
Electric production fuel and purchased power	300	342	386
Electric transmission service	151	141	143
Cost of gas sold	83	102	103
Other operation and maintenance	254	261	242
Depreciation and amortization	254	236	219
Taxes other than income taxes	47	47	47
Total operating expenses	1,089	1,129	1,140
Operating income (loss)	306	347	313
Other (income) and deductions:
Interest expense	104	102	98
Allowance for funds used during construction	(31)	(43)	(34)
Other	3	6	5
Total other (income) and deductions	76	65	69
Income before income taxes	230	282	244
Income tax expense (benefit)	(19)	49	36
Net income	$ 249	$ 233	$ 208

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 54	$ 16
Accounts receivable, less allowance for expected credit losses	412	402
Production fuel, at weighted average cost	66	78
Gas stored underground, at weighted average cost	46	49
Materials and supplies, at weighted average cost	105	100
Regulatory assets	81	87
Other	123	144
Total current assets	887	876
Property, plant and equipment, net	14,336	13,527
Investments:
ATC Holdings	331	320
Other	154	148
Total investments	485	468
Other assets:
Regulatory assets	1,929	1,758
Deferred charges and other	73	72
Total other assets	2,002	1,830
Total assets	17,710	16,701
Current liabilities:
Current maturities of long-term debt	8	657
Commercial paper	389	337
Accounts payable	377	422
Accrued taxes	67	70
Regulatory liabilities	249	212
Other	207	356
Total current liabilities	1,297	2,054
Long-term debt, net (excluding current portion)	6,769	5,533
Other liabilities:
Deferred tax liabilities	1,814	1,714
Regulatory liabilities	1,057	1,212
Pension and other benefit obligations	511	484
Other	374	299
Total other liabilities	3,756	3,709
Commitments and contingencies (Note 17)
Common equity:
Common stock	2	2
Additional paid-in capital	2,704	2,446
Retained earnings	2,994	2,766
Accumulated other comprehensive income (loss)	(1)	1
Shares in deferred compensation trust - 380,542 and 381,232 shares at a weighted average cost of $28.73 and $26.24 per share	(11)	(10)
Total common equity	5,688	5,205
Cumulative preferred stock of Interstate Power and Light Company	200	200
Total equity	5,888	5,405
Total liabilities and equity	17,710	16,701
IPL [Member]
Current assets:
Cash and cash equivalents	50	9
Accounts receivable, less allowance for expected credit losses	210	203
Income taxes refunds receivable	26	11
Production fuel, at weighted average cost	48	47
Gas stored underground, at weighted average cost	20	22
Materials and supplies, at weighted average cost	63	55
Regulatory assets	52	44
Other	27	18
Total current assets	496	409
Property, plant and equipment, net	7,889	7,481
Other assets:
Regulatory assets	1,431	1,356
Deferred charges and other	33	31
Total other assets	1,464	1,387
Total assets	9,849	9,277
Current liabilities:
Current maturities of long-term debt	0	200
Commercial paper	0	0
Accounts payable	162	207
Accounts payable to associated companies	45	39
Accrued taxes	54	63
Regulatory liabilities	103	116
Accrued interest	34	37
Other	49	168
Total current liabilities	447	830
Long-term debt, net (excluding current portion)	3,345	2,947
Other liabilities:
Deferred tax liabilities	1,035	1,008
Regulatory liabilities	573	599
Pension and other benefit obligations	186	168
Other	299	253
Total other liabilities	2,093	2,028
Commitments and contingencies (Note 17)
Common equity:
Common stock	33	33
Additional paid-in capital	2,752	2,348
Retained earnings	979	891
Total common equity	3,764	3,272
Cumulative preferred stock of Interstate Power and Light Company	200	200
Total equity	3,964	3,472
Total liabilities and equity	9,849	9,277
WPL [Member]
Current assets:
Cash and cash equivalents	3	4
Accounts receivable, less allowance for expected credit losses	192	191
Production fuel, at weighted average cost	18	31
Gas stored underground, at weighted average cost	26	27
Materials and supplies, at weighted average cost	40	43
Regulatory assets	29	43
Prepaid gross receipts tax	42	42
Other	12	62
Total current assets	362	443
Property, plant and equipment, net	6,022	5,638
Other assets:
Regulatory assets	498	402
Deferred charges and other	30	24
Total other assets	528	426
Total assets	6,912	6,507
Current liabilities:
Current maturities of long-term debt	0	150
Commercial paper	257	168
Accounts payable	154	160
Accounts payable to associated companies	35	42
Regulatory liabilities	146	96
Other	82	74
Total current liabilities	674	690
Long-term debt, net (excluding current portion)	2,130	1,783
Other liabilities:
Deferred tax liabilities	702	626
Regulatory liabilities	484	613
Finance lease obligations - Sheboygan Falls Energy Facility	42	51
Pension and other benefit obligations	222	211
Other	180	169
Total other liabilities	1,630	1,670
Commitments and contingencies (Note 17)
Common equity:
Common stock	66	66
Additional paid-in capital	1,459	1,434
Retained earnings	953	864
Total common equity	2,478	2,364
Total equity	2,478	2,364
Total liabilities and equity	$ 6,912	$ 6,507

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
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)	249,868,415	245,022,800
Shares in deferred compensation trust (in shares)	380,542	381,232
Shares in deferred compensation trust, weighted average cost per share (in dollars per share)	$ 28.73	$ 26.24
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, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from (used for) operating activities:
Net income	$ 624	$ 567	$ 522
Adjustments to reconcile net income to net cash flows from (used for) operating activities:
Depreciation and amortization	615	567	507
Deferred tax expense (benefit) and tax credits	(66)	56	67
Equity component of allowance for funds used during construction	(39)	(66)	(51)
Other	37	20	8
Other changes in assets and liabilities:
Accounts receivable	(468)	(472)	(475)
Regulatory assets	(130)	(16)	(16)
Regulatory liabilities	(113)	(40)	1
Deferred income taxes	171	54	56
DAEC PPA amendment buyout payment	(110)	0	0
Other	(20)	(10)	(91)
Net cash flows from (used for) operating activities	501	660	528
Cash flows used for investing activities:
Utility construction and acquisition expenditures	(1,293)	(1,539)	(1,569)
Other construction and acquisition expenditures	(73)	(101)	(65)
Cash receipts on sold receivables	458	413	605
Other	(43)	(60)	(38)
Net cash flows used for investing activities	(951)	(1,287)	(1,067)
Cash flows from financing activities:
Common stock dividends	(377)	(337)	(312)
Proceeds from issuance of common stock, net	247	390	197
Proceeds from issuance of long-term debt	1,250	950	1,500
Payments to retire long-term debt	(657)	(256)	(856)
Net change in commercial paper and other short-term borrowings	52	(104)	26
Other	(27)	(24)	(24)
Net cash flows from (used for) financing activities	488	619	531
Net increase (decrease) in cash, cash equivalents and restricted cash	38	(8)	(8)
Cash, cash equivalents and restricted cash at beginning of period	18	26	34
Cash, cash equivalents and restricted cash at end of period	56	18	26
Supplemental cash flows information:
Interest	(274)	(268)	(247)
Income taxes, net	5	21	(5)
Significant non-cash investing and financing activities:
Accrued capital expenditures	131	196	299
Beneficial interest obtained in exchange for securitized accounts receivable	188	188	119
IPL [Member]
Cash flows from (used for) operating activities:
Net income	334	294	274
Adjustments to reconcile net income to net cash flows from (used for) operating activities:
Depreciation and amortization	356	327	283
Deferred tax expense (benefit) and tax credits	(52)	15	2
Equity component of allowance for funds used during construction	(17)	(35)	(29)
Other	12	1	4
Other changes in assets and liabilities:
Accounts receivable	(466)	(467)	(494)
Regulatory assets	(93)	(11)	(20)
Accounts payable	6	(21)	(25)
Regulatory liabilities	(20)	2	1
Deferred income taxes	79	35	44
DAEC PPA amendment buyout payment	(110)	0	0
Other	(35)	33	(45)
Net cash flows from (used for) operating activities	(6)	173	(5)
Cash flows used for investing activities:
Utility construction and acquisition expenditures	(687)	(1,020)	(991)
Cash receipts on sold receivables	458	413	605
Other	(72)	(60)	(44)
Net cash flows used for investing activities	(301)	(667)	(430)
Cash flows from financing activities:
Common stock dividends	(236)	(168)	(168)
Capital contributions from parent	404	125	425
Proceeds from issuance of long-term debt	400	600	500
Payments to retire long-term debt	(200)	0	(350)
Net change in commercial paper and other short-term borrowings	0	(50)	50
Other	(20)	(16)	(17)
Net cash flows from (used for) financing activities	348	491	440
Net increase (decrease) in cash, cash equivalents and restricted cash	41	(3)	5
Cash, cash equivalents and restricted cash at beginning of period	9	12	7
Cash, cash equivalents and restricted cash at end of period	50	9	12
Supplemental cash flows information:
Interest	(141)	(122)	(120)
Income taxes, net	(18)	7	(24)
Significant non-cash investing and financing activities:
Accrued capital expenditures	73	112	187
Beneficial interest obtained in exchange for securitized accounts receivable	188	188	119
WPL [Member]
Cash flows from (used for) operating activities:
Net income	249	233	208
Adjustments to reconcile net income to net cash flows from (used for) operating activities:
Depreciation and amortization	254	236	219
Deferred tax expense (benefit) and tax credits	(15)	24	50
Other	2	(12)	(17)
Other changes in assets and liabilities:
Regulatory liabilities	(93)	(42)	1
Deferred income taxes	90	20	9
Other	(21)	(36)	(13)
Net cash flows from (used for) operating activities	466	423	457
Cash flows used for investing activities:
Utility construction and acquisition expenditures	(606)	(519)	(578)
Other	(7)	(38)	(30)
Net cash flows used for investing activities	(613)	(557)	(608)
Cash flows from financing activities:
Common stock dividends	(160)	(144)	(140)
Capital contributions from parent	25	125	200
Proceeds from issuance of long-term debt	350	350	0
Payments to retire long-term debt	(150)	(250)	0
Net change in commercial paper and other short-term borrowings	89	63	81
Other	(8)	(15)	(5)
Net cash flows from (used for) financing activities	146	129	136
Net increase (decrease) in cash, cash equivalents and restricted cash	(1)	(5)	(15)
Cash, cash equivalents and restricted cash at beginning of period	4	9	24
Cash, cash equivalents and restricted cash at end of period	3	4	9
Supplemental cash flows information:
Interest	(102)	(103)	(98)
Income taxes, net	13	(29)	14
Significant non-cash investing and financing activities:
Accrued capital expenditures	$ 55	$ 82	$ 102

Consolidated Statements of Comm

Consolidated Statements of Common Equity - USD ($) $ in Millions	Total	Cumulative Effect, Period of Adoption, Adjustment [Member]	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]Cumulative Effect, Period of Adoption, Adjustment [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, 2017					$ 2	$ 33	$ 66	$ 1,845	$ 1,798	$ 1,109	$ 2,346		$ 679	$ 707	$ 0	$ (11)	$ 200	$ 200
Beginning balance at Dec. 31, 2017	$ 4,382		$ 2,710	$ 1,882
Increase (Decrease) in Shareowners' Equity [Roll Forward]
Net income attributable to common shareowners	512		264								512		264
Net income	522		274	208										208
Common stock dividends	(312)		(168)	(140)							(312)		(168)	(140)
Equity forward settlements, At-the-market offering program and Shareowner Direct Plan issuances	197							197
Capital contributions from parent			425	200					425	200
Equity-based compensation plans and other	5							4								1
Other comprehensive income (loss), net of tax	2														2
Ending balance at Dec. 31, 2018					2	33	66	2,046	2,223	1,309	2,546		775	775	2	(10)	200	200
Ending balance at Dec. 31, 2018	4,786		3,231	2,150
Increase (Decrease) in Shareowners' Equity [Roll Forward]
Net income attributable to common shareowners	557		284								557		284
Net income	567		294	233										233
Common stock dividends	(337)		(168)	(144)							(337)		(168)	(144)
Equity forward settlements, At-the-market offering program and Shareowner Direct Plan issuances	390							390
Capital contributions from parent			125	125					125	125
Equity-based compensation plans and other	10							10
Other comprehensive income (loss), net of tax	(1)														(1)
Ending balance at Dec. 31, 2019	5,205		3,272	2,364	2	33	66	2,446	2,348	1,434	2,766		891	864	1	(10)	200	200
Ending balance at Dec. 31, 2019	5,405		3,472	2,364
Increase (Decrease) in Shareowners' Equity [Roll Forward]
Retained earnings	2,766	$ (9)	891	864								$ (9)
Net income attributable to common shareowners	614		324								614		324
Net income	624		334	249										249
Common stock dividends	(377)		(236)	(160)							(377)		(236)	(160)
Equity forward settlements, At-the-market offering program and Shareowner Direct Plan issuances	247							247
Capital contributions from parent			404	25					404	25
Equity-based compensation plans and other	10							11								(1)
Other comprehensive income (loss), net of tax	(2)														(2)
Ending balance at Dec. 31, 2020	5,688		3,764	2,478	$ 2	$ 33	$ 66	$ 2,704	$ 2,752	$ 1,459	$ 2,994		$ 979	$ 953	$ (1)	$ (11)	$ 200	$ 200
Ending balance at Dec. 31, 2020	5,888		3,964	2,478
Increase (Decrease) in Shareowners' Equity [Roll Forward]
Retained earnings	$ 2,994		$ 979	$ 953

Consolidated Statements of Co_2

Consolidated Statements of Common Equity (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Stockholders' Equity [Abstract]
Common stock dividends (in dollars per share)	$ 1.52	$ 1.42	$ 1.34

Summary Of Significant Accounti

Summary Of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
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 Travero (formerly known as Transportation), ATI, a non-utility wind farm, the Sheboygan Falls Energy Facility and other non-utility holdings. Travero includes a short-line rail freight service in Iowa; a barge, rail and truck freight terminal on the Mississippi River; and freight brokerage and logistics management 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. In March 2020, COVID-19 was declared a global pandemic, which has resulted in widespread travel restrictions, closures of commercial spaces and industrial facilities, and more people working from home in Alliant Energy’s service territories. For 2020, Alliant Energy, IPL and WPL considered the impact of COVID-19 on their overall business operations, financial condition, results of operations and cash flows, along with assumptions and estimates used. While the total expected impact of COVID-19 on future sales is currently unknown, Alliant Energy, IPL and WPL have experienced higher electric residential sales and lower electric commercial and industrial sales since the outset of the pandemic. The degree to which the COVID-19 pandemic may impact Alliant Energy, IPL and WPL in the future is currently unknown and will depend on future developments of the pandemic as well as possible additional actions by government and regulatory authorities. 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. 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. 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 allowed unutilized alternative minimum tax credits to be refunded over four tax years beginning with the U.S. federal tax return for calendar year 2018. Pursuant to the Coronavirus Aid, Relief, and Economic Security Act, Alliant Energy received the remaining alternative minimum tax credits refunds in 2020. 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 2020, 2019 and 2018. 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 2020 2019 2018 2020 2019 2018 Electric - generation 3.5% 3.8% 3.6% 3.5% 3.6% 3.6% Electric - distribution 2.8% 2.9% 2.8% 2.6% 2.6% 2.6% Electric - other 5.2% 5.3% 4.7% 6.1% 5.8% 5.7% Gas 3.3% 3.3% 3.2% 2.4% 2.5% 2.5% Other 6.3% 5.9% 5.2% 5.9% 5.6% 5.8% 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: 2020 2019 2018 IPL (Wind generation CWIP) 7.1% 7.4% 7.5% IPL (other CWIP) 7.2% 7.5% 7.5% WPL (retail jurisdiction) 7.0% 6.8% 7.7% WPL (wholesale jurisdiction) 6.3% 6.9% 7.2% 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 then 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 component 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, 2020, 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 Travero 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 periodic 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 energy efficiency and demand response cost recovery factor tariffs, which are revised annually and include 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 recovers a return of, as well as earn a return on, its 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, are also 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 are 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 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. 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 qualified for and were designated under the normal purchase and sale exception, and were 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. Credit Losses - In 2016, the Financial Accounting Standards Board 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, certain other assets and off-balance sheet credit exposures. Alliant Energy, IPL and WPL adopted this standard on January 1, 2020 using a modified retrospective method of adoption, which required cumulative effect adjustments to retained earnings on January 1, 2020. IPL and WPL did not record a cumulative effect adjustment to retained earnings and Alliant Energy recorded a pre-tax $12 million (after-tax $9 million) cumulative effect adjustment to decrease retained earnings related to Alliant Energy’s guarantees in the partnership obligations of an affiliate of Whiting Petroleum (refer to Note 17( d) for further discussion). This adjustment is included in “Adoption of new accounting standard” in Alliant Energy’s equity statement for 2020.
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 Travero (formerly known as Transportation), ATI, a non-utility wind farm, the Sheboygan Falls Energy Facility and other non-utility holdings. Travero includes a short-line rail freight service in Iowa; a barge, rail and truck freight terminal on the Mississippi River; and freight brokerage and logistics management 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. In March 2020, COVID-19 was declared a global pandemic, which has resulted in widespread travel restrictions, closures of commercial spaces and industrial facilities, and more people working from home in Alliant Energy’s service territories. For 2020, Alliant Energy, IPL and WPL considered the impact of COVID-19 on their overall business operations, financial condition, results of operations and cash flows, along with assumptions and estimates used. While the total expected impact of COVID-19 on future sales is currently unknown, Alliant Energy, IPL and WPL have experienced higher electric residential sales and lower electric commercial and industrial sales since the outset of the pandemic. The degree to which the COVID-19 pandemic may impact Alliant Energy, IPL and WPL in the future is currently unknown and will depend on future developments of the pandemic as well as possible additional actions by government and regulatory authorities. 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. 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. 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 allowed unutilized alternative minimum tax credits to be refunded over four tax years beginning with the U.S. federal tax return for calendar year 2018. Pursuant to the Coronavirus Aid, Relief, and Economic Security Act, Alliant Energy received the remaining alternative minimum tax credits refunds in 2020. 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 2020, 2019 and 2018. 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 2020 2019 2018 2020 2019 2018 Electric - generation 3.5% 3.8% 3.6% 3.5% 3.6% 3.6% Electric - distribution 2.8% 2.9% 2.8% 2.6% 2.6% 2.6% Electric - other 5.2% 5.3% 4.7% 6.1% 5.8% 5.7% Gas 3.3% 3.3% 3.2% 2.4% 2.5% 2.5% Other 6.3% 5.9% 5.2% 5.9% 5.6% 5.8% 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: 2020 2019 2018 IPL (Wind generation CWIP) 7.1% 7.4% 7.5% IPL (other CWIP) 7.2% 7.5% 7.5% WPL (retail jurisdiction) 7.0% 6.8% 7.7% WPL (wholesale jurisdiction) 6.3% 6.9% 7.2% 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 then 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 component 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, 2020, 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 Travero 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 periodic 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 energy efficiency and demand response cost recovery factor tariffs, which are revised annually and include 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 recovers a return of, as well as earn a return on, its 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, are also 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 are 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 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. 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 qualified for and were designated under the normal purchase and sale exception, and were 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. Credit Losses - In 2016, the Financial Accounting Standards Board 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, certain other assets and off-balance sheet credit exposures. Alliant Energy, IPL and WPL adopted this standard on January 1, 2020 using a modified retrospective method of adoption, which required cumulative effect adjustments to retained earnings on January 1, 2020. IPL and WPL did not record a cumulative effect adjustment to retained earnings and Alliant Energy recorded a pre-tax $12 million (after-tax $9 million) cumulative effect adjustment to decrease retained earnings related to Alliant Energy’s guarantees in the partnership obligations of an affiliate of Whiting Petroleum (refer to Note 17( d) for further discussion). This adjustment is included in “Adoption of new accounting standard” in Alliant Energy’s equity statement for 2020.
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 Travero (formerly known as Transportation), ATI, a non-utility wind farm, the Sheboygan Falls Energy Facility and other non-utility holdings. Travero includes a short-line rail freight service in Iowa; a barge, rail and truck freight terminal on the Mississippi River; and freight brokerage and logistics management 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. In March 2020, COVID-19 was declared a global pandemic, which has resulted in widespread travel restrictions, closures of commercial spaces and industrial facilities, and more people working from home in Alliant Energy’s service territories. For 2020, Alliant Energy, IPL and WPL considered the impact of COVID-19 on their overall business operations, financial condition, results of operations and cash flows, along with assumptions and estimates used. While the total expected impact of COVID-19 on future sales is currently unknown, Alliant Energy, IPL and WPL have experienced higher electric residential sales and lower electric commercial and industrial sales since the outset of the pandemic. The degree to which the COVID-19 pandemic may impact Alliant Energy, IPL and WPL in the future is currently unknown and will depend on future developments of the pandemic as well as possible additional actions by government and regulatory authorities. 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. 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. 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 allowed unutilized alternative minimum tax credits to be refunded over four tax years beginning with the U.S. federal tax return for calendar year 2018. Pursuant to the Coronavirus Aid, Relief, and Economic Security Act, Alliant Energy received the remaining alternative minimum tax credits refunds in 2020. 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 2020, 2019 and 2018. 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 2020 2019 2018 2020 2019 2018 Electric - generation 3.5% 3.8% 3.6% 3.5% 3.6% 3.6% Electric - distribution 2.8% 2.9% 2.8% 2.6% 2.6% 2.6% Electric - other 5.2% 5.3% 4.7% 6.1% 5.8% 5.7% Gas 3.3% 3.3% 3.2% 2.4% 2.5% 2.5% Other 6.3% 5.9% 5.2% 5.9% 5.6% 5.8% 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: 2020 2019 2018 IPL (Wind generation CWIP) 7.1% 7.4% 7.5% IPL (other CWIP) 7.2% 7.5% 7.5% WPL (retail jurisdiction) 7.0% 6.8% 7.7% WPL (wholesale jurisdiction) 6.3% 6.9% 7.2% 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 then 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 component 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, 2020, 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 Travero 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 periodic 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 energy efficiency and demand response cost recovery factor tariffs, which are revised annually and include 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 recovers a return of, as well as earn a return on, its 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, are also 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 are 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 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. 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 qualified for and were designated under the normal purchase and sale exception, and were 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. Credit Losses - In 2016, the Financial Accounting Standards Board 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, certain other assets and off-balance sheet credit exposures. Alliant Energy, IPL and WPL adopted this standard on January 1, 2020 using a modified retrospective method of adoption, which required cumulative effect adjustments to retained earnings on January 1, 2020. IPL and WPL did not record a cumulative effect adjustment to retained earnings and Alliant Energy recorded a pre-tax $12 million (after-tax $9 million) cumulative effect adjustment to decrease retained earnings related to Alliant Energy’s guarantees in the partnership obligations of an affiliate of Whiting Petroleum (refer to Note 17( d) for further discussion). This adjustment is included in “Adoption of new accounting standard” in Alliant Energy’s equity statement for 2020.

Regulatory Matters

Regulatory Matters	12 Months Ended
	Dec. 31, 2020
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, 2020 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 2020 2019 2020 2019 2020 2019 Tax-related $890 $818 $843 $777 $47 $41 Pension and OPEB costs 580 524 291 263 289 261 AROs 119 112 81 76 38 36 Assets retired early 113 134 77 88 36 46 IPL’s DAEC PPA amendment 110 108 110 108 — — WPL’s Western Wisconsin gas distribution expansion investments 55 — — — 55 — Derivatives 26 39 13 18 13 21 Other 117 110 68 70 49 40 $2,010 $1,845 $1,483 $1,400 $527 $445 At December 31, 2020, IPL and WPL had $81 million and $11 million, respectively, of regulatory assets that were not earning a return. IPL’s regulatory assets that were not earning a return consisted primarily of retired analog electric meters, emission allowances, clean air compliance projects and debt redemption costs. WPL’s regulatory assets that were not earning a return consisted primarily of costs for future expansion projects and environmental-related costs. The other regulatory assets reported in the above table either earn a return or the cash has not yet been expended, in which case the assets are offset by liabilities that also do not incur a carrying cost. Tax-related - IPL and WPL record regulatory assets for certain temporary differences (primarily related to utility property, plant and equipment at IPL) that result in a decrease in current rates charged to customers and an increase in future rates charged to customers based on the timing of income tax expense that is used to determine such rates. These temporary differences for IPL include the impacts of qualifying deductions for repairs expenditures, allocation of mixed service costs, and Iowa accelerated tax depreciation, which all contribute to lower current income tax expense during the first part of an asset’s useful life and higher current 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. 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. 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 (dollars in millions): Entity Asset Retirement Date Regulatory Asset Balance as of Dec. 31, 2020 Recovery Regulatory Approval IPL Sutherland Units 1 and 3 2017 $23 Return of and return on remaining net book value through 2027 IUB and FERC IPL M.L. Kapp Unit 2 2018 21 Return of and return on remaining net book value through 2029 IUB and FERC IPL Analog electric meters 2019 33 Return of remaining net book value through 2028 IUB and FERC WPL Nelson Dewey Units 1 and 2 and Edgewater Unit 3 2015 13 Return of and return on remaining net book value through 2022 PSCW and FERC WPL Edgewater Unit 4 2018 23 Return of and return on remaining net book value through 2028 PSCW and FERC IPL’s DAEC PPA Amendment - In September 2020, IPL made a buyout payment of $110 million in exchange for shortening the term of its DAEC PPA by 5 years. The payment was recorded as a reduction to “Other current liabilities” on Alliant Energy’s and IPL’s balance sheets and was included in “DAEC PPA amendment buyout payment” in Alliant Energy’s and IPL’s cash flows used for operating activities in 2020. The buyout payment, including a return on, will be recovered from IPL’s retail and wholesale customers from 2021 through the end of 2025. WPL’s Western Wisconsin gas distribution expansion investments - WPL made contributions in aid of construction to a third party for investments as part of its Western Wisconsin gas distribution expansion project. Pursuant to authorization by the PSCW, Alliant Energy and WPL have recorded a regulatory asset for these costs, and are authorized by the PSCW to recover these amounts from WPL’s retail gas customers in base rates beginning in 2021. 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. Regulatory Liabilities - At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2020 2019 2020 2019 2020 2019 Tax-related $732 $836 $331 $351 $401 $485 Cost of removal obligations 367 388 238 257 129 131 Electric transmission cost recovery 68 88 39 51 29 37 WPL’s West Riverside liquidated damages 38 — — — 38 — Derivatives 28 20 25 17 3 3 Other 73 92 43 39 30 53 $1,306 $1,424 $676 $715 $630 $709 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 is 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. Cash payments related to cost of removal obligations are included in “Other” in cash flows used for investing activities. Electric transmission cost recovery - Refer to Note 1(g) for details of IPL’s and WPL’s electric transmission cost recovery mechanisms. In 2020, pursuant to a June 2020 IUB order, IPL issued $42 million of credits to its retail electric customers through its transmission cost rider for amounts previously collected in rates, which resulted in a reduction in regulatory liabilities and a corresponding reduction to “Electric transmission service” expense in Alliant Energy’s and IPL’s income statements during 2020. Refer to Note 17(g) for discussion of refunds received by IPL and WPL in 2020 related to MISO transmission owner return on equity complaints, which were recorded to regulatory liabilities in 2020. WPL’s West Riverside liquidated damages - Pursuant to terms included in the related West Riverside construction procurement contracts, WPL reached agreement with the contractor on liquidated damages in 2020. A significant portion of the liquidated damages was settled by WPL offsetting amounts owed to the contractor that were previously withheld for payment, which were non-cash investing activities. In December 2020, the PSCW authorized WPL to record the liquidated damages as a regulatory liability, which is expected to be returned to WPL’s customers as determined in future regulatory proceedings. Derecho Windstorm - In August 2020, a derecho windstorm caused considerable damage to IPL’s electric distribution system in its service territory, and over 250,000 of its customers lost power. IPL completed its initial restoration and rebuilding efforts in August 2020 and permanent repairs to the system will continue into 2021. IPL’s current estimate of the total cost of the windstorm is approximately $140 million, and as of December 31, 2020, approximately $135 million was recorded substantially to “Property, plant and equipment, net” on Alliant Energy’s and IPL’s balance sheets. In December 2020, IPL received approval from the IUB for utilization of a regulatory account to track certain incremental costs and benefits incurred resulting from the windstorm until IPL’s next rate proceeding. Tax benefits and the incremental operation and maintenance expenses resulting from the windstorm were deferred and recorded as a net regulatory liability of $7 million as of December 31, 2020, which is included in “Other” regulatory liabilities in the above table. 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. In January 2020, the IUB issued an order approving the settlement with final rates, which were effective February 26, 2020. 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 is being recovered from its retail electric customers through the renewable energy rider. The agreement also includes IPL providing retail electric billing credits, which began in the third quarter of 2020 and will continue through June 2021, and in aggregate include $27 million of excess deferred tax benefits and $8 million from a partial refund of interim rates implemented in 2019. 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 Forward-looking 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 did not change from then current levels through the end of 2020. In September 2020, pursuant to an August 2020 PSCW order, WPL refunded $12 million of 2019 fuel-related cost over-collections to its retail electric customers. In addition, WPL’s amortization of excess deferred taxes resulting from the remeasurement of accumulated deferred income taxes caused by Federal Tax Reform was used to offset increases in WPL’s 2020 increased revenue requirements. WPL’s Retail Fuel-related Rate Filing (2020 Forward-looking Test Period) - In December 2019, WPL received an order from the PSCW authorizing an annual retail electric rate decrease of $29 million, or approximately 2%, effective January 1, 2020. The decrease primarily reflected a change in expected fuel-related costs in 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, 2020 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 2020 2019 2020 2019 2020 2019 Tax-related $890 $818 $843 $777 $47 $41 Pension and OPEB costs 580 524 291 263 289 261 AROs 119 112 81 76 38 36 Assets retired early 113 134 77 88 36 46 IPL’s DAEC PPA amendment 110 108 110 108 — — WPL’s Western Wisconsin gas distribution expansion investments 55 — — — 55 — Derivatives 26 39 13 18 13 21 Other 117 110 68 70 49 40 $2,010 $1,845 $1,483 $1,400 $527 $445 At December 31, 2020, IPL and WPL had $81 million and $11 million, respectively, of regulatory assets that were not earning a return. IPL’s regulatory assets that were not earning a return consisted primarily of retired analog electric meters, emission allowances, clean air compliance projects and debt redemption costs. WPL’s regulatory assets that were not earning a return consisted primarily of costs for future expansion projects and environmental-related costs. The other regulatory assets reported in the above table either earn a return or the cash has not yet been expended, in which case the assets are offset by liabilities that also do not incur a carrying cost. Tax-related - IPL and WPL record regulatory assets for certain temporary differences (primarily related to utility property, plant and equipment at IPL) that result in a decrease in current rates charged to customers and an increase in future rates charged to customers based on the timing of income tax expense that is used to determine such rates. These temporary differences for IPL include the impacts of qualifying deductions for repairs expenditures, allocation of mixed service costs, and Iowa accelerated tax depreciation, which all contribute to lower current income tax expense during the first part of an asset’s useful life and higher current 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. 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. 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 (dollars in millions): Entity Asset Retirement Date Regulatory Asset Balance as of Dec. 31, 2020 Recovery Regulatory Approval IPL Sutherland Units 1 and 3 2017 $23 Return of and return on remaining net book value through 2027 IUB and FERC IPL M.L. Kapp Unit 2 2018 21 Return of and return on remaining net book value through 2029 IUB and FERC IPL Analog electric meters 2019 33 Return of remaining net book value through 2028 IUB and FERC WPL Nelson Dewey Units 1 and 2 and Edgewater Unit 3 2015 13 Return of and return on remaining net book value through 2022 PSCW and FERC WPL Edgewater Unit 4 2018 23 Return of and return on remaining net book value through 2028 PSCW and FERC IPL’s DAEC PPA Amendment - In September 2020, IPL made a buyout payment of $110 million in exchange for shortening the term of its DAEC PPA by 5 years. The payment was recorded as a reduction to “Other current liabilities” on Alliant Energy’s and IPL’s balance sheets and was included in “DAEC PPA amendment buyout payment” in Alliant Energy’s and IPL’s cash flows used for operating activities in 2020. The buyout payment, including a return on, will be recovered from IPL’s retail and wholesale customers from 2021 through the end of 2025. WPL’s Western Wisconsin gas distribution expansion investments - WPL made contributions in aid of construction to a third party for investments as part of its Western Wisconsin gas distribution expansion project. Pursuant to authorization by the PSCW, Alliant Energy and WPL have recorded a regulatory asset for these costs, and are authorized by the PSCW to recover these amounts from WPL’s retail gas customers in base rates beginning in 2021. 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. Regulatory Liabilities - At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2020 2019 2020 2019 2020 2019 Tax-related $732 $836 $331 $351 $401 $485 Cost of removal obligations 367 388 238 257 129 131 Electric transmission cost recovery 68 88 39 51 29 37 WPL’s West Riverside liquidated damages 38 — — — 38 — Derivatives 28 20 25 17 3 3 Other 73 92 43 39 30 53 $1,306 $1,424 $676 $715 $630 $709 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 is 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. Cash payments related to cost of removal obligations are included in “Other” in cash flows used for investing activities. Electric transmission cost recovery - Refer to Note 1(g) for details of IPL’s and WPL’s electric transmission cost recovery mechanisms. In 2020, pursuant to a June 2020 IUB order, IPL issued $42 million of credits to its retail electric customers through its transmission cost rider for amounts previously collected in rates, which resulted in a reduction in regulatory liabilities and a corresponding reduction to “Electric transmission service” expense in Alliant Energy’s and IPL’s income statements during 2020. Refer to Note 17(g) for discussion of refunds received by IPL and WPL in 2020 related to MISO transmission owner return on equity complaints, which were recorded to regulatory liabilities in 2020. WPL’s West Riverside liquidated damages - Pursuant to terms included in the related West Riverside construction procurement contracts, WPL reached agreement with the contractor on liquidated damages in 2020. A significant portion of the liquidated damages was settled by WPL offsetting amounts owed to the contractor that were previously withheld for payment, which were non-cash investing activities. In December 2020, the PSCW authorized WPL to record the liquidated damages as a regulatory liability, which is expected to be returned to WPL’s customers as determined in future regulatory proceedings. Derecho Windstorm - In August 2020, a derecho windstorm caused considerable damage to IPL’s electric distribution system in its service territory, and over 250,000 of its customers lost power. IPL completed its initial restoration and rebuilding efforts in August 2020 and permanent repairs to the system will continue into 2021. IPL’s current estimate of the total cost of the windstorm is approximately $140 million, and as of December 31, 2020, approximately $135 million was recorded substantially to “Property, plant and equipment, net” on Alliant Energy’s and IPL’s balance sheets. In December 2020, IPL received approval from the IUB for utilization of a regulatory account to track certain incremental costs and benefits incurred resulting from the windstorm until IPL’s next rate proceeding. Tax benefits and the incremental operation and maintenance expenses resulting from the windstorm were deferred and recorded as a net regulatory liability of $7 million as of December 31, 2020, which is included in “Other” regulatory liabilities in the above table. 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. In January 2020, the IUB issued an order approving the settlement with final rates, which were effective February 26, 2020. 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 is being recovered from its retail electric customers through the renewable energy rider. The agreement also includes IPL providing retail electric billing credits, which began in the third quarter of 2020 and will continue through June 2021, and in aggregate include $27 million of excess deferred tax benefits and $8 million from a partial refund of interim rates implemented in 2019. 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 Forward-looking 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 did not change from then current levels through the end of 2020. In September 2020, pursuant to an August 2020 PSCW order, WPL refunded $12 million of 2019 fuel-related cost over-collections to its retail electric customers. In addition, WPL’s amortization of excess deferred taxes resulting from the remeasurement of accumulated deferred income taxes caused by Federal Tax Reform was used to offset increases in WPL’s 2020 increased revenue requirements. WPL’s Retail Fuel-related Rate Filing (2020 Forward-looking Test Period) - In December 2019, WPL received an order from the PSCW authorizing an annual retail electric rate decrease of $29 million, or approximately 2%, effective January 1, 2020. The decrease primarily reflected a change in expected fuel-related costs in 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, 2020 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 2020 2019 2020 2019 2020 2019 Tax-related $890 $818 $843 $777 $47 $41 Pension and OPEB costs 580 524 291 263 289 261 AROs 119 112 81 76 38 36 Assets retired early 113 134 77 88 36 46 IPL’s DAEC PPA amendment 110 108 110 108 — — WPL’s Western Wisconsin gas distribution expansion investments 55 — — — 55 — Derivatives 26 39 13 18 13 21 Other 117 110 68 70 49 40 $2,010 $1,845 $1,483 $1,400 $527 $445 At December 31, 2020, IPL and WPL had $81 million and $11 million, respectively, of regulatory assets that were not earning a return. IPL’s regulatory assets that were not earning a return consisted primarily of retired analog electric meters, emission allowances, clean air compliance projects and debt redemption costs. WPL’s regulatory assets that were not earning a return consisted primarily of costs for future expansion projects and environmental-related costs. The other regulatory assets reported in the above table either earn a return or the cash has not yet been expended, in which case the assets are offset by liabilities that also do not incur a carrying cost. Tax-related - IPL and WPL record regulatory assets for certain temporary differences (primarily related to utility property, plant and equipment at IPL) that result in a decrease in current rates charged to customers and an increase in future rates charged to customers based on the timing of income tax expense that is used to determine such rates. These temporary differences for IPL include the impacts of qualifying deductions for repairs expenditures, allocation of mixed service costs, and Iowa accelerated tax depreciation, which all contribute to lower current income tax expense during the first part of an asset’s useful life and higher current 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. 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. 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 (dollars in millions): Entity Asset Retirement Date Regulatory Asset Balance as of Dec. 31, 2020 Recovery Regulatory Approval IPL Sutherland Units 1 and 3 2017 $23 Return of and return on remaining net book value through 2027 IUB and FERC IPL M.L. Kapp Unit 2 2018 21 Return of and return on remaining net book value through 2029 IUB and FERC IPL Analog electric meters 2019 33 Return of remaining net book value through 2028 IUB and FERC WPL Nelson Dewey Units 1 and 2 and Edgewater Unit 3 2015 13 Return of and return on remaining net book value through 2022 PSCW and FERC WPL Edgewater Unit 4 2018 23 Return of and return on remaining net book value through 2028 PSCW and FERC IPL’s DAEC PPA Amendment - In September 2020, IPL made a buyout payment of $110 million in exchange for shortening the term of its DAEC PPA by 5 years. The payment was recorded as a reduction to “Other current liabilities” on Alliant Energy’s and IPL’s balance sheets and was included in “DAEC PPA amendment buyout payment” in Alliant Energy’s and IPL’s cash flows used for operating activities in 2020. The buyout payment, including a return on, will be recovered from IPL’s retail and wholesale customers from 2021 through the end of 2025. WPL’s Western Wisconsin gas distribution expansion investments - WPL made contributions in aid of construction to a third party for investments as part of its Western Wisconsin gas distribution expansion project. Pursuant to authorization by the PSCW, Alliant Energy and WPL have recorded a regulatory asset for these costs, and are authorized by the PSCW to recover these amounts from WPL’s retail gas customers in base rates beginning in 2021. 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. Regulatory Liabilities - At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2020 2019 2020 2019 2020 2019 Tax-related $732 $836 $331 $351 $401 $485 Cost of removal obligations 367 388 238 257 129 131 Electric transmission cost recovery 68 88 39 51 29 37 WPL’s West Riverside liquidated damages 38 — — — 38 — Derivatives 28 20 25 17 3 3 Other 73 92 43 39 30 53 $1,306 $1,424 $676 $715 $630 $709 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 is 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. Cash payments related to cost of removal obligations are included in “Other” in cash flows used for investing activities. Electric transmission cost recovery - Refer to Note 1(g) for details of IPL’s and WPL’s electric transmission cost recovery mechanisms. In 2020, pursuant to a June 2020 IUB order, IPL issued $42 million of credits to its retail electric customers through its transmission cost rider for amounts previously collected in rates, which resulted in a reduction in regulatory liabilities and a corresponding reduction to “Electric transmission service” expense in Alliant Energy’s and IPL’s income statements during 2020. Refer to Note 17(g) for discussion of refunds received by IPL and WPL in 2020 related to MISO transmission owner return on equity complaints, which were recorded to regulatory liabilities in 2020. WPL’s West Riverside liquidated damages - Pursuant to terms included in the related West Riverside construction procurement contracts, WPL reached agreement with the contractor on liquidated damages in 2020. A significant portion of the liquidated damages was settled by WPL offsetting amounts owed to the contractor that were previously withheld for payment, which were non-cash investing activities. In December 2020, the PSCW authorized WPL to record the liquidated damages as a regulatory liability, which is expected to be returned to WPL’s customers as determined in future regulatory proceedings. Derecho Windstorm - In August 2020, a derecho windstorm caused considerable damage to IPL’s electric distribution system in its service territory, and over 250,000 of its customers lost power. IPL completed its initial restoration and rebuilding efforts in August 2020 and permanent repairs to the system will continue into 2021. IPL’s current estimate of the total cost of the windstorm is approximately $140 million, and as of December 31, 2020, approximately $135 million was recorded substantially to “Property, plant and equipment, net” on Alliant Energy’s and IPL’s balance sheets. In December 2020, IPL received approval from the IUB for utilization of a regulatory account to track certain incremental costs and benefits incurred resulting from the windstorm until IPL’s next rate proceeding. Tax benefits and the incremental operation and maintenance expenses resulting from the windstorm were deferred and recorded as a net regulatory liability of $7 million as of December 31, 2020, which is included in “Other” regulatory liabilities in the above table. 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. In January 2020, the IUB issued an order approving the settlement with final rates, which were effective February 26, 2020. 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 is being recovered from its retail electric customers through the renewable energy rider. The agreement also includes IPL providing retail electric billing credits, which began in the third quarter of 2020 and will continue through June 2021, and in aggregate include $27 million of excess deferred tax benefits and $8 million from a partial refund of interim rates implemented in 2019. 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 Forward-looking 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 did not change from then current levels through the end of 2020. In September 2020, pursuant to an August 2020 PSCW order, WPL refunded $12 million of 2019 fuel-related cost over-collections to its retail electric customers. In addition, WPL’s amortization of excess deferred taxes resulting from the remeasurement of accumulated deferred income taxes caused by Federal Tax Reform was used to offset increases in WPL’s 2020 increased revenue requirements. WPL’s Retail Fuel-related Rate Filing (2020 Forward-looking Test Period) - In December 2019, WPL received an order from the PSCW authorizing an annual retail electric rate decrease of $29 million, or approximately 2%, effective January 1, 2020. The decrease primarily reflected a change in expected fuel-related costs in 2020.

Property, Plant and Equipment

Property, Plant and Equipment	12 Months Ended
	Dec. 31, 2020
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 2020 2019 2020 2019 2020 2019 Utility: Electric plant: Generation in service $8,222 $7,625 $4,884 $4,432 $3,338 $3,193 Distribution in service 6,216 5,783 3,470 3,190 2,746 2,593 Other in service 536 457 358 299 178 158 Anticipated to be retired early (a) 1,269 — 459 — 810 — Total electric plant 16,243 13,865 9,171 7,921 7,072 5,944 Gas plant in service 1,563 1,463 844 802 719 661 Other plant in service 534 520 354 345 180 175 Accumulated depreciation (4,868) (4,602) (2,671) (2,499) (2,197) (2,103) Net plant 13,472 11,246 7,698 6,569 5,774 4,677 Leased Sheboygan Falls Energy Facility, net (b) — — — — 27 32 Construction work in progress 405 1,835 185 907 220 928 Other, net 7 6 6 5 1 1 Total utility 13,884 13,087 7,889 7,481 6,022 5,638 Non-utility and other: Non-utility Generation, net (c) 79 83 — — — — Corporate Services and other, net (d) 373 357 — — — — Total non-utility and other 452 440 — — — — Total property, plant and equipment $14,336 $13,527 $7,889 $7,481 $6,022 $5,638 (a) In 2020, IPL and WPL received approval from MISO to retire Lansing and Edgewater Unit 5, respectively, and currently anticipate retiring these EGUs by the end of 2022. Alliant Energy and IPL concluded that Lansing, and Alliant Energy and WPL concluded that Edgewater Unit 5, met the criteria to be considered probable of abandonment as of December 31, 2020. IPL and WPL are currently allowed a full recovery of and a full return on its respective EGU from both its retail and wholesale customers. (b) Less accumulated amortization of $95 million and $89 million for WPL as of December 31, 2020 and 2019, respectively. The leased Sheboygan Falls Energy Facility is eliminated from WPL upon consolidation and is included in the “Non-utility Generation, net” line within Alliant Energy’s consolidated property, plant and equipment. (c) Less accumulated depreciation of $63 million and $59 million for Alliant Energy as of December 31, 2020 and 2019, respectively. (d) Less accumulated depreciation of $210 million and $172 million for Alliant Energy as of December 31, 2020 and 2019, 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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Equity $39 $66 $51 $17 $35 $29 $22 $31 $22 Debt 16 27 25 7 15 13 9 12 12 $55 $93 $76 $24 $50 $42 $31 $43 $34 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 Travero 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.
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 2020 2019 2020 2019 2020 2019 Utility: Electric plant: Generation in service $8,222 $7,625 $4,884 $4,432 $3,338 $3,193 Distribution in service 6,216 5,783 3,470 3,190 2,746 2,593 Other in service 536 457 358 299 178 158 Anticipated to be retired early (a) 1,269 — 459 — 810 — Total electric plant 16,243 13,865 9,171 7,921 7,072 5,944 Gas plant in service 1,563 1,463 844 802 719 661 Other plant in service 534 520 354 345 180 175 Accumulated depreciation (4,868) (4,602) (2,671) (2,499) (2,197) (2,103) Net plant 13,472 11,246 7,698 6,569 5,774 4,677 Leased Sheboygan Falls Energy Facility, net (b) — — — — 27 32 Construction work in progress 405 1,835 185 907 220 928 Other, net 7 6 6 5 1 1 Total utility 13,884 13,087 7,889 7,481 6,022 5,638 Non-utility and other: Non-utility Generation, net (c) 79 83 — — — — Corporate Services and other, net (d) 373 357 — — — — Total non-utility and other 452 440 — — — — Total property, plant and equipment $14,336 $13,527 $7,889 $7,481 $6,022 $5,638 (a) In 2020, IPL and WPL received approval from MISO to retire Lansing and Edgewater Unit 5, respectively, and currently anticipate retiring these EGUs by the end of 2022. Alliant Energy and IPL concluded that Lansing, and Alliant Energy and WPL concluded that Edgewater Unit 5, met the criteria to be considered probable of abandonment as of December 31, 2020. IPL and WPL are currently allowed a full recovery of and a full return on its respective EGU from both its retail and wholesale customers. (b) Less accumulated amortization of $95 million and $89 million for WPL as of December 31, 2020 and 2019, respectively. The leased Sheboygan Falls Energy Facility is eliminated from WPL upon consolidation and is included in the “Non-utility Generation, net” line within Alliant Energy’s consolidated property, plant and equipment. (c) Less accumulated depreciation of $63 million and $59 million for Alliant Energy as of December 31, 2020 and 2019, respectively. (d) Less accumulated depreciation of $210 million and $172 million for Alliant Energy as of December 31, 2020 and 2019, 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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Equity $39 $66 $51 $17 $35 $29 $22 $31 $22 Debt 16 27 25 7 15 13 9 12 12 $55 $93 $76 $24 $50 $42 $31 $43 $34
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 2020 2019 2020 2019 2020 2019 Utility: Electric plant: Generation in service $8,222 $7,625 $4,884 $4,432 $3,338 $3,193 Distribution in service 6,216 5,783 3,470 3,190 2,746 2,593 Other in service 536 457 358 299 178 158 Anticipated to be retired early (a) 1,269 — 459 — 810 — Total electric plant 16,243 13,865 9,171 7,921 7,072 5,944 Gas plant in service 1,563 1,463 844 802 719 661 Other plant in service 534 520 354 345 180 175 Accumulated depreciation (4,868) (4,602) (2,671) (2,499) (2,197) (2,103) Net plant 13,472 11,246 7,698 6,569 5,774 4,677 Leased Sheboygan Falls Energy Facility, net (b) — — — — 27 32 Construction work in progress 405 1,835 185 907 220 928 Other, net 7 6 6 5 1 1 Total utility 13,884 13,087 7,889 7,481 6,022 5,638 Non-utility and other: Non-utility Generation, net (c) 79 83 — — — — Corporate Services and other, net (d) 373 357 — — — — Total non-utility and other 452 440 — — — — Total property, plant and equipment $14,336 $13,527 $7,889 $7,481 $6,022 $5,638 (a) In 2020, IPL and WPL received approval from MISO to retire Lansing and Edgewater Unit 5, respectively, and currently anticipate retiring these EGUs by the end of 2022. Alliant Energy and IPL concluded that Lansing, and Alliant Energy and WPL concluded that Edgewater Unit 5, met the criteria to be considered probable of abandonment as of December 31, 2020. IPL and WPL are currently allowed a full recovery of and a full return on its respective EGU from both its retail and wholesale customers. (b) Less accumulated amortization of $95 million and $89 million for WPL as of December 31, 2020 and 2019, respectively. The leased Sheboygan Falls Energy Facility is eliminated from WPL upon consolidation and is included in the “Non-utility Generation, net” line within Alliant Energy’s consolidated property, plant and equipment. (c) Less accumulated depreciation of $63 million and $59 million for Alliant Energy as of December 31, 2020 and 2019, respectively. (d) Less accumulated depreciation of $210 million and $172 million for Alliant Energy as of December 31, 2020 and 2019, 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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Equity $39 $66 $51 $17 $35 $29 $22 $31 $22 Debt 16 27 25 7 15 13 9 12 12 $55 $93 $76 $24 $50 $42 $31 $43 $34

Jointly-Owned Electric Utility

Jointly-Owned Electric Utility Plant	12 Months Ended
	Dec. 31, 2020
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. 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 the income statements. Information relative to IPL’s and WPL’s ownership interest in these jointly-owned EGUs at December 31, 2020 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0% $580 $195 $22 George Neal Unit 4 25.7% 192 96 2 George Neal Unit 3 28.0% 171 67 — Louisa Unit 1 4.0% 39 22 — 982 380 24 WPL Columbia Units 1-2 53.5% 786 289 4 West Riverside 91.8% 686 12 13 Forward Wind Energy Center 42.6% 120 46 — 1,592 347 17 Alliant Energy $2,574 $727 $41
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. 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 the income statements. Information relative to IPL’s and WPL’s ownership interest in these jointly-owned EGUs at December 31, 2020 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0% $580 $195 $22 George Neal Unit 4 25.7% 192 96 2 George Neal Unit 3 28.0% 171 67 — Louisa Unit 1 4.0% 39 22 — 982 380 24 WPL Columbia Units 1-2 53.5% 786 289 4 West Riverside 91.8% 686 12 13 Forward Wind Energy Center 42.6% 120 46 — 1,592 347 17 Alliant Energy $2,574 $727 $41
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. 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 the income statements. Information relative to IPL’s and WPL’s ownership interest in these jointly-owned EGUs at December 31, 2020 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0% $580 $195 $22 George Neal Unit 4 25.7% 192 96 2 George Neal Unit 3 28.0% 171 67 — Louisa Unit 1 4.0% 39 22 — 982 380 24 WPL Columbia Units 1-2 53.5% 786 289 4 West Riverside 91.8% 686 12 13 Forward Wind Energy Center 42.6% 120 46 — 1,592 347 17 Alliant Energy $2,574 $727 $41

Receivables

Receivables	12 Months Ended
	Dec. 31, 2020
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 2020 2019 2020 2019 2020 2019 Customer $101 $92 $— $— $92 $84 Unbilled utility revenues 82 82 — — 82 82 Deferred proceeds 188 188 188 188 — — Other 59 47 23 16 35 31 Allowance for expected credit losses (18) (7) (1) (1) (17) (6) $412 $402 $210 $203 $192 $191 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 currently expects to amend and extend the purchase commitment. IPL pays a monthly fee to the third party that varies based on interest rates, limits on cash proceeds and cash amounts received from the third party. Deferred proceeds represent IPL’s interest in the receivables sold to the third party. At IPL’s request, deferred proceeds are paid to IPL from collections of receivables, after paying any required expenses incurred by the third party and the collection agent. Corporate Services acts as collection agent for the third party and receives a fee for collection services. The Receivables Agreement can be terminated by the third party if arrears or write-offs exceed certain levels. 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 expected credit losses 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, 2020, IPL had $109 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 2020 2019 2018 2020 2019 2018 Outstanding aggregate cash proceeds $96 $108 $116 $9 $36 $53 As of December 31, the attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2020 2019 Customer accounts receivable $114 $125 Unbilled utility revenues 92 95 Other receivables — 1 Receivables sold to third party 206 221 Less: cash proceeds 1 27 Deferred proceeds 205 194 Less: allowance for expected credit losses 17 6 Fair value of deferred proceeds $188 $188 Outstanding receivables past due $27 $26 Additional attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2020 2019 2018 Collections $2,025 $2,193 $2,077 Write-offs, net of recoveries 7 19 21
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 2020 2019 2020 2019 2020 2019 Customer $101 $92 $— $— $92 $84 Unbilled utility revenues 82 82 — — 82 82 Deferred proceeds 188 188 188 188 — — Other 59 47 23 16 35 31 Allowance for expected credit losses (18) (7) (1) (1) (17) (6) $412 $402 $210 $203 $192 $191 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 currently expects to amend and extend the purchase commitment. IPL pays a monthly fee to the third party that varies based on interest rates, limits on cash proceeds and cash amounts received from the third party. Deferred proceeds represent IPL’s interest in the receivables sold to the third party. At IPL’s request, deferred proceeds are paid to IPL from collections of receivables, after paying any required expenses incurred by the third party and the collection agent. Corporate Services acts as collection agent for the third party and receives a fee for collection services. The Receivables Agreement can be terminated by the third party if arrears or write-offs exceed certain levels. 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 expected credit losses 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, 2020, IPL had $109 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 2020 2019 2018 2020 2019 2018 Outstanding aggregate cash proceeds $96 $108 $116 $9 $36 $53 As of December 31, the attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2020 2019 Customer accounts receivable $114 $125 Unbilled utility revenues 92 95 Other receivables — 1 Receivables sold to third party 206 221 Less: cash proceeds 1 27 Deferred proceeds 205 194 Less: allowance for expected credit losses 17 6 Fair value of deferred proceeds $188 $188 Outstanding receivables past due $27 $26 Additional attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2020 2019 2018 Collections $2,025 $2,193 $2,077 Write-offs, net of recoveries 7 19 21
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 2020 2019 2020 2019 2020 2019 Customer $101 $92 $— $— $92 $84 Unbilled utility revenues 82 82 — — 82 82 Deferred proceeds 188 188 188 188 — — Other 59 47 23 16 35 31 Allowance for expected credit losses (18) (7) (1) (1) (17) (6) $412 $402 $210 $203 $192 $191

Investments

Investments	12 Months Ended
	Dec. 31, 2020
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, 2020 2020 2019 2020 2019 2018 ATC Holdings 16%, 20% $331 $320 ($47) ($45) ($38) Non-utility wind farm in Oklahoma 50% 103 105 (8) (5) (16) Other Various 42 33 (6) (3) (1) $476 $458 ($61) ($53) ($55) Summary aggregate financial information from the financial statements of these holdings is as follows (in millions): Alliant Energy 2020 2019 2018 Revenues $801 $783 $724 Operating income 376 359 325 Net income 343 340 217 As of December 31: Current assets 121 139 Non-current assets 6,388 5,964 Current liabilities 317 507 Non-current liabilities 2,997 2,659 Noncontrolling interest 192 208 ATC Holdings - As of December 31, 2020, 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, 2020
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: 2020 2019 2018 Shares outstanding, January 1 245,022,800 236,063,279 231,348,646 Equity forward agreements 4,275,127 8,358,973 — At-the-market offering programs — — 4,171,013 Shareowner Direct Plan 472,283 501,808 576,965 Equity-based compensation plans 98,205 101,478 5,078 Other — (2,738) (38,423) Shares outstanding, December 31 249,868,415 245,022,800 236,063,279 At December 31, 2020, Alliant Energy had a total of 15 million shares available for issuance in the aggregate, pursuant to its 2020 OIP, Shareowner Direct Plan and 401(k) Savings Plan. Equity Forward Agreements - In 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 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 2020, Alliant Energy settled $222 million under the forward sale agreements by delivering 4,275,127 shares of newly issued Alliant Energy common stock at a weighted average forward sale price of $51.98 per share. Alliant Energy used the net proceeds from the settlements for general corporate purposes. At-the-Market Offering Program - In 2018, Alliant Energy filed a prospectus supplement under which it could sell up to $175 million of its common stock through an at-the-market offering program. 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. The 2018 at-the-market offering program has 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.

Preferred Stock

Preferred Stock	12 Months Ended
	Dec. 31, 2020
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 2020 2019 (in millions) 5.1% $25 8,000,000 8,000,000 $200 $200 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 2020 2019 (in millions) 5.1% $25 8,000,000 8,000,000 $200 $200 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, 2020
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, 2020, 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 Alliant Energy’s, IPL’s and WPL’s commercial paper, and Alliant Energy’s and WPL’s borrowings under the single credit facility, classified as short-term debt was as follows (dollars in millions): Alliant Energy IPL WPL December 31 2020 2019 2020 2019 2020 2019 Amount outstanding $389 $337 $— $— $257 $168 Weighted average interest rates 0.2% 1.9% N/A N/A 0.1% 1.8% Available credit facility capacity $611 $663 $250 $250 $43 $132 Alliant Energy IPL WPL For the year ended 2020 2019 2020 2019 2020 2019 Maximum amount outstanding (based on daily outstanding balances) $495 $601 $8 $50 $266 $195 Average amount outstanding (based on daily outstanding balances) $292 $453 $— $— $117 $93 Weighted average interest rates 0.7% 2.5% 0.5% 2.8% 0.7% 2.4% 2020 2019 Alliant Energy IPL WPL Alliant Energy IPL WPL Senior Debentures (a): 3.25%, due 2024 $500 $500 $— $500 $500 $— 3.4%, due 2025 250 250 — 250 250 — 5.5%, due 2025 50 50 — 50 50 — 4.1%, due 2028 500 500 — 500 500 — 3.6%, due 2029 300 300 — 300 300 — 2.3%, due 2030 (b) 400 400 — — — — 6.45%, due 2033 100 100 — 100 100 — 6.3%, due 2034 125 125 — 125 125 — 6.25%, due 2039 300 300 — 300 300 — 4.7%, due 2043 250 250 — 250 250 — 3.7%, due 2046 300 300 — 300 300 — 3.5%, due 2049 300 300 — 300 300 — 3.65% (Retired in 2020) — — — 200 200 — 3,375 3,375 — 3,175 3,175 — Debentures (a): 2.25%, due 2022 250 — 250 250 — 250 3.05%, due 2027 300 — 300 300 — 300 3%, due 2029 350 — 350 350 — 350 6.25%, due 2034 100 — 100 100 — 100 6.375%, due 2037 300 — 300 300 — 300 7.6%, due 2038 250 — 250 250 — 250 4.1%, due 2044 250 — 250 250 — 250 3.65%, due 2050 (c) 350 — 350 — — — 4.6% (Retired in 2020) — — — 150 — 150 2,150 — 2,150 1,950 — 1,950 Other: AEF term loan credit agreement through March 2022, 1% at December 31, 2020 (with Alliant Energy as guarantor) (d) 300 — — — — — Corporate Services 3.45% senior notes, due 2022 (a) 75 — — 75 — — AEF 3.75% senior notes, due 2023 (with Alliant Energy as guarantor) (a) 400 — — 400 — — AEF 1.4% senior notes, due 2026 (with Alliant Energy as guarantor) (a)(e) 200 — — — — — AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a) 300 — — 300 — — Sheboygan Power, LLC 5.06% senior secured notes, due 2021 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 31 — — 38 — — AEF term loan credit agreement, 2% at December 31, 2019 (with Alliant Energy as guarantor) (Retired in 2020) — — — 300 — — Other, 1% at December 31, 2020, due 2021 to 2025 2 — — 2 — — 1,308 — — 1,115 — — Subtotal 6,833 3,375 2,150 6,240 3,175 1,950 Current maturities (8) — — (657) (200) (150) Unamortized debt issuance costs (41) (22) (14) (37) (21) (11) Unamortized debt (discount) and premium, net (15) (8) (6) (13) (7) (6) Long-term debt, net (f) $6,769 $3,345 $2,130 $5,533 $2,947 $1,783 (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 June 2020, IPL issued $400 million of 2.3% senior debentures due 2030. The net proceeds from the issuance were used by IPL to retire its $200 million 3.65% senior debentures that would have matured in September 2020 and for general corporate purposes. (c) In April 2020, WPL issued $350 million of 3.65% debentures due 2050. The net proceeds from the issuance were used by WPL to reduce borrowings under the single credit facility and for general corporate purposes. (d) In March 2020, AEF entered into a $300 million variable-rate term loan credit agreement and used the borrowings under this agreement to retire its $300 million variable-rate term loan credit agreement that would have expired in April 2020. (e) In November 2020, AEF issued $200 million of 1.4% senior notes due 2026. The net proceeds from the issuance were used to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes. (f) 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, 2020, long-term debt maturities for 2021 through 2025 were as follows (in millions): 2021 2022 2023 2024 2025 IPL $— $— $— $500 $300 WPL — 250 — — — Corporate Services — 75 — — — AEF 8 308 408 9 — Alliant Energy $8 $633 $408 $509 $300 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, 2020, 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 Alliant Energy’s, IPL’s and WPL’s commercial paper, and Alliant Energy’s and WPL’s borrowings under the single credit facility, classified as short-term debt was as follows (dollars in millions): Alliant Energy IPL WPL December 31 2020 2019 2020 2019 2020 2019 Amount outstanding $389 $337 $— $— $257 $168 Weighted average interest rates 0.2% 1.9% N/A N/A 0.1% 1.8% Available credit facility capacity $611 $663 $250 $250 $43 $132 Alliant Energy IPL WPL For the year ended 2020 2019 2020 2019 2020 2019 Maximum amount outstanding (based on daily outstanding balances) $495 $601 $8 $50 $266 $195 Average amount outstanding (based on daily outstanding balances) $292 $453 $— $— $117 $93 Weighted average interest rates 0.7% 2.5% 0.5% 2.8% 0.7% 2.4% 2020 2019 Alliant Energy IPL WPL Alliant Energy IPL WPL Senior Debentures (a): 3.25%, due 2024 $500 $500 $— $500 $500 $— 3.4%, due 2025 250 250 — 250 250 — 5.5%, due 2025 50 50 — 50 50 — 4.1%, due 2028 500 500 — 500 500 — 3.6%, due 2029 300 300 — 300 300 — 2.3%, due 2030 (b) 400 400 — — — — 6.45%, due 2033 100 100 — 100 100 — 6.3%, due 2034 125 125 — 125 125 — 6.25%, due 2039 300 300 — 300 300 — 4.7%, due 2043 250 250 — 250 250 — 3.7%, due 2046 300 300 — 300 300 — 3.5%, due 2049 300 300 — 300 300 — 3.65% (Retired in 2020) — — — 200 200 — 3,375 3,375 — 3,175 3,175 — Debentures (a): 2.25%, due 2022 250 — 250 250 — 250 3.05%, due 2027 300 — 300 300 — 300 3%, due 2029 350 — 350 350 — 350 6.25%, due 2034 100 — 100 100 — 100 6.375%, due 2037 300 — 300 300 — 300 7.6%, due 2038 250 — 250 250 — 250 4.1%, due 2044 250 — 250 250 — 250 3.65%, due 2050 (c) 350 — 350 — — — 4.6% (Retired in 2020) — — — 150 — 150 2,150 — 2,150 1,950 — 1,950 Other: AEF term loan credit agreement through March 2022, 1% at December 31, 2020 (with Alliant Energy as guarantor) (d) 300 — — — — — Corporate Services 3.45% senior notes, due 2022 (a) 75 — — 75 — — AEF 3.75% senior notes, due 2023 (with Alliant Energy as guarantor) (a) 400 — — 400 — — AEF 1.4% senior notes, due 2026 (with Alliant Energy as guarantor) (a)(e) 200 — — — — — AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a) 300 — — 300 — — Sheboygan Power, LLC 5.06% senior secured notes, due 2021 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 31 — — 38 — — AEF term loan credit agreement, 2% at December 31, 2019 (with Alliant Energy as guarantor) (Retired in 2020) — — — 300 — — Other, 1% at December 31, 2020, due 2021 to 2025 2 — — 2 — — 1,308 — — 1,115 — — Subtotal 6,833 3,375 2,150 6,240 3,175 1,950 Current maturities (8) — — (657) (200) (150) Unamortized debt issuance costs (41) (22) (14) (37) (21) (11) Unamortized debt (discount) and premium, net (15) (8) (6) (13) (7) (6) Long-term debt, net (f) $6,769 $3,345 $2,130 $5,533 $2,947 $1,783 (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 June 2020, IPL issued $400 million of 2.3% senior debentures due 2030. The net proceeds from the issuance were used by IPL to retire its $200 million 3.65% senior debentures that would have matured in September 2020 and for general corporate purposes. (c) In April 2020, WPL issued $350 million of 3.65% debentures due 2050. The net proceeds from the issuance were used by WPL to reduce borrowings under the single credit facility and for general corporate purposes. (d) In March 2020, AEF entered into a $300 million variable-rate term loan credit agreement and used the borrowings under this agreement to retire its $300 million variable-rate term loan credit agreement that would have expired in April 2020. (e) In November 2020, AEF issued $200 million of 1.4% senior notes due 2026. The net proceeds from the issuance were used to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes. (f) 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, 2020, long-term debt maturities for 2021 through 2025 were as follows (in millions): 2021 2022 2023 2024 2025 IPL $— $— $— $500 $300 WPL — 250 — — — Corporate Services — 75 — — — AEF 8 308 408 9 — Alliant Energy $8 $633 $408 $509 $300 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, 2020, 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 Alliant Energy’s, IPL’s and WPL’s commercial paper, and Alliant Energy’s and WPL’s borrowings under the single credit facility, classified as short-term debt was as follows (dollars in millions): Alliant Energy IPL WPL December 31 2020 2019 2020 2019 2020 2019 Amount outstanding $389 $337 $— $— $257 $168 Weighted average interest rates 0.2% 1.9% N/A N/A 0.1% 1.8% Available credit facility capacity $611 $663 $250 $250 $43 $132 Alliant Energy IPL WPL For the year ended 2020 2019 2020 2019 2020 2019 Maximum amount outstanding (based on daily outstanding balances) $495 $601 $8 $50 $266 $195 Average amount outstanding (based on daily outstanding balances) $292 $453 $— $— $117 $93 Weighted average interest rates 0.7% 2.5% 0.5% 2.8% 0.7% 2.4% 2020 2019 Alliant Energy IPL WPL Alliant Energy IPL WPL Senior Debentures (a): 3.25%, due 2024 $500 $500 $— $500 $500 $— 3.4%, due 2025 250 250 — 250 250 — 5.5%, due 2025 50 50 — 50 50 — 4.1%, due 2028 500 500 — 500 500 — 3.6%, due 2029 300 300 — 300 300 — 2.3%, due 2030 (b) 400 400 — — — — 6.45%, due 2033 100 100 — 100 100 — 6.3%, due 2034 125 125 — 125 125 — 6.25%, due 2039 300 300 — 300 300 — 4.7%, due 2043 250 250 — 250 250 — 3.7%, due 2046 300 300 — 300 300 — 3.5%, due 2049 300 300 — 300 300 — 3.65% (Retired in 2020) — — — 200 200 — 3,375 3,375 — 3,175 3,175 — Debentures (a): 2.25%, due 2022 250 — 250 250 — 250 3.05%, due 2027 300 — 300 300 — 300 3%, due 2029 350 — 350 350 — 350 6.25%, due 2034 100 — 100 100 — 100 6.375%, due 2037 300 — 300 300 — 300 7.6%, due 2038 250 — 250 250 — 250 4.1%, due 2044 250 — 250 250 — 250 3.65%, due 2050 (c) 350 — 350 — — — 4.6% (Retired in 2020) — — — 150 — 150 2,150 — 2,150 1,950 — 1,950 Other: AEF term loan credit agreement through March 2022, 1% at December 31, 2020 (with Alliant Energy as guarantor) (d) 300 — — — — — Corporate Services 3.45% senior notes, due 2022 (a) 75 — — 75 — — AEF 3.75% senior notes, due 2023 (with Alliant Energy as guarantor) (a) 400 — — 400 — — AEF 1.4% senior notes, due 2026 (with Alliant Energy as guarantor) (a)(e) 200 — — — — — AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a) 300 — — 300 — — Sheboygan Power, LLC 5.06% senior secured notes, due 2021 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 31 — — 38 — — AEF term loan credit agreement, 2% at December 31, 2019 (with Alliant Energy as guarantor) (Retired in 2020) — — — 300 — — Other, 1% at December 31, 2020, due 2021 to 2025 2 — — 2 — — 1,308 — — 1,115 — — Subtotal 6,833 3,375 2,150 6,240 3,175 1,950 Current maturities (8) — — (657) (200) (150) Unamortized debt issuance costs (41) (22) (14) (37) (21) (11) Unamortized debt (discount) and premium, net (15) (8) (6) (13) (7) (6) Long-term debt, net (f) $6,769 $3,345 $2,130 $5,533 $2,947 $1,783 (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 June 2020, IPL issued $400 million of 2.3% senior debentures due 2030. The net proceeds from the issuance were used by IPL to retire its $200 million 3.65% senior debentures that would have matured in September 2020 and for general corporate purposes. (c) In April 2020, WPL issued $350 million of 3.65% debentures due 2050. The net proceeds from the issuance were used by WPL to reduce borrowings under the single credit facility and for general corporate purposes. (d) In March 2020, AEF entered into a $300 million variable-rate term loan credit agreement and used the borrowings under this agreement to retire its $300 million variable-rate term loan credit agreement that would have expired in April 2020. (e) In November 2020, AEF issued $200 million of 1.4% senior notes due 2026. The net proceeds from the issuance were used to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes. (f) 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, 2020, long-term debt maturities for 2021 through 2025 were as follows (in millions): 2021 2022 2023 2024 2025 IPL $— $— $— $500 $300 WPL — 250 — — — Corporate Services — 75 — — — AEF 8 308 408 9 — Alliant Energy $8 $633 $408 $509 $300 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, 2020
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. At December 31, operating lease details are as follows (dollars in millions): 2020 2019 Alliant Energy IPL WPL Alliant Energy IPL WPL Property, plant and equipment, net $15 $9 $6 $16 $10 $6 Other current liabilities $2 $1 $1 $2 $1 $1 Other liabilities 13 8 5 14 9 5 Total operating lease liabilities $15 $9 $6 $16 $10 $6 Weighted average remaining lease term 10 years 11 years 9 years 11 years 12 years 10 years Weighted average discount rate 4% 4% 4% 4% 4% 4% 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, 2020 December 31, 2019 Property, plant and equipment, net $27 $32 Other current liabilities $9 $9 Finance lease obligations - Sheboygan Falls Energy Facility 42 51 Total finance lease liabilities $51 $60 Remaining lease term 4 years 5 years Discount rate 11% 11% 2020 2019 Depreciation expense $6 $6 Interest expense 6 7 Total finance lease expense $12 $13 Expected Maturities - As of December 31, 2020, expected maturities of lease liabilities were as follows (in millions): 2021 2022 2023 2024 2025 Thereafter Total Less: amount representing interest Present value of minimum lease payments Operating Leases: Alliant Energy $2 $2 $2 $2 $2 $8 $18 $3 $15 IPL 1 1 1 1 1 6 11 2 9 WPL 1 1 1 1 1 2 7 1 6 WPL’s Finance Lease: Sheboygan Falls Energy Facility 15 15 15 15 5 — 65 14 51
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. At December 31, operating lease details are as follows (dollars in millions): 2020 2019 Alliant Energy IPL WPL Alliant Energy IPL WPL Property, plant and equipment, net $15 $9 $6 $16 $10 $6 Other current liabilities $2 $1 $1 $2 $1 $1 Other liabilities 13 8 5 14 9 5 Total operating lease liabilities $15 $9 $6 $16 $10 $6 Weighted average remaining lease term 10 years 11 years 9 years 11 years 12 years 10 years Weighted average discount rate 4% 4% 4% 4% 4% 4% 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, 2020 December 31, 2019 Property, plant and equipment, net $27 $32 Other current liabilities $9 $9 Finance lease obligations - Sheboygan Falls Energy Facility 42 51 Total finance lease liabilities $51 $60 Remaining lease term 4 years 5 years Discount rate 11% 11% 2020 2019 Depreciation expense $6 $6 Interest expense 6 7 Total finance lease expense $12 $13 Expected Maturities - As of December 31, 2020, expected maturities of lease liabilities were as follows (in millions): 2021 2022 2023 2024 2025 Thereafter Total Less: amount representing interest Present value of minimum lease payments Operating Leases: Alliant Energy $2 $2 $2 $2 $2 $8 $18 $3 $15 IPL 1 1 1 1 1 6 11 2 9 WPL 1 1 1 1 1 2 7 1 6 WPL’s Finance Lease: Sheboygan Falls Energy Facility 15 15 15 15 5 — 65 14 51
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. At December 31, operating lease details are as follows (dollars in millions): 2020 2019 Alliant Energy IPL WPL Alliant Energy IPL WPL Property, plant and equipment, net $15 $9 $6 $16 $10 $6 Other current liabilities $2 $1 $1 $2 $1 $1 Other liabilities 13 8 5 14 9 5 Total operating lease liabilities $15 $9 $6 $16 $10 $6 Weighted average remaining lease term 10 years 11 years 9 years 11 years 12 years 10 years Weighted average discount rate 4% 4% 4% 4% 4% 4% 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, 2020 December 31, 2019 Property, plant and equipment, net $27 $32 Other current liabilities $9 $9 Finance lease obligations - Sheboygan Falls Energy Facility 42 51 Total finance lease liabilities $51 $60 Remaining lease term 4 years 5 years Discount rate 11% 11% 2020 2019 Depreciation expense $6 $6 Interest expense 6 7 Total finance lease expense $12 $13 Expected Maturities - As of December 31, 2020, expected maturities of lease liabilities were as follows (in millions): 2021 2022 2023 2024 2025 Thereafter Total Less: amount representing interest Present value of minimum lease payments Operating Leases: Alliant Energy $2 $2 $2 $2 $2 $8 $18 $3 $15 IPL 1 1 1 1 1 6 11 2 9 WPL 1 1 1 1 1 2 7 1 6 WPL’s Finance Lease: Sheboygan Falls Energy Facility 15 15 15 15 5 — 65 14 51
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. At December 31, operating lease details are as follows (dollars in millions): 2020 2019 Alliant Energy IPL WPL Alliant Energy IPL WPL Property, plant and equipment, net $15 $9 $6 $16 $10 $6 Other current liabilities $2 $1 $1 $2 $1 $1 Other liabilities 13 8 5 14 9 5 Total operating lease liabilities $15 $9 $6 $16 $10 $6 Weighted average remaining lease term 10 years 11 years 9 years 11 years 12 years 10 years Weighted average discount rate 4% 4% 4% 4% 4% 4% 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, 2020 December 31, 2019 Property, plant and equipment, net $27 $32 Other current liabilities $9 $9 Finance lease obligations - Sheboygan Falls Energy Facility 42 51 Total finance lease liabilities $51 $60 Remaining lease term 4 years 5 years Discount rate 11% 11% 2020 2019 Depreciation expense $6 $6 Interest expense 6 7 Total finance lease expense $12 $13 Expected Maturities - As of December 31, 2020, expected maturities of lease liabilities were as follows (in millions): 2021 2022 2023 2024 2025 Thereafter Total Less: amount representing interest Present value of minimum lease payments Operating Leases: Alliant Energy $2 $2 $2 $2 $2 $8 $18 $3 $15 IPL 1 1 1 1 1 6 11 2 9 WPL 1 1 1 1 1 2 7 1 6 WPL’s Finance Lease: Sheboygan Falls Energy Facility 15 15 15 15 5 — 65 14 51

Revenues

Revenues	12 Months Ended
	Dec. 31, 2020
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 Travero business, which includes a short-line rail freight service in Iowa; a barge, rail and truck freight terminal on the Mississippi River; and freight brokerage and logistics management 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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Electric Utility: Retail - residential $1,093 $1,092 $1,063 $602 $601 $590 $491 $491 $473 Retail - commercial 718 770 736 474 510 487 244 260 249 Retail - industrial 841 889 889 488 504 501 353 385 388 Wholesale 168 177 188 57 64 71 111 113 117 Bulk power and other 100 136 124 74 102 82 26 34 42 Total Electric Utility 2,920 3,064 3,000 1,695 1,781 1,731 1,225 1,283 1,269 Gas Utility: Retail - residential 214 259 254 116 149 152 98 110 102 Retail - commercial 107 133 133 59 75 76 48 58 57 Retail - industrial 12 16 15 8 12 10 4 4 5 Transportation/other 40 47 45 25 28 28 15 19 17 Total Gas Utility 373 455 447 208 264 266 165 191 181 Other Utility: Steam 36 37 35 36 37 35 — — — Other utility 13 9 13 8 7 10 5 2 3 Total Other Utility 49 46 48 44 44 45 5 2 3 Non-Utility and Other: Travero and other 74 83 39 — — — — — — Total Non-Utility and Other 74 83 39 — — — — — — Total revenues $3,416 $3,648 $3,534 $1,947 $2,089 $2,042 $1,395 $1,476 $1,453
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 Travero business, which includes a short-line rail freight service in Iowa; a barge, rail and truck freight terminal on the Mississippi River; and freight brokerage and logistics management 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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Electric Utility: Retail - residential $1,093 $1,092 $1,063 $602 $601 $590 $491 $491 $473 Retail - commercial 718 770 736 474 510 487 244 260 249 Retail - industrial 841 889 889 488 504 501 353 385 388 Wholesale 168 177 188 57 64 71 111 113 117 Bulk power and other 100 136 124 74 102 82 26 34 42 Total Electric Utility 2,920 3,064 3,000 1,695 1,781 1,731 1,225 1,283 1,269 Gas Utility: Retail - residential 214 259 254 116 149 152 98 110 102 Retail - commercial 107 133 133 59 75 76 48 58 57 Retail - industrial 12 16 15 8 12 10 4 4 5 Transportation/other 40 47 45 25 28 28 15 19 17 Total Gas Utility 373 455 447 208 264 266 165 191 181 Other Utility: Steam 36 37 35 36 37 35 — — — Other utility 13 9 13 8 7 10 5 2 3 Total Other Utility 49 46 48 44 44 45 5 2 3 Non-Utility and Other: Travero and other 74 83 39 — — — — — — Total Non-Utility and Other 74 83 39 — — — — — — Total revenues $3,416 $3,648 $3,534 $1,947 $2,089 $2,042 $1,395 $1,476 $1,453
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 Travero business, which includes a short-line rail freight service in Iowa; a barge, rail and truck freight terminal on the Mississippi River; and freight brokerage and logistics management 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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Electric Utility: Retail - residential $1,093 $1,092 $1,063 $602 $601 $590 $491 $491 $473 Retail - commercial 718 770 736 474 510 487 244 260 249 Retail - industrial 841 889 889 488 504 501 353 385 388 Wholesale 168 177 188 57 64 71 111 113 117 Bulk power and other 100 136 124 74 102 82 26 34 42 Total Electric Utility 2,920 3,064 3,000 1,695 1,781 1,731 1,225 1,283 1,269 Gas Utility: Retail - residential 214 259 254 116 149 152 98 110 102 Retail - commercial 107 133 133 59 75 76 48 58 57 Retail - industrial 12 16 15 8 12 10 4 4 5 Transportation/other 40 47 45 25 28 28 15 19 17 Total Gas Utility 373 455 447 208 264 266 165 191 181 Other Utility: Steam 36 37 35 36 37 35 — — — Other utility 13 9 13 8 7 10 5 2 3 Total Other Utility 49 46 48 44 44 45 5 2 3 Non-Utility and Other: Travero and other 74 83 39 — — — — — — Total Non-Utility and Other 74 83 39 — — — — — — Total revenues $3,416 $3,648 $3,534 $1,947 $2,089 $2,042 $1,395 $1,476 $1,453

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Current tax expense (benefit): Federal $1 ($7) ($1) $6 ($11) $15 ($11) $12 ($9) State 8 24 (5) (1) 24 (7) 7 13 (5) IPL’s tax benefit riders — (4) (13) — (4) (13) — — — Deferred tax expense (benefit): Federal 22 70 68 30 26 10 (9) 31 44 State 8 42 30 (2) 31 7 10 6 22 Production tax credits (95) (55) (30) (80) (42) (14) (15) (13) (15) Investment tax credits (1) (1) (1) — — (1) (1) — (1) ($57) $69 $48 ($47) $24 ($3) ($19) $49 $36 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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Statutory federal income tax rate 21 % 21 % 21 % 21 % 21 % 21 % 21 % 21 % 21 % State income taxes, net of federal benefits 2 7 7 (1) 8 8 6 6 6 Production tax credits (17) (9) (5) (28) (13) (5) (7) (5) (7) Amortization of excess deferred taxes (Refer to Note 2 ) (13) (1) (1) (5) — — (26) (2) — Effect of rate-making on property-related differences (3) (6) (8) (4) (10) (14) (2) (3) (2) Adjustment for prior period taxes 1 1 (2) 1 3 (5) — — — IPL’s tax benefit riders — (1) (2) — (1) (5) — — — Federal Tax Reform adjustments — — (1) — — — — — (2) Other items, net (1) (1) (1) — — (1) — — (1) Overall income tax rate (10 %) 11 % 8 % (16 %) 8 % (1 %) (8 %) 17 % 15 % 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 2020 2019 2020 2019 2020 2019 Deferred tax liabilities: Property $2,232 $2,022 $1,312 $1,184 $854 $770 ATC Holdings 116 111 — — — — Other 101 84 83 76 41 32 Total deferred tax liabilities 2,449 2,217 1,395 1,260 895 802 Deferred tax assets: Federal credit carryforwards 454 355 258 175 175 160 Net operating losses carryforwards - federal 77 60 71 56 1 1 Net operating losses carryforwards - state 37 37 1 1 — — Other 73 61 30 20 18 16 Subtotal deferred tax assets 641 513 360 252 194 177 Valuation allowances (6) (10) — — (1) (1) Total deferred tax assets 635 503 360 252 193 176 Total deferred tax liabilities, net $1,814 $1,714 $1,035 $1,008 $702 $626 Carryforwards - At December 31, 2020, carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2037 $366 $339 $3 State net operating losses 2021-2040 622 10 2 Federal tax credits 2022-2040 454 258 175 Valuation Allowances - Alliant Energy currently expects its federal net operating losses carryforwards will not be fully utilized until 2023. 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 vintage federal credit carryforwards prior to their expiration in 2022, resulting in valuation allowances that remain as of December 31, 2020. Alliant Energy currently expects to be able to utilize 2003 vintage federal credit carryforwards prior to their expiration in 2023, and as a result, in 2020, Alliant Energy reversed $4 million of previously recorded valuation allowances related to these federal credit carryforwards. Uncertain Tax Positions - At December 31, 2020, 2019 and 2018, there were no uncertain tax positions or penalties accrued related to uncertain tax positions. As of December 31, 2020, 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) 2017 - 2019 Consolidated Iowa income tax returns (b) 2017 - 2019 Wisconsin combined tax returns (c) 2016 - 2019 (a) The 2017 federal tax return is 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 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 $6 million, $1 million and $5 million, respectively, in 2018. Iowa Tax Reform - In 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, for taxes related to 2020 and prior.
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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Current tax expense (benefit): Federal $1 ($7) ($1) $6 ($11) $15 ($11) $12 ($9) State 8 24 (5) (1) 24 (7) 7 13 (5) IPL’s tax benefit riders — (4) (13) — (4) (13) — — — Deferred tax expense (benefit): Federal 22 70 68 30 26 10 (9) 31 44 State 8 42 30 (2) 31 7 10 6 22 Production tax credits (95) (55) (30) (80) (42) (14) (15) (13) (15) Investment tax credits (1) (1) (1) — — (1) (1) — (1) ($57) $69 $48 ($47) $24 ($3) ($19) $49 $36 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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Statutory federal income tax rate 21 % 21 % 21 % 21 % 21 % 21 % 21 % 21 % 21 % State income taxes, net of federal benefits 2 7 7 (1) 8 8 6 6 6 Production tax credits (17) (9) (5) (28) (13) (5) (7) (5) (7) Amortization of excess deferred taxes (Refer to Note 2 ) (13) (1) (1) (5) — — (26) (2) — Effect of rate-making on property-related differences (3) (6) (8) (4) (10) (14) (2) (3) (2) Adjustment for prior period taxes 1 1 (2) 1 3 (5) — — — IPL’s tax benefit riders — (1) (2) — (1) (5) — — — Federal Tax Reform adjustments — — (1) — — — — — (2) Other items, net (1) (1) (1) — — (1) — — (1) Overall income tax rate (10 %) 11 % 8 % (16 %) 8 % (1 %) (8 %) 17 % 15 % 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 2020 2019 2020 2019 2020 2019 Deferred tax liabilities: Property $2,232 $2,022 $1,312 $1,184 $854 $770 ATC Holdings 116 111 — — — — Other 101 84 83 76 41 32 Total deferred tax liabilities 2,449 2,217 1,395 1,260 895 802 Deferred tax assets: Federal credit carryforwards 454 355 258 175 175 160 Net operating losses carryforwards - federal 77 60 71 56 1 1 Net operating losses carryforwards - state 37 37 1 1 — — Other 73 61 30 20 18 16 Subtotal deferred tax assets 641 513 360 252 194 177 Valuation allowances (6) (10) — — (1) (1) Total deferred tax assets 635 503 360 252 193 176 Total deferred tax liabilities, net $1,814 $1,714 $1,035 $1,008 $702 $626 Carryforwards - At December 31, 2020, carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2037 $366 $339 $3 State net operating losses 2021-2040 622 10 2 Federal tax credits 2022-2040 454 258 175 Valuation Allowances - Alliant Energy currently expects its federal net operating losses carryforwards will not be fully utilized until 2023. 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 vintage federal credit carryforwards prior to their expiration in 2022, resulting in valuation allowances that remain as of December 31, 2020. Alliant Energy currently expects to be able to utilize 2003 vintage federal credit carryforwards prior to their expiration in 2023, and as a result, in 2020, Alliant Energy reversed $4 million of previously recorded valuation allowances related to these federal credit carryforwards. Uncertain Tax Positions - At December 31, 2020, 2019 and 2018, there were no uncertain tax positions or penalties accrued related to uncertain tax positions. As of December 31, 2020, 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) 2017 - 2019 Consolidated Iowa income tax returns (b) 2017 - 2019 Wisconsin combined tax returns (c) 2016 - 2019 (a) The 2017 federal tax return is 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 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 $6 million, $1 million and $5 million, respectively, in 2018. Iowa Tax Reform - In 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, for taxes related to 2020 and prior.
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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Current tax expense (benefit): Federal $1 ($7) ($1) $6 ($11) $15 ($11) $12 ($9) State 8 24 (5) (1) 24 (7) 7 13 (5) IPL’s tax benefit riders — (4) (13) — (4) (13) — — — Deferred tax expense (benefit): Federal 22 70 68 30 26 10 (9) 31 44 State 8 42 30 (2) 31 7 10 6 22 Production tax credits (95) (55) (30) (80) (42) (14) (15) (13) (15) Investment tax credits (1) (1) (1) — — (1) (1) — (1) ($57) $69 $48 ($47) $24 ($3) ($19) $49 $36 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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Statutory federal income tax rate 21 % 21 % 21 % 21 % 21 % 21 % 21 % 21 % 21 % State income taxes, net of federal benefits 2 7 7 (1) 8 8 6 6 6 Production tax credits (17) (9) (5) (28) (13) (5) (7) (5) (7) Amortization of excess deferred taxes (Refer to Note 2 ) (13) (1) (1) (5) — — (26) (2) — Effect of rate-making on property-related differences (3) (6) (8) (4) (10) (14) (2) (3) (2) Adjustment for prior period taxes 1 1 (2) 1 3 (5) — — — IPL’s tax benefit riders — (1) (2) — (1) (5) — — — Federal Tax Reform adjustments — — (1) — — — — — (2) Other items, net (1) (1) (1) — — (1) — — (1) Overall income tax rate (10 %) 11 % 8 % (16 %) 8 % (1 %) (8 %) 17 % 15 % 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 2020 2019 2020 2019 2020 2019 Deferred tax liabilities: Property $2,232 $2,022 $1,312 $1,184 $854 $770 ATC Holdings 116 111 — — — — Other 101 84 83 76 41 32 Total deferred tax liabilities 2,449 2,217 1,395 1,260 895 802 Deferred tax assets: Federal credit carryforwards 454 355 258 175 175 160 Net operating losses carryforwards - federal 77 60 71 56 1 1 Net operating losses carryforwards - state 37 37 1 1 — — Other 73 61 30 20 18 16 Subtotal deferred tax assets 641 513 360 252 194 177 Valuation allowances (6) (10) — — (1) (1) Total deferred tax assets 635 503 360 252 193 176 Total deferred tax liabilities, net $1,814 $1,714 $1,035 $1,008 $702 $626 Carryforwards - At December 31, 2020, carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2037 $366 $339 $3 State net operating losses 2021-2040 622 10 2 Federal tax credits 2022-2040 454 258 175 Valuation Allowances - Alliant Energy currently expects its federal net operating losses carryforwards will not be fully utilized until 2023. 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 vintage federal credit carryforwards prior to their expiration in 2022, resulting in valuation allowances that remain as of December 31, 2020. Alliant Energy currently expects to be able to utilize 2003 vintage federal credit carryforwards prior to their expiration in 2023, and as a result, in 2020, Alliant Energy reversed $4 million of previously recorded valuation allowances related to these federal credit carryforwards. Uncertain Tax Positions - At December 31, 2020, 2019 and 2018, there were no uncertain tax positions or penalties accrued related to uncertain tax positions. As of December 31, 2020, 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) 2017 - 2019 Consolidated Iowa income tax returns (b) 2017 - 2019 Wisconsin combined tax returns (c) 2016 - 2019 (a) The 2017 federal tax return is 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 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 $6 million, $1 million and $5 million, respectively, in 2018. Iowa Tax Reform - In 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, for taxes related to 2020 and prior.

Benefit Plans

Benefit Plans	12 Months Ended
	Dec. 31, 2020
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 2020 2019 2018 2020 2019 2018 Discount rate for benefit obligations 2.57% 3.48% 4.34% 2.31% 3.40% 4.24% Discount rate for net periodic cost 3.48% 4.34% 3.66% 3.40% 4.24% 3.53% Expected rate of return on plan assets 7.10% 7.60% 7.60% 4.50% 5.44% 5.44% Interest crediting rate for Alliant Energy Cash Balance Pension Plan 4.76% 5.52% 5.04% 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 2020 2019 2018 2020 2019 2018 Discount rate for benefit obligations 2.61% 3.51% 4.35% 2.28% 3.39% 4.23% Discount rate for net periodic cost 3.51% 4.35% 3.68% 3.39% 4.23% 3.51% Expected rate of return on plan assets 7.10% 7.60% 7.60% 4.50% 5.60% 5.60% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Qualified Defined Benefit Pension Plan OPEB Plans WPL 2020 2019 2018 2020 2019 2018 Discount rate for benefit obligations 2.64% 3.50% 4.35% 2.27% 3.39% 4.23% Discount rate for net periodic cost 3.50% 4.35% 3.69% 3.39% 4.23% 3.51% Expected rate of return on plan assets 7.10% 7.60% 7.60% 3.28% 3.81% 3.84% 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 and mortality projection tables that were released in 2020 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 2020 2019 2018 2020 2019 2018 Service cost $11 $10 $12 $3 $3 $4 Interest cost 43 50 47 7 9 8 Expected return on plan assets (a) (70) (60) (69) (5) (5) (6) Amortization of prior service credit (b) — (1) (1) — — — Amortization of actuarial loss (c) 34 36 35 3 3 3 Settlement losses (d) 12 — — — — — $30 $35 $24 $8 $10 $9 IPL Defined Benefit Pension Plans OPEB Plans 2020 2019 2018 2020 2019 2018 Service cost $7 $6 $7 $1 $1 $2 Interest cost 20 23 21 3 3 3 Expected return on plan assets (a) (33) (28) (32) (4) (4) (4) Amortization of actuarial loss (c) 15 15 15 1 2 1 Settlement losses (d) 7 — — — — — $16 $16 $11 $1 $2 $2 WPL Defined Benefit Pension Plans OPEB Plans 2020 2019 2018 2020 2019 2018 Service cost $4 $3 $4 $2 $1 $2 Interest cost 19 21 20 3 3 3 Expected return on plan assets (a) (31) (26) (30) (1) (1) (1) Amortization of prior service credit (b) — — — (1) — — Amortization of actuarial loss (c) 16 18 17 2 2 2 $8 $16 $11 $5 $5 $6 (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 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 and lump sum payments related to IPL’s qualified defined benefit pension plan. 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 2020 2019 2020 2019 Change in benefit obligation: Net benefit obligation at January 1 $1,280 $1,175 $214 $206 Service cost 11 10 3 3 Interest cost 43 50 7 9 Plan participants’ contributions — — 4 3 Actuarial loss 132 124 18 14 Gross benefits paid (115) (79) (19) (21) Net benefit obligation at December 31 1,351 1,280 227 214 Change in plan assets: Fair value of plan assets at January 1 931 809 105 99 Actual return on plan assets 108 168 11 15 Employer contributions 60 33 7 9 Plan participants’ contributions — — 4 3 Gross benefits paid (115) (79) (19) (21) Fair value of plan assets at December 31 984 931 108 105 Under funded status at December 31 ($367) ($349) ($119) ($109) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2020 2019 2020 2019 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $8 $10 Current liabilities (2) (14) (8) (8) Pension and other benefit obligations (365) (335) (119) (111) Net amounts recognized at December 31 ($367) ($349) ($119) ($109) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $533 $485 $54 $45 Prior service credit (5) (5) (1) (1) $528 $480 $53 $44 Defined Benefit Pension Plans OPEB Plans IPL 2020 2019 2020 2019 Change in benefit obligation: Net benefit obligation at January 1 $581 $534 $86 $83 Service cost 7 6 1 1 Interest cost 20 23 3 3 Plan participants’ contributions — — 1 1 Actuarial loss 60 56 8 6 Gross benefits paid (53) (38) (8) (8) Net benefit obligation at December 31 615 581 91 86 Change in plan assets: Fair value of plan assets at January 1 436 378 72 67 Actual return on plan assets 51 79 7 10 Employer contributions 19 17 2 2 Plan participants’ contributions — — 1 1 Gross benefits paid (53) (38) (8) (8) Fair value of plan assets at December 31 453 436 74 72 Under funded status at December 31 ($162) ($145) ($17) ($14) Defined Benefit Pension Plans OPEB Plans IPL 2020 2019 2020 2019 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $4 $7 Current liabilities (1) (1) (2) (2) Pension and other benefit obligations (161) (144) (19) (19) Net amounts recognized at December 31 ($162) ($145) ($17) ($14) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $228 $208 $20 $17 Prior service credit (1) (1) — — $227 $207 $20 $17 Defined Benefit Pension Plans OPEB Plans WPL 2020 2019 2020 2019 Change in benefit obligation: Net benefit obligation at January 1 $550 $507 $85 $83 Service cost 4 3 2 1 Interest cost 19 21 3 3 Plan participants’ contributions — — 2 2 Actuarial loss 55 54 5 5 Gross benefits paid (40) (35) (8) (9) Net benefit obligation at December 31 588 550 89 85 Change in plan assets: Fair value of plan assets at January 1 406 352 17 17 Actual return on plan assets 48 73 2 1 Employer contributions 22 16 5 6 Plan participants’ contributions — — 2 2 Gross benefits paid (40) (35) (8) (9) Fair value of plan assets at December 31 436 406 18 17 Under funded status at December 31 ($152) ($144) ($71) ($68) Defined Benefit Pension Plans OPEB Plans WPL 2020 2019 2020 2019 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $3 $3 Current liabilities — — (6) (6) Pension and other benefit obligations (152) (144) (68) (65) Net amounts recognized at December 31 ($152) ($144) ($71) ($68) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $233 $212 $25 $22 Prior service credit (1) (1) (1) (1) $232 $211 $24 $21 Actuarial losses related to benefit obligations in 2020 for defined benefit pension and OPEB plans were primarily due to decreases in the discount rates, partially offset by the impact of updated mortality projection tables. 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. 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 are as follows (in millions): Defined Benefit Pension Plans OPEB Plans Alliant Energy 2020 2019 2020 2019 Accumulated benefit obligations $1,305 $1,234 $227 $214 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,305 1,234 227 214 Fair value of plan assets 984 931 108 105 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,351 1,280 N/A N/A Fair value of plan assets 984 931 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2020 2019 2020 2019 Accumulated benefit obligations $589 $558 $91 $86 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 589 558 91 86 Fair value of plan assets 453 436 74 72 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 615 581 N/A N/A Fair value of plan assets 453 436 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2020 2019 2020 2019 Accumulated benefit obligations $573 $536 $89 $85 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 573 536 89 85 Fair value of plan assets 436 406 18 17 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 588 550 N/A N/A Fair value of plan assets 436 406 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 2020 2019 2020 2019 Regulatory assets $44 $39 $34 $29 Estimated Future Employer Contributions and Benefit Payments - Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2021 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $39 $17 $18 OPEB plans 8 2 6 (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 2021 2022 2023 2024 2025 2026 - 2030 Defined benefit pension benefits $76 $78 $79 $79 $80 $398 OPEB 18 18 17 17 17 74 $94 $96 $96 $96 $97 $472 IPL 2021 2022 2023 2024 2025 2026 - 2030 Defined benefit pension benefits $38 $38 $39 $38 $38 $181 OPEB 7 7 7 7 7 29 $45 $45 $46 $45 $45 $210 WPL 2021 2022 2023 2024 2025 2026 - 2030 Defined benefit pension benefits $32 $32 $32 $33 $33 $162 OPEB 7 7 7 7 6 29 $39 $39 $39 $40 $39 $191 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, 2020, 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. 11% - 41% 26% Equity securities - international 14% - 34% 23% Global asset securities 5% - 15% 9% Risk parity securities 5% - 15% 9% Fixed income securities 20% - 40% 28% 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, 2020, the current target ranges and actual allocations for VEBA trusts with assets greater than $5 million and the WPL 401(h) assets were as follows: Target Range Actual Allocation Allocation Cash and equivalents 0% - 5% 1% Equity securities - U.S. 0% - 46% 25% Equity securities - international 0% - 34% 1% Fixed income securities 20% - 100% 73% Fair Value Measurements - Fair value measurement accounting establishes three levels of fair value hierarchy that prioritize the inputs to valuation techniques used to measure fair value. Refer to Note 16 for discussion of levels within the fair value hierarchy. Level 1 items include investments in securities held in registered investment companies 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): 2020 2019 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $46 $— $46 $— $46 $4 $42 $— Equity securities - U.S. 182 182 — — 161 161 — — Equity securities - international 151 151 — — 136 136 — — Global asset securities 45 45 — — 47 47 — — Fixed income securities 134 56 78 — 131 58 73 — Total assets in fair value hierarchy 558 $434 $124 $— 521 $406 $115 $— Assets measured at net asset value 426 410 Accrued investment income 1 1 Due to brokers, net (pending trades with brokers) (1) (1) Total pension plan assets $984 $931 2020 2019 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $21 $— $21 $— $22 $2 $20 $— Equity securities - U.S. 84 84 — — 75 75 — — Equity securities - international 69 69 — — 64 64 — — Global asset securities 21 21 — — 22 22 — — Fixed income securities 62 26 36 — 61 27 34 — Total assets in fair value hierarchy 257 $200 $57 $— 244 $190 $54 $— Assets measured at net asset value 196 192 Total pension plan assets $453 $436 2020 2019 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $20 $— $20 $— $20 $2 $18 $— Equity securities - U.S. 81 81 — — 70 70 — — Equity securities - international 67 67 — — 59 59 — — Global asset securities 20 20 — — 21 21 — — Fixed income securities 59 25 34 — 57 25 32 — Total assets in fair value hierarchy 247 $193 $54 $— 227 $177 $50 $— Assets measured at net asset value 189 179 Total pension plan assets $436 $406 At December 31, the fair values of OPEB plan assets were as follows (in millions): 2020 2019 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $2 $— $2 $— $3 $3 $— $— Equity securities - U.S. 4 4 — — 4 4 — — Equity securities - international 3 3 — — 3 3 — — Fixed income securities 72 71 1 — 69 68 1 — Total assets in fair value hierarchy 81 $78 $3 $— 79 $78 $1 $— Assets measured at net asset value 27 26 Total OPEB plan assets $108 $105 2020 2019 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $1 $— $1 $— $2 $2 $— $— Fixed income securities 51 51 — — 48 48 — — Total assets in fair value hierarchy 52 $51 $1 $— 50 $50 $— $— Assets measured at net asset value 22 22 Total OPEB plan assets $74 $72 2020 2019 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Fixed income securities $18 $18 $— $— $17 $17 $— $— Total OPEB plan assets $18 $18 $— $— $17 $17 $— $— 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, 2020 and 2019. 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 9% and 11% of total assets in the 401(k) savings plans at December 31, 2020 and 2019, 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 2020 2019 2018 2020 2019 2018 2020 2019 2018 401(k) costs $25 $25 $25 $13 $13 $13 $12 $12 $11 Alliant Energy IPL WPL 2020 2019 2018 2020 2019 2018 2020 2019 2018 Compensation expense $16 $26 $17 $9 $15 $9 $6 $10 $7 Income tax benefits 4 7 5 2 4 3 2 3 2 As of December 31, 2020, Alliant Energy’s, IPL’s and WPL’s total unrecognized compensation cost related to share-based compensation awards was $5 million, $3 million and $2 million, respectively, which is expected to be recognized over a weighted average period of between one year 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, 2020, 537,485 shares were included in the calculation of diluted EPS related to the nonvested equity awards. Performance Shares - Equity Awards - Payouts of 2020 and 2019 performance shares 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 2020 and 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 2020 and 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 2020 and 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. If minimum performance targets are not met during the performance period, these performance shares are forfeited. 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 2020 and 2019 performance shares activity, with amounts representing the target number of awards, was as follows: 2020 2019 Performance Shares - Equity Weighted Average Performance Shares - Equity Weighted Average Nonvested awards, January 1 74,193 $47.44 — $— Granted 56,204 64.04 91,816 47.23 Forfeited (1,241) 50.94 (17,623) 46.35 Nonvested awards, December 31 129,156 54.63 74,193 47.44 Performance Shares and Performance Units - Liability Awards - Payouts of performance shares granted prior to 2019 under the Amended and Restated 2010 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) 2020 2019 2018 2020 2019 2018 Nonvested awards, January 1 131,872 203,188 223,511 34,574 57,761 71,737 Granted — — 74,163 — — 19,840 Vested (63,565) (66,322) (90,806) (16,661) (20,131) (31,910) Forfeited — (4,994) (3,680) (1,112) (3,056) (1,906) Nonvested awards, December 31 68,307 131,872 203,188 16,801 34,574 57,761 Fair Value of Awards - The fair value of each performance share and performance unit accounted for as a liability award 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, 2020, the Alliant Energy common stock closing price was $51.53, and additional information related to the fair value of nonvested performance shares and performance units granted in 2018 was as follows: Performance Shares Performance Units Nonvested awards at target 68,307 16,801 Estimated payout percentage based on performance criteria 172.5% 172.5% Fair values of each nonvested award $88.89 $88.89 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) 2020 2019 2018 2020 2019 2018 2017 Grant 2016 Grant 2015 Grant 2017 Grant 2016 Grant 2015 Grant Performance awards vested 63,565 66,322 90,806 16,661 20,131 31,910 Percentage of target number of performance awards 155.0% 142.5% 137.5% 155.0% 142.5% 137.5% Aggregate payout value (in millions) $6 $4 $5 $2 $1 $1 Payout - cash (in millions) $5 $4 $5 $2 $1 $1 Payout - common stock shares issued 9,543 6,447 5,078 N/A N/A N/A Restricted Stock Units - Equity Awards - Payouts of 2020 and 2019 restricted stock units are based on the expiration of a three-year time-vesting period. Restricted stock units granted in 2020 and 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 2020 and 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 2020 and 2019 restricted stock units activity was as follows: 2020 2019 Restricted Stock Units - Equity Weighted Average Restricted Stock Units - Equity Weighted Average Nonvested units, January 1 89,281 $46.04 — $— Granted 61,056 59.42 105,348 45.98 Forfeited (3,788) 49.01 (16,067) 45.63 Nonvested units, December 31 146,549 51.54 89,281 46.04 Restricted Stock Units - Liability Awards - Payouts of restricted stock units granted prior to 2019 are based on the expiration of a three-year time-vesting period and 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: 2020 2019 2018 Nonvested units, January 1 113,033 174,163 113,749 Granted — — 63,568 Vested (54,483) (56,849) — Forfeited — (4,281) (3,154) Nonvested units, December 31 58,550 113,033 174,163 Performance Restricted Stock Units - Equity Awards - Payouts of performance restricted stock units 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 minimum 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: 2020 2019 2018 Units Weighted Average Units Weighted Average Units Weighted Average Nonvested units, January 1 206,065 $41.50 203,188 $37.23 132,705 $36.50 Granted 56,204 59.37 91,816 46.10 74,163 38.60 Vested (63,565) 39.12 (66,322) 33.93 — — Forfeited (1,241) 49.25 (22,617) 44.00 (3,680) 38.60 Nonvested units, December 31 197,463 47.31 206,065 41.50 203,188 37.23 Company Stock Account - The DCP does not permit diversification of deferrals credited to the company stock account and all distributions from participants’ company stock accounts are made in the form of shares of Alliant Energy common stock. The deferred compensation obligations for participants’ company stock accounts are recorded in “Additional paid-in capital” and the shares of Alliant Energy common stock held in a rabbi trust to satisfy this obligation are recorded in “Shares in deferred compensation trust” on Alliant Energy’s balance sheets. At December 31, the carrying value of the deferred 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): 2020 2019 Carrying value $11 $10 Fair market value 20 21 Interest, Equity and Mutual Fund Accounts - Distributions from participants’ interest, equity and mutual fund accounts are in the form of cash payments. The deferred compensation obligations for participants’ interest, equity and mutual fund accounts are recorded in “Pension and other benefit obligations” on the balance sheets. At December 31, 2020 and 2019, the carrying value of Alliant Energy’s deferred compensation obligations for participants’ interest, equity and mutual fund accounts, which approximates fair market value, was $22 million and $21 million, respectively.
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 2020 2019 2018 2020 2019 2018 Discount rate for benefit obligations 2.57% 3.48% 4.34% 2.31% 3.40% 4.24% Discount rate for net periodic cost 3.48% 4.34% 3.66% 3.40% 4.24% 3.53% Expected rate of return on plan assets 7.10% 7.60% 7.60% 4.50% 5.44% 5.44% Interest crediting rate for Alliant Energy Cash Balance Pension Plan 4.76% 5.52% 5.04% 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 2020 2019 2018 2020 2019 2018 Discount rate for benefit obligations 2.61% 3.51% 4.35% 2.28% 3.39% 4.23% Discount rate for net periodic cost 3.51% 4.35% 3.68% 3.39% 4.23% 3.51% Expected rate of return on plan assets 7.10% 7.60% 7.60% 4.50% 5.60% 5.60% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Qualified Defined Benefit Pension Plan OPEB Plans WPL 2020 2019 2018 2020 2019 2018 Discount rate for benefit obligations 2.64% 3.50% 4.35% 2.27% 3.39% 4.23% Discount rate for net periodic cost 3.50% 4.35% 3.69% 3.39% 4.23% 3.51% Expected rate of return on plan assets 7.10% 7.60% 7.60% 3.28% 3.81% 3.84% 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 and mortality projection tables that were released in 2020 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 2020 2019 2018 2020 2019 2018 Service cost $11 $10 $12 $3 $3 $4 Interest cost 43 50 47 7 9 8 Expected return on plan assets (a) (70) (60) (69) (5) (5) (6) Amortization of prior service credit (b) — (1) (1) — — — Amortization of actuarial loss (c) 34 36 35 3 3 3 Settlement losses (d) 12 — — — — — $30 $35 $24 $8 $10 $9 IPL Defined Benefit Pension Plans OPEB Plans 2020 2019 2018 2020 2019 2018 Service cost $7 $6 $7 $1 $1 $2 Interest cost 20 23 21 3 3 3 Expected return on plan assets (a) (33) (28) (32) (4) (4) (4) Amortization of actuarial loss (c) 15 15 15 1 2 1 Settlement losses (d) 7 — — — — — $16 $16 $11 $1 $2 $2 WPL Defined Benefit Pension Plans OPEB Plans 2020 2019 2018 2020 2019 2018 Service cost $4 $3 $4 $2 $1 $2 Interest cost 19 21 20 3 3 3 Expected return on plan assets (a) (31) (26) (30) (1) (1) (1) Amortization of prior service credit (b) — — — (1) — — Amortization of actuarial loss (c) 16 18 17 2 2 2 $8 $16 $11 $5 $5 $6 (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 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 and lump sum payments related to IPL’s qualified defined benefit pension plan. 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 2020 2019 2020 2019 Change in benefit obligation: Net benefit obligation at January 1 $1,280 $1,175 $214 $206 Service cost 11 10 3 3 Interest cost 43 50 7 9 Plan participants’ contributions — — 4 3 Actuarial loss 132 124 18 14 Gross benefits paid (115) (79) (19) (21) Net benefit obligation at December 31 1,351 1,280 227 214 Change in plan assets: Fair value of plan assets at January 1 931 809 105 99 Actual return on plan assets 108 168 11 15 Employer contributions 60 33 7 9 Plan participants’ contributions — — 4 3 Gross benefits paid (115) (79) (19) (21) Fair value of plan assets at December 31 984 931 108 105 Under funded status at December 31 ($367) ($349) ($119) ($109) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2020 2019 2020 2019 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $8 $10 Current liabilities (2) (14) (8) (8) Pension and other benefit obligations (365) (335) (119) (111) Net amounts recognized at December 31 ($367) ($349) ($119) ($109) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $533 $485 $54 $45 Prior service credit (5) (5) (1) (1) $528 $480 $53 $44 Defined Benefit Pension Plans OPEB Plans IPL 2020 2019 2020 2019 Change in benefit obligation: Net benefit obligation at January 1 $581 $534 $86 $83 Service cost 7 6 1 1 Interest cost 20 23 3 3 Plan participants’ contributions — — 1 1 Actuarial loss 60 56 8 6 Gross benefits paid (53) (38) (8) (8) Net benefit obligation at December 31 615 581 91 86 Change in plan assets: Fair value of plan assets at January 1 436 378 72 67 Actual return on plan assets 51 79 7 10 Employer contributions 19 17 2 2 Plan participants’ contributions — — 1 1 Gross benefits paid (53) (38) (8) (8) Fair value of plan assets at December 31 453 436 74 72 Under funded status at December 31 ($162) ($145) ($17) ($14) Defined Benefit Pension Plans OPEB Plans IPL 2020 2019 2020 2019 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $4 $7 Current liabilities (1) (1) (2) (2) Pension and other benefit obligations (161) (144) (19) (19) Net amounts recognized at December 31 ($162) ($145) ($17) ($14) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $228 $208 $20 $17 Prior service credit (1) (1) — — $227 $207 $20 $17 Defined Benefit Pension Plans OPEB Plans WPL 2020 2019 2020 2019 Change in benefit obligation: Net benefit obligation at January 1 $550 $507 $85 $83 Service cost 4 3 2 1 Interest cost 19 21 3 3 Plan participants’ contributions — — 2 2 Actuarial loss 55 54 5 5 Gross benefits paid (40) (35) (8) (9) Net benefit obligation at December 31 588 550 89 85 Change in plan assets: Fair value of plan assets at January 1 406 352 17 17 Actual return on plan assets 48 73 2 1 Employer contributions 22 16 5 6 Plan participants’ contributions — — 2 2 Gross benefits paid (40) (35) (8) (9) Fair value of plan assets at December 31 436 406 18 17 Under funded status at December 31 ($152) ($144) ($71) ($68) Defined Benefit Pension Plans OPEB Plans WPL 2020 2019 2020 2019 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $3 $3 Current liabilities — — (6) (6) Pension and other benefit obligations (152) (144) (68) (65) Net amounts recognized at December 31 ($152) ($144) ($71) ($68) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $233 $212 $25 $22 Prior service credit (1) (1) (1) (1) $232 $211 $24 $21 Actuarial losses related to benefit obligations in 2020 for defined benefit pension and OPEB plans were primarily due to decreases in the discount rates, partially offset by the impact of updated mortality projection tables. 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. 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 are as follows (in millions): Defined Benefit Pension Plans OPEB Plans Alliant Energy 2020 2019 2020 2019 Accumulated benefit obligations $1,305 $1,234 $227 $214 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,305 1,234 227 214 Fair value of plan assets 984 931 108 105 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,351 1,280 N/A N/A Fair value of plan assets 984 931 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2020 2019 2020 2019 Accumulated benefit obligations $589 $558 $91 $86 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 589 558 91 86 Fair value of plan assets 453 436 74 72 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 615 581 N/A N/A Fair value of plan assets 453 436 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2020 2019 2020 2019 Accumulated benefit obligations $573 $536 $89 $85 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 573 536 89 85 Fair value of plan assets 436 406 18 17 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 588 550 N/A N/A Fair value of plan assets 436 406 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 2020 2019 2020 2019 Regulatory assets $44 $39 $34 $29 Estimated Future Employer Contributions and Benefit Payments - Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2021 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $39 $17 $18 OPEB plans 8 2 6 (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 2021 2022 2023 2024 2025 2026 - 2030 Defined benefit pension benefits $76 $78 $79 $79 $80 $398 OPEB 18 18 17 17 17 74 $94 $96 $96 $96 $97 $472 IPL 2021 2022 2023 2024 2025 2026 - 2030 Defined benefit pension benefits $38 $38 $39 $38 $38 $181 OPEB 7 7 7 7 7 29 $45 $45 $46 $45 $45 $210 WPL 2021 2022 2023 2024 2025 2026 - 2030 Defined benefit pension benefits $32 $32 $32 $33 $33 $162 OPEB 7 7 7 7 6 29 $39 $39 $39 $40 $39 $191 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, 2020, 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. 11% - 41% 26% Equity securities - international 14% - 34% 23% Global asset securities 5% - 15% 9% Risk parity securities 5% - 15% 9% Fixed income securities 20% - 40% 28% 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, 2020, the current target ranges and actual allocations for VEBA trusts with assets greater than $5 million and the WPL 401(h) assets were as follows: Target Range Actual Allocation Allocation Cash and equivalents 0% - 5% 1% Equity securities - U.S. 0% - 46% 25% Equity securities - international 0% - 34% 1% Fixed income securities 20% - 100% 73% Fair Value Measurements - Fair value measurement accounting establishes three levels of fair value hierarchy that prioritize the inputs to valuation techniques used to measure fair value. Refer to Note 16 for discussion of levels within the fair value hierarchy. Level 1 items include investments in securities held in registered investment companies 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): 2020 2019 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $46 $— $46 $— $46 $4 $42 $— Equity securities - U.S. 182 182 — — 161 161 — — Equity securities - international 151 151 — — 136 136 — — Global asset securities 45 45 — — 47 47 — — Fixed income securities 134 56 78 — 131 58 73 — Total assets in fair value hierarchy 558 $434 $124 $— 521 $406 $115 $— Assets measured at net asset value 426 410 Accrued investment income 1 1 Due to brokers, net (pending trades with brokers) (1) (1) Total pension plan assets $984 $931 2020 2019 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $21 $— $21 $— $22 $2 $20 $— Equity securities - U.S. 84 84 — — 75 75 — — Equity securities - international 69 69 — — 64 64 — — Global asset securities 21 21 — — 22 22 — — Fixed income securities 62 26 36 — 61 27 34 — Total assets in fair value hierarchy 257 $200 $57 $— 244 $190 $54 $— Assets measured at net asset value 196 192 Total pension plan assets $453 $436 2020 2019 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $20 $— $20 $— $20 $2 $18 $— Equity securities - U.S. 81 81 — — 70 70 — — Equity securities - international 67 67 — — 59 59 — — Global asset securities 20 20 — — 21 21 — — Fixed income securities 59 25 34 — 57 25 32 — Total assets in fair value hierarchy 247 $193 $54 $— 227 $177 $50 $— Assets measured at net asset value 189 179 Total pension plan assets $436 $406 At December 31, the fair values of OPEB plan assets were as follows (in millions): 2020 2019 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $2 $— $2 $— $3 $3 $— $— Equity securities - U.S. 4 4 — — 4 4 — — Equity securities - international 3 3 — — 3 3 — — Fixed income securities 72 71 1 — 69 68 1 — Total assets in fair value hierarchy 81 $78 $3 $— 79 $78 $1 $— Assets measured at net asset value 27 26 Total OPEB plan assets $108 $105 2020 2019 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $1 $— $1 $— $2 $2 $— $— Fixed income securities 51 51 — — 48 48 — — Total assets in fair value hierarchy 52 $51 $1 $— 50 $50 $— $— Assets measured at net asset value 22 22 Total OPEB plan assets $74 $72 2020 2019 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Fixed income securities $18 $18 $— $— $17 $17 $— $— Total OPEB plan assets $18 $18 $— $— $17 $17 $— $— 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, 2020 and 2019. 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 9% and 11% of total assets in the 401(k) savings plans at December 31, 2020 and 2019, 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 2020 2019 2018 2020 2019 2018 2020 2019 2018 401(k) costs $25 $25 $25 $13 $13 $13 $12 $12 $11 Alliant Energy IPL WPL 2020 2019 2018 2020 2019 2018 2020 2019 2018 Compensation expense $16 $26 $17 $9 $15 $9 $6 $10 $7 Income tax benefits 4 7 5 2 4 3 2 3 2 As of December 31, 2020, Alliant Energy’s, IPL’s and WPL’s total unrecognized compensation cost related to share-based compensation awards was $5 million, $3 million and $2 million, respectively, which is expected to be recognized over a weighted average period of between one year 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, 2020, 537,485 shares were included in the calculation of diluted EPS related to the nonvested equity awards. Performance Shares - Equity Awards - Payouts of 2020 and 2019 performance shares 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 2020 and 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 2020 and 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 2020 and 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. If minimum performance targets are not met during the performance period, these performance shares are forfeited. 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 2020 and 2019 performance shares activity, with amounts representing the target number of awards, was as follows: 2020 2019 Performance Shares - Equity Weighted Average Performance Shares - Equity Weighted Average Nonvested awards, January 1 74,193 $47.44 — $— Granted 56,204 64.04 91,816 47.23 Forfeited (1,241) 50.94 (17,623) 46.35 Nonvested awards, December 31 129,156 54.63 74,193 47.44 Performance Shares and Performance Units - Liability Awards - Payouts of performance shares granted prior to 2019 under the Amended and Restated 2010 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) 2020 2019 2018 2020 2019 2018 Nonvested awards, January 1 131,872 203,188 223,511 34,574 57,761 71,737 Granted — — 74,163 — — 19,840 Vested (63,565) (66,322) (90,806) (16,661) (20,131) (31,910) Forfeited — (4,994) (3,680) (1,112) (3,056) (1,906) Nonvested awards, December 31 68,307 131,872 203,188 16,801 34,574 57,761 Fair Value of Awards - The fair value of each performance share and performance unit accounted for as a liability award 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, 2020, the Alliant Energy common stock closing price was $51.53, and additional information related to the fair value of nonvested performance shares and performance units granted in 2018 was as follows: Performance Shares Performance Units Nonvested awards at target 68,307 16,801 Estimated payout percentage based on performance criteria 172.5% 172.5% Fair values of each nonvested award $88.89 $88.89 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) 2020 2019 2018 2020 2019 2018 2017 Grant 2016 Grant 2015 Grant 2017 Grant 2016 Grant 2015 Grant Performance awards vested 63,565 66,322 90,806 16,661 20,131 31,910 Percentage of target number of performance awards 155.0% 142.5% 137.5% 155.0% 142.5% 137.5% Aggregate payout value (in millions) $6 $4 $5 $2 $1 $1 Payout - cash (in millions) $5 $4 $5 $2 $1 $1 Payout - common stock shares issued 9,543 6,447 5,078 N/A N/A N/A Restricted Stock Units - Equity Awards - Payouts of 2020 and 2019 restricted stock units are based on the expiration of a three-year time-vesting period. Restricted stock units granted in 2020 and 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 2020 and 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 2020 and 2019 restricted stock units activity was as follows: 2020 2019 Restricted Stock Units - Equity Weighted Average Restricted Stock Units - Equity Weighted Average Nonvested units, January 1 89,281 $46.04 — $— Granted 61,056 59.42 105,348 45.98 Forfeited (3,788) 49.01 (16,067) 45.63 Nonvested units, December 31 146,549 51.54 89,281 46.04 Restricted Stock Units - Liability Awards - Payouts of restricted stock units granted prior to 2019 are based on the expiration of a three-year time-vesting period and 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: 2020 2019 2018 Nonvested units, January 1 113,033 174,163 113,749 Granted — — 63,568 Vested (54,483) (56,849) — Forfeited — (4,281) (3,154) Nonvested units, December 31 58,550 113,033 174,163 Performance Restricted Stock Units - Equity Awards - Payouts of performance restricted stock units 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 minimum 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: 2020 2019 2018 Units Weighted Average Units Weighted Average Units Weighted Average Nonvested units, January 1 206,065 $41.50 203,188 $37.23 132,705 $36.50 Granted 56,204 59.37 91,816 46.10 74,163 38.60 Vested (63,565) 39.12 (66,322) 33.93 — — Forfeited (1,241) 49.25 (22,617) 44.00 (3,680) 38.60 Nonvested units, December 31 197,463 47.31 206,065 41.50 203,188 37.23
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 2020 2019 2018 2020 2019 2018 Discount rate for benefit obligations 2.57% 3.48% 4.34% 2.31% 3.40% 4.24% Discount rate for net periodic cost 3.48% 4.34% 3.66% 3.40% 4.24% 3.53% Expected rate of return on plan assets 7.10% 7.60% 7.60% 4.50% 5.44% 5.44% Interest crediting rate for Alliant Energy Cash Balance Pension Plan 4.76% 5.52% 5.04% 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 2020 2019 2018 2020 2019 2018 Discount rate for benefit obligations 2.61% 3.51% 4.35% 2.28% 3.39% 4.23% Discount rate for net periodic cost 3.51% 4.35% 3.68% 3.39% 4.23% 3.51% Expected rate of return on plan assets 7.10% 7.60% 7.60% 4.50% 5.60% 5.60% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Qualified Defined Benefit Pension Plan OPEB Plans WPL 2020 2019 2018 2020 2019 2018 Discount rate for benefit obligations 2.64% 3.50% 4.35% 2.27% 3.39% 4.23% Discount rate for net periodic cost 3.50% 4.35% 3.69% 3.39% 4.23% 3.51% Expected rate of return on plan assets 7.10% 7.60% 7.60% 3.28% 3.81% 3.84% 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 and mortality projection tables that were released in 2020 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 2020 2019 2018 2020 2019 2018 Service cost $11 $10 $12 $3 $3 $4 Interest cost 43 50 47 7 9 8 Expected return on plan assets (a) (70) (60) (69) (5) (5) (6) Amortization of prior service credit (b) — (1) (1) — — — Amortization of actuarial loss (c) 34 36 35 3 3 3 Settlement losses (d) 12 — — — — — $30 $35 $24 $8 $10 $9 IPL Defined Benefit Pension Plans OPEB Plans 2020 2019 2018 2020 2019 2018 Service cost $7 $6 $7 $1 $1 $2 Interest cost 20 23 21 3 3 3 Expected return on plan assets (a) (33) (28) (32) (4) (4) (4) Amortization of actuarial loss (c) 15 15 15 1 2 1 Settlement losses (d) 7 — — — — — $16 $16 $11 $1 $2 $2 WPL Defined Benefit Pension Plans OPEB Plans 2020 2019 2018 2020 2019 2018 Service cost $4 $3 $4 $2 $1 $2 Interest cost 19 21 20 3 3 3 Expected return on plan assets (a) (31) (26) (30) (1) (1) (1) Amortization of prior service credit (b) — — — (1) — — Amortization of actuarial loss (c) 16 18 17 2 2 2 $8 $16 $11 $5 $5 $6 (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 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 and lump sum payments related to IPL’s qualified defined benefit pension plan. 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 2020 2019 2020 2019 Change in benefit obligation: Net benefit obligation at January 1 $1,280 $1,175 $214 $206 Service cost 11 10 3 3 Interest cost 43 50 7 9 Plan participants’ contributions — — 4 3 Actuarial loss 132 124 18 14 Gross benefits paid (115) (79) (19) (21) Net benefit obligation at December 31 1,351 1,280 227 214 Change in plan assets: Fair value of plan assets at January 1 931 809 105 99 Actual return on plan assets 108 168 11 15 Employer contributions 60 33 7 9 Plan participants’ contributions — — 4 3 Gross benefits paid (115) (79) (19) (21) Fair value of plan assets at December 31 984 931 108 105 Under funded status at December 31 ($367) ($349) ($119) ($109) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2020 2019 2020 2019 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $8 $10 Current liabilities (2) (14) (8) (8) Pension and other benefit obligations (365) (335) (119) (111) Net amounts recognized at December 31 ($367) ($349) ($119) ($109) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $533 $485 $54 $45 Prior service credit (5) (5) (1) (1) $528 $480 $53 $44 Defined Benefit Pension Plans OPEB Plans IPL 2020 2019 2020 2019 Change in benefit obligation: Net benefit obligation at January 1 $581 $534 $86 $83 Service cost 7 6 1 1 Interest cost 20 23 3 3 Plan participants’ contributions — — 1 1 Actuarial loss 60 56 8 6 Gross benefits paid (53) (38) (8) (8) Net benefit obligation at December 31 615 581 91 86 Change in plan assets: Fair value of plan assets at January 1 436 378 72 67 Actual return on plan assets 51 79 7 10 Employer contributions 19 17 2 2 Plan participants’ contributions — — 1 1 Gross benefits paid (53) (38) (8) (8) Fair value of plan assets at December 31 453 436 74 72 Under funded status at December 31 ($162) ($145) ($17) ($14) Defined Benefit Pension Plans OPEB Plans IPL 2020 2019 2020 2019 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $4 $7 Current liabilities (1) (1) (2) (2) Pension and other benefit obligations (161) (144) (19) (19) Net amounts recognized at December 31 ($162) ($145) ($17) ($14) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $228 $208 $20 $17 Prior service credit (1) (1) — — $227 $207 $20 $17 Defined Benefit Pension Plans OPEB Plans WPL 2020 2019 2020 2019 Change in benefit obligation: Net benefit obligation at January 1 $550 $507 $85 $83 Service cost 4 3 2 1 Interest cost 19 21 3 3 Plan participants’ contributions — — 2 2 Actuarial loss 55 54 5 5 Gross benefits paid (40) (35) (8) (9) Net benefit obligation at December 31 588 550 89 85 Change in plan assets: Fair value of plan assets at January 1 406 352 17 17 Actual return on plan assets 48 73 2 1 Employer contributions 22 16 5 6 Plan participants’ contributions — — 2 2 Gross benefits paid (40) (35) (8) (9) Fair value of plan assets at December 31 436 406 18 17 Under funded status at December 31 ($152) ($144) ($71) ($68) Defined Benefit Pension Plans OPEB Plans WPL 2020 2019 2020 2019 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $3 $3 Current liabilities — — (6) (6) Pension and other benefit obligations (152) (144) (68) (65) Net amounts recognized at December 31 ($152) ($144) ($71) ($68) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $233 $212 $25 $22 Prior service credit (1) (1) (1) (1) $232 $211 $24 $21 Actuarial losses related to benefit obligations in 2020 for defined benefit pension and OPEB plans were primarily due to decreases in the discount rates, partially offset by the impact of updated mortality projection tables. 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. 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 are as follows (in millions): Defined Benefit Pension Plans OPEB Plans Alliant Energy 2020 2019 2020 2019 Accumulated benefit obligations $1,305 $1,234 $227 $214 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,305 1,234 227 214 Fair value of plan assets 984 931 108 105 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,351 1,280 N/A N/A Fair value of plan assets 984 931 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2020 2019 2020 2019 Accumulated benefit obligations $589 $558 $91 $86 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 589 558 91 86 Fair value of plan assets 453 436 74 72 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 615 581 N/A N/A Fair value of plan assets 453 436 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2020 2019 2020 2019 Accumulated benefit obligations $573 $536 $89 $85 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 573 536 89 85 Fair value of plan assets 436 406 18 17 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 588 550 N/A N/A Fair value of plan assets 436 406 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 2020 2019 2020 2019 Regulatory assets $44 $39 $34 $29 Estimated Future Employer Contributions and Benefit Payments - Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2021 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $39 $17 $18 OPEB plans 8 2 6 (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 2021 2022 2023 2024 2025 2026 - 2030 Defined benefit pension benefits $76 $78 $79 $79 $80 $398 OPEB 18 18 17 17 17 74 $94 $96 $96 $96 $97 $472 IPL 2021 2022 2023 2024 2025 2026 - 2030 Defined benefit pension benefits $38 $38 $39 $38 $38 $181 OPEB 7 7 7 7 7 29 $45 $45 $46 $45 $45 $210 WPL 2021 2022 2023 2024 2025 2026 - 2030 Defined benefit pension benefits $32 $32 $32 $33 $33 $162 OPEB 7 7 7 7 6 29 $39 $39 $39 $40 $39 $191 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, 2020, 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. 11% - 41% 26% Equity securities - international 14% - 34% 23% Global asset securities 5% - 15% 9% Risk parity securities 5% - 15% 9% Fixed income securities 20% - 40% 28% 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, 2020, the current target ranges and actual allocations for VEBA trusts with assets greater than $5 million and the WPL 401(h) assets were as follows: Target Range Actual Allocation Allocation Cash and equivalents 0% - 5% 1% Equity securities - U.S. 0% - 46% 25% Equity securities - international 0% - 34% 1% Fixed income securities 20% - 100% 73% Fair Value Measurements - Fair value measurement accounting establishes three levels of fair value hierarchy that prioritize the inputs to valuation techniques used to measure fair value. Refer to Note 16 for discussion of levels within the fair value hierarchy. Level 1 items include investments in securities held in registered investment companies 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): 2020 2019 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $46 $— $46 $— $46 $4 $42 $— Equity securities - U.S. 182 182 — — 161 161 — — Equity securities - international 151 151 — — 136 136 — — Global asset securities 45 45 — — 47 47 — — Fixed income securities 134 56 78 — 131 58 73 — Total assets in fair value hierarchy 558 $434 $124 $— 521 $406 $115 $— Assets measured at net asset value 426 410 Accrued investment income 1 1 Due to brokers, net (pending trades with brokers) (1) (1) Total pension plan assets $984 $931 2020 2019 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $21 $— $21 $— $22 $2 $20 $— Equity securities - U.S. 84 84 — — 75 75 — — Equity securities - international 69 69 — — 64 64 — — Global asset securities 21 21 — — 22 22 — — Fixed income securities 62 26 36 — 61 27 34 — Total assets in fair value hierarchy 257 $200 $57 $— 244 $190 $54 $— Assets measured at net asset value 196 192 Total pension plan assets $453 $436 2020 2019 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $20 $— $20 $— $20 $2 $18 $— Equity securities - U.S. 81 81 — — 70 70 — — Equity securities - international 67 67 — — 59 59 — — Global asset securities 20 20 — — 21 21 — — Fixed income securities 59 25 34 — 57 25 32 — Total assets in fair value hierarchy 247 $193 $54 $— 227 $177 $50 $— Assets measured at net asset value 189 179 Total pension plan assets $436 $406 At December 31, the fair values of OPEB plan assets were as follows (in millions): 2020 2019 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $2 $— $2 $— $3 $3 $— $— Equity securities - U.S. 4 4 — — 4 4 — — Equity securities - international 3 3 — — 3 3 — — Fixed income securities 72 71 1 — 69 68 1 — Total assets in fair value hierarchy 81 $78 $3 $— 79 $78 $1 $— Assets measured at net asset value 27 26 Total OPEB plan assets $108 $105 2020 2019 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $1 $— $1 $— $2 $2 $— $— Fixed income securities 51 51 — — 48 48 — — Total assets in fair value hierarchy 52 $51 $1 $— 50 $50 $— $— Assets measured at net asset value 22 22 Total OPEB plan assets $74 $72 2020 2019 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Fixed income securities $18 $18 $— $— $17 $17 $— $— Total OPEB plan assets $18 $18 $— $— $17 $17 $— $— 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, 2020 and 2019. 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 9% and 11% of total assets in the 401(k) savings plans at December 31, 2020 and 2019, 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 2020 2019 2018 2020 2019 2018 2020 2019 2018 401(k) costs $25 $25 $25 $13 $13 $13 $12 $12 $11 Alliant Energy IPL WPL 2020 2019 2018 2020 2019 2018 2020 2019 2018 Compensation expense $16 $26 $17 $9 $15 $9 $6 $10 $7 Income tax benefits 4 7 5 2 4 3 2 3 2 As of December 31, 2020, Alliant Energy’s, IPL’s and WPL’s total unrecognized compensation cost related to share-based compensation awards was $5 million, $3 million and $2 million, respectively, which is expected to be recognized over a weighted average period of between one year 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, 2020, 537,485 shares were included in the calculation of diluted EPS related to the nonvested equity awards. Performance Shares - Equity Awards - Payouts of 2020 and 2019 performance shares 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 2020 and 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 2020 and 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 2020 and 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. If minimum performance targets are not met during the performance period, these performance shares are forfeited. 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 2020 and 2019 performance shares activity, with amounts representing the target number of awards, was as follows: 2020 2019 Performance Shares - Equity Weighted Average Performance Shares - Equity Weighted Average Nonvested awards, January 1 74,193 $47.44 — $— Granted 56,204 64.04 91,816 47.23 Forfeited (1,241) 50.94 (17,623) 46.35 Nonvested awards, December 31 129,156 54.63 74,193 47.44 Performance Shares and Performance Units - Liability Awards - Payouts of performance shares granted prior to 2019 under the Amended and Restated 2010 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) 2020 2019 2018 2020 2019 2018 Nonvested awards, January 1 131,872 203,188 223,511 34,574 57,761 71,737 Granted — — 74,163 — — 19,840 Vested (63,565) (66,322) (90,806) (16,661) (20,131) (31,910) Forfeited — (4,994) (3,680) (1,112) (3,056) (1,906) Nonvested awards, December 31 68,307 131,872 203,188 16,801 34,574 57,761 Fair Value of Awards - The fair value of each performance share and performance unit accounted for as a liability award 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, 2020, the Alliant Energy common stock closing price was $51.53, and additional information related to the fair value of nonvested performance shares and performance units granted in 2018 was as follows: Performance Shares Performance Units Nonvested awards at target 68,307 16,801 Estimated payout percentage based on performance criteria 172.5% 172.5% Fair values of each nonvested award $88.89 $88.89 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) 2020 2019 2018 2020 2019 2018 2017 Grant 2016 Grant 2015 Grant 2017 Grant 2016 Grant 2015 Grant Performance awards vested 63,565 66,322 90,806 16,661 20,131 31,910 Percentage of target number of performance awards 155.0% 142.5% 137.5% 155.0% 142.5% 137.5% Aggregate payout value (in millions) $6 $4 $5 $2 $1 $1 Payout - cash (in millions) $5 $4 $5 $2 $1 $1 Payout - common stock shares issued 9,543 6,447 5,078 N/A N/A N/A Restricted Stock Units - Equity Awards - Payouts of 2020 and 2019 restricted stock units are based on the expiration of a three-year time-vesting period. Restricted stock units granted in 2020 and 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 2020 and 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 2020 and 2019 restricted stock units activity was as follows: 2020 2019 Restricted Stock Units - Equity Weighted Average Restricted Stock Units - Equity Weighted Average Nonvested units, January 1 89,281 $46.04 — $— Granted 61,056 59.42 105,348 45.98 Forfeited (3,788) 49.01 (16,067) 45.63 Nonvested units, December 31 146,549 51.54 89,281 46.04 Restricted Stock Units - Liability Awards - Payouts of restricted stock units granted prior to 2019 are based on the expiration of a three-year time-vesting period and 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: 2020 2019 2018 Nonvested units, January 1 113,033 174,163 113,749 Granted — — 63,568 Vested (54,483) (56,849) — Forfeited — (4,281) (3,154) Nonvested units, December 31 58,550 113,033 174,163 Performance Restricted Stock Units - Equity Awards - Payouts of performance restricted stock units 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 minimum 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: 2020 2019 2018 Units Weighted Average Units Weighted Average Units Weighted Average Nonvested units, January 1 206,065 $41.50 203,188 $37.23 132,705 $36.50 Granted 56,204 59.37 91,816 46.10 74,163 38.60 Vested (63,565) 39.12 (66,322) 33.93 — — Forfeited (1,241) 49.25 (22,617) 44.00 (3,680) 38.60 Nonvested units, December 31 197,463 47.31 206,065 41.50 203,188 37.23

Asset Retirement Obligations

Asset Retirement Obligations	12 Months Ended
	Dec. 31, 2020
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, coal yards 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 2020 2019 2020 2019 2020 2019 Balance, January 1 $196 $177 $134 $118 $62 $59 Revisions in estimated cash flows 13 (6) 9 (7) 4 1 Liabilities settled (13) (8) (9) (8) (4) — Liabilities incurred (a) 48 26 38 26 10 — Accretion expense 7 7 5 5 2 2 Balance, December 31 $251 $196 $177 $134 $74 $62 (a) In 2020, IPL placed in service the Whispering Willow North, Golden Plains and Richland wind sites, and WPL placed in service the Kossuth wind site. As a result, Alliant Energy, IPL and WPL recognized additional AROs in 2020, which 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, coal yards 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 2020 2019 2020 2019 2020 2019 Balance, January 1 $196 $177 $134 $118 $62 $59 Revisions in estimated cash flows 13 (6) 9 (7) 4 1 Liabilities settled (13) (8) (9) (8) (4) — Liabilities incurred (a) 48 26 38 26 10 — Accretion expense 7 7 5 5 2 2 Balance, December 31 $251 $196 $177 $134 $74 $62 (a) In 2020, IPL placed in service the Whispering Willow North, Golden Plains and Richland wind sites, and WPL placed in service the Kossuth wind site. As a result, Alliant Energy, IPL and WPL recognized additional AROs in 2020, which 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, coal yards 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 2020 2019 2020 2019 2020 2019 Balance, January 1 $196 $177 $134 $118 $62 $59 Revisions in estimated cash flows 13 (6) 9 (7) 4 1 Liabilities settled (13) (8) (9) (8) (4) — Liabilities incurred (a) 48 26 38 26 10 — Accretion expense 7 7 5 5 2 2 Balance, December 31 $251 $196 $177 $134 $74 $62 (a) In 2020, IPL placed in service the Whispering Willow North, Golden Plains and Richland wind sites, and WPL placed in service the Kossuth wind site. As a result, Alliant Energy, IPL and WPL recognized additional AROs in 2020, which resulted in corresponding increases in property, plant and equipment, net on the respective balance sheets.

Derivative Instruments

Derivative Instruments	12 Months Ended
	Dec. 31, 2020
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, 2020, 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 9,285 2021 208,542 2021-2028 5,648 2021-2023 5,544 2021-2022 IPL 3,398 2021 109,063 2021-2028 2,548 2021-2023 — — WPL 5,887 2021 99,479 2021-2027 3,100 2021-2023 5,544 2021-2022 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 2020 2019 2020 2019 2020 2019 Current derivative assets $24 $16 $20 $12 $4 $4 Non-current derivative assets 10 11 9 10 1 1 Current derivative liabilities 9 19 3 9 6 10 Non-current derivative liabilities 16 19 9 9 7 10 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, 2020 and 2019, 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, 2020 and 2019. 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, 2020, 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 9,285 2021 208,542 2021-2028 5,648 2021-2023 5,544 2021-2022 IPL 3,398 2021 109,063 2021-2028 2,548 2021-2023 — — WPL 5,887 2021 99,479 2021-2027 3,100 2021-2023 5,544 2021-2022 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 2020 2019 2020 2019 2020 2019 Current derivative assets $24 $16 $20 $12 $4 $4 Non-current derivative assets 10 11 9 10 1 1 Current derivative liabilities 9 19 3 9 6 10 Non-current derivative liabilities 16 19 9 9 7 10 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, 2020 and 2019, 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, 2020 and 2019. 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, 2020, 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 9,285 2021 208,542 2021-2028 5,648 2021-2023 5,544 2021-2022 IPL 3,398 2021 109,063 2021-2028 2,548 2021-2023 — — WPL 5,887 2021 99,479 2021-2027 3,100 2021-2023 5,544 2021-2022 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 2020 2019 2020 2019 2020 2019 Current derivative assets $24 $16 $20 $12 $4 $4 Non-current derivative assets 10 11 9 10 1 1 Current derivative liabilities 9 19 3 9 6 10 Non-current derivative liabilities 16 19 9 9 7 10 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, 2020 and 2019, 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, 2020 and 2019. 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, 2020
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 expected credit losses 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 2020 2019 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Money market fund investments $44 $44 $— $— $44 $5 $5 $— $— $5 Derivatives 34 — 5 29 34 27 — 5 22 27 Deferred proceeds 188 — — 188 188 188 — — 188 188 Liabilities and equity: Derivatives 25 — 25 — 25 38 — 37 1 38 Long-term debt (incl. current maturities) 6,777 — 8,107 2 8,109 6,190 — 6,918 2 6,920 IPL 2020 2019 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Money market fund investments $44 $44 $— $— $44 $5 $5 $— $— $5 Derivatives 29 — 3 26 29 22 — 3 19 22 Deferred proceeds 188 — — 188 188 188 — — 188 188 Liabilities and equity: Derivatives 12 — 12 — 12 18 — 17 1 18 Long-term debt (incl. current maturities) 3,345 — 4,021 — 4,021 3,147 — 3,489 — 3,489 WPL 2020 2019 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $5 $— $2 $3 $5 $5 $— $2 $3 $5 Liabilities and equity: Derivatives 13 — 13 — 13 20 — 20 — 20 Long-term debt (incl. current maturities) 2,130 — 2,690 — 2,690 1,933 — 2,268 — 2,268 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 2020 2019 2020 2019 Beginning balance, January 1 $21 $12 $188 $119 Total net gains included in changes in net assets (realized/unrealized) 11 8 — — Transfers out of Level 3 (a) — 4 — — Purchases 14 14 — — Sales (1) — — — Settlements (b) (16) (17) — 69 Ending balance, December 31 $29 $21 $188 $188 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 $11 $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2020 2019 2020 2019 Beginning balance, January 1 $18 $9 $188 $119 Total net gains included in changes in net assets (realized/unrealized) 10 11 — — Transfers out of Level 3 (a) — 2 — — Purchases 11 9 — — Sales (1) — — — Settlements (b) (12) (13) — 69 Ending balance, December 31 $26 $18 $188 $188 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 $10 $12 $— $— (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 expected credit losses 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 2020 $18 $11 $17 $9 $1 $2 2019 15 7 14 5 1 2
IPL [Member]
Fair Value Measurements	FAIR VALUE MEASUREMENTS Valuation Hierarchy - Fair value measurement accounting establishes three levels of fair value hierarchy that prioritize the inputs to valuation techniques used to measure fair value. Level 1 pricing inputs are quoted prices available in active markets for identical assets or liabilities as of the reporting date. Level 2 pricing inputs are quoted prices for similar assets or liabilities in active markets or quoted prices for identical or similar assets or liabilities in markets that are not active as of the reporting date. Level 3 pricing inputs are unobservable inputs for assets or liabilities for which little or no market data exist and require significant management judgment or estimation. The fair value hierarchy gives the highest priority to quoted prices in active markets (Level 1) and the lowest priority to unobservable data (Level 3). In some cases, the inputs used to measure fair value might fall in different levels of the fair value hierarchy. The lowest level input that is significant to a fair value measurement in its entirety determines the applicable level in the fair value hierarchy. Assessing the significance of a particular input to the fair value measurement in its entirety requires judgment, considering factors specific to the asset or liability. Valuation Techniques - Derivative assets and derivative liabilities - Swap, option and physical forward commodity contracts were non-exchange-based derivative instruments and were valued using indicative price quotations from a pricing vendor that provides daily exchange forward price settlements, from broker or dealer quotations, from market publications or from on-line exchanges. The indicative price quotations reflected the average of the bid-ask mid-point prices and were obtained from sources believed to provide the most liquid market for the commodity. A portion of these indicative price quotations were corroborated using quoted prices for similar assets or liabilities in active markets and categorized derivative instruments based on such indicative price quotations as Level 2. Commodity contracts that were valued using indicative price quotations based on significant assumptions such as seasonal or monthly shaping and indicative price quotations that could not be readily corroborated were categorized as Level 3. Swap, option and physical forward commodity contracts were predominately at liquid trading points. FTRs were valued using auction prices and were categorized as Level 3. Refer to Note 15 for additional details of derivative assets and derivative liabilities. Deferred proceeds (sales of receivables) - The fair value of IPL’s deferred proceeds related to its sales of accounts receivable program was calculated each reporting date using the cost approach valuation technique. The fair value represents the carrying amount of receivables sold less the allowance for expected credit losses 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 2020 2019 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Money market fund investments $44 $44 $— $— $44 $5 $5 $— $— $5 Derivatives 34 — 5 29 34 27 — 5 22 27 Deferred proceeds 188 — — 188 188 188 — — 188 188 Liabilities and equity: Derivatives 25 — 25 — 25 38 — 37 1 38 Long-term debt (incl. current maturities) 6,777 — 8,107 2 8,109 6,190 — 6,918 2 6,920 IPL 2020 2019 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Money market fund investments $44 $44 $— $— $44 $5 $5 $— $— $5 Derivatives 29 — 3 26 29 22 — 3 19 22 Deferred proceeds 188 — — 188 188 188 — — 188 188 Liabilities and equity: Derivatives 12 — 12 — 12 18 — 17 1 18 Long-term debt (incl. current maturities) 3,345 — 4,021 — 4,021 3,147 — 3,489 — 3,489 WPL 2020 2019 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $5 $— $2 $3 $5 $5 $— $2 $3 $5 Liabilities and equity: Derivatives 13 — 13 — 13 20 — 20 — 20 Long-term debt (incl. current maturities) 2,130 — 2,690 — 2,690 1,933 — 2,268 — 2,268 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 2020 2019 2020 2019 Beginning balance, January 1 $21 $12 $188 $119 Total net gains included in changes in net assets (realized/unrealized) 11 8 — — Transfers out of Level 3 (a) — 4 — — Purchases 14 14 — — Sales (1) — — — Settlements (b) (16) (17) — 69 Ending balance, December 31 $29 $21 $188 $188 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 $11 $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2020 2019 2020 2019 Beginning balance, January 1 $18 $9 $188 $119 Total net gains included in changes in net assets (realized/unrealized) 10 11 — — Transfers out of Level 3 (a) — 2 — — Purchases 11 9 — — Sales (1) — — — Settlements (b) (12) (13) — 69 Ending balance, December 31 $26 $18 $188 $188 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 $10 $12 $— $— (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 expected credit losses 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 2020 $18 $11 $17 $9 $1 $2 2019 15 7 14 5 1 2
WPL [Member]
Fair Value Measurements	FAIR VALUE MEASUREMENTS Valuation Hierarchy - Fair value measurement accounting establishes three levels of fair value hierarchy that prioritize the inputs to valuation techniques used to measure fair value. Level 1 pricing inputs are quoted prices available in active markets for identical assets or liabilities as of the reporting date. Level 2 pricing inputs are quoted prices for similar assets or liabilities in active markets or quoted prices for identical or similar assets or liabilities in markets that are not active as of the reporting date. Level 3 pricing inputs are unobservable inputs for assets or liabilities for which little or no market data exist and require significant management judgment or estimation. The fair value hierarchy gives the highest priority to quoted prices in active markets (Level 1) and the lowest priority to unobservable data (Level 3). In some cases, the inputs used to measure fair value might fall in different levels of the fair value hierarchy. The lowest level input that is significant to a fair value measurement in its entirety determines the applicable level in the fair value hierarchy. Assessing the significance of a particular input to the fair value measurement in its entirety requires judgment, considering factors specific to the asset or liability. Valuation Techniques - Derivative assets and derivative liabilities - Swap, option and physical forward commodity contracts were non-exchange-based derivative instruments and were valued using indicative price quotations from a pricing vendor that provides daily exchange forward price settlements, from broker or dealer quotations, from market publications or from on-line exchanges. The indicative price quotations reflected the average of the bid-ask mid-point prices and were obtained from sources believed to provide the most liquid market for the commodity. A portion of these indicative price quotations were corroborated using quoted prices for similar assets or liabilities in active markets and categorized derivative instruments based on such indicative price quotations as Level 2. Commodity contracts that were valued using indicative price quotations based on significant assumptions such as seasonal or monthly shaping and indicative price quotations that could not be readily corroborated were categorized as Level 3. Swap, option and physical forward commodity contracts were predominately at liquid trading points. FTRs were valued using auction prices and were categorized as Level 3. Refer to Note 15 for additional details of derivative assets and derivative liabilities. Deferred proceeds (sales of receivables) - The fair value of IPL’s deferred proceeds related to its sales of accounts receivable program was calculated each reporting date using the cost approach valuation technique. The fair value represents the carrying amount of receivables sold less the allowance for expected credit losses 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 2020 2019 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Money market fund investments $44 $44 $— $— $44 $5 $5 $— $— $5 Derivatives 34 — 5 29 34 27 — 5 22 27 Deferred proceeds 188 — — 188 188 188 — — 188 188 Liabilities and equity: Derivatives 25 — 25 — 25 38 — 37 1 38 Long-term debt (incl. current maturities) 6,777 — 8,107 2 8,109 6,190 — 6,918 2 6,920 IPL 2020 2019 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Money market fund investments $44 $44 $— $— $44 $5 $5 $— $— $5 Derivatives 29 — 3 26 29 22 — 3 19 22 Deferred proceeds 188 — — 188 188 188 — — 188 188 Liabilities and equity: Derivatives 12 — 12 — 12 18 — 17 1 18 Long-term debt (incl. current maturities) 3,345 — 4,021 — 4,021 3,147 — 3,489 — 3,489 WPL 2020 2019 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $5 $— $2 $3 $5 $5 $— $2 $3 $5 Liabilities and equity: Derivatives 13 — 13 — 13 20 — 20 — 20 Long-term debt (incl. current maturities) 2,130 — 2,690 — 2,690 1,933 — 2,268 — 2,268 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 2020 2019 2020 2019 Beginning balance, January 1 $21 $12 $188 $119 Total net gains included in changes in net assets (realized/unrealized) 11 8 — — Transfers out of Level 3 (a) — 4 — — Purchases 14 14 — — Sales (1) — — — Settlements (b) (16) (17) — 69 Ending balance, December 31 $29 $21 $188 $188 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 $11 $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2020 2019 2020 2019 Beginning balance, January 1 $18 $9 $188 $119 Total net gains included in changes in net assets (realized/unrealized) 10 11 — — Transfers out of Level 3 (a) — 2 — — Purchases 11 9 — — Sales (1) — — — Settlements (b) (12) (13) — 69 Ending balance, December 31 $26 $18 $188 $188 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 $10 $12 $— $— (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 expected credit losses 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 2020 $18 $11 $17 $9 $1 $2 2019 15 7 14 5 1 2

Commitments And Contingencies

Commitments And Contingencies	12 Months Ended
	Dec. 31, 2020
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, including WPL’s expansion of solar generation. At December 31, 2020, Alliant Energy’s and WPL’s minimum future commitments in 2021 for these projects were $8 million and $7 million, respectively.(b) Other Purchase Commitments - Various commodity supply, transportation and storage contracts help meet obligations to provide electricity and natural gas to utility customers. In addition, there are various purchase commitments associated with other goods and services. At December 31, 2020, the related minimum future commitments were as follows (in millions): Alliant Energy 2021 2022 2023 2024 2025 Thereafter Total Natural gas 252 174 137 105 68 185 921 Coal 70 27 17 — — — 114 Other (a) 61 17 11 8 3 29 129 $383 $218 $165 $113 $71 $214 $1,164 IPL 2021 2022 2023 2024 2025 Thereafter Total Natural gas 130 88 79 68 38 71 474 Coal 32 16 13 — — — 61 Other (a) 25 5 4 2 2 27 65 $187 $109 $96 $70 $40 $98 $600 WPL 2021 2022 2023 2024 2025 Thereafter Total Natural gas 122 86 58 37 30 114 447 Coal 38 11 4 — — — 53 Other (a) 26 2 1 1 1 1 32 $186 $99 $63 $38 $31 $115 $532 (a) Includes individual commitments incurred during the normal course of business that exceeded $1 million at December 31, 2020. Whiting Petroleum - Whiting Petroleum is an independent oil and gas company. In 2004, Alliant Energy sold its remaining interest in Whiting Petroleum. 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 multiple general partnership agreements in the oil and gas industry. The guarantees do not include a maximum limit. Based on information made available to Alliant Energy by Whiting Petroleum, the Whiting Petroleum affiliate holds an approximate 6% share in the partnerships, and currently known obligations include costs associated with the future abandonment of certain facilities owned by the partnerships. The general partnerships were formed under California law, and Alliant Energy Resources, LLC may need to perform under the guarantees if the affiliate of Whiting Petroleum is unable to meet its partnership obligations. As of December 31, 2020, the currently known partnership obligations for the abandonment obligations are estimated at $68 million, which represents Alliant Energy’s currently estimated maximum exposure under the guarantees. Alliant Energy estimates its expected loss to be a portion of the $68 million of known partnership abandonment obligations of the Whiting Petroleum affiliate and the other partners. Alliant Energy is not aware of any material liabilities related to these guarantees that it is probable that it will be obligated to pay; however, the new credit loss accounting standard adopted on January 1, 2020 requires recognition of a liability for expected credit losses related to the contingent obligations that are in the scope of these guarantees. With the adoption of this standard, Alliant Energy recorded a pre-tax $12 million cumulative effect adjustment to decrease the opening balance of retained earnings as of January 1, 2020. In April 2020, Whiting Petroleum filed for bankruptcy, and in September 2020, Whiting Petroleum completed its reorganization and emerged from bankruptcy. As a result of Whiting Petroleum’s completed bankruptcy proceedings, as well as additional information regarding the guarantees obtained from the bankruptcy proceedings, the credit loss liability was decreased to $5 million in 2020. The credit loss liability is recorded in “Other liabilities” on Alliant Energy’s balance sheet as of December 31, 2020. In 2020, the pre-tax credit loss adjustment of $7 million was recorded as a reduction in Alliant Energy’s “Other operation and maintenance” expenses. 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 $74 million as of December 31, 2020 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, 2020 and 2019. 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, 2020, 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, 2020, such amounts for WPL were not material. Alliant Energy IPL Range of estimated future costs $11 - $28 $8 - $22 Current and non-current environmental liabilities 15 12 IPL Consent Decree - In 2015, the U.S. District Court for the Northern District of Iowa approved a Consent Decree that IPL entered into with the EPA, the Sierra Club, the State of Iowa and Linn County in Iowa, thereby resolving potential CAA issues associated with emissions from IPL’s coal-fired generating facilities in Iowa. IPL has completed various requirements under the Consent Decree. IPL’s remaining requirements include 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 and electric and gas distribution systems 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. 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 expected credit losses and credit risk-related contingent features, respectively.
IPL [Member]
Commitments And Contingencies	COMMITMENTS AND CONTINGENCIES(b) Other Purchase Commitments - Various commodity supply, transportation and storage contracts help meet obligations to provide electricity and natural gas to utility customers. In addition, there are various purchase commitments associated with other goods and services. At December 31, 2020, the related minimum future commitments were as follows (in millions): Alliant Energy 2021 2022 2023 2024 2025 Thereafter Total Natural gas 252 174 137 105 68 185 921 Coal 70 27 17 — — — 114 Other (a) 61 17 11 8 3 29 129 $383 $218 $165 $113 $71 $214 $1,164 IPL 2021 2022 2023 2024 2025 Thereafter Total Natural gas 130 88 79 68 38 71 474 Coal 32 16 13 — — — 61 Other (a) 25 5 4 2 2 27 65 $187 $109 $96 $70 $40 $98 $600 WPL 2021 2022 2023 2024 2025 Thereafter Total Natural gas 122 86 58 37 30 114 447 Coal 38 11 4 — — — 53 Other (a) 26 2 1 1 1 1 32 $186 $99 $63 $38 $31 $115 $532 (a) Includes individual commitments incurred during the normal course of business that exceeded $1 million at December 31, 2020. 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, 2020, 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, 2020, such amounts for WPL were not material. Alliant Energy IPL Range of estimated future costs $11 - $28 $8 - $22 Current and non-current environmental liabilities 15 12 IPL Consent Decree - In 2015, the U.S. District Court for the Northern District of Iowa approved a Consent Decree that IPL entered into with the EPA, the Sierra Club, the State of Iowa and Linn County in Iowa, thereby resolving potential CAA issues associated with emissions from IPL’s coal-fired generating facilities in Iowa. IPL has completed various requirements under the Consent Decree. IPL’s remaining requirements include 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 and electric and gas distribution systems 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. 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 expected credit losses and credit risk-related contingent features, respectively.
WPL [Member]
Commitments And Contingencies	COMMITMENTS AND CONTINGENCIES(a) Capital Purchase Commitments - Various contractual obligations contain minimum future commitments related to capital expenditures for certain construction projects, including WPL’s expansion of solar generation. At December 31, 2020, Alliant Energy’s and WPL’s minimum future commitments in 2021 for these projects were $8 million and $7 million, respectively.(b) Other Purchase Commitments - Various commodity supply, transportation and storage contracts help meet obligations to provide electricity and natural gas to utility customers. In addition, there are various purchase commitments associated with other goods and services. At December 31, 2020, the related minimum future commitments were as follows (in millions): Alliant Energy 2021 2022 2023 2024 2025 Thereafter Total Natural gas 252 174 137 105 68 185 921 Coal 70 27 17 — — — 114 Other (a) 61 17 11 8 3 29 129 $383 $218 $165 $113 $71 $214 $1,164 IPL 2021 2022 2023 2024 2025 Thereafter Total Natural gas 130 88 79 68 38 71 474 Coal 32 16 13 — — — 61 Other (a) 25 5 4 2 2 27 65 $187 $109 $96 $70 $40 $98 $600 WPL 2021 2022 2023 2024 2025 Thereafter Total Natural gas 122 86 58 37 30 114 447 Coal 38 11 4 — — — 53 Other (a) 26 2 1 1 1 1 32 $186 $99 $63 $38 $31 $115 $532 (a) Includes individual commitments incurred during the normal course of business that exceeded $1 million at December 31, 2020. 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, 2020, 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, 2020, such amounts for WPL were not material. Alliant Energy IPL Range of estimated future costs $11 - $28 $8 - $22 Current and non-current environmental liabilities 15 12 IPL Consent Decree - In 2015, the U.S. District Court for the Northern District of Iowa approved a Consent Decree that IPL entered into with the EPA, the Sierra Club, the State of Iowa and Linn County in Iowa, thereby resolving potential CAA issues associated with emissions from IPL’s coal-fired generating facilities in Iowa. IPL has completed various requirements under the Consent Decree. IPL’s remaining requirements include 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 and electric and gas distribution systems 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. 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 expected credit losses and credit risk-related contingent features, respectively.

Segments Of Business

Segments Of Business	12 Months Ended
	Dec. 31, 2020
Segment Reporting Information [Line Items]
Segments Of Business	SEGMENTS OF BUSINESS Alliant Energy - Alliant Energy’s principal businesses as of December 31, 2020 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, Travero, 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 2020 Electric Gas Other Total Parent and Other Consolidated Revenues $2,920 $373 $49 $3,342 $74 $3,416 Depreciation and amortization 556 49 5 610 5 615 Operating income (loss) 643 74 (1) 716 24 740 Interest expense 243 32 275 Equity income from unconsolidated investments, net (2) — — (2) (59) (61) Income tax expense (benefit) (66) 9 (57) Net income attributable to Alliant Energy common shareowners 573 41 614 Total assets 14,358 1,413 990 16,761 949 17,710 Investments in equity method subsidiaries 11 — — 11 465 476 Construction and acquisition expenditures 1,109 182 2 1,293 73 1,366 ATC Holdings, Utility Non-utility, Alliant Energy 2019 Electric Gas Other Total Parent and Other Consolidated Revenues $3,064 $455 $46 $3,565 $83 $3,648 Depreciation and amortization 513 47 3 563 4 567 Operating income 679 70 1 750 28 778 Interest expense 229 44 273 Equity income from unconsolidated investments, net (1) — — (1) (52) (53) Income tax expense (benefit) 73 (4) 69 Net income attributable to Alliant Energy common shareowners 517 40 557 Total assets 13,659 1,269 856 15,784 917 16,701 Investments in equity method subsidiaries 9 — — 9 449 458 Construction and acquisition expenditures 1,439 100 — 1,539 101 1,640 ATC Holdings, Utility Non-utility, Alliant Energy 2018 Electric Gas Other Total Parent and Other Consolidated Revenues $3,000 $447 $48 $3,495 $39 $3,534 Depreciation and amortization 457 42 3 502 5 507 Operating income 610 53 1 664 30 694 Interest expense 217 30 247 Equity income from unconsolidated investments, net (1) — — (1) (54) (55) Income taxes 33 15 48 Net income attributable to Alliant Energy common shareowners 472 40 512 Total assets 12,486 1,184 893 14,563 863 15,426 Investments in equity method subsidiaries 8 — — 8 413 421 Construction and acquisition expenditures 1,422 147 — 1,569 65 1,634 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): 2020 Electric Gas Other Total Revenues $1,695 $208 $44 $1,947 Depreciation and amortization 321 30 5 356 Operating income 358 50 2 410 Interest expense 139 Income tax benefit (47) Net income available for common stock 324 Total assets 8,518 766 565 9,849 Construction and acquisition expenditures 626 59 2 687 2019 Electric Gas Other Total Revenues $1,781 $264 $44 $2,089 Depreciation and amortization 295 29 3 327 Operating income 360 39 4 403 Interest expense 127 Income taxes 24 Net income available for common stock 284 Total assets 8,075 734 468 9,277 Construction and acquisition expenditures 964 56 — 1,020 2018 Electric Gas Other Total Revenues $1,731 $266 $45 $2,042 Depreciation and amortization 255 25 3 283 Operating income 318 28 5 351 Interest expense 119 Income tax benefit (3) Net income available for common stock 264 Total assets 7,220 687 504 8,411 Construction and acquisition expenditures 891 100 — 991 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): 2020 Electric Gas Other Total Revenues $1,225 $165 $5 $1,395 Depreciation and amortization 235 19 — 254 Operating income (loss) 285 24 (3) 306 Interest expense 104 Income tax benefit (19) Net income 249 Total assets 5,840 647 425 6,912 Construction and acquisition expenditures 483 123 — 606 2019 Electric Gas Other Total Revenues $1,283 $191 $2 $1,476 Depreciation and amortization 218 18 — 236 Operating income (loss) 319 31 (3) 347 Interest expense 102 Income taxes 49 Net income 233 Total assets 5,584 535 388 6,507 Construction and acquisition expenditures 475 44 — 519 2018 Electric Gas Other Total Revenues $1,269 $181 $3 $1,453 Depreciation and amortization 202 17 — 219 Operating income (loss) 292 25 (4) 313 Interest expense 98 Income taxes 36 Net income 208 Total assets 5,266 497 389 6,152 Construction and acquisition expenditures 531 47 — 578
IPL [Member]
Segment Reporting Information [Line Items]
Segments Of Business	SEGMENTS OF BUSINESS Alliant Energy - Alliant Energy’s principal businesses as of December 31, 2020 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, Travero, 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 2020 Electric Gas Other Total Parent and Other Consolidated Revenues $2,920 $373 $49 $3,342 $74 $3,416 Depreciation and amortization 556 49 5 610 5 615 Operating income (loss) 643 74 (1) 716 24 740 Interest expense 243 32 275 Equity income from unconsolidated investments, net (2) — — (2) (59) (61) Income tax expense (benefit) (66) 9 (57) Net income attributable to Alliant Energy common shareowners 573 41 614 Total assets 14,358 1,413 990 16,761 949 17,710 Investments in equity method subsidiaries 11 — — 11 465 476 Construction and acquisition expenditures 1,109 182 2 1,293 73 1,366 ATC Holdings, Utility Non-utility, Alliant Energy 2019 Electric Gas Other Total Parent and Other Consolidated Revenues $3,064 $455 $46 $3,565 $83 $3,648 Depreciation and amortization 513 47 3 563 4 567 Operating income 679 70 1 750 28 778 Interest expense 229 44 273 Equity income from unconsolidated investments, net (1) — — (1) (52) (53) Income tax expense (benefit) 73 (4) 69 Net income attributable to Alliant Energy common shareowners 517 40 557 Total assets 13,659 1,269 856 15,784 917 16,701 Investments in equity method subsidiaries 9 — — 9 449 458 Construction and acquisition expenditures 1,439 100 — 1,539 101 1,640 ATC Holdings, Utility Non-utility, Alliant Energy 2018 Electric Gas Other Total Parent and Other Consolidated Revenues $3,000 $447 $48 $3,495 $39 $3,534 Depreciation and amortization 457 42 3 502 5 507 Operating income 610 53 1 664 30 694 Interest expense 217 30 247 Equity income from unconsolidated investments, net (1) — — (1) (54) (55) Income taxes 33 15 48 Net income attributable to Alliant Energy common shareowners 472 40 512 Total assets 12,486 1,184 893 14,563 863 15,426 Investments in equity method subsidiaries 8 — — 8 413 421 Construction and acquisition expenditures 1,422 147 — 1,569 65 1,634 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): 2020 Electric Gas Other Total Revenues $1,695 $208 $44 $1,947 Depreciation and amortization 321 30 5 356 Operating income 358 50 2 410 Interest expense 139 Income tax benefit (47) Net income available for common stock 324 Total assets 8,518 766 565 9,849 Construction and acquisition expenditures 626 59 2 687 2019 Electric Gas Other Total Revenues $1,781 $264 $44 $2,089 Depreciation and amortization 295 29 3 327 Operating income 360 39 4 403 Interest expense 127 Income taxes 24 Net income available for common stock 284 Total assets 8,075 734 468 9,277 Construction and acquisition expenditures 964 56 — 1,020 2018 Electric Gas Other Total Revenues $1,731 $266 $45 $2,042 Depreciation and amortization 255 25 3 283 Operating income 318 28 5 351 Interest expense 119 Income tax benefit (3) Net income available for common stock 264 Total assets 7,220 687 504 8,411 Construction and acquisition expenditures 891 100 — 991 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): 2020 Electric Gas Other Total Revenues $1,225 $165 $5 $1,395 Depreciation and amortization 235 19 — 254 Operating income (loss) 285 24 (3) 306 Interest expense 104 Income tax benefit (19) Net income 249 Total assets 5,840 647 425 6,912 Construction and acquisition expenditures 483 123 — 606 2019 Electric Gas Other Total Revenues $1,283 $191 $2 $1,476 Depreciation and amortization 218 18 — 236 Operating income (loss) 319 31 (3) 347 Interest expense 102 Income taxes 49 Net income 233 Total assets 5,584 535 388 6,507 Construction and acquisition expenditures 475 44 — 519 2018 Electric Gas Other Total Revenues $1,269 $181 $3 $1,453 Depreciation and amortization 202 17 — 219 Operating income (loss) 292 25 (4) 313 Interest expense 98 Income taxes 36 Net income 208 Total assets 5,266 497 389 6,152 Construction and acquisition expenditures 531 47 — 578
WPL [Member]
Segment Reporting Information [Line Items]
Segments Of Business	SEGMENTS OF BUSINESS Alliant Energy - Alliant Energy’s principal businesses as of December 31, 2020 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, Travero, 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 2020 Electric Gas Other Total Parent and Other Consolidated Revenues $2,920 $373 $49 $3,342 $74 $3,416 Depreciation and amortization 556 49 5 610 5 615 Operating income (loss) 643 74 (1) 716 24 740 Interest expense 243 32 275 Equity income from unconsolidated investments, net (2) — — (2) (59) (61) Income tax expense (benefit) (66) 9 (57) Net income attributable to Alliant Energy common shareowners 573 41 614 Total assets 14,358 1,413 990 16,761 949 17,710 Investments in equity method subsidiaries 11 — — 11 465 476 Construction and acquisition expenditures 1,109 182 2 1,293 73 1,366 ATC Holdings, Utility Non-utility, Alliant Energy 2019 Electric Gas Other Total Parent and Other Consolidated Revenues $3,064 $455 $46 $3,565 $83 $3,648 Depreciation and amortization 513 47 3 563 4 567 Operating income 679 70 1 750 28 778 Interest expense 229 44 273 Equity income from unconsolidated investments, net (1) — — (1) (52) (53) Income tax expense (benefit) 73 (4) 69 Net income attributable to Alliant Energy common shareowners 517 40 557 Total assets 13,659 1,269 856 15,784 917 16,701 Investments in equity method subsidiaries 9 — — 9 449 458 Construction and acquisition expenditures 1,439 100 — 1,539 101 1,640 ATC Holdings, Utility Non-utility, Alliant Energy 2018 Electric Gas Other Total Parent and Other Consolidated Revenues $3,000 $447 $48 $3,495 $39 $3,534 Depreciation and amortization 457 42 3 502 5 507 Operating income 610 53 1 664 30 694 Interest expense 217 30 247 Equity income from unconsolidated investments, net (1) — — (1) (54) (55) Income taxes 33 15 48 Net income attributable to Alliant Energy common shareowners 472 40 512 Total assets 12,486 1,184 893 14,563 863 15,426 Investments in equity method subsidiaries 8 — — 8 413 421 Construction and acquisition expenditures 1,422 147 — 1,569 65 1,634 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): 2020 Electric Gas Other Total Revenues $1,695 $208 $44 $1,947 Depreciation and amortization 321 30 5 356 Operating income 358 50 2 410 Interest expense 139 Income tax benefit (47) Net income available for common stock 324 Total assets 8,518 766 565 9,849 Construction and acquisition expenditures 626 59 2 687 2019 Electric Gas Other Total Revenues $1,781 $264 $44 $2,089 Depreciation and amortization 295 29 3 327 Operating income 360 39 4 403 Interest expense 127 Income taxes 24 Net income available for common stock 284 Total assets 8,075 734 468 9,277 Construction and acquisition expenditures 964 56 — 1,020 2018 Electric Gas Other Total Revenues $1,731 $266 $45 $2,042 Depreciation and amortization 255 25 3 283 Operating income 318 28 5 351 Interest expense 119 Income tax benefit (3) Net income available for common stock 264 Total assets 7,220 687 504 8,411 Construction and acquisition expenditures 891 100 — 991 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): 2020 Electric Gas Other Total Revenues $1,225 $165 $5 $1,395 Depreciation and amortization 235 19 — 254 Operating income (loss) 285 24 (3) 306 Interest expense 104 Income tax benefit (19) Net income 249 Total assets 5,840 647 425 6,912 Construction and acquisition expenditures 483 123 — 606 2019 Electric Gas Other Total Revenues $1,283 $191 $2 $1,476 Depreciation and amortization 218 18 — 236 Operating income (loss) 319 31 (3) 347 Interest expense 102 Income taxes 49 Net income 233 Total assets 5,584 535 388 6,507 Construction and acquisition expenditures 475 44 — 519 2018 Electric Gas Other Total Revenues $1,269 $181 $3 $1,453 Depreciation and amortization 202 17 — 219 Operating income (loss) 292 25 (4) 313 Interest expense 98 Income taxes 36 Net income 208 Total assets 5,266 497 389 6,152 Construction and acquisition expenditures 531 47 — 578

Related Parties

Related Parties	12 Months Ended
	Dec. 31, 2020
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 2020 2019 2018 2020 2019 2018 Corporate Services billings $176 $185 $170 $142 $142 $132 Sales credited 35 68 48 3 7 28 Purchases billed 329 331 358 108 120 81 As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2020 2019 IPL $110 $112 WPL 73 85 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): 2020 2019 2018 ATC billings to WPL $108 $109 $106 WPL billings to ATC 10 13 11 As of December 31, 2020 and 2019, WPL owed ATC net amounts of $9 million and $9 million, respectively. In 2020, WPL received $46 million from ATC related to construction deposits WPL previously provided ATC for transmission network upgrades for West Riverside, which is substantially recorded in “Other” in Alliant Energy’s and WPL’s cash flows from investing activities. 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 2020 2019 2018 2020 2019 2018 Corporate Services billings $176 $185 $170 $142 $142 $132 Sales credited 35 68 48 3 7 28 Purchases billed 329 331 358 108 120 81 As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2020 2019 IPL $110 $112 WPL 73 85
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 2020 2019 2018 2020 2019 2018 Corporate Services billings $176 $185 $170 $142 $142 $132 Sales credited 35 68 48 3 7 28 Purchases billed 329 331 358 108 120 81 As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2020 2019 IPL $110 $112 WPL 73 85 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): 2020 2019 2018 ATC billings to WPL $108 $109 $106 WPL billings to ATC 10 13 11 As of December 31, 2020 and 2019, WPL owed ATC net amounts of $9 million and $9 million, respectively. In 2020, WPL received $46 million from ATC related to construction deposits WPL previously provided ATC for transmission network upgrades for West Riverside, which is substantially recorded in “Other” in Alliant Energy’s and WPL’s cash flows from investing activities. WPL’s Sheboygan Falls Energy Facility Lease - Refer to Note 10 for discussion of WPL’s Sheboygan Falls Energy Facility lease.

Condensed Parent Company Financ

Condensed Parent Company Financial Statements	12 Months Ended
	Dec. 31, 2020
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 2020 2019 2018 (in millions) Operating expenses $7 $2 $5 Operating loss (7) (2) (5) Other (income) and deductions: Equity earnings from consolidated subsidiaries (625) (562) (523) Interest expense 2 9 4 Other 4 7 2 Total other (income) and deductions (619) (546) (517) Income before income taxes 612 544 512 Income tax benefit (4) (15) (1) Net income $616 $559 $513 Refer to accompanying Notes to Condensed Financial Statements. ALLIANT ENERGY CORPORATION (Parent Company Only) December 31, CONDENSED BALANCE SHEETS 2020 2019 (in millions) ASSETS Current assets: Notes receivable from affiliated companies $32 $27 Other 5 6 Total current assets 37 33 Investments: Investments in consolidated subsidiaries 6,664 6,017 Other 2 1 Total investments 6,666 6,018 Other assets 88 89 Total assets $6,791 $6,140 LIABILITIES AND EQUITY Current liabilities: Commercial paper $132 $169 Notes payable to affiliated companies 937 731 Other 29 19 Total current liabilities 1,098 919 Other liabilities 2 15 Common equity: Common stock and additional paid-in capital 2,706 2,448 Retained earnings 2,996 2,766 Accumulated other comprehensive income — 2 Shares in deferred compensation trust (11) (10) Total common equity 5,691 5,206 Total liabilities and equity $6,791 $6,140 Refer to accompanying Notes to Condensed Financial Statements. ALLIANT ENERGY CORPORATION (Parent Company Only) Year Ended December 31, CONDENSED STATEMENTS OF CASH FLOWS 2020 2019 2018 (in millions) Net cash flows from operating activities $396 $305 $311 Cash flows used for investing activities: Capital contributions to consolidated subsidiaries (429) (250) (625) Net change in notes receivable from and payable to affiliates 201 8 441 Net cash flows used for investing activities (228) (242) (184) Cash flows used for financing activities: Common stock dividends (377) (338) (312) Proceeds from issuance of common stock, net 247 390 197 Net change in commercial paper (37) (116) (10) Other (1) 1 (2) Net cash flows used for financing activities (168) (63) (127) 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 ($2) ($9) ($4) Income taxes, net 10 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 2020 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, 2020
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, 2020 $7 $25 $9 $23 $18 Year ended December 31, 2019 10 17 2 22 7 Year ended December 31, 2018 12 21 1 24 10 IPL (c) Year ended December 31, 2020 $1 $18 $— $18 $1 Year ended December 31, 2019 3 16 — 18 1 Year ended December 31, 2018 1 21 — 19 3 WPL Year ended December 31, 2020 $6 $7 $9 $5 $17 Year ended December 31, 2019 7 1 2 4 6 Year ended December 31, 2018 11 — 1 5 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. (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, 2020 $7 $25 $9 $23 $18 Year ended December 31, 2019 10 17 2 22 7 Year ended December 31, 2018 12 21 1 24 10 IPL (c) Year ended December 31, 2020 $1 $18 $— $18 $1 Year ended December 31, 2019 3 16 — 18 1 Year ended December 31, 2018 1 21 — 19 3 WPL Year ended December 31, 2020 $6 $7 $9 $5 $17 Year ended December 31, 2019 7 1 2 4 6 Year ended December 31, 2018 11 — 1 5 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. (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, 2020 $7 $25 $9 $23 $18 Year ended December 31, 2019 10 17 2 22 7 Year ended December 31, 2018 12 21 1 24 10 IPL (c) Year ended December 31, 2020 $1 $18 $— $18 $1 Year ended December 31, 2019 3 16 — 18 1 Year ended December 31, 2018 1 21 — 19 3 WPL Year ended December 31, 2020 $6 $7 $9 $5 $17 Year ended December 31, 2019 7 1 2 4 6 Year ended December 31, 2018 11 — 1 5 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. (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, 2020
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.All intercompany balances and transactions, other than certain transactions affecting the rate-making process at IPL and WPL, have been eliminated from the financial statements. Such transactions not eliminated include costs that are recoverable from customers through rate-making processes.
General, Basis of Accounting	The financial statements are prepared in conformity with GAAP, which give recognition to the rate-making practices of FERC and state commissions having regulatory jurisdiction.
General, Reclassification	Certain prior period amounts in the Financial Statements and Notes have been reclassified to conform to the current period presentation for comparative purposes.
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 allowed unutilized alternative minimum tax credits to be refunded over four tax years beginning with the U.S. federal tax return for calendar year 2018. Pursuant to the Coronavirus Aid, Relief, and Economic Security Act, Alliant Energy received the remaining alternative minimum tax credits refunds in 2020. 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 2020, 2019 and 2018.
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, 2020, Alliant Energy’s and IPL’s cash and cash equivalents included $44 million and $44 million of money market fund investments, with interest rates of 0.04% and 0.04%, respectively. At December 31, 2020 and 2019, 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. Non-utility and Other Property - General - Non-utility property is recorded at the original cost of acquisition or construction, which includes material, labor and contractor services. Repairs, replacements and renewals of items of property determined to be less than a unit of property or that do not increase the property’s life or functionality are charged to maintenance expense. Upon retirement or sale of non-utility property, the original cost and related accumulated depreciation are removed from the accounts and any gain or loss is included in the income statements. Costs related to software developed or obtained for internal use are capitalized and amortized on a straight-line basis over the estimated useful life of the related software. If software is retired prior to being fully amortized, the remaining book value is recorded as a loss in the income statements.
Depreciation	IPL and WPL use a combination of remaining life and straight-line depreciation methods as approved by their respective regulatory commissions. The composite or group method of depreciation is used, in which a single depreciation rate is applied to the gross investment in a particular class of property. This method pools similar assets and then depreciates each group as a whole. Periodic depreciation studies are performed to determine the appropriate group lives, net salvage, estimated cost of removal and group depreciation rates. These depreciation studies are subject to review and approval by IPL’s and WPL’s respective regulatory commissions. Depreciation expense is included within the recoverable cost of service component of rates collected from customers. The average rates of depreciation for electric, gas and other properties, consistent with current rate-making practices, were as follows: IPL WPL 2020 2019 2018 2020 2019 2018 Electric - generation 3.5% 3.8% 3.6% 3.5% 3.6% 3.6% Electric - distribution 2.8% 2.9% 2.8% 2.6% 2.6% 2.6% Electric - other 5.2% 5.3% 4.7% 6.1% 5.8% 5.7% Gas 3.3% 3.3% 3.2% 2.4% 2.5% 2.5% Other 6.3% 5.9% 5.2% 5.9% 5.6% 5.8%
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: 2020 2019 2018 IPL (Wind generation CWIP) 7.1% 7.4% 7.5% IPL (other CWIP) 7.2% 7.5% 7.5% WPL (retail jurisdiction) 7.0% 6.8% 7.7% WPL (wholesale jurisdiction) 6.3% 6.9% 7.2% 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 then 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 component 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, 2020, 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 Travero 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 periodic 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 energy efficiency and demand response cost recovery factor tariffs, which are revised annually and include 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 recovers a return of, as well as earn a return on, its 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, are also 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 are 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 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.
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 qualified for and were designated under the normal purchase and sale exception, and were 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 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 sheets 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.
Current Expected Credit Losses Estimates	Current expected credit losses are estimated for trade and other receivables and credit exposures on guarantees of the performance by third parties. The current expected credit losses for short-term trade receivables are based on estimates of losses resulting from the inability of customers to make required payments. The methodology used to estimate losses is based on historical write-offs, regional economic conditions, significant events that could impact collectability, such as impacts related to COVID-19, the derecho windstorm and related regulatory actions, and forecasted changes to the accounts receivable aging portfolio and write-offs. The current expected credit losses related to guarantees of the performance by third parties are estimated using both quantitative and qualitative information, which utilizes potential outcomes in a range of possible estimated amounts.
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 2016, the Financial Accounting Standards Board 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, certain other assets and off-balance sheet credit exposures. Alliant Energy, IPL and WPL adopted this standard on January 1, 2020 using a modified retrospective method of adoption, which required cumulative effect adjustments to retained earnings on January 1, 2020. IPL and WPL did not record a cumulative effect adjustment to retained earnings and Alliant Energy recorded a pre-tax $12 million (after-tax $9 million) cumulative effect adjustment to decrease retained earnings related to Alliant Energy’s guarantees in the partnership obligations of an affiliate of Whiting Petroleum (refer to Note 17( d) for further discussion). This adjustment is included in “Adoption of new accounting standard” in Alliant Energy’s equity statement for 2020.
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.All intercompany balances and transactions, other than certain transactions affecting the rate-making process at IPL and WPL, have been eliminated from the financial statements. Such transactions not eliminated include costs that are recoverable from customers through rate-making processes.
General, Basis of Accounting	The financial statements are prepared in conformity with GAAP, which give recognition to the rate-making practices of FERC and state commissions having regulatory jurisdiction.
General, Reclassification	Certain prior period amounts in the Financial Statements and Notes have been reclassified to conform to the current period presentation for comparative purposes.
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 allowed unutilized alternative minimum tax credits to be refunded over four tax years beginning with the U.S. federal tax return for calendar year 2018. Pursuant to the Coronavirus Aid, Relief, and Economic Security Act, Alliant Energy received the remaining alternative minimum tax credits refunds in 2020. 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 2020, 2019 and 2018.
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, 2020, Alliant Energy’s and IPL’s cash and cash equivalents included $44 million and $44 million of money market fund investments, with interest rates of 0.04% and 0.04%, respectively. At December 31, 2020 and 2019, 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. Non-utility and Other Property - General - Non-utility property is recorded at the original cost of acquisition or construction, which includes material, labor and contractor services. Repairs, replacements and renewals of items of property determined to be less than a unit of property or that do not increase the property’s life or functionality are charged to maintenance expense. Upon retirement or sale of non-utility property, the original cost and related accumulated depreciation are removed from the accounts and any gain or loss is included in the income statements. Costs related to software developed or obtained for internal use are capitalized and amortized on a straight-line basis over the estimated useful life of the related software. If software is retired prior to being fully amortized, the remaining book value is recorded as a loss in the income statements.
Depreciation	IPL and WPL use a combination of remaining life and straight-line depreciation methods as approved by their respective regulatory commissions. The composite or group method of depreciation is used, in which a single depreciation rate is applied to the gross investment in a particular class of property. This method pools similar assets and then depreciates each group as a whole. Periodic depreciation studies are performed to determine the appropriate group lives, net salvage, estimated cost of removal and group depreciation rates. These depreciation studies are subject to review and approval by IPL’s and WPL’s respective regulatory commissions. Depreciation expense is included within the recoverable cost of service component of rates collected from customers. The average rates of depreciation for electric, gas and other properties, consistent with current rate-making practices, were as follows: IPL WPL 2020 2019 2018 2020 2019 2018 Electric - generation 3.5% 3.8% 3.6% 3.5% 3.6% 3.6% Electric - distribution 2.8% 2.9% 2.8% 2.6% 2.6% 2.6% Electric - other 5.2% 5.3% 4.7% 6.1% 5.8% 5.7% Gas 3.3% 3.3% 3.2% 2.4% 2.5% 2.5% Other 6.3% 5.9% 5.2% 5.9% 5.6% 5.8%
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: 2020 2019 2018 IPL (Wind generation CWIP) 7.1% 7.4% 7.5% IPL (other CWIP) 7.2% 7.5% 7.5% WPL (retail jurisdiction) 7.0% 6.8% 7.7% WPL (wholesale jurisdiction) 6.3% 6.9% 7.2% 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 then 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 component 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, 2020, 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 Travero 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 periodic 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 energy efficiency and demand response cost recovery factor tariffs, which are revised annually and include 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 recovers a return of, as well as earn a return on, its 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, are also 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 are 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 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.
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 qualified for and were designated under the normal purchase and sale exception, and were 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 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 sheets 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.
Current Expected Credit Losses Estimates	Current expected credit losses are estimated for trade and other receivables and credit exposures on guarantees of the performance by third parties. The current expected credit losses for short-term trade receivables are based on estimates of losses resulting from the inability of customers to make required payments. The methodology used to estimate losses is based on historical write-offs, regional economic conditions, significant events that could impact collectability, such as impacts related to COVID-19, the derecho windstorm and related regulatory actions, and forecasted changes to the accounts receivable aging portfolio and write-offs. The current expected credit losses related to guarantees of the performance by third parties are estimated using both quantitative and qualitative information, which utilizes potential outcomes in a range of possible estimated amounts.
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 2016, the Financial Accounting Standards Board 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, certain other assets and off-balance sheet credit exposures. Alliant Energy, IPL and WPL adopted this standard on January 1, 2020 using a modified retrospective method of adoption, which required cumulative effect adjustments to retained earnings on January 1, 2020. IPL and WPL did not record a cumulative effect adjustment to retained earnings and Alliant Energy recorded a pre-tax $12 million (after-tax $9 million) cumulative effect adjustment to decrease retained earnings related to Alliant Energy’s guarantees in the partnership obligations of an affiliate of Whiting Petroleum (refer to Note 17( d) for further discussion). This adjustment is included in “Adoption of new accounting standard” in Alliant Energy’s equity statement for 2020.
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.All intercompany balances and transactions, other than certain transactions affecting the rate-making process at IPL and WPL, have been eliminated from the financial statements. Such transactions not eliminated include costs that are recoverable from customers through rate-making processes.
General, Basis of Accounting	The financial statements are prepared in conformity with GAAP, which give recognition to the rate-making practices of FERC and state commissions having regulatory jurisdiction.
General, Reclassification	Certain prior period amounts in the Financial Statements and Notes have been reclassified to conform to the current period presentation for comparative purposes.
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 allowed unutilized alternative minimum tax credits to be refunded over four tax years beginning with the U.S. federal tax return for calendar year 2018. Pursuant to the Coronavirus Aid, Relief, and Economic Security Act, Alliant Energy received the remaining alternative minimum tax credits refunds in 2020. 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 2020, 2019 and 2018.
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, 2020, Alliant Energy’s and IPL’s cash and cash equivalents included $44 million and $44 million of money market fund investments, with interest rates of 0.04% and 0.04%, respectively. At December 31, 2020 and 2019, 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. Non-utility and Other Property - General - Non-utility property is recorded at the original cost of acquisition or construction, which includes material, labor and contractor services. Repairs, replacements and renewals of items of property determined to be less than a unit of property or that do not increase the property’s life or functionality are charged to maintenance expense. Upon retirement or sale of non-utility property, the original cost and related accumulated depreciation are removed from the accounts and any gain or loss is included in the income statements. Costs related to software developed or obtained for internal use are capitalized and amortized on a straight-line basis over the estimated useful life of the related software. If software is retired prior to being fully amortized, the remaining book value is recorded as a loss in the income statements.
Depreciation	IPL and WPL use a combination of remaining life and straight-line depreciation methods as approved by their respective regulatory commissions. The composite or group method of depreciation is used, in which a single depreciation rate is applied to the gross investment in a particular class of property. This method pools similar assets and then depreciates each group as a whole. Periodic depreciation studies are performed to determine the appropriate group lives, net salvage, estimated cost of removal and group depreciation rates. These depreciation studies are subject to review and approval by IPL’s and WPL’s respective regulatory commissions. Depreciation expense is included within the recoverable cost of service component of rates collected from customers. The average rates of depreciation for electric, gas and other properties, consistent with current rate-making practices, were as follows: IPL WPL 2020 2019 2018 2020 2019 2018 Electric - generation 3.5% 3.8% 3.6% 3.5% 3.6% 3.6% Electric - distribution 2.8% 2.9% 2.8% 2.6% 2.6% 2.6% Electric - other 5.2% 5.3% 4.7% 6.1% 5.8% 5.7% Gas 3.3% 3.3% 3.2% 2.4% 2.5% 2.5% Other 6.3% 5.9% 5.2% 5.9% 5.6% 5.8%
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: 2020 2019 2018 IPL (Wind generation CWIP) 7.1% 7.4% 7.5% IPL (other CWIP) 7.2% 7.5% 7.5% WPL (retail jurisdiction) 7.0% 6.8% 7.7% WPL (wholesale jurisdiction) 6.3% 6.9% 7.2% 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 then 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 component 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, 2020, 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 Travero 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 periodic 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 energy efficiency and demand response cost recovery factor tariffs, which are revised annually and include 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 recovers a return of, as well as earn a return on, its 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, are also 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 are 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 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.
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 qualified for and were designated under the normal purchase and sale exception, and were 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 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 sheets 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.
Current Expected Credit Losses Estimates	Current expected credit losses are estimated for trade and other receivables and credit exposures on guarantees of the performance by third parties. The current expected credit losses for short-term trade receivables are based on estimates of losses resulting from the inability of customers to make required payments. The methodology used to estimate losses is based on historical write-offs, regional economic conditions, significant events that could impact collectability, such as impacts related to COVID-19, the derecho windstorm and related regulatory actions, and forecasted changes to the accounts receivable aging portfolio and write-offs. The current expected credit losses related to guarantees of the performance by third parties are estimated using both quantitative and qualitative information, which utilizes potential outcomes in a range of possible estimated amounts.
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 2016, the Financial Accounting Standards Board 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, certain other assets and off-balance sheet credit exposures. Alliant Energy, IPL and WPL adopted this standard on January 1, 2020 using a modified retrospective method of adoption, which required cumulative effect adjustments to retained earnings on January 1, 2020. IPL and WPL did not record a cumulative effect adjustment to retained earnings and Alliant Energy recorded a pre-tax $12 million (after-tax $9 million) cumulative effect adjustment to decrease retained earnings related to Alliant Energy’s guarantees in the partnership obligations of an affiliate of Whiting Petroleum (refer to Note 17( d) for further discussion). This adjustment is included in “Adoption of new accounting standard” in Alliant Energy’s equity statement for 2020.

Summary Of Significant Accoun_3

Summary Of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
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 2020 2019 2018 2020 2019 2018 Electric - generation 3.5% 3.8% 3.6% 3.5% 3.6% 3.6% Electric - distribution 2.8% 2.9% 2.8% 2.6% 2.6% 2.6% Electric - other 5.2% 5.3% 4.7% 6.1% 5.8% 5.7% Gas 3.3% 3.3% 3.2% 2.4% 2.5% 2.5% Other 6.3% 5.9% 5.2% 5.9% 5.6% 5.8%
Schedule of Allowance for Funds Used During Construction Rate	The AFUDC rates, computed in accordance with the prescribed regulatory formula, were as follows: 2020 2019 2018 IPL (Wind generation CWIP) 7.1% 7.4% 7.5% IPL (other CWIP) 7.2% 7.5% 7.5% WPL (retail jurisdiction) 7.0% 6.8% 7.7% WPL (wholesale jurisdiction) 6.3% 6.9% 7.2%
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 2020 2019 2018 2020 2019 2018 Electric - generation 3.5% 3.8% 3.6% 3.5% 3.6% 3.6% Electric - distribution 2.8% 2.9% 2.8% 2.6% 2.6% 2.6% Electric - other 5.2% 5.3% 4.7% 6.1% 5.8% 5.7% Gas 3.3% 3.3% 3.2% 2.4% 2.5% 2.5% Other 6.3% 5.9% 5.2% 5.9% 5.6% 5.8%
Schedule of Allowance for Funds Used During Construction Rate	The AFUDC rates, computed in accordance with the prescribed regulatory formula, were as follows: 2020 2019 2018 IPL (Wind generation CWIP) 7.1% 7.4% 7.5% IPL (other CWIP) 7.2% 7.5% 7.5% WPL (retail jurisdiction) 7.0% 6.8% 7.7% WPL (wholesale jurisdiction) 6.3% 6.9% 7.2%
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 2020 2019 2018 2020 2019 2018 Electric - generation 3.5% 3.8% 3.6% 3.5% 3.6% 3.6% Electric - distribution 2.8% 2.9% 2.8% 2.6% 2.6% 2.6% Electric - other 5.2% 5.3% 4.7% 6.1% 5.8% 5.7% Gas 3.3% 3.3% 3.2% 2.4% 2.5% 2.5% Other 6.3% 5.9% 5.2% 5.9% 5.6% 5.8%
Schedule of Allowance for Funds Used During Construction Rate	The AFUDC rates, computed in accordance with the prescribed regulatory formula, were as follows: 2020 2019 2018 IPL (Wind generation CWIP) 7.1% 7.4% 7.5% IPL (other CWIP) 7.2% 7.5% 7.5% WPL (retail jurisdiction) 7.0% 6.8% 7.7% WPL (wholesale jurisdiction) 6.3% 6.9% 7.2%

Regulatory Matters (Tables)

Regulatory Matters (Tables)	12 Months Ended
	Dec. 31, 2020
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 2020 2019 2020 2019 2020 2019 Tax-related $890 $818 $843 $777 $47 $41 Pension and OPEB costs 580 524 291 263 289 261 AROs 119 112 81 76 38 36 Assets retired early 113 134 77 88 36 46 IPL’s DAEC PPA amendment 110 108 110 108 — — WPL’s Western Wisconsin gas distribution expansion investments 55 — — — 55 — Derivatives 26 39 13 18 13 21 Other 117 110 68 70 49 40 $2,010 $1,845 $1,483 $1,400 $527 $445
Assets 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 (dollars in millions): Entity Asset Retirement Date Regulatory Asset Balance as of Dec. 31, 2020 Recovery Regulatory Approval IPL Sutherland Units 1 and 3 2017 $23 Return of and return on remaining net book value through 2027 IUB and FERC IPL M.L. Kapp Unit 2 2018 21 Return of and return on remaining net book value through 2029 IUB and FERC IPL Analog electric meters 2019 33 Return of remaining net book value through 2028 IUB and FERC WPL Nelson Dewey Units 1 and 2 and Edgewater Unit 3 2015 13 Return of and return on remaining net book value through 2022 PSCW and FERC WPL Edgewater Unit 4 2018 23 Return of and return on remaining net book value through 2028 PSCW and FERC
Schedule of Regulatory Liabilities	At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2020 2019 2020 2019 2020 2019 Tax-related $732 $836 $331 $351 $401 $485 Cost of removal obligations 367 388 238 257 129 131 Electric transmission cost recovery 68 88 39 51 29 37 WPL’s West Riverside liquidated damages 38 — — — 38 — Derivatives 28 20 25 17 3 3 Other 73 92 43 39 30 53 $1,306 $1,424 $676 $715 $630 $709
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 2020 2019 2020 2019 2020 2019 Tax-related $890 $818 $843 $777 $47 $41 Pension and OPEB costs 580 524 291 263 289 261 AROs 119 112 81 76 38 36 Assets retired early 113 134 77 88 36 46 IPL’s DAEC PPA amendment 110 108 110 108 — — WPL’s Western Wisconsin gas distribution expansion investments 55 — — — 55 — Derivatives 26 39 13 18 13 21 Other 117 110 68 70 49 40 $2,010 $1,845 $1,483 $1,400 $527 $445
Assets 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 (dollars in millions): Entity Asset Retirement Date Regulatory Asset Balance as of Dec. 31, 2020 Recovery Regulatory Approval IPL Sutherland Units 1 and 3 2017 $23 Return of and return on remaining net book value through 2027 IUB and FERC IPL M.L. Kapp Unit 2 2018 21 Return of and return on remaining net book value through 2029 IUB and FERC IPL Analog electric meters 2019 33 Return of remaining net book value through 2028 IUB and FERC WPL Nelson Dewey Units 1 and 2 and Edgewater Unit 3 2015 13 Return of and return on remaining net book value through 2022 PSCW and FERC WPL Edgewater Unit 4 2018 23 Return of and return on remaining net book value through 2028 PSCW and FERC
Schedule of Regulatory Liabilities	At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2020 2019 2020 2019 2020 2019 Tax-related $732 $836 $331 $351 $401 $485 Cost of removal obligations 367 388 238 257 129 131 Electric transmission cost recovery 68 88 39 51 29 37 WPL’s West Riverside liquidated damages 38 — — — 38 — Derivatives 28 20 25 17 3 3 Other 73 92 43 39 30 53 $1,306 $1,424 $676 $715 $630 $709
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 2020 2019 2020 2019 2020 2019 Tax-related $890 $818 $843 $777 $47 $41 Pension and OPEB costs 580 524 291 263 289 261 AROs 119 112 81 76 38 36 Assets retired early 113 134 77 88 36 46 IPL’s DAEC PPA amendment 110 108 110 108 — — WPL’s Western Wisconsin gas distribution expansion investments 55 — — — 55 — Derivatives 26 39 13 18 13 21 Other 117 110 68 70 49 40 $2,010 $1,845 $1,483 $1,400 $527 $445
Assets 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 (dollars in millions): Entity Asset Retirement Date Regulatory Asset Balance as of Dec. 31, 2020 Recovery Regulatory Approval IPL Sutherland Units 1 and 3 2017 $23 Return of and return on remaining net book value through 2027 IUB and FERC IPL M.L. Kapp Unit 2 2018 21 Return of and return on remaining net book value through 2029 IUB and FERC IPL Analog electric meters 2019 33 Return of remaining net book value through 2028 IUB and FERC WPL Nelson Dewey Units 1 and 2 and Edgewater Unit 3 2015 13 Return of and return on remaining net book value through 2022 PSCW and FERC WPL Edgewater Unit 4 2018 23 Return of and return on remaining net book value through 2028 PSCW and FERC
Schedule of Regulatory Liabilities	At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2020 2019 2020 2019 2020 2019 Tax-related $732 $836 $331 $351 $401 $485 Cost of removal obligations 367 388 238 257 129 131 Electric transmission cost recovery 68 88 39 51 29 37 WPL’s West Riverside liquidated damages 38 — — — 38 — Derivatives 28 20 25 17 3 3 Other 73 92 43 39 30 53 $1,306 $1,424 $676 $715 $630 $709

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	12 Months Ended
	Dec. 31, 2020
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 2020 2019 2020 2019 2020 2019 Utility: Electric plant: Generation in service $8,222 $7,625 $4,884 $4,432 $3,338 $3,193 Distribution in service 6,216 5,783 3,470 3,190 2,746 2,593 Other in service 536 457 358 299 178 158 Anticipated to be retired early (a) 1,269 — 459 — 810 — Total electric plant 16,243 13,865 9,171 7,921 7,072 5,944 Gas plant in service 1,563 1,463 844 802 719 661 Other plant in service 534 520 354 345 180 175 Accumulated depreciation (4,868) (4,602) (2,671) (2,499) (2,197) (2,103) Net plant 13,472 11,246 7,698 6,569 5,774 4,677 Leased Sheboygan Falls Energy Facility, net (b) — — — — 27 32 Construction work in progress 405 1,835 185 907 220 928 Other, net 7 6 6 5 1 1 Total utility 13,884 13,087 7,889 7,481 6,022 5,638 Non-utility and other: Non-utility Generation, net (c) 79 83 — — — — Corporate Services and other, net (d) 373 357 — — — — Total non-utility and other 452 440 — — — — Total property, plant and equipment $14,336 $13,527 $7,889 $7,481 $6,022 $5,638 (a) In 2020, IPL and WPL received approval from MISO to retire Lansing and Edgewater Unit 5, respectively, and currently anticipate retiring these EGUs by the end of 2022. Alliant Energy and IPL concluded that Lansing, and Alliant Energy and WPL concluded that Edgewater Unit 5, met the criteria to be considered probable of abandonment as of December 31, 2020. IPL and WPL are currently allowed a full recovery of and a full return on its respective EGU from both its retail and wholesale customers. (b) Less accumulated amortization of $95 million and $89 million for WPL as of December 31, 2020 and 2019, respectively. The leased Sheboygan Falls Energy Facility is eliminated from WPL upon consolidation and is included in the “Non-utility Generation, net” line within Alliant Energy’s consolidated property, plant and equipment. (c) Less accumulated depreciation of $63 million and $59 million for Alliant Energy as of December 31, 2020 and 2019, respectively. (d) Less accumulated depreciation of $210 million and $172 million for Alliant Energy as of December 31, 2020 and 2019, 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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Equity $39 $66 $51 $17 $35 $29 $22 $31 $22 Debt 16 27 25 7 15 13 9 12 12 $55 $93 $76 $24 $50 $42 $31 $43 $34
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 2020 2019 2020 2019 2020 2019 Utility: Electric plant: Generation in service $8,222 $7,625 $4,884 $4,432 $3,338 $3,193 Distribution in service 6,216 5,783 3,470 3,190 2,746 2,593 Other in service 536 457 358 299 178 158 Anticipated to be retired early (a) 1,269 — 459 — 810 — Total electric plant 16,243 13,865 9,171 7,921 7,072 5,944 Gas plant in service 1,563 1,463 844 802 719 661 Other plant in service 534 520 354 345 180 175 Accumulated depreciation (4,868) (4,602) (2,671) (2,499) (2,197) (2,103) Net plant 13,472 11,246 7,698 6,569 5,774 4,677 Leased Sheboygan Falls Energy Facility, net (b) — — — — 27 32 Construction work in progress 405 1,835 185 907 220 928 Other, net 7 6 6 5 1 1 Total utility 13,884 13,087 7,889 7,481 6,022 5,638 Non-utility and other: Non-utility Generation, net (c) 79 83 — — — — Corporate Services and other, net (d) 373 357 — — — — Total non-utility and other 452 440 — — — — Total property, plant and equipment $14,336 $13,527 $7,889 $7,481 $6,022 $5,638 (a) In 2020, IPL and WPL received approval from MISO to retire Lansing and Edgewater Unit 5, respectively, and currently anticipate retiring these EGUs by the end of 2022. Alliant Energy and IPL concluded that Lansing, and Alliant Energy and WPL concluded that Edgewater Unit 5, met the criteria to be considered probable of abandonment as of December 31, 2020. IPL and WPL are currently allowed a full recovery of and a full return on its respective EGU from both its retail and wholesale customers. (b) Less accumulated amortization of $95 million and $89 million for WPL as of December 31, 2020 and 2019, respectively. The leased Sheboygan Falls Energy Facility is eliminated from WPL upon consolidation and is included in the “Non-utility Generation, net” line within Alliant Energy’s consolidated property, plant and equipment. (c) Less accumulated depreciation of $63 million and $59 million for Alliant Energy as of December 31, 2020 and 2019, respectively. (d) Less accumulated depreciation of $210 million and $172 million for Alliant Energy as of December 31, 2020 and 2019, 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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Equity $39 $66 $51 $17 $35 $29 $22 $31 $22 Debt 16 27 25 7 15 13 9 12 12 $55 $93 $76 $24 $50 $42 $31 $43 $34
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 2020 2019 2020 2019 2020 2019 Utility: Electric plant: Generation in service $8,222 $7,625 $4,884 $4,432 $3,338 $3,193 Distribution in service 6,216 5,783 3,470 3,190 2,746 2,593 Other in service 536 457 358 299 178 158 Anticipated to be retired early (a) 1,269 — 459 — 810 — Total electric plant 16,243 13,865 9,171 7,921 7,072 5,944 Gas plant in service 1,563 1,463 844 802 719 661 Other plant in service 534 520 354 345 180 175 Accumulated depreciation (4,868) (4,602) (2,671) (2,499) (2,197) (2,103) Net plant 13,472 11,246 7,698 6,569 5,774 4,677 Leased Sheboygan Falls Energy Facility, net (b) — — — — 27 32 Construction work in progress 405 1,835 185 907 220 928 Other, net 7 6 6 5 1 1 Total utility 13,884 13,087 7,889 7,481 6,022 5,638 Non-utility and other: Non-utility Generation, net (c) 79 83 — — — — Corporate Services and other, net (d) 373 357 — — — — Total non-utility and other 452 440 — — — — Total property, plant and equipment $14,336 $13,527 $7,889 $7,481 $6,022 $5,638 (a) In 2020, IPL and WPL received approval from MISO to retire Lansing and Edgewater Unit 5, respectively, and currently anticipate retiring these EGUs by the end of 2022. Alliant Energy and IPL concluded that Lansing, and Alliant Energy and WPL concluded that Edgewater Unit 5, met the criteria to be considered probable of abandonment as of December 31, 2020. IPL and WPL are currently allowed a full recovery of and a full return on its respective EGU from both its retail and wholesale customers. (b) Less accumulated amortization of $95 million and $89 million for WPL as of December 31, 2020 and 2019, respectively. The leased Sheboygan Falls Energy Facility is eliminated from WPL upon consolidation and is included in the “Non-utility Generation, net” line within Alliant Energy’s consolidated property, plant and equipment. (c) Less accumulated depreciation of $63 million and $59 million for Alliant Energy as of December 31, 2020 and 2019, respectively. (d) Less accumulated depreciation of $210 million and $172 million for Alliant Energy as of December 31, 2020 and 2019, 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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Equity $39 $66 $51 $17 $35 $29 $22 $31 $22 Debt 16 27 25 7 15 13 9 12 12 $55 $93 $76 $24 $50 $42 $31 $43 $34

Jointly-Owned Electric Utilit_2

Jointly-Owned Electric Utility Plant (Tables)	12 Months Ended
	Dec. 31, 2020
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, 2020 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0% $580 $195 $22 George Neal Unit 4 25.7% 192 96 2 George Neal Unit 3 28.0% 171 67 — Louisa Unit 1 4.0% 39 22 — 982 380 24 WPL Columbia Units 1-2 53.5% 786 289 4 West Riverside 91.8% 686 12 13 Forward Wind Energy Center 42.6% 120 46 — 1,592 347 17 Alliant Energy $2,574 $727 $41
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, 2020 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0% $580 $195 $22 George Neal Unit 4 25.7% 192 96 2 George Neal Unit 3 28.0% 171 67 — Louisa Unit 1 4.0% 39 22 — 982 380 24 WPL Columbia Units 1-2 53.5% 786 289 4 West Riverside 91.8% 686 12 13 Forward Wind Energy Center 42.6% 120 46 — 1,592 347 17 Alliant Energy $2,574 $727 $41
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, 2020 was as follows (dollars in millions): Ownership Electric Accumulated Provision Construction Interest % Plant for Depreciation Work in Progress IPL Ottumwa Unit 1 48.0% $580 $195 $22 George Neal Unit 4 25.7% 192 96 2 George Neal Unit 3 28.0% 171 67 — Louisa Unit 1 4.0% 39 22 — 982 380 24 WPL Columbia Units 1-2 53.5% 786 289 4 West Riverside 91.8% 686 12 13 Forward Wind Energy Center 42.6% 120 46 — 1,592 347 17 Alliant Energy $2,574 $727 $41

Receivables (Tables)

Receivables (Tables)	12 Months Ended
	Dec. 31, 2020
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 2020 2019 2020 2019 2020 2019 Customer $101 $92 $— $— $92 $84 Unbilled utility revenues 82 82 — — 82 82 Deferred proceeds 188 188 188 188 — — Other 59 47 23 16 35 31 Allowance for expected credit losses (18) (7) (1) (1) (17) (6) $412 $402 $210 $203 $192 $191
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 2020 2019 2018 2020 2019 2018 Outstanding aggregate cash proceeds $96 $108 $116 $9 $36 $53
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): 2020 2019 Customer accounts receivable $114 $125 Unbilled utility revenues 92 95 Other receivables — 1 Receivables sold to third party 206 221 Less: cash proceeds 1 27 Deferred proceeds 205 194 Less: allowance for expected credit losses 17 6 Fair value of deferred proceeds $188 $188 Outstanding receivables past due $27 $26
Additional Attributes Of Receivables Sold Under The Agreement	Additional attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2020 2019 2018 Collections $2,025 $2,193 $2,077 Write-offs, net of recoveries 7 19 21
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 2020 2019 2020 2019 2020 2019 Customer $101 $92 $— $— $92 $84 Unbilled utility revenues 82 82 — — 82 82 Deferred proceeds 188 188 188 188 — — Other 59 47 23 16 35 31 Allowance for expected credit losses (18) (7) (1) (1) (17) (6) $412 $402 $210 $203 $192 $191
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 2020 2019 2018 2020 2019 2018 Outstanding aggregate cash proceeds $96 $108 $116 $9 $36 $53
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): 2020 2019 Customer accounts receivable $114 $125 Unbilled utility revenues 92 95 Other receivables — 1 Receivables sold to third party 206 221 Less: cash proceeds 1 27 Deferred proceeds 205 194 Less: allowance for expected credit losses 17 6 Fair value of deferred proceeds $188 $188 Outstanding receivables past due $27 $26
Additional Attributes Of Receivables Sold Under The Agreement	Additional attributes of IPL’s receivables sold under the Receivables Agreement were as follows (in millions): 2020 2019 2018 Collections $2,025 $2,193 $2,077 Write-offs, net of recoveries 7 19 21
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 2020 2019 2020 2019 2020 2019 Customer $101 $92 $— $— $92 $84 Unbilled utility revenues 82 82 — — 82 82 Deferred proceeds 188 188 188 188 — — Other 59 47 23 16 35 31 Allowance for expected credit losses (18) (7) (1) (1) (17) (6) $412 $402 $210 $203 $192 $191

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2020
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, 2020 2020 2019 2020 2019 2018 ATC Holdings 16%, 20% $331 $320 ($47) ($45) ($38) Non-utility wind farm in Oklahoma 50% 103 105 (8) (5) (16) Other Various 42 33 (6) (3) (1) $476 $458 ($61) ($53) ($55)
Summary Financial Information	Summary aggregate financial information from the financial statements of these holdings is as follows (in millions): Alliant Energy 2020 2019 2018 Revenues $801 $783 $724 Operating income 376 359 325 Net income 343 340 217 As of December 31: Current assets 121 139 Non-current assets 6,388 5,964 Current liabilities 317 507 Non-current liabilities 2,997 2,659 Noncontrolling interest 192 208

Common Equity (Tables)

Common Equity (Tables)	12 Months Ended
	Dec. 31, 2020
Schedule of Common Equity [Line Items]
Common Stock Activity	A summary of Alliant Energy’s common stock activity was as follows: 2020 2019 2018 Shares outstanding, January 1 245,022,800 236,063,279 231,348,646 Equity forward agreements 4,275,127 8,358,973 — At-the-market offering programs — — 4,171,013 Shareowner Direct Plan 472,283 501,808 576,965 Equity-based compensation plans 98,205 101,478 5,078 Other — (2,738) (38,423) Shares outstanding, December 31 249,868,415 245,022,800 236,063,279

Preferred Stock (Tables)

Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2020
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 2020 2019 (in millions) 5.1% $25 8,000,000 8,000,000 $200 $200
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 2020 2019 (in millions) 5.1% $25 8,000,000 8,000,000 $200 $200

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2020
Debt Instrument [Line Items]
Schedule of Other Short-term Borrowings	Information regarding Alliant Energy’s, IPL’s and WPL’s commercial paper, and Alliant Energy’s and WPL’s borrowings under the single credit facility, classified as short-term debt was as follows (dollars in millions): Alliant Energy IPL WPL December 31 2020 2019 2020 2019 2020 2019 Amount outstanding $389 $337 $— $— $257 $168 Weighted average interest rates 0.2% 1.9% N/A N/A 0.1% 1.8% Available credit facility capacity $611 $663 $250 $250 $43 $132 Alliant Energy IPL WPL For the year ended 2020 2019 2020 2019 2020 2019 Maximum amount outstanding (based on daily outstanding balances) $495 $601 $8 $50 $266 $195 Average amount outstanding (based on daily outstanding balances) $292 $453 $— $— $117 $93 Weighted average interest rates 0.7% 2.5% 0.5% 2.8% 0.7% 2.4%
Schedule of Long-term Debt	Long-term debt, net as of December 31 was as follows (dollars in millions): 2020 2019 Alliant Energy IPL WPL Alliant Energy IPL WPL Senior Debentures (a): 3.25%, due 2024 $500 $500 $— $500 $500 $— 3.4%, due 2025 250 250 — 250 250 — 5.5%, due 2025 50 50 — 50 50 — 4.1%, due 2028 500 500 — 500 500 — 3.6%, due 2029 300 300 — 300 300 — 2.3%, due 2030 (b) 400 400 — — — — 6.45%, due 2033 100 100 — 100 100 — 6.3%, due 2034 125 125 — 125 125 — 6.25%, due 2039 300 300 — 300 300 — 4.7%, due 2043 250 250 — 250 250 — 3.7%, due 2046 300 300 — 300 300 — 3.5%, due 2049 300 300 — 300 300 — 3.65% (Retired in 2020) — — — 200 200 — 3,375 3,375 — 3,175 3,175 — Debentures (a): 2.25%, due 2022 250 — 250 250 — 250 3.05%, due 2027 300 — 300 300 — 300 3%, due 2029 350 — 350 350 — 350 6.25%, due 2034 100 — 100 100 — 100 6.375%, due 2037 300 — 300 300 — 300 7.6%, due 2038 250 — 250 250 — 250 4.1%, due 2044 250 — 250 250 — 250 3.65%, due 2050 (c) 350 — 350 — — — 4.6% (Retired in 2020) — — — 150 — 150 2,150 — 2,150 1,950 — 1,950 Other: AEF term loan credit agreement through March 2022, 1% at December 31, 2020 (with Alliant Energy as guarantor) (d) 300 — — — — — Corporate Services 3.45% senior notes, due 2022 (a) 75 — — 75 — — AEF 3.75% senior notes, due 2023 (with Alliant Energy as guarantor) (a) 400 — — 400 — — AEF 1.4% senior notes, due 2026 (with Alliant Energy as guarantor) (a)(e) 200 — — — — — AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a) 300 — — 300 — — Sheboygan Power, LLC 5.06% senior secured notes, due 2021 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 31 — — 38 — — AEF term loan credit agreement, 2% at December 31, 2019 (with Alliant Energy as guarantor) (Retired in 2020) — — — 300 — — Other, 1% at December 31, 2020, due 2021 to 2025 2 — — 2 — — 1,308 — — 1,115 — — Subtotal 6,833 3,375 2,150 6,240 3,175 1,950 Current maturities (8) — — (657) (200) (150) Unamortized debt issuance costs (41) (22) (14) (37) (21) (11) Unamortized debt (discount) and premium, net (15) (8) (6) (13) (7) (6) Long-term debt, net (f) $6,769 $3,345 $2,130 $5,533 $2,947 $1,783 (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 June 2020, IPL issued $400 million of 2.3% senior debentures due 2030. The net proceeds from the issuance were used by IPL to retire its $200 million 3.65% senior debentures that would have matured in September 2020 and for general corporate purposes. (c) In April 2020, WPL issued $350 million of 3.65% debentures due 2050. The net proceeds from the issuance were used by WPL to reduce borrowings under the single credit facility and for general corporate purposes. (d) In March 2020, AEF entered into a $300 million variable-rate term loan credit agreement and used the borrowings under this agreement to retire its $300 million variable-rate term loan credit agreement that would have expired in April 2020. (e) In November 2020, AEF issued $200 million of 1.4% senior notes due 2026. The net proceeds from the issuance were used to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes. (f) There were no significant sinking fund requirements related to the outstanding long-term debt.
Schedule of Maturities of Long-term Debt	At December 31, 2020, long-term debt maturities for 2021 through 2025 were as follows (in millions): 2021 2022 2023 2024 2025 IPL $— $— $— $500 $300 WPL — 250 — — — Corporate Services — 75 — — — AEF 8 308 408 9 — Alliant Energy $8 $633 $408 $509 $300
IPL [Member]
Debt Instrument [Line Items]
Schedule of Other Short-term Borrowings	Information regarding Alliant Energy’s, IPL’s and WPL’s commercial paper, and Alliant Energy’s and WPL’s borrowings under the single credit facility, classified as short-term debt was as follows (dollars in millions): Alliant Energy IPL WPL December 31 2020 2019 2020 2019 2020 2019 Amount outstanding $389 $337 $— $— $257 $168 Weighted average interest rates 0.2% 1.9% N/A N/A 0.1% 1.8% Available credit facility capacity $611 $663 $250 $250 $43 $132 Alliant Energy IPL WPL For the year ended 2020 2019 2020 2019 2020 2019 Maximum amount outstanding (based on daily outstanding balances) $495 $601 $8 $50 $266 $195 Average amount outstanding (based on daily outstanding balances) $292 $453 $— $— $117 $93 Weighted average interest rates 0.7% 2.5% 0.5% 2.8% 0.7% 2.4%
Schedule of Long-term Debt	Long-term debt, net as of December 31 was as follows (dollars in millions): 2020 2019 Alliant Energy IPL WPL Alliant Energy IPL WPL Senior Debentures (a): 3.25%, due 2024 $500 $500 $— $500 $500 $— 3.4%, due 2025 250 250 — 250 250 — 5.5%, due 2025 50 50 — 50 50 — 4.1%, due 2028 500 500 — 500 500 — 3.6%, due 2029 300 300 — 300 300 — 2.3%, due 2030 (b) 400 400 — — — — 6.45%, due 2033 100 100 — 100 100 — 6.3%, due 2034 125 125 — 125 125 — 6.25%, due 2039 300 300 — 300 300 — 4.7%, due 2043 250 250 — 250 250 — 3.7%, due 2046 300 300 — 300 300 — 3.5%, due 2049 300 300 — 300 300 — 3.65% (Retired in 2020) — — — 200 200 — 3,375 3,375 — 3,175 3,175 — Debentures (a): 2.25%, due 2022 250 — 250 250 — 250 3.05%, due 2027 300 — 300 300 — 300 3%, due 2029 350 — 350 350 — 350 6.25%, due 2034 100 — 100 100 — 100 6.375%, due 2037 300 — 300 300 — 300 7.6%, due 2038 250 — 250 250 — 250 4.1%, due 2044 250 — 250 250 — 250 3.65%, due 2050 (c) 350 — 350 — — — 4.6% (Retired in 2020) — — — 150 — 150 2,150 — 2,150 1,950 — 1,950 Other: AEF term loan credit agreement through March 2022, 1% at December 31, 2020 (with Alliant Energy as guarantor) (d) 300 — — — — — Corporate Services 3.45% senior notes, due 2022 (a) 75 — — 75 — — AEF 3.75% senior notes, due 2023 (with Alliant Energy as guarantor) (a) 400 — — 400 — — AEF 1.4% senior notes, due 2026 (with Alliant Energy as guarantor) (a)(e) 200 — — — — — AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a) 300 — — 300 — — Sheboygan Power, LLC 5.06% senior secured notes, due 2021 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 31 — — 38 — — AEF term loan credit agreement, 2% at December 31, 2019 (with Alliant Energy as guarantor) (Retired in 2020) — — — 300 — — Other, 1% at December 31, 2020, due 2021 to 2025 2 — — 2 — — 1,308 — — 1,115 — — Subtotal 6,833 3,375 2,150 6,240 3,175 1,950 Current maturities (8) — — (657) (200) (150) Unamortized debt issuance costs (41) (22) (14) (37) (21) (11) Unamortized debt (discount) and premium, net (15) (8) (6) (13) (7) (6) Long-term debt, net (f) $6,769 $3,345 $2,130 $5,533 $2,947 $1,783 (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 June 2020, IPL issued $400 million of 2.3% senior debentures due 2030. The net proceeds from the issuance were used by IPL to retire its $200 million 3.65% senior debentures that would have matured in September 2020 and for general corporate purposes. (c) In April 2020, WPL issued $350 million of 3.65% debentures due 2050. The net proceeds from the issuance were used by WPL to reduce borrowings under the single credit facility and for general corporate purposes. (d) In March 2020, AEF entered into a $300 million variable-rate term loan credit agreement and used the borrowings under this agreement to retire its $300 million variable-rate term loan credit agreement that would have expired in April 2020. (e) In November 2020, AEF issued $200 million of 1.4% senior notes due 2026. The net proceeds from the issuance were used to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes. (f) There were no significant sinking fund requirements related to the outstanding long-term debt.
Schedule of Maturities of Long-term Debt	At December 31, 2020, long-term debt maturities for 2021 through 2025 were as follows (in millions): 2021 2022 2023 2024 2025 IPL $— $— $— $500 $300 WPL — 250 — — — Corporate Services — 75 — — — AEF 8 308 408 9 — Alliant Energy $8 $633 $408 $509 $300
WPL [Member]
Debt Instrument [Line Items]
Schedule of Other Short-term Borrowings	Information regarding Alliant Energy’s, IPL’s and WPL’s commercial paper, and Alliant Energy’s and WPL’s borrowings under the single credit facility, classified as short-term debt was as follows (dollars in millions): Alliant Energy IPL WPL December 31 2020 2019 2020 2019 2020 2019 Amount outstanding $389 $337 $— $— $257 $168 Weighted average interest rates 0.2% 1.9% N/A N/A 0.1% 1.8% Available credit facility capacity $611 $663 $250 $250 $43 $132 Alliant Energy IPL WPL For the year ended 2020 2019 2020 2019 2020 2019 Maximum amount outstanding (based on daily outstanding balances) $495 $601 $8 $50 $266 $195 Average amount outstanding (based on daily outstanding balances) $292 $453 $— $— $117 $93 Weighted average interest rates 0.7% 2.5% 0.5% 2.8% 0.7% 2.4%
Schedule of Long-term Debt	Long-term debt, net as of December 31 was as follows (dollars in millions): 2020 2019 Alliant Energy IPL WPL Alliant Energy IPL WPL Senior Debentures (a): 3.25%, due 2024 $500 $500 $— $500 $500 $— 3.4%, due 2025 250 250 — 250 250 — 5.5%, due 2025 50 50 — 50 50 — 4.1%, due 2028 500 500 — 500 500 — 3.6%, due 2029 300 300 — 300 300 — 2.3%, due 2030 (b) 400 400 — — — — 6.45%, due 2033 100 100 — 100 100 — 6.3%, due 2034 125 125 — 125 125 — 6.25%, due 2039 300 300 — 300 300 — 4.7%, due 2043 250 250 — 250 250 — 3.7%, due 2046 300 300 — 300 300 — 3.5%, due 2049 300 300 — 300 300 — 3.65% (Retired in 2020) — — — 200 200 — 3,375 3,375 — 3,175 3,175 — Debentures (a): 2.25%, due 2022 250 — 250 250 — 250 3.05%, due 2027 300 — 300 300 — 300 3%, due 2029 350 — 350 350 — 350 6.25%, due 2034 100 — 100 100 — 100 6.375%, due 2037 300 — 300 300 — 300 7.6%, due 2038 250 — 250 250 — 250 4.1%, due 2044 250 — 250 250 — 250 3.65%, due 2050 (c) 350 — 350 — — — 4.6% (Retired in 2020) — — — 150 — 150 2,150 — 2,150 1,950 — 1,950 Other: AEF term loan credit agreement through March 2022, 1% at December 31, 2020 (with Alliant Energy as guarantor) (d) 300 — — — — — Corporate Services 3.45% senior notes, due 2022 (a) 75 — — 75 — — AEF 3.75% senior notes, due 2023 (with Alliant Energy as guarantor) (a) 400 — — 400 — — AEF 1.4% senior notes, due 2026 (with Alliant Energy as guarantor) (a)(e) 200 — — — — — AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a) 300 — — 300 — — Sheboygan Power, LLC 5.06% senior secured notes, due 2021 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) 31 — — 38 — — AEF term loan credit agreement, 2% at December 31, 2019 (with Alliant Energy as guarantor) (Retired in 2020) — — — 300 — — Other, 1% at December 31, 2020, due 2021 to 2025 2 — — 2 — — 1,308 — — 1,115 — — Subtotal 6,833 3,375 2,150 6,240 3,175 1,950 Current maturities (8) — — (657) (200) (150) Unamortized debt issuance costs (41) (22) (14) (37) (21) (11) Unamortized debt (discount) and premium, net (15) (8) (6) (13) (7) (6) Long-term debt, net (f) $6,769 $3,345 $2,130 $5,533 $2,947 $1,783 (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 June 2020, IPL issued $400 million of 2.3% senior debentures due 2030. The net proceeds from the issuance were used by IPL to retire its $200 million 3.65% senior debentures that would have matured in September 2020 and for general corporate purposes. (c) In April 2020, WPL issued $350 million of 3.65% debentures due 2050. The net proceeds from the issuance were used by WPL to reduce borrowings under the single credit facility and for general corporate purposes. (d) In March 2020, AEF entered into a $300 million variable-rate term loan credit agreement and used the borrowings under this agreement to retire its $300 million variable-rate term loan credit agreement that would have expired in April 2020. (e) In November 2020, AEF issued $200 million of 1.4% senior notes due 2026. The net proceeds from the issuance were used to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes. (f) There were no significant sinking fund requirements related to the outstanding long-term debt.
Schedule of Maturities of Long-term Debt	At December 31, 2020, long-term debt maturities for 2021 through 2025 were as follows (in millions): 2021 2022 2023 2024 2025 IPL $— $— $— $500 $300 WPL — 250 — — — Corporate Services — 75 — — — AEF 8 308 408 9 — Alliant Energy $8 $633 $408 $509 $300

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2020
Leases [Line Items]
Schedule of Lease Details	At December 31, operating lease details are as follows (dollars in millions): 2020 2019 Alliant Energy IPL WPL Alliant Energy IPL WPL Property, plant and equipment, net $15 $9 $6 $16 $10 $6 Other current liabilities $2 $1 $1 $2 $1 $1 Other liabilities 13 8 5 14 9 5 Total operating lease liabilities $15 $9 $6 $16 $10 $6 Weighted average remaining lease term 10 years 11 years 9 years 11 years 12 years 10 years Weighted average discount rate 4% 4% 4% 4% 4% 4%
Schedule of Operating Lease Liability Maturities	As of December 31, 2020, expected maturities of lease liabilities were as follows (in millions): 2021 2022 2023 2024 2025 Thereafter Total Less: amount representing interest Present value of minimum lease payments Operating Leases: Alliant Energy $2 $2 $2 $2 $2 $8 $18 $3 $15 IPL 1 1 1 1 1 6 11 2 9 WPL 1 1 1 1 1 2 7 1 6 WPL’s Finance Lease: Sheboygan Falls Energy Facility 15 15 15 15 5 — 65 14 51
IPL [Member]
Leases [Line Items]
Schedule of Lease Details	At December 31, operating lease details are as follows (dollars in millions): 2020 2019 Alliant Energy IPL WPL Alliant Energy IPL WPL Property, plant and equipment, net $15 $9 $6 $16 $10 $6 Other current liabilities $2 $1 $1 $2 $1 $1 Other liabilities 13 8 5 14 9 5 Total operating lease liabilities $15 $9 $6 $16 $10 $6 Weighted average remaining lease term 10 years 11 years 9 years 11 years 12 years 10 years Weighted average discount rate 4% 4% 4% 4% 4% 4%
Schedule of Operating Lease Liability Maturities	As of December 31, 2020, expected maturities of lease liabilities were as follows (in millions): 2021 2022 2023 2024 2025 Thereafter Total Less: amount representing interest Present value of minimum lease payments Operating Leases: Alliant Energy $2 $2 $2 $2 $2 $8 $18 $3 $15 IPL 1 1 1 1 1 6 11 2 9 WPL 1 1 1 1 1 2 7 1 6 WPL’s Finance Lease: Sheboygan Falls Energy Facility 15 15 15 15 5 — 65 14 51
WPL [Member]
Leases [Line Items]
Schedule of Lease Details	At December 31, operating lease details are as follows (dollars in millions): 2020 2019 Alliant Energy IPL WPL Alliant Energy IPL WPL Property, plant and equipment, net $15 $9 $6 $16 $10 $6 Other current liabilities $2 $1 $1 $2 $1 $1 Other liabilities 13 8 5 14 9 5 Total operating lease liabilities $15 $9 $6 $16 $10 $6 Weighted average remaining lease term 10 years 11 years 9 years 11 years 12 years 10 years Weighted average discount rate 4% 4% 4% 4% 4% 4% December 31, 2020 December 31, 2019 Property, plant and equipment, net $27 $32 Other current liabilities $9 $9 Finance lease obligations - Sheboygan Falls Energy Facility 42 51 Total finance lease liabilities $51 $60 Remaining lease term 4 years 5 years Discount rate 11% 11% 2020 2019 Depreciation expense $6 $6 Interest expense 6 7 Total finance lease expense $12 $13
Schedule of Operating Lease Liability Maturities	As of December 31, 2020, expected maturities of lease liabilities were as follows (in millions): 2021 2022 2023 2024 2025 Thereafter Total Less: amount representing interest Present value of minimum lease payments Operating Leases: Alliant Energy $2 $2 $2 $2 $2 $8 $18 $3 $15 IPL 1 1 1 1 1 6 11 2 9 WPL 1 1 1 1 1 2 7 1 6 WPL’s Finance Lease: Sheboygan Falls Energy Facility 15 15 15 15 5 — 65 14 51
Schedule of Finance Lease Liability Maturities	As of December 31, 2020, expected maturities of lease liabilities were as follows (in millions): 2021 2022 2023 2024 2025 Thereafter Total Less: amount representing interest Present value of minimum lease payments Operating Leases: Alliant Energy $2 $2 $2 $2 $2 $8 $18 $3 $15 IPL 1 1 1 1 1 6 11 2 9 WPL 1 1 1 1 1 2 7 1 6 WPL’s Finance Lease: Sheboygan Falls Energy Facility 15 15 15 15 5 — 65 14 51

Revenues (Tables)

Revenues (Tables)	12 Months Ended
	Dec. 31, 2020
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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Electric Utility: Retail - residential $1,093 $1,092 $1,063 $602 $601 $590 $491 $491 $473 Retail - commercial 718 770 736 474 510 487 244 260 249 Retail - industrial 841 889 889 488 504 501 353 385 388 Wholesale 168 177 188 57 64 71 111 113 117 Bulk power and other 100 136 124 74 102 82 26 34 42 Total Electric Utility 2,920 3,064 3,000 1,695 1,781 1,731 1,225 1,283 1,269 Gas Utility: Retail - residential 214 259 254 116 149 152 98 110 102 Retail - commercial 107 133 133 59 75 76 48 58 57 Retail - industrial 12 16 15 8 12 10 4 4 5 Transportation/other 40 47 45 25 28 28 15 19 17 Total Gas Utility 373 455 447 208 264 266 165 191 181 Other Utility: Steam 36 37 35 36 37 35 — — — Other utility 13 9 13 8 7 10 5 2 3 Total Other Utility 49 46 48 44 44 45 5 2 3 Non-Utility and Other: Travero and other 74 83 39 — — — — — — Total Non-Utility and Other 74 83 39 — — — — — — Total revenues $3,416 $3,648 $3,534 $1,947 $2,089 $2,042 $1,395 $1,476 $1,453
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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Electric Utility: Retail - residential $1,093 $1,092 $1,063 $602 $601 $590 $491 $491 $473 Retail - commercial 718 770 736 474 510 487 244 260 249 Retail - industrial 841 889 889 488 504 501 353 385 388 Wholesale 168 177 188 57 64 71 111 113 117 Bulk power and other 100 136 124 74 102 82 26 34 42 Total Electric Utility 2,920 3,064 3,000 1,695 1,781 1,731 1,225 1,283 1,269 Gas Utility: Retail - residential 214 259 254 116 149 152 98 110 102 Retail - commercial 107 133 133 59 75 76 48 58 57 Retail - industrial 12 16 15 8 12 10 4 4 5 Transportation/other 40 47 45 25 28 28 15 19 17 Total Gas Utility 373 455 447 208 264 266 165 191 181 Other Utility: Steam 36 37 35 36 37 35 — — — Other utility 13 9 13 8 7 10 5 2 3 Total Other Utility 49 46 48 44 44 45 5 2 3 Non-Utility and Other: Travero and other 74 83 39 — — — — — — Total Non-Utility and Other 74 83 39 — — — — — — Total revenues $3,416 $3,648 $3,534 $1,947 $2,089 $2,042 $1,395 $1,476 $1,453
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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Electric Utility: Retail - residential $1,093 $1,092 $1,063 $602 $601 $590 $491 $491 $473 Retail - commercial 718 770 736 474 510 487 244 260 249 Retail - industrial 841 889 889 488 504 501 353 385 388 Wholesale 168 177 188 57 64 71 111 113 117 Bulk power and other 100 136 124 74 102 82 26 34 42 Total Electric Utility 2,920 3,064 3,000 1,695 1,781 1,731 1,225 1,283 1,269 Gas Utility: Retail - residential 214 259 254 116 149 152 98 110 102 Retail - commercial 107 133 133 59 75 76 48 58 57 Retail - industrial 12 16 15 8 12 10 4 4 5 Transportation/other 40 47 45 25 28 28 15 19 17 Total Gas Utility 373 455 447 208 264 266 165 191 181 Other Utility: Steam 36 37 35 36 37 35 — — — Other utility 13 9 13 8 7 10 5 2 3 Total Other Utility 49 46 48 44 44 45 5 2 3 Non-Utility and Other: Travero and other 74 83 39 — — — — — — Total Non-Utility and Other 74 83 39 — — — — — — Total revenues $3,416 $3,648 $3,534 $1,947 $2,089 $2,042 $1,395 $1,476 $1,453

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Current tax expense (benefit): Federal $1 ($7) ($1) $6 ($11) $15 ($11) $12 ($9) State 8 24 (5) (1) 24 (7) 7 13 (5) IPL’s tax benefit riders — (4) (13) — (4) (13) — — — Deferred tax expense (benefit): Federal 22 70 68 30 26 10 (9) 31 44 State 8 42 30 (2) 31 7 10 6 22 Production tax credits (95) (55) (30) (80) (42) (14) (15) (13) (15) Investment tax credits (1) (1) (1) — — (1) (1) — (1) ($57) $69 $48 ($47) $24 ($3) ($19) $49 $36
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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Statutory federal income tax rate 21 % 21 % 21 % 21 % 21 % 21 % 21 % 21 % 21 % State income taxes, net of federal benefits 2 7 7 (1) 8 8 6 6 6 Production tax credits (17) (9) (5) (28) (13) (5) (7) (5) (7) Amortization of excess deferred taxes (Refer to Note 2 ) (13) (1) (1) (5) — — (26) (2) — Effect of rate-making on property-related differences (3) (6) (8) (4) (10) (14) (2) (3) (2) Adjustment for prior period taxes 1 1 (2) 1 3 (5) — — — IPL’s tax benefit riders — (1) (2) — (1) (5) — — — Federal Tax Reform adjustments — — (1) — — — — — (2) Other items, net (1) (1) (1) — — (1) — — (1) Overall income tax rate (10 %) 11 % 8 % (16 %) 8 % (1 %) (8 %) 17 % 15 %
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 2020 2019 2020 2019 2020 2019 Deferred tax liabilities: Property $2,232 $2,022 $1,312 $1,184 $854 $770 ATC Holdings 116 111 — — — — Other 101 84 83 76 41 32 Total deferred tax liabilities 2,449 2,217 1,395 1,260 895 802 Deferred tax assets: Federal credit carryforwards 454 355 258 175 175 160 Net operating losses carryforwards - federal 77 60 71 56 1 1 Net operating losses carryforwards - state 37 37 1 1 — — Other 73 61 30 20 18 16 Subtotal deferred tax assets 641 513 360 252 194 177 Valuation allowances (6) (10) — — (1) (1) Total deferred tax assets 635 503 360 252 193 176 Total deferred tax liabilities, net $1,814 $1,714 $1,035 $1,008 $702 $626
Summary Of Tax Credit Carryforwards	At December 31, 2020, carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2037 $366 $339 $3 State net operating losses 2021-2040 622 10 2 Federal tax credits 2022-2040 454 258 175
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) 2017 - 2019 Consolidated Iowa income tax returns (b) 2017 - 2019 Wisconsin combined tax returns (c) 2016 - 2019 (a) The 2017 federal tax return is 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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Current tax expense (benefit): Federal $1 ($7) ($1) $6 ($11) $15 ($11) $12 ($9) State 8 24 (5) (1) 24 (7) 7 13 (5) IPL’s tax benefit riders — (4) (13) — (4) (13) — — — Deferred tax expense (benefit): Federal 22 70 68 30 26 10 (9) 31 44 State 8 42 30 (2) 31 7 10 6 22 Production tax credits (95) (55) (30) (80) (42) (14) (15) (13) (15) Investment tax credits (1) (1) (1) — — (1) (1) — (1) ($57) $69 $48 ($47) $24 ($3) ($19) $49 $36
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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Statutory federal income tax rate 21 % 21 % 21 % 21 % 21 % 21 % 21 % 21 % 21 % State income taxes, net of federal benefits 2 7 7 (1) 8 8 6 6 6 Production tax credits (17) (9) (5) (28) (13) (5) (7) (5) (7) Amortization of excess deferred taxes (Refer to Note 2 ) (13) (1) (1) (5) — — (26) (2) — Effect of rate-making on property-related differences (3) (6) (8) (4) (10) (14) (2) (3) (2) Adjustment for prior period taxes 1 1 (2) 1 3 (5) — — — IPL’s tax benefit riders — (1) (2) — (1) (5) — — — Federal Tax Reform adjustments — — (1) — — — — — (2) Other items, net (1) (1) (1) — — (1) — — (1) Overall income tax rate (10 %) 11 % 8 % (16 %) 8 % (1 %) (8 %) 17 % 15 %
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 2020 2019 2020 2019 2020 2019 Deferred tax liabilities: Property $2,232 $2,022 $1,312 $1,184 $854 $770 ATC Holdings 116 111 — — — — Other 101 84 83 76 41 32 Total deferred tax liabilities 2,449 2,217 1,395 1,260 895 802 Deferred tax assets: Federal credit carryforwards 454 355 258 175 175 160 Net operating losses carryforwards - federal 77 60 71 56 1 1 Net operating losses carryforwards - state 37 37 1 1 — — Other 73 61 30 20 18 16 Subtotal deferred tax assets 641 513 360 252 194 177 Valuation allowances (6) (10) — — (1) (1) Total deferred tax assets 635 503 360 252 193 176 Total deferred tax liabilities, net $1,814 $1,714 $1,035 $1,008 $702 $626
Summary Of Tax Credit Carryforwards	At December 31, 2020, carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2037 $366 $339 $3 State net operating losses 2021-2040 622 10 2 Federal tax credits 2022-2040 454 258 175
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) 2017 - 2019 Consolidated Iowa income tax returns (b) 2017 - 2019 Wisconsin combined tax returns (c) 2016 - 2019 (a) The 2017 federal tax return is 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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Current tax expense (benefit): Federal $1 ($7) ($1) $6 ($11) $15 ($11) $12 ($9) State 8 24 (5) (1) 24 (7) 7 13 (5) IPL’s tax benefit riders — (4) (13) — (4) (13) — — — Deferred tax expense (benefit): Federal 22 70 68 30 26 10 (9) 31 44 State 8 42 30 (2) 31 7 10 6 22 Production tax credits (95) (55) (30) (80) (42) (14) (15) (13) (15) Investment tax credits (1) (1) (1) — — (1) (1) — (1) ($57) $69 $48 ($47) $24 ($3) ($19) $49 $36
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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Statutory federal income tax rate 21 % 21 % 21 % 21 % 21 % 21 % 21 % 21 % 21 % State income taxes, net of federal benefits 2 7 7 (1) 8 8 6 6 6 Production tax credits (17) (9) (5) (28) (13) (5) (7) (5) (7) Amortization of excess deferred taxes (Refer to Note 2 ) (13) (1) (1) (5) — — (26) (2) — Effect of rate-making on property-related differences (3) (6) (8) (4) (10) (14) (2) (3) (2) Adjustment for prior period taxes 1 1 (2) 1 3 (5) — — — IPL’s tax benefit riders — (1) (2) — (1) (5) — — — Federal Tax Reform adjustments — — (1) — — — — — (2) Other items, net (1) (1) (1) — — (1) — — (1) Overall income tax rate (10 %) 11 % 8 % (16 %) 8 % (1 %) (8 %) 17 % 15 %
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 2020 2019 2020 2019 2020 2019 Deferred tax liabilities: Property $2,232 $2,022 $1,312 $1,184 $854 $770 ATC Holdings 116 111 — — — — Other 101 84 83 76 41 32 Total deferred tax liabilities 2,449 2,217 1,395 1,260 895 802 Deferred tax assets: Federal credit carryforwards 454 355 258 175 175 160 Net operating losses carryforwards - federal 77 60 71 56 1 1 Net operating losses carryforwards - state 37 37 1 1 — — Other 73 61 30 20 18 16 Subtotal deferred tax assets 641 513 360 252 194 177 Valuation allowances (6) (10) — — (1) (1) Total deferred tax assets 635 503 360 252 193 176 Total deferred tax liabilities, net $1,814 $1,714 $1,035 $1,008 $702 $626
Summary Of Tax Credit Carryforwards	At December 31, 2020, carryforwards and expiration dates were estimated as follows (in millions): Range of Expiration Dates Alliant Energy IPL WPL Federal net operating losses 2037 $366 $339 $3 State net operating losses 2021-2040 622 10 2 Federal tax credits 2022-2040 454 258 175
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) 2017 - 2019 Consolidated Iowa income tax returns (b) 2017 - 2019 Wisconsin combined tax returns (c) 2016 - 2019 (a) The 2017 federal tax return is 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, 2020
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 2020 2019 2018 2020 2019 2018 Discount rate for benefit obligations 2.57% 3.48% 4.34% 2.31% 3.40% 4.24% Discount rate for net periodic cost 3.48% 4.34% 3.66% 3.40% 4.24% 3.53% Expected rate of return on plan assets 7.10% 7.60% 7.60% 4.50% 5.44% 5.44% Interest crediting rate for Alliant Energy Cash Balance Pension Plan 4.76% 5.52% 5.04% 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 2020 2019 2018 2020 2019 2018 Discount rate for benefit obligations 2.61% 3.51% 4.35% 2.28% 3.39% 4.23% Discount rate for net periodic cost 3.51% 4.35% 3.68% 3.39% 4.23% 3.51% Expected rate of return on plan assets 7.10% 7.60% 7.60% 4.50% 5.60% 5.60% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Qualified Defined Benefit Pension Plan OPEB Plans WPL 2020 2019 2018 2020 2019 2018 Discount rate for benefit obligations 2.64% 3.50% 4.35% 2.27% 3.39% 4.23% Discount rate for net periodic cost 3.50% 4.35% 3.69% 3.39% 4.23% 3.51% Expected rate of return on plan assets 7.10% 7.60% 7.60% 3.28% 3.81% 3.84% 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 2020 2019 2018 2020 2019 2018 Service cost $11 $10 $12 $3 $3 $4 Interest cost 43 50 47 7 9 8 Expected return on plan assets (a) (70) (60) (69) (5) (5) (6) Amortization of prior service credit (b) — (1) (1) — — — Amortization of actuarial loss (c) 34 36 35 3 3 3 Settlement losses (d) 12 — — — — — $30 $35 $24 $8 $10 $9 IPL Defined Benefit Pension Plans OPEB Plans 2020 2019 2018 2020 2019 2018 Service cost $7 $6 $7 $1 $1 $2 Interest cost 20 23 21 3 3 3 Expected return on plan assets (a) (33) (28) (32) (4) (4) (4) Amortization of actuarial loss (c) 15 15 15 1 2 1 Settlement losses (d) 7 — — — — — $16 $16 $11 $1 $2 $2 WPL Defined Benefit Pension Plans OPEB Plans 2020 2019 2018 2020 2019 2018 Service cost $4 $3 $4 $2 $1 $2 Interest cost 19 21 20 3 3 3 Expected return on plan assets (a) (31) (26) (30) (1) (1) (1) Amortization of prior service credit (b) — — — (1) — — Amortization of actuarial loss (c) 16 18 17 2 2 2 $8 $16 $11 $5 $5 $6 (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 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 and lump sum payments related to IPL’s qualified defined benefit pension plan.
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 2020 2019 2020 2019 Change in benefit obligation: Net benefit obligation at January 1 $1,280 $1,175 $214 $206 Service cost 11 10 3 3 Interest cost 43 50 7 9 Plan participants’ contributions — — 4 3 Actuarial loss 132 124 18 14 Gross benefits paid (115) (79) (19) (21) Net benefit obligation at December 31 1,351 1,280 227 214 Change in plan assets: Fair value of plan assets at January 1 931 809 105 99 Actual return on plan assets 108 168 11 15 Employer contributions 60 33 7 9 Plan participants’ contributions — — 4 3 Gross benefits paid (115) (79) (19) (21) Fair value of plan assets at December 31 984 931 108 105 Under funded status at December 31 ($367) ($349) ($119) ($109) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2020 2019 2020 2019 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $8 $10 Current liabilities (2) (14) (8) (8) Pension and other benefit obligations (365) (335) (119) (111) Net amounts recognized at December 31 ($367) ($349) ($119) ($109) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $533 $485 $54 $45 Prior service credit (5) (5) (1) (1) $528 $480 $53 $44 Defined Benefit Pension Plans OPEB Plans IPL 2020 2019 2020 2019 Change in benefit obligation: Net benefit obligation at January 1 $581 $534 $86 $83 Service cost 7 6 1 1 Interest cost 20 23 3 3 Plan participants’ contributions — — 1 1 Actuarial loss 60 56 8 6 Gross benefits paid (53) (38) (8) (8) Net benefit obligation at December 31 615 581 91 86 Change in plan assets: Fair value of plan assets at January 1 436 378 72 67 Actual return on plan assets 51 79 7 10 Employer contributions 19 17 2 2 Plan participants’ contributions — — 1 1 Gross benefits paid (53) (38) (8) (8) Fair value of plan assets at December 31 453 436 74 72 Under funded status at December 31 ($162) ($145) ($17) ($14) Defined Benefit Pension Plans OPEB Plans IPL 2020 2019 2020 2019 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $4 $7 Current liabilities (1) (1) (2) (2) Pension and other benefit obligations (161) (144) (19) (19) Net amounts recognized at December 31 ($162) ($145) ($17) ($14) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $228 $208 $20 $17 Prior service credit (1) (1) — — $227 $207 $20 $17 Defined Benefit Pension Plans OPEB Plans WPL 2020 2019 2020 2019 Change in benefit obligation: Net benefit obligation at January 1 $550 $507 $85 $83 Service cost 4 3 2 1 Interest cost 19 21 3 3 Plan participants’ contributions — — 2 2 Actuarial loss 55 54 5 5 Gross benefits paid (40) (35) (8) (9) Net benefit obligation at December 31 588 550 89 85 Change in plan assets: Fair value of plan assets at January 1 406 352 17 17 Actual return on plan assets 48 73 2 1 Employer contributions 22 16 5 6 Plan participants’ contributions — — 2 2 Gross benefits paid (40) (35) (8) (9) Fair value of plan assets at December 31 436 406 18 17 Under funded status at December 31 ($152) ($144) ($71) ($68) Defined Benefit Pension Plans OPEB Plans WPL 2020 2019 2020 2019 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $3 $3 Current liabilities — — (6) (6) Pension and other benefit obligations (152) (144) (68) (65) Net amounts recognized at December 31 ($152) ($144) ($71) ($68) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $233 $212 $25 $22 Prior service credit (1) (1) (1) (1) $232 $211 $24 $21
Accumulated Benefit Obligations	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 are as follows (in millions): Defined Benefit Pension Plans OPEB Plans Alliant Energy 2020 2019 2020 2019 Accumulated benefit obligations $1,305 $1,234 $227 $214 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,305 1,234 227 214 Fair value of plan assets 984 931 108 105 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,351 1,280 N/A N/A Fair value of plan assets 984 931 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2020 2019 2020 2019 Accumulated benefit obligations $589 $558 $91 $86 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 589 558 91 86 Fair value of plan assets 453 436 74 72 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 615 581 N/A N/A Fair value of plan assets 453 436 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2020 2019 2020 2019 Accumulated benefit obligations $573 $536 $89 $85 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 573 536 89 85 Fair value of plan assets 436 406 18 17 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 588 550 N/A N/A Fair value of plan assets 436 406 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 2020 2019 2020 2019 Regulatory assets $44 $39 $34 $29
Estimated Future Employer Contributions	Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2021 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $39 $17 $18 OPEB plans 8 2 6 (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 2021 2022 2023 2024 2025 2026 - 2030 Defined benefit pension benefits $76 $78 $79 $79 $80 $398 OPEB 18 18 17 17 17 74 $94 $96 $96 $96 $97 $472 IPL 2021 2022 2023 2024 2025 2026 - 2030 Defined benefit pension benefits $38 $38 $39 $38 $38 $181 OPEB 7 7 7 7 7 29 $45 $45 $46 $45 $45 $210 WPL 2021 2022 2023 2024 2025 2026 - 2030 Defined benefit pension benefits $32 $32 $32 $33 $33 $162 OPEB 7 7 7 7 6 29 $39 $39 $39 $40 $39 $191
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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Compensation expense $16 $26 $17 $9 $15 $9 $6 $10 $7 Income tax benefits 4 7 5 2 4 3 2 3 2
Schedule of Carrying Value and Fair Market Value of the Deferred Compensation Obligation	At December 31, the carrying value of the deferred compensation obligation for the company stock account and the shares in the deferred compensation trust based on the historical value of the shares of Alliant Energy common stock contributed to the rabbi trust, and the fair market value of the shares held in the rabbi trust, were as follows (in millions): 2020 2019 Carrying value $11 $10 Fair market value 20 21
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 2020 2019 2018 2020 2019 2018 Discount rate for benefit obligations 2.57% 3.48% 4.34% 2.31% 3.40% 4.24% Discount rate for net periodic cost 3.48% 4.34% 3.66% 3.40% 4.24% 3.53% Expected rate of return on plan assets 7.10% 7.60% 7.60% 4.50% 5.44% 5.44% Interest crediting rate for Alliant Energy Cash Balance Pension Plan 4.76% 5.52% 5.04% 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 2020 2019 2018 2020 2019 2018 Discount rate for benefit obligations 2.61% 3.51% 4.35% 2.28% 3.39% 4.23% Discount rate for net periodic cost 3.51% 4.35% 3.68% 3.39% 4.23% 3.51% Expected rate of return on plan assets 7.10% 7.60% 7.60% 4.50% 5.60% 5.60% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Qualified Defined Benefit Pension Plan OPEB Plans WPL 2020 2019 2018 2020 2019 2018 Discount rate for benefit obligations 2.64% 3.50% 4.35% 2.27% 3.39% 4.23% Discount rate for net periodic cost 3.50% 4.35% 3.69% 3.39% 4.23% 3.51% Expected rate of return on plan assets 7.10% 7.60% 7.60% 3.28% 3.81% 3.84% 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 2020 2019 2018 2020 2019 2018 Service cost $11 $10 $12 $3 $3 $4 Interest cost 43 50 47 7 9 8 Expected return on plan assets (a) (70) (60) (69) (5) (5) (6) Amortization of prior service credit (b) — (1) (1) — — — Amortization of actuarial loss (c) 34 36 35 3 3 3 Settlement losses (d) 12 — — — — — $30 $35 $24 $8 $10 $9 IPL Defined Benefit Pension Plans OPEB Plans 2020 2019 2018 2020 2019 2018 Service cost $7 $6 $7 $1 $1 $2 Interest cost 20 23 21 3 3 3 Expected return on plan assets (a) (33) (28) (32) (4) (4) (4) Amortization of actuarial loss (c) 15 15 15 1 2 1 Settlement losses (d) 7 — — — — — $16 $16 $11 $1 $2 $2 WPL Defined Benefit Pension Plans OPEB Plans 2020 2019 2018 2020 2019 2018 Service cost $4 $3 $4 $2 $1 $2 Interest cost 19 21 20 3 3 3 Expected return on plan assets (a) (31) (26) (30) (1) (1) (1) Amortization of prior service credit (b) — — — (1) — — Amortization of actuarial loss (c) 16 18 17 2 2 2 $8 $16 $11 $5 $5 $6 (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 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 and lump sum payments related to IPL’s qualified defined benefit pension plan.
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 2020 2019 2020 2019 Change in benefit obligation: Net benefit obligation at January 1 $1,280 $1,175 $214 $206 Service cost 11 10 3 3 Interest cost 43 50 7 9 Plan participants’ contributions — — 4 3 Actuarial loss 132 124 18 14 Gross benefits paid (115) (79) (19) (21) Net benefit obligation at December 31 1,351 1,280 227 214 Change in plan assets: Fair value of plan assets at January 1 931 809 105 99 Actual return on plan assets 108 168 11 15 Employer contributions 60 33 7 9 Plan participants’ contributions — — 4 3 Gross benefits paid (115) (79) (19) (21) Fair value of plan assets at December 31 984 931 108 105 Under funded status at December 31 ($367) ($349) ($119) ($109) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2020 2019 2020 2019 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $8 $10 Current liabilities (2) (14) (8) (8) Pension and other benefit obligations (365) (335) (119) (111) Net amounts recognized at December 31 ($367) ($349) ($119) ($109) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $533 $485 $54 $45 Prior service credit (5) (5) (1) (1) $528 $480 $53 $44 Defined Benefit Pension Plans OPEB Plans IPL 2020 2019 2020 2019 Change in benefit obligation: Net benefit obligation at January 1 $581 $534 $86 $83 Service cost 7 6 1 1 Interest cost 20 23 3 3 Plan participants’ contributions — — 1 1 Actuarial loss 60 56 8 6 Gross benefits paid (53) (38) (8) (8) Net benefit obligation at December 31 615 581 91 86 Change in plan assets: Fair value of plan assets at January 1 436 378 72 67 Actual return on plan assets 51 79 7 10 Employer contributions 19 17 2 2 Plan participants’ contributions — — 1 1 Gross benefits paid (53) (38) (8) (8) Fair value of plan assets at December 31 453 436 74 72 Under funded status at December 31 ($162) ($145) ($17) ($14) Defined Benefit Pension Plans OPEB Plans IPL 2020 2019 2020 2019 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $4 $7 Current liabilities (1) (1) (2) (2) Pension and other benefit obligations (161) (144) (19) (19) Net amounts recognized at December 31 ($162) ($145) ($17) ($14) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $228 $208 $20 $17 Prior service credit (1) (1) — — $227 $207 $20 $17 Defined Benefit Pension Plans OPEB Plans WPL 2020 2019 2020 2019 Change in benefit obligation: Net benefit obligation at January 1 $550 $507 $85 $83 Service cost 4 3 2 1 Interest cost 19 21 3 3 Plan participants’ contributions — — 2 2 Actuarial loss 55 54 5 5 Gross benefits paid (40) (35) (8) (9) Net benefit obligation at December 31 588 550 89 85 Change in plan assets: Fair value of plan assets at January 1 406 352 17 17 Actual return on plan assets 48 73 2 1 Employer contributions 22 16 5 6 Plan participants’ contributions — — 2 2 Gross benefits paid (40) (35) (8) (9) Fair value of plan assets at December 31 436 406 18 17 Under funded status at December 31 ($152) ($144) ($71) ($68) Defined Benefit Pension Plans OPEB Plans WPL 2020 2019 2020 2019 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $3 $3 Current liabilities — — (6) (6) Pension and other benefit obligations (152) (144) (68) (65) Net amounts recognized at December 31 ($152) ($144) ($71) ($68) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $233 $212 $25 $22 Prior service credit (1) (1) (1) (1) $232 $211 $24 $21
Accumulated Benefit Obligations	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 are as follows (in millions): Defined Benefit Pension Plans OPEB Plans Alliant Energy 2020 2019 2020 2019 Accumulated benefit obligations $1,305 $1,234 $227 $214 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,305 1,234 227 214 Fair value of plan assets 984 931 108 105 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,351 1,280 N/A N/A Fair value of plan assets 984 931 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2020 2019 2020 2019 Accumulated benefit obligations $589 $558 $91 $86 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 589 558 91 86 Fair value of plan assets 453 436 74 72 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 615 581 N/A N/A Fair value of plan assets 453 436 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2020 2019 2020 2019 Accumulated benefit obligations $573 $536 $89 $85 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 573 536 89 85 Fair value of plan assets 436 406 18 17 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 588 550 N/A N/A Fair value of plan assets 436 406 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 2020 2019 2020 2019 Regulatory assets $44 $39 $34 $29
Estimated Future Employer Contributions	Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2021 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $39 $17 $18 OPEB plans 8 2 6 (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 2021 2022 2023 2024 2025 2026 - 2030 Defined benefit pension benefits $76 $78 $79 $79 $80 $398 OPEB 18 18 17 17 17 74 $94 $96 $96 $96 $97 $472 IPL 2021 2022 2023 2024 2025 2026 - 2030 Defined benefit pension benefits $38 $38 $39 $38 $38 $181 OPEB 7 7 7 7 7 29 $45 $45 $46 $45 $45 $210 WPL 2021 2022 2023 2024 2025 2026 - 2030 Defined benefit pension benefits $32 $32 $32 $33 $33 $162 OPEB 7 7 7 7 6 29 $39 $39 $39 $40 $39 $191
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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Compensation expense $16 $26 $17 $9 $15 $9 $6 $10 $7 Income tax benefits 4 7 5 2 4 3 2 3 2
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumptions Used To Measure Benefit Plans	The assumptions for defined benefit pension and OPEB plans at the measurement date of December 31 were as follows: Defined Benefit Pension Plans OPEB Plans Alliant Energy 2020 2019 2018 2020 2019 2018 Discount rate for benefit obligations 2.57% 3.48% 4.34% 2.31% 3.40% 4.24% Discount rate for net periodic cost 3.48% 4.34% 3.66% 3.40% 4.24% 3.53% Expected rate of return on plan assets 7.10% 7.60% 7.60% 4.50% 5.44% 5.44% Interest crediting rate for Alliant Energy Cash Balance Pension Plan 4.76% 5.52% 5.04% 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 2020 2019 2018 2020 2019 2018 Discount rate for benefit obligations 2.61% 3.51% 4.35% 2.28% 3.39% 4.23% Discount rate for net periodic cost 3.51% 4.35% 3.68% 3.39% 4.23% 3.51% Expected rate of return on plan assets 7.10% 7.60% 7.60% 4.50% 5.60% 5.60% Rate of compensation increase 3.65% 3.65% 3.65% N/A N/A N/A Qualified Defined Benefit Pension Plan OPEB Plans WPL 2020 2019 2018 2020 2019 2018 Discount rate for benefit obligations 2.64% 3.50% 4.35% 2.27% 3.39% 4.23% Discount rate for net periodic cost 3.50% 4.35% 3.69% 3.39% 4.23% 3.51% Expected rate of return on plan assets 7.10% 7.60% 7.60% 3.28% 3.81% 3.84% 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 2020 2019 2018 2020 2019 2018 Service cost $11 $10 $12 $3 $3 $4 Interest cost 43 50 47 7 9 8 Expected return on plan assets (a) (70) (60) (69) (5) (5) (6) Amortization of prior service credit (b) — (1) (1) — — — Amortization of actuarial loss (c) 34 36 35 3 3 3 Settlement losses (d) 12 — — — — — $30 $35 $24 $8 $10 $9 IPL Defined Benefit Pension Plans OPEB Plans 2020 2019 2018 2020 2019 2018 Service cost $7 $6 $7 $1 $1 $2 Interest cost 20 23 21 3 3 3 Expected return on plan assets (a) (33) (28) (32) (4) (4) (4) Amortization of actuarial loss (c) 15 15 15 1 2 1 Settlement losses (d) 7 — — — — — $16 $16 $11 $1 $2 $2 WPL Defined Benefit Pension Plans OPEB Plans 2020 2019 2018 2020 2019 2018 Service cost $4 $3 $4 $2 $1 $2 Interest cost 19 21 20 3 3 3 Expected return on plan assets (a) (31) (26) (30) (1) (1) (1) Amortization of prior service credit (b) — — — (1) — — Amortization of actuarial loss (c) 16 18 17 2 2 2 $8 $16 $11 $5 $5 $6 (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 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 and lump sum payments related to IPL’s qualified defined benefit pension plan.
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 2020 2019 2020 2019 Change in benefit obligation: Net benefit obligation at January 1 $1,280 $1,175 $214 $206 Service cost 11 10 3 3 Interest cost 43 50 7 9 Plan participants’ contributions — — 4 3 Actuarial loss 132 124 18 14 Gross benefits paid (115) (79) (19) (21) Net benefit obligation at December 31 1,351 1,280 227 214 Change in plan assets: Fair value of plan assets at January 1 931 809 105 99 Actual return on plan assets 108 168 11 15 Employer contributions 60 33 7 9 Plan participants’ contributions — — 4 3 Gross benefits paid (115) (79) (19) (21) Fair value of plan assets at December 31 984 931 108 105 Under funded status at December 31 ($367) ($349) ($119) ($109) Defined Benefit Pension Plans OPEB Plans Alliant Energy 2020 2019 2020 2019 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $8 $10 Current liabilities (2) (14) (8) (8) Pension and other benefit obligations (365) (335) (119) (111) Net amounts recognized at December 31 ($367) ($349) ($119) ($109) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $533 $485 $54 $45 Prior service credit (5) (5) (1) (1) $528 $480 $53 $44 Defined Benefit Pension Plans OPEB Plans IPL 2020 2019 2020 2019 Change in benefit obligation: Net benefit obligation at January 1 $581 $534 $86 $83 Service cost 7 6 1 1 Interest cost 20 23 3 3 Plan participants’ contributions — — 1 1 Actuarial loss 60 56 8 6 Gross benefits paid (53) (38) (8) (8) Net benefit obligation at December 31 615 581 91 86 Change in plan assets: Fair value of plan assets at January 1 436 378 72 67 Actual return on plan assets 51 79 7 10 Employer contributions 19 17 2 2 Plan participants’ contributions — — 1 1 Gross benefits paid (53) (38) (8) (8) Fair value of plan assets at December 31 453 436 74 72 Under funded status at December 31 ($162) ($145) ($17) ($14) Defined Benefit Pension Plans OPEB Plans IPL 2020 2019 2020 2019 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $4 $7 Current liabilities (1) (1) (2) (2) Pension and other benefit obligations (161) (144) (19) (19) Net amounts recognized at December 31 ($162) ($145) ($17) ($14) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $228 $208 $20 $17 Prior service credit (1) (1) — — $227 $207 $20 $17 Defined Benefit Pension Plans OPEB Plans WPL 2020 2019 2020 2019 Change in benefit obligation: Net benefit obligation at January 1 $550 $507 $85 $83 Service cost 4 3 2 1 Interest cost 19 21 3 3 Plan participants’ contributions — — 2 2 Actuarial loss 55 54 5 5 Gross benefits paid (40) (35) (8) (9) Net benefit obligation at December 31 588 550 89 85 Change in plan assets: Fair value of plan assets at January 1 406 352 17 17 Actual return on plan assets 48 73 2 1 Employer contributions 22 16 5 6 Plan participants’ contributions — — 2 2 Gross benefits paid (40) (35) (8) (9) Fair value of plan assets at December 31 436 406 18 17 Under funded status at December 31 ($152) ($144) ($71) ($68) Defined Benefit Pension Plans OPEB Plans WPL 2020 2019 2020 2019 Amounts recognized on the balance sheets consist of: Non-current assets $— $— $3 $3 Current liabilities — — (6) (6) Pension and other benefit obligations (152) (144) (68) (65) Net amounts recognized at December 31 ($152) ($144) ($71) ($68) Amounts recognized in Regulatory Assets consist of: Net actuarial loss $233 $212 $25 $22 Prior service credit (1) (1) (1) (1) $232 $211 $24 $21
Accumulated Benefit Obligations	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 are as follows (in millions): Defined Benefit Pension Plans OPEB Plans Alliant Energy 2020 2019 2020 2019 Accumulated benefit obligations $1,305 $1,234 $227 $214 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 1,305 1,234 227 214 Fair value of plan assets 984 931 108 105 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 1,351 1,280 N/A N/A Fair value of plan assets 984 931 N/A N/A Defined Benefit Pension Plans OPEB Plans IPL 2020 2019 2020 2019 Accumulated benefit obligations $589 $558 $91 $86 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 589 558 91 86 Fair value of plan assets 453 436 74 72 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 615 581 N/A N/A Fair value of plan assets 453 436 N/A N/A Defined Benefit Pension Plans OPEB Plans WPL 2020 2019 2020 2019 Accumulated benefit obligations $573 $536 $89 $85 Plans with accumulated benefit obligations in excess of plan assets: Accumulated benefit obligations 573 536 89 85 Fair value of plan assets 436 406 18 17 Plans with projected benefit obligations in excess of plan assets: Projected benefit obligations 588 550 N/A N/A Fair value of plan assets 436 406 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 2020 2019 2020 2019 Regulatory assets $44 $39 $34 $29
Estimated Future Employer Contributions	Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2021 is as follows (in millions): Alliant Energy IPL WPL Defined benefit pension plans (a) $39 $17 $18 OPEB plans 8 2 6 (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 2021 2022 2023 2024 2025 2026 - 2030 Defined benefit pension benefits $76 $78 $79 $79 $80 $398 OPEB 18 18 17 17 17 74 $94 $96 $96 $96 $97 $472 IPL 2021 2022 2023 2024 2025 2026 - 2030 Defined benefit pension benefits $38 $38 $39 $38 $38 $181 OPEB 7 7 7 7 7 29 $45 $45 $46 $45 $45 $210 WPL 2021 2022 2023 2024 2025 2026 - 2030 Defined benefit pension benefits $32 $32 $32 $33 $33 $162 OPEB 7 7 7 7 6 29 $39 $39 $39 $40 $39 $191
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 2020 2019 2018 2020 2019 2018 2020 2019 2018 Compensation expense $16 $26 $17 $9 $15 $9 $6 $10 $7 Income tax benefits 4 7 5 2 4 3 2 3 2
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value Of Plan Assets By Asset Category, Fair Value Hierarchy Level and Allocations	At December 31, 2020, 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. 11% - 41% 26% Equity securities - international 14% - 34% 23% Global asset securities 5% - 15% 9% Risk parity securities 5% - 15% 9% Fixed income securities 20% - 40% 28% At December 31, the fair values of qualified and non-qualified defined benefit pension plan assets were as follows (in millions): 2020 2019 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $46 $— $46 $— $46 $4 $42 $— Equity securities - U.S. 182 182 — — 161 161 — — Equity securities - international 151 151 — — 136 136 — — Global asset securities 45 45 — — 47 47 — — Fixed income securities 134 56 78 — 131 58 73 — Total assets in fair value hierarchy 558 $434 $124 $— 521 $406 $115 $— Assets measured at net asset value 426 410 Accrued investment income 1 1 Due to brokers, net (pending trades with brokers) (1) (1) Total pension plan assets $984 $931 2020 2019 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $21 $— $21 $— $22 $2 $20 $— Equity securities - U.S. 84 84 — — 75 75 — — Equity securities - international 69 69 — — 64 64 — — Global asset securities 21 21 — — 22 22 — — Fixed income securities 62 26 36 — 61 27 34 — Total assets in fair value hierarchy 257 $200 $57 $— 244 $190 $54 $— Assets measured at net asset value 196 192 Total pension plan assets $453 $436 2020 2019 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $20 $— $20 $— $20 $2 $18 $— Equity securities - U.S. 81 81 — — 70 70 — — Equity securities - international 67 67 — — 59 59 — — Global asset securities 20 20 — — 21 21 — — Fixed income securities 59 25 34 — 57 25 32 — Total assets in fair value hierarchy 247 $193 $54 $— 227 $177 $50 $— Assets measured at net asset value 189 179 Total pension plan assets $436 $406
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, 2020, 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. 11% - 41% 26% Equity securities - international 14% - 34% 23% Global asset securities 5% - 15% 9% Risk parity securities 5% - 15% 9% Fixed income securities 20% - 40% 28% At December 31, the fair values of qualified and non-qualified defined benefit pension plan assets were as follows (in millions): 2020 2019 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $46 $— $46 $— $46 $4 $42 $— Equity securities - U.S. 182 182 — — 161 161 — — Equity securities - international 151 151 — — 136 136 — — Global asset securities 45 45 — — 47 47 — — Fixed income securities 134 56 78 — 131 58 73 — Total assets in fair value hierarchy 558 $434 $124 $— 521 $406 $115 $— Assets measured at net asset value 426 410 Accrued investment income 1 1 Due to brokers, net (pending trades with brokers) (1) (1) Total pension plan assets $984 $931 2020 2019 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $21 $— $21 $— $22 $2 $20 $— Equity securities - U.S. 84 84 — — 75 75 — — Equity securities - international 69 69 — — 64 64 — — Global asset securities 21 21 — — 22 22 — — Fixed income securities 62 26 36 — 61 27 34 — Total assets in fair value hierarchy 257 $200 $57 $— 244 $190 $54 $— Assets measured at net asset value 196 192 Total pension plan assets $453 $436 2020 2019 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $20 $— $20 $— $20 $2 $18 $— Equity securities - U.S. 81 81 — — 70 70 — — Equity securities - international 67 67 — — 59 59 — — Global asset securities 20 20 — — 21 21 — — Fixed income securities 59 25 34 — 57 25 32 — Total assets in fair value hierarchy 247 $193 $54 $— 227 $177 $50 $— Assets measured at net asset value 189 179 Total pension plan assets $436 $406
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, 2020, 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. 11% - 41% 26% Equity securities - international 14% - 34% 23% Global asset securities 5% - 15% 9% Risk parity securities 5% - 15% 9% Fixed income securities 20% - 40% 28% At December 31, the fair values of qualified and non-qualified defined benefit pension plan assets were as follows (in millions): 2020 2019 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $46 $— $46 $— $46 $4 $42 $— Equity securities - U.S. 182 182 — — 161 161 — — Equity securities - international 151 151 — — 136 136 — — Global asset securities 45 45 — — 47 47 — — Fixed income securities 134 56 78 — 131 58 73 — Total assets in fair value hierarchy 558 $434 $124 $— 521 $406 $115 $— Assets measured at net asset value 426 410 Accrued investment income 1 1 Due to brokers, net (pending trades with brokers) (1) (1) Total pension plan assets $984 $931 2020 2019 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $21 $— $21 $— $22 $2 $20 $— Equity securities - U.S. 84 84 — — 75 75 — — Equity securities - international 69 69 — — 64 64 — — Global asset securities 21 21 — — 22 22 — — Fixed income securities 62 26 36 — 61 27 34 — Total assets in fair value hierarchy 257 $200 $57 $— 244 $190 $54 $— Assets measured at net asset value 196 192 Total pension plan assets $453 $436 2020 2019 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Cash and equivalents $20 $— $20 $— $20 $2 $18 $— Equity securities - U.S. 81 81 — — 70 70 — — Equity securities - international 67 67 — — 59 59 — — Global asset securities 20 20 — — 21 21 — — Fixed income securities 59 25 34 — 57 25 32 — Total assets in fair value hierarchy 247 $193 $54 $— 227 $177 $50 $— Assets measured at net asset value 189 179 Total pension plan assets $436 $406
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, 2020, the current target ranges and actual allocations for VEBA trusts with assets greater than $5 million and the WPL 401(h) assets were as follows: Target Range Actual Allocation Allocation Cash and equivalents 0% - 5% 1% Equity securities - U.S. 0% - 46% 25% Equity securities - international 0% - 34% 1% Fixed income securities 20% - 100% 73% At December 31, the fair values of OPEB plan assets were as follows (in millions): 2020 2019 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $2 $— $2 $— $3 $3 $— $— Equity securities - U.S. 4 4 — — 4 4 — — Equity securities - international 3 3 — — 3 3 — — Fixed income securities 72 71 1 — 69 68 1 — Total assets in fair value hierarchy 81 $78 $3 $— 79 $78 $1 $— Assets measured at net asset value 27 26 Total OPEB plan assets $108 $105 2020 2019 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $1 $— $1 $— $2 $2 $— $— Fixed income securities 51 51 — — 48 48 — — Total assets in fair value hierarchy 52 $51 $1 $— 50 $50 $— $— Assets measured at net asset value 22 22 Total OPEB plan assets $74 $72 2020 2019 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Fixed income securities $18 $18 $— $— $17 $17 $— $— Total OPEB plan assets $18 $18 $— $— $17 $17 $— $—
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, 2020, the current target ranges and actual allocations for VEBA trusts with assets greater than $5 million and the WPL 401(h) assets were as follows: Target Range Actual Allocation Allocation Cash and equivalents 0% - 5% 1% Equity securities - U.S. 0% - 46% 25% Equity securities - international 0% - 34% 1% Fixed income securities 20% - 100% 73% At December 31, the fair values of OPEB plan assets were as follows (in millions): 2020 2019 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $2 $— $2 $— $3 $3 $— $— Equity securities - U.S. 4 4 — — 4 4 — — Equity securities - international 3 3 — — 3 3 — — Fixed income securities 72 71 1 — 69 68 1 — Total assets in fair value hierarchy 81 $78 $3 $— 79 $78 $1 $— Assets measured at net asset value 27 26 Total OPEB plan assets $108 $105 2020 2019 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $1 $— $1 $— $2 $2 $— $— Fixed income securities 51 51 — — 48 48 — — Total assets in fair value hierarchy 52 $51 $1 $— 50 $50 $— $— Assets measured at net asset value 22 22 Total OPEB plan assets $74 $72 2020 2019 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Fixed income securities $18 $18 $— $— $17 $17 $— $— Total OPEB plan assets $18 $18 $— $— $17 $17 $— $—
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, 2020, the current target ranges and actual allocations for VEBA trusts with assets greater than $5 million and the WPL 401(h) assets were as follows: Target Range Actual Allocation Allocation Cash and equivalents 0% - 5% 1% Equity securities - U.S. 0% - 46% 25% Equity securities - international 0% - 34% 1% Fixed income securities 20% - 100% 73% At December 31, the fair values of OPEB plan assets were as follows (in millions): 2020 2019 Fair Level Level Level Fair Level Level Level Alliant Energy Value 1 2 3 Value 1 2 3 Cash and equivalents $2 $— $2 $— $3 $3 $— $— Equity securities - U.S. 4 4 — — 4 4 — — Equity securities - international 3 3 — — 3 3 — — Fixed income securities 72 71 1 — 69 68 1 — Total assets in fair value hierarchy 81 $78 $3 $— 79 $78 $1 $— Assets measured at net asset value 27 26 Total OPEB plan assets $108 $105 2020 2019 Fair Level Level Level Fair Level Level Level IPL Value 1 2 3 Value 1 2 3 Cash and equivalents $1 $— $1 $— $2 $2 $— $— Fixed income securities 51 51 — — 48 48 — — Total assets in fair value hierarchy 52 $51 $1 $— 50 $50 $— $— Assets measured at net asset value 22 22 Total OPEB plan assets $74 $72 2020 2019 Fair Level Level Level Fair Level Level Level WPL Value 1 2 3 Value 1 2 3 Fixed income securities $18 $18 $— $— $17 $17 $— $— Total OPEB plan assets $18 $18 $— $— $17 $17 $— $—
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 2020 2019 2018 2020 2019 2018 2020 2019 2018 401(k) costs $25 $25 $25 $13 $13 $13 $12 $12 $11
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 2020 2019 2018 2020 2019 2018 2020 2019 2018 401(k) costs $25 $25 $25 $13 $13 $13 $12 $12 $11
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 2020 2019 2018 2020 2019 2018 2020 2019 2018 401(k) costs $25 $25 $25 $13 $13 $13 $12 $12 $11
Performance Shares Equity Awards [Member]
Defined Benefit Plan Disclosure [Line Items]
Schedule Of Equity-based Compensation Plans Activity	A summary of the 2020 and 2019 performance shares activity, with amounts representing the target number of awards, was as follows: 2020 2019 Performance Shares - Equity Weighted Average Performance Shares - Equity Weighted Average Nonvested awards, January 1 74,193 $47.44 — $— Granted 56,204 64.04 91,816 47.23 Forfeited (1,241) 50.94 (17,623) 46.35 Nonvested awards, December 31 129,156 54.63 74,193 47.44
Performance Shares and Performance Units Liability Awards [Member]
Defined Benefit Plan Disclosure [Line Items]
Schedule Of Equity-based Compensation Plans Activity	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) 2020 2019 2018 2020 2019 2018 Nonvested awards, January 1 131,872 203,188 223,511 34,574 57,761 71,737 Granted — — 74,163 — — 19,840 Vested (63,565) (66,322) (90,806) (16,661) (20,131) (31,910) Forfeited — (4,994) (3,680) (1,112) (3,056) (1,906) Nonvested awards, December 31 68,307 131,872 203,188 16,801 34,574 57,761 Performance Shares Performance Units Nonvested awards at target 68,307 16,801 Estimated payout percentage based on performance criteria 172.5% 172.5% Fair values of each nonvested award $88.89 $88.89 Performance Shares (granted prior to 2019) Performance Units (granted prior to 2019) 2020 2019 2018 2020 2019 2018 2017 Grant 2016 Grant 2015 Grant 2017 Grant 2016 Grant 2015 Grant Performance awards vested 63,565 66,322 90,806 16,661 20,131 31,910 Percentage of target number of performance awards 155.0% 142.5% 137.5% 155.0% 142.5% 137.5% Aggregate payout value (in millions) $6 $4 $5 $2 $1 $1 Payout - cash (in millions) $5 $4 $5 $2 $1 $1 Payout - common stock shares issued 9,543 6,447 5,078 N/A N/A N/A
Restricted Stock Units Equity Awards [Member]
Defined Benefit Plan Disclosure [Line Items]
Schedule Of Equity-based Compensation Plans Activity	A summary of the 2020 and 2019 restricted stock units activity was as follows: 2020 2019 Restricted Stock Units - Equity Weighted Average Restricted Stock Units - Equity Weighted Average Nonvested units, January 1 89,281 $46.04 — $— Granted 61,056 59.42 105,348 45.98 Forfeited (3,788) 49.01 (16,067) 45.63 Nonvested units, December 31 146,549 51.54 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: 2020 2019 2018 Nonvested units, January 1 113,033 174,163 113,749 Granted — — 63,568 Vested (54,483) (56,849) — Forfeited — (4,281) (3,154) Nonvested units, December 31 58,550 113,033 174,163
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: 2020 2019 2018 Units Weighted Average Units Weighted Average Units Weighted Average Nonvested units, January 1 206,065 $41.50 203,188 $37.23 132,705 $36.50 Granted 56,204 59.37 91,816 46.10 74,163 38.60 Vested (63,565) 39.12 (66,322) 33.93 — — Forfeited (1,241) 49.25 (22,617) 44.00 (3,680) 38.60 Nonvested units, December 31 197,463 47.31 206,065 41.50 203,188 37.23

Asset Retirement Obligations (T

Asset Retirement Obligations (Tables)	12 Months Ended
	Dec. 31, 2020
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 2020 2019 2020 2019 2020 2019 Balance, January 1 $196 $177 $134 $118 $62 $59 Revisions in estimated cash flows 13 (6) 9 (7) 4 1 Liabilities settled (13) (8) (9) (8) (4) — Liabilities incurred (a) 48 26 38 26 10 — Accretion expense 7 7 5 5 2 2 Balance, December 31 $251 $196 $177 $134 $74 $62 (a) In 2020, IPL placed in service the Whispering Willow North, Golden Plains and Richland wind sites, and WPL placed in service the Kossuth wind site. As a result, Alliant Energy, IPL and WPL recognized additional AROs in 2020, which 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 2020 2019 2020 2019 2020 2019 Balance, January 1 $196 $177 $134 $118 $62 $59 Revisions in estimated cash flows 13 (6) 9 (7) 4 1 Liabilities settled (13) (8) (9) (8) (4) — Liabilities incurred (a) 48 26 38 26 10 — Accretion expense 7 7 5 5 2 2 Balance, December 31 $251 $196 $177 $134 $74 $62 (a) In 2020, IPL placed in service the Whispering Willow North, Golden Plains and Richland wind sites, and WPL placed in service the Kossuth wind site. As a result, Alliant Energy, IPL and WPL recognized additional AROs in 2020, which 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 2020 2019 2020 2019 2020 2019 Balance, January 1 $196 $177 $134 $118 $62 $59 Revisions in estimated cash flows 13 (6) 9 (7) 4 1 Liabilities settled (13) (8) (9) (8) (4) — Liabilities incurred (a) 48 26 38 26 10 — Accretion expense 7 7 5 5 2 2 Balance, December 31 $251 $196 $177 $134 $74 $62 (a) In 2020, IPL placed in service the Whispering Willow North, Golden Plains and Richland wind sites, and WPL placed in service the Kossuth wind site. As a result, Alliant Energy, IPL and WPL recognized additional AROs in 2020, which 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, 2020
Derivative [Line Items]
Notional Amounts of Derivative Instruments	As of December 31, 2020, 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 9,285 2021 208,542 2021-2028 5,648 2021-2023 5,544 2021-2022 IPL 3,398 2021 109,063 2021-2028 2,548 2021-2023 — — WPL 5,887 2021 99,479 2021-2027 3,100 2021-2023 5,544 2021-2022
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 2020 2019 2020 2019 2020 2019 Current derivative assets $24 $16 $20 $12 $4 $4 Non-current derivative assets 10 11 9 10 1 1 Current derivative liabilities 9 19 3 9 6 10 Non-current derivative liabilities 16 19 9 9 7 10
IPL [Member]
Derivative [Line Items]
Notional Amounts of Derivative Instruments	As of December 31, 2020, 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 9,285 2021 208,542 2021-2028 5,648 2021-2023 5,544 2021-2022 IPL 3,398 2021 109,063 2021-2028 2,548 2021-2023 — — WPL 5,887 2021 99,479 2021-2027 3,100 2021-2023 5,544 2021-2022
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 2020 2019 2020 2019 2020 2019 Current derivative assets $24 $16 $20 $12 $4 $4 Non-current derivative assets 10 11 9 10 1 1 Current derivative liabilities 9 19 3 9 6 10 Non-current derivative liabilities 16 19 9 9 7 10
WPL [Member]
Derivative [Line Items]
Notional Amounts of Derivative Instruments	As of December 31, 2020, 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 9,285 2021 208,542 2021-2028 5,648 2021-2023 5,544 2021-2022 IPL 3,398 2021 109,063 2021-2028 2,548 2021-2023 — — WPL 5,887 2021 99,479 2021-2027 3,100 2021-2023 5,544 2021-2022
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 2020 2019 2020 2019 2020 2019 Current derivative assets $24 $16 $20 $12 $4 $4 Non-current derivative assets 10 11 9 10 1 1 Current derivative liabilities 9 19 3 9 6 10 Non-current derivative liabilities 16 19 9 9 7 10

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2020
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 2020 2019 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Money market fund investments $44 $44 $— $— $44 $5 $5 $— $— $5 Derivatives 34 — 5 29 34 27 — 5 22 27 Deferred proceeds 188 — — 188 188 188 — — 188 188 Liabilities and equity: Derivatives 25 — 25 — 25 38 — 37 1 38 Long-term debt (incl. current maturities) 6,777 — 8,107 2 8,109 6,190 — 6,918 2 6,920 IPL 2020 2019 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Money market fund investments $44 $44 $— $— $44 $5 $5 $— $— $5 Derivatives 29 — 3 26 29 22 — 3 19 22 Deferred proceeds 188 — — 188 188 188 — — 188 188 Liabilities and equity: Derivatives 12 — 12 — 12 18 — 17 1 18 Long-term debt (incl. current maturities) 3,345 — 4,021 — 4,021 3,147 — 3,489 — 3,489 WPL 2020 2019 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $5 $— $2 $3 $5 $5 $— $2 $3 $5 Liabilities and equity: Derivatives 13 — 13 — 13 20 — 20 — 20 Long-term debt (incl. current maturities) 2,130 — 2,690 — 2,690 1,933 — 2,268 — 2,268
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 2020 2019 2020 2019 Beginning balance, January 1 $21 $12 $188 $119 Total net gains included in changes in net assets (realized/unrealized) 11 8 — — Transfers out of Level 3 (a) — 4 — — Purchases 14 14 — — Sales (1) — — — Settlements (b) (16) (17) — 69 Ending balance, December 31 $29 $21 $188 $188 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 $11 $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2020 2019 2020 2019 Beginning balance, January 1 $18 $9 $188 $119 Total net gains included in changes in net assets (realized/unrealized) 10 11 — — Transfers out of Level 3 (a) — 2 — — Purchases 11 9 — — Sales (1) — — — Settlements (b) (12) (13) — 69 Ending balance, December 31 $26 $18 $188 $188 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 $10 $12 $— $— (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 expected credit losses 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 2020 $18 $11 $17 $9 $1 $2 2019 15 7 14 5 1 2
IPL [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Recurring Fair Value Measurements	Carrying amounts and the related estimated fair values of other financial instruments at December 31 were as follows (in millions): Alliant Energy 2020 2019 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Money market fund investments $44 $44 $— $— $44 $5 $5 $— $— $5 Derivatives 34 — 5 29 34 27 — 5 22 27 Deferred proceeds 188 — — 188 188 188 — — 188 188 Liabilities and equity: Derivatives 25 — 25 — 25 38 — 37 1 38 Long-term debt (incl. current maturities) 6,777 — 8,107 2 8,109 6,190 — 6,918 2 6,920 IPL 2020 2019 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Money market fund investments $44 $44 $— $— $44 $5 $5 $— $— $5 Derivatives 29 — 3 26 29 22 — 3 19 22 Deferred proceeds 188 — — 188 188 188 — — 188 188 Liabilities and equity: Derivatives 12 — 12 — 12 18 — 17 1 18 Long-term debt (incl. current maturities) 3,345 — 4,021 — 4,021 3,147 — 3,489 — 3,489 WPL 2020 2019 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $5 $— $2 $3 $5 $5 $— $2 $3 $5 Liabilities and equity: Derivatives 13 — 13 — 13 20 — 20 — 20 Long-term debt (incl. current maturities) 2,130 — 2,690 — 2,690 1,933 — 2,268 — 2,268
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 2020 2019 2020 2019 Beginning balance, January 1 $21 $12 $188 $119 Total net gains included in changes in net assets (realized/unrealized) 11 8 — — Transfers out of Level 3 (a) — 4 — — Purchases 14 14 — — Sales (1) — — — Settlements (b) (16) (17) — 69 Ending balance, December 31 $29 $21 $188 $188 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 $11 $— $— IPL Commodity Contract Derivative Assets and (Liabilities), net Deferred Proceeds 2020 2019 2020 2019 Beginning balance, January 1 $18 $9 $188 $119 Total net gains included in changes in net assets (realized/unrealized) 10 11 — — Transfers out of Level 3 (a) — 2 — — Purchases 11 9 — — Sales (1) — — — Settlements (b) (12) (13) — 69 Ending balance, December 31 $26 $18 $188 $188 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 $10 $12 $— $— (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 expected credit losses 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 2020 $18 $11 $17 $9 $1 $2 2019 15 7 14 5 1 2
WPL [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Recurring Fair Value Measurements	Carrying amounts and the related estimated fair values of other financial instruments at December 31 were as follows (in millions): Alliant Energy 2020 2019 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Money market fund investments $44 $44 $— $— $44 $5 $5 $— $— $5 Derivatives 34 — 5 29 34 27 — 5 22 27 Deferred proceeds 188 — — 188 188 188 — — 188 188 Liabilities and equity: Derivatives 25 — 25 — 25 38 — 37 1 38 Long-term debt (incl. current maturities) 6,777 — 8,107 2 8,109 6,190 — 6,918 2 6,920 IPL 2020 2019 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Money market fund investments $44 $44 $— $— $44 $5 $5 $— $— $5 Derivatives 29 — 3 26 29 22 — 3 19 22 Deferred proceeds 188 — — 188 188 188 — — 188 188 Liabilities and equity: Derivatives 12 — 12 — 12 18 — 17 1 18 Long-term debt (incl. current maturities) 3,345 — 4,021 — 4,021 3,147 — 3,489 — 3,489 WPL 2020 2019 Fair Value Fair Value Carrying Level Level Level Carrying Level Level Level Amount 1 2 3 Total Amount 1 2 3 Total Assets: Derivatives $5 $— $2 $3 $5 $5 $— $2 $3 $5 Liabilities and equity: Derivatives 13 — 13 — 13 20 — 20 — 20 Long-term debt (incl. current maturities) 2,130 — 2,690 — 2,690 1,933 — 2,268 — 2,268
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 2020 $18 $11 $17 $9 $1 $2 2019 15 7 14 5 1 2

Commitments And Contingencies (

Commitments And Contingencies (Tables)	12 Months Ended
	Dec. 31, 2020
Long-term Purchase Commitment [Line Items]
Other Purchase Commitments	At December 31, 2020, the related minimum future commitments were as follows (in millions): Alliant Energy 2021 2022 2023 2024 2025 Thereafter Total Natural gas 252 174 137 105 68 185 921 Coal 70 27 17 — — — 114 Other (a) 61 17 11 8 3 29 129 $383 $218 $165 $113 $71 $214 $1,164 IPL 2021 2022 2023 2024 2025 Thereafter Total Natural gas 130 88 79 68 38 71 474 Coal 32 16 13 — — — 61 Other (a) 25 5 4 2 2 27 65 $187 $109 $96 $70 $40 $98 $600 WPL 2021 2022 2023 2024 2025 Thereafter Total Natural gas 122 86 58 37 30 114 447 Coal 38 11 4 — — — 53 Other (a) 26 2 1 1 1 1 32 $186 $99 $63 $38 $31 $115 $532 (a) Includes individual commitments incurred during the normal course of business that exceeded $1 million at December 31, 2020.
Schedule of Environmental Liabilities	At December 31, 2020, 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, 2020, such amounts for WPL were not material. Alliant Energy IPL Range of estimated future costs $11 - $28 $8 - $22 Current and non-current environmental liabilities 15 12
IPL [Member]
Long-term Purchase Commitment [Line Items]
Other Purchase Commitments	At December 31, 2020, the related minimum future commitments were as follows (in millions): Alliant Energy 2021 2022 2023 2024 2025 Thereafter Total Natural gas 252 174 137 105 68 185 921 Coal 70 27 17 — — — 114 Other (a) 61 17 11 8 3 29 129 $383 $218 $165 $113 $71 $214 $1,164 IPL 2021 2022 2023 2024 2025 Thereafter Total Natural gas 130 88 79 68 38 71 474 Coal 32 16 13 — — — 61 Other (a) 25 5 4 2 2 27 65 $187 $109 $96 $70 $40 $98 $600 WPL 2021 2022 2023 2024 2025 Thereafter Total Natural gas 122 86 58 37 30 114 447 Coal 38 11 4 — — — 53 Other (a) 26 2 1 1 1 1 32 $186 $99 $63 $38 $31 $115 $532 (a) Includes individual commitments incurred during the normal course of business that exceeded $1 million at December 31, 2020.
Schedule of Environmental Liabilities	At December 31, 2020, 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, 2020, such amounts for WPL were not material. Alliant Energy IPL Range of estimated future costs $11 - $28 $8 - $22 Current and non-current environmental liabilities 15 12
WPL [Member]
Long-term Purchase Commitment [Line Items]
Other Purchase Commitments	At December 31, 2020, the related minimum future commitments were as follows (in millions): Alliant Energy 2021 2022 2023 2024 2025 Thereafter Total Natural gas 252 174 137 105 68 185 921 Coal 70 27 17 — — — 114 Other (a) 61 17 11 8 3 29 129 $383 $218 $165 $113 $71 $214 $1,164 IPL 2021 2022 2023 2024 2025 Thereafter Total Natural gas 130 88 79 68 38 71 474 Coal 32 16 13 — — — 61 Other (a) 25 5 4 2 2 27 65 $187 $109 $96 $70 $40 $98 $600 WPL 2021 2022 2023 2024 2025 Thereafter Total Natural gas 122 86 58 37 30 114 447 Coal 38 11 4 — — — 53 Other (a) 26 2 1 1 1 1 32 $186 $99 $63 $38 $31 $115 $532 (a) Includes individual commitments incurred during the normal course of business that exceeded $1 million at December 31, 2020.

Segments Of Business (Tables)

Segments Of Business (Tables)	12 Months Ended
	Dec. 31, 2020
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 2020 Electric Gas Other Total Parent and Other Consolidated Revenues $2,920 $373 $49 $3,342 $74 $3,416 Depreciation and amortization 556 49 5 610 5 615 Operating income (loss) 643 74 (1) 716 24 740 Interest expense 243 32 275 Equity income from unconsolidated investments, net (2) — — (2) (59) (61) Income tax expense (benefit) (66) 9 (57) Net income attributable to Alliant Energy common shareowners 573 41 614 Total assets 14,358 1,413 990 16,761 949 17,710 Investments in equity method subsidiaries 11 — — 11 465 476 Construction and acquisition expenditures 1,109 182 2 1,293 73 1,366 ATC Holdings, Utility Non-utility, Alliant Energy 2019 Electric Gas Other Total Parent and Other Consolidated Revenues $3,064 $455 $46 $3,565 $83 $3,648 Depreciation and amortization 513 47 3 563 4 567 Operating income 679 70 1 750 28 778 Interest expense 229 44 273 Equity income from unconsolidated investments, net (1) — — (1) (52) (53) Income tax expense (benefit) 73 (4) 69 Net income attributable to Alliant Energy common shareowners 517 40 557 Total assets 13,659 1,269 856 15,784 917 16,701 Investments in equity method subsidiaries 9 — — 9 449 458 Construction and acquisition expenditures 1,439 100 — 1,539 101 1,640 ATC Holdings, Utility Non-utility, Alliant Energy 2018 Electric Gas Other Total Parent and Other Consolidated Revenues $3,000 $447 $48 $3,495 $39 $3,534 Depreciation and amortization 457 42 3 502 5 507 Operating income 610 53 1 664 30 694 Interest expense 217 30 247 Equity income from unconsolidated investments, net (1) — — (1) (54) (55) Income taxes 33 15 48 Net income attributable to Alliant Energy common shareowners 472 40 512 Total assets 12,486 1,184 893 14,563 863 15,426 Investments in equity method subsidiaries 8 — — 8 413 421 Construction and acquisition expenditures 1,422 147 — 1,569 65 1,634
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): 2020 Electric Gas Other Total Revenues $1,695 $208 $44 $1,947 Depreciation and amortization 321 30 5 356 Operating income 358 50 2 410 Interest expense 139 Income tax benefit (47) Net income available for common stock 324 Total assets 8,518 766 565 9,849 Construction and acquisition expenditures 626 59 2 687 2019 Electric Gas Other Total Revenues $1,781 $264 $44 $2,089 Depreciation and amortization 295 29 3 327 Operating income 360 39 4 403 Interest expense 127 Income taxes 24 Net income available for common stock 284 Total assets 8,075 734 468 9,277 Construction and acquisition expenditures 964 56 — 1,020 2018 Electric Gas Other Total Revenues $1,731 $266 $45 $2,042 Depreciation and amortization 255 25 3 283 Operating income 318 28 5 351 Interest expense 119 Income tax benefit (3) Net income available for common stock 264 Total assets 7,220 687 504 8,411 Construction and acquisition expenditures 891 100 — 991
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): 2020 Electric Gas Other Total Revenues $1,225 $165 $5 $1,395 Depreciation and amortization 235 19 — 254 Operating income (loss) 285 24 (3) 306 Interest expense 104 Income tax benefit (19) Net income 249 Total assets 5,840 647 425 6,912 Construction and acquisition expenditures 483 123 — 606 2019 Electric Gas Other Total Revenues $1,283 $191 $2 $1,476 Depreciation and amortization 218 18 — 236 Operating income (loss) 319 31 (3) 347 Interest expense 102 Income taxes 49 Net income 233 Total assets 5,584 535 388 6,507 Construction and acquisition expenditures 475 44 — 519 2018 Electric Gas Other Total Revenues $1,269 $181 $3 $1,453 Depreciation and amortization 202 17 — 219 Operating income (loss) 292 25 (4) 313 Interest expense 98 Income taxes 36 Net income 208 Total assets 5,266 497 389 6,152 Construction and acquisition expenditures 531 47 — 578

Related Parties (Tables)

Related Parties (Tables)	12 Months Ended
	Dec. 31, 2020
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 2020 2019 2018 2020 2019 2018 Corporate Services billings $176 $185 $170 $142 $142 $132 Sales credited 35 68 48 3 7 28 Purchases billed 329 331 358 108 120 81
Net Intercompany Payables	As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2020 2019 IPL $110 $112 WPL 73 85
Related Amounts Billed Between Parties	The related amounts billed between the parties were as follows (in millions): 2020 2019 2018 ATC billings to WPL $108 $109 $106 WPL billings to ATC 10 13 11
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 2020 2019 2018 2020 2019 2018 Corporate Services billings $176 $185 $170 $142 $142 $132 Sales credited 35 68 48 3 7 28 Purchases billed 329 331 358 108 120 81
Net Intercompany Payables	As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2020 2019 IPL $110 $112 WPL 73 85
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 2020 2019 2018 2020 2019 2018 Corporate Services billings $176 $185 $170 $142 $142 $132 Sales credited 35 68 48 3 7 28 Purchases billed 329 331 358 108 120 81
Net Intercompany Payables	As of December 31, net intercompany payables to Corporate Services were as follows (in millions): 2020 2019 IPL $110 $112 WPL 73 85
Related Amounts Billed Between Parties	The related amounts billed between the parties were as follows (in millions): 2020 2019 2018 ATC billings to WPL $108 $109 $106 WPL billings to ATC 10 13 11

Condensed Parent Company Fina_2

Condensed Parent Company Financial Statements (Tables)	12 Months Ended
	Dec. 31, 2020
Condensed Financial Information Disclosure [Abstract]
Condensed Statements of Income	ALLIANT ENERGY CORPORATION (Parent Company Only) Year Ended December 31, CONDENSED STATEMENTS OF INCOME 2020 2019 2018 (in millions) Operating expenses $7 $2 $5 Operating loss (7) (2) (5) Other (income) and deductions: Equity earnings from consolidated subsidiaries (625) (562) (523) Interest expense 2 9 4 Other 4 7 2 Total other (income) and deductions (619) (546) (517) Income before income taxes 612 544 512 Income tax benefit (4) (15) (1) Net income $616 $559 $513 Refer to accompanying Notes to Condensed Financial Statements.
Condensed Balance Sheets	ALLIANT ENERGY CORPORATION (Parent Company Only) December 31, CONDENSED BALANCE SHEETS 2020 2019 (in millions) ASSETS Current assets: Notes receivable from affiliated companies $32 $27 Other 5 6 Total current assets 37 33 Investments: Investments in consolidated subsidiaries 6,664 6,017 Other 2 1 Total investments 6,666 6,018 Other assets 88 89 Total assets $6,791 $6,140 LIABILITIES AND EQUITY Current liabilities: Commercial paper $132 $169 Notes payable to affiliated companies 937 731 Other 29 19 Total current liabilities 1,098 919 Other liabilities 2 15 Common equity: Common stock and additional paid-in capital 2,706 2,448 Retained earnings 2,996 2,766 Accumulated other comprehensive income — 2 Shares in deferred compensation trust (11) (10) Total common equity 5,691 5,206 Total liabilities and equity $6,791 $6,140 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 2020 2019 2018 (in millions) Net cash flows from operating activities $396 $305 $311 Cash flows used for investing activities: Capital contributions to consolidated subsidiaries (429) (250) (625) Net change in notes receivable from and payable to affiliates 201 8 441 Net cash flows used for investing activities (228) (242) (184) Cash flows used for financing activities: Common stock dividends (377) (338) (312) Proceeds from issuance of common stock, net 247 390 197 Net change in commercial paper (37) (116) (10) Other (1) 1 (2) Net cash flows used for financing activities (168) (63) (127) 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 ($2) ($9) ($4) Income taxes, net 10 14 5 Refer to accompanying Notes to Condensed Financial Statements.

Summary Of Significant Accoun_4

Summary Of Significant Accounting Policies (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)customerMW	Dec. 31, 2019USD ($)
Cash, Cash Equivalents and Restricted Cash [Abstract]
Money market fund investments	$ 44	$ 5
Money market fund investments interest rate, percentage	0.04%
New Accounting Standards:
New accounting standard, cumulative effect adjustment to retained earnings, after-tax	$ (2,994)	(2,766)
Cumulative Effect, Period of Adoption, Adjustment [Member]
New Accounting Standards:
New accounting standard, cumulative effect adjustment to retained earnings, pre-tax		12
New accounting standard, cumulative effect adjustment to retained earnings, after-tax		9
IPL [Member]
General:
Generation and distribution of steam, number of customers served (in customers) | customer	2
Electric capacity of wind farm (in megawatts) | MW	1,000
Cash, Cash Equivalents and Restricted Cash [Abstract]
Money market fund investments	$ 44	5
Money market fund investments interest rate, percentage	0.04%
Property, Plant and Equipment:
AFUDC accrual recorded, percentage of estimated CWIP	100.00%
New Accounting Standards:
New accounting standard, cumulative effect adjustment to retained earnings, after-tax	$ (979)	(891)
WPL [Member]
Property, Plant and Equipment:
AFUDC accrual recorded, percentage of estimated CWIP	50.00%
AFUDC rates, projects with approval	100.00%
New Accounting Standards:
New accounting standard, cumulative effect adjustment to retained earnings, after-tax	$ (953)	$ (864)
Alliant Energy Finance, LLC [Member]
General:
Ownership interest	50.00%
Electric capacity of wind farm (in megawatts) | MW	225
Sheboygan Falls Energy Facility [Member]
General:
Fossil-fueled EGU capacity (in megawatts) | MW	347
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, 2020	Dec. 31, 2019	Dec. 31, 2018
Electric - generation [Member] | IPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	3.50%	3.80%	3.60%
Electric - generation [Member] | WPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	3.50%	3.60%	3.60%
Electric - distribution [Member] | IPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	2.80%	2.90%	2.80%
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	5.20%	5.30%	4.70%
Electric - other [Member] | WPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	6.10%	5.80%	5.70%
Gas [Member] | IPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	3.30%	3.30%	3.20%
Gas [Member] | WPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	2.40%	2.50%	2.50%
Other [Member] | IPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	6.30%	5.90%	5.20%
Other [Member] | WPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average rate of depreciation	5.90%	5.60%	5.80%

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, 2020	Dec. 31, 2019	Dec. 31, 2018
IPL [Member] | IPL (Wind generation CWIP) [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Recovery rates	7.10%	7.40%	7.50%
IPL [Member] | IPL (other CWIP) [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Recovery rates	7.20%	7.50%	7.50%
WPL [Member] | WPL (retail jurisdiction) [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Recovery rates	7.00%	6.80%	7.70%
WPL [Member] | WPL (wholesale jurisdiction) [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Recovery rates	6.30%	6.90%	7.20%

Regulatory Matters (Narrative)

Regulatory Matters (Narrative) (Details) $ in Millions	1 Months Ended		12 Months Ended
	Sep. 30, 2020USD ($)	Aug. 31, 2020customer	Dec. 31, 2020USD ($)MW	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2021USD ($)
Regulatory Matters [Line Items]
Credits issued to customers			$ (113)	$ (40)	$ 1
Regulatory liabilities			1,306	1,424
Electric transmission cost recovery [Member]
Regulatory Matters [Line Items]
Regulatory liabilities			68	88
IPL [Member]
Regulatory Matters [Line Items]
Regulatory assets not earning a return			81
Credits issued to customers			$ (20)	2	1
Electric capacity of wind farm (in megawatts) | MW			1,000
Regulatory liabilities			$ 676	715
IPL [Member] | Electric transmission cost recovery [Member]
Regulatory Matters [Line Items]
Credits issued to customers			42
Regulatory liabilities			39	51
IPL [Member] | Derecho windstorm [Member]
Regulatory Matters [Line Items]
Number of customers without power, derecho windstorm (number of customers) | customer		250,000
Derecho windstorm repair and rebuilding costs			135
Net regulatory liability, derecho windstorm			7
WPL [Member]
Regulatory Matters [Line Items]
Regulatory assets not earning a return			11
Credits issued to customers			(93)	(42)	1
Regulatory liabilities			630	709
WPL [Member] | Electric transmission cost recovery [Member]
Regulatory Matters [Line Items]
Regulatory liabilities			29	37
2020 Test Period Retail Electric [Member] | IPL [Member]
Regulatory Matters [Line Items]
Interim rate increase (decrease), amount				$ 90
Authorized increase (decrease) in final rates, amount			127
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
2020 Test Period Retail Electric [Member] | WPL [Member]
Regulatory Matters [Line Items]
Authorized increase (decrease) in final rates, amount			$ 29
Authorized increase (decrease) in final rates, percentage			2.00%
2020 Test Period Retail Gas [Member] | IPL [Member]
Regulatory Matters [Line Items]
Authorized increase (decrease) in final rates, amount			$ 12
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
2019 Test Period Retail Electric [Member] | WPL [Member] | Revenue Subject to Refund [Member]
Regulatory Matters [Line Items]
Credits issued to customers			$ 12
DAEC PPA [Member]
Regulatory Matters [Line Items]
Buyout payment	$ 110
PPA term amendment			5 years
Forecast [Member] | IPL [Member] | Derecho windstorm [Member]
Regulatory Matters [Line Items]
Derecho windstorm repair and rebuilding costs							$ 140

Regulatory Matters (Regulatory

Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Regulatory Assets [Line Items]
Regulatory assets	$ 2,010	$ 1,845
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	890	818
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	580	524
AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	119	112
Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	113	134
IPL's DAEC PPA amendment [Member]
Regulatory Assets [Line Items]
Regulatory assets	110	108
WPL's Western Wisconsin gas distribution expansion investments [Member]
Regulatory Assets [Line Items]
Regulatory assets	55	0
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	26	39
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	117	110
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,483	1,400
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	843	777
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	291	263
IPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	81	76
IPL [Member] | Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	77	88
IPL [Member] | IPL's DAEC PPA amendment [Member]
Regulatory Assets [Line Items]
Regulatory assets	110	108
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	13	18
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	68	70
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	527	445
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	47	41
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	289	261
WPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	38	36
WPL [Member] | Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	36	46
WPL [Member] | WPL's Western Wisconsin gas distribution expansion investments [Member]
Regulatory Assets [Line Items]
Regulatory assets	55	0
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	13	21
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 49	$ 40

Regulatory Matters (Assets Reti

Regulatory Matters (Assets Retired Early) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Regulatory Assets [Line Items]
Regulatory assets	$ 2,010	$ 1,845
Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	113	134
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,483	1,400
IPL [Member] | Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	77	88
IPL [Member] | Assets retired early [Member] | Sutherland Units 1 and 3 [Member]
Regulatory Assets [Line Items]
Regulatory assets	23
IPL [Member] | Assets retired early [Member] | M.L. Kapp Unit 2 [Member]
Regulatory Assets [Line Items]
Regulatory assets	21
IPL [Member] | Assets retired early [Member] | Analog Electric Meters [Member]
Regulatory Assets [Line Items]
Regulatory assets	33
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	527	445
WPL [Member] | Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	36	$ 46
WPL [Member] | Assets retired early [Member] | Nelson Dewey Units 1 and 2 and Edgewater Unit 3 [Member]
Regulatory Assets [Line Items]
Regulatory assets	13
WPL [Member] | Assets retired early [Member] | Edgewater Unit 4 [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 23

Regulatory Matters (Regulator_2

Regulatory Matters (Regulatory Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 1,306	$ 1,424
Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	732	836
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	367	388
Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	68	88
West Riverside liquidated damages [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	38	0
Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	28	20
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	73	92
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	676	715
IPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	331	351
IPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	238	257
IPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	39	51
IPL [Member] | Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	25	17
IPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	43	39
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	630	709
WPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	401	485
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	129	131
WPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	29	37
WPL [Member] | West Riverside liquidated damages [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	38	0
WPL [Member] | Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	3	3
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 30	$ 53

Property, Plant and Equipment_2

Property, Plant and Equipment (Narrative) (Details)	12 Months Ended
	Dec. 31, 2020
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, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Electric plant anticipated to be retired early	$ 1,269	$ 0
Total electric plant	16,243	13,865
Gas plant in service	1,563	1,463
Other plant in service	534	520
Accumulated depreciation	(4,868)	(4,602)
Net plant	13,472	11,246
Construction work in progress	405	1,835
Other, net	7	6
Total utility	13,884	13,087
Non-utility Generation, net	79	83
Corporate Services and other, net	373	357
Total non-utility and other	452	440
Total property, plant and equipment	14,336	13,527
Non-utility Generation, accumulated depreciation	63	59
Corporate Services and other, accumulated depreciation	210	172
Generation [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service	8,222	7,625
Distribution [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service	6,216	5,783
Other [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service	536	457
IPL [Member]
Property, Plant and Equipment [Line Items]
Electric plant anticipated to be retired early	459	0
Total electric plant	9,171	7,921
Gas plant in service	844	802
Other plant in service	354	345
Accumulated depreciation	(2,671)	(2,499)
Net plant	7,698	6,569
Construction work in progress	185	907
Other, net	6	5
Total utility	7,889	7,481
Total property, plant and equipment	7,889	7,481
IPL [Member] | Generation [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service	4,884	4,432
IPL [Member] | Distribution [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service	3,470	3,190
IPL [Member] | Other [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service	358	299
WPL [Member]
Property, Plant and Equipment [Line Items]
Electric plant anticipated to be retired early	810	0
Total electric plant	7,072	5,944
Gas plant in service	719	661
Other plant in service	180	175
Accumulated depreciation	(2,197)	(2,103)
Net plant	5,774	4,677
Leased Sheboygan Falls Energy Facility, net	27	32
Construction work in progress	220	928
Other, net	1	1
Total utility	6,022	5,638
Total property, plant and equipment	6,022	5,638
Leased Sheboygan Falls Energy Facility, accumulated depreciation	95	89
WPL [Member] | Generation [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service	3,338	3,193
WPL [Member] | Distribution [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service	2,746	2,593
WPL [Member] | Other [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service	$ 178	$ 158

Property, Plant and Equipment_4

Property, Plant and Equipment (Equity and Debt AFUDC) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 55	$ 93	$ 76
Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	39	66	51
Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	16	27	25
IPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	24	50	42
IPL [Member] | Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	17	35	29
IPL [Member] | Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	7	15	13
WPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	31	43	34
WPL [Member] | Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	22	31	22
WPL [Member] | Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 9	$ 12	$ 12

Jointly-Owned Electric Utilit_3

Jointly-Owned Electric Utility Plant (Details) $ in Millions	Dec. 31, 2020USD ($)
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	$ 2,574
Accumulated Provision for Depreciation	727
Construction Work in Progress	41
IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	982
Accumulated Provision for Depreciation	380
Construction Work in Progress	24
WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	1,592
Accumulated Provision for Depreciation	347
Construction Work in Progress	$ 17
Ottumwa Unit 1 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	48.00%
Electric Plant	$ 580
Accumulated Provision for Depreciation	195
Construction Work in Progress	$ 22
George Neal Unit 4 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	25.70%
Electric Plant	$ 192
Accumulated Provision for Depreciation	96
Construction Work in Progress	$ 2
George Neal Unit 3 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	28.00%
Electric Plant	$ 171
Accumulated Provision for Depreciation	67
Construction Work in Progress	$ 0
Louisa Unit 1 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	4.00%
Electric Plant	$ 39
Accumulated Provision for Depreciation	22
Construction Work in Progress	$ 0
Columbia Units 1-2 [Member] | WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	53.50%
Electric Plant	$ 786
Accumulated Provision for Depreciation	289
Construction Work in Progress	$ 4
West Riverside [Member] | WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	91.80%
Electric Plant	$ 686
Accumulated Provision for Depreciation	12
Construction Work in Progress	$ 13
Forward Wind Energy Center [Member] | WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	42.60%
Electric Plant	$ 120
Accumulated Provision for Depreciation	46
Construction Work in Progress	$ 0

Receivables (Narrative) (Detail

Receivables (Narrative) (Details) $ in Millions	Dec. 31, 2020USD ($)
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	$ 109

Receivables (Details of Account

Receivables (Details of Accounts Receivable) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Accounts Receivable [Line Items]
Customer	$ 101	$ 92
Unbilled utility revenues	82	82
Deferred proceeds	188	188
Other	59	47
Allowance for expected credit losses	(18)	(7)
Accounts receivable, less allowance for expected credit losses	412	402
IPL [Member]
Accounts Receivable [Line Items]
Customer	0	0
Unbilled utility revenues	0	0
Deferred proceeds	188	188
Other	23	16
Allowance for expected credit losses	(1)	(1)
Accounts receivable, less allowance for expected credit losses	210	203
WPL [Member]
Accounts Receivable [Line Items]
Customer	92	84
Unbilled utility revenues	82	82
Other	35	31
Allowance for expected credit losses	(17)	(6)
Accounts receivable, less allowance for expected credit losses	$ 192	$ 191

Receivables (Maximum And Averag

Receivables (Maximum And Average Outstanding Cash Proceeds) (Details) - IPL [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Maximum [Member]
Accounts Receivable [Line Items]
Outstanding aggregate cash proceeds (based on daily outstanding balances)	$ 96	$ 108	$ 116
Average [Member]
Accounts Receivable [Line Items]
Outstanding aggregate cash proceeds (based on daily outstanding balances)	$ 9	$ 36	$ 53

Receivables (Receivables Sold U

Receivables (Receivables Sold Under The Agreement) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Accounts Receivable [Line Items]
Fair value of deferred proceeds	$ 188	$ 188
IPL [Member]
Accounts Receivable [Line Items]
Fair value of deferred proceeds	188	188
Financing Receivable [Member] | IPL [Member]
Accounts Receivable [Line Items]
Customer accounts receivable	114	125
Unbilled utility revenues	92	95
Other receivables	0	1
Receivables sold to third party	206	221
Less: cash proceeds	1	27
Deferred proceeds	205	194
Less: allowance for expected credit losses	17	6
Fair value of deferred proceeds	188	188
Outstanding receivables past due	$ 27	$ 26

Receivables (Additional Attribu

Receivables (Additional Attributes Of Receivables Sold Under The Agreement) (Details) - IPL [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounts Receivable [Line Items]
Collections	$ 2,025	$ 2,193	$ 2,077
Write-offs, net of recoveries	$ 7	$ 19	$ 21

Investments (Narrative) (Detail

Investments (Narrative) (Details)	Dec. 31, 2020
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, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Carrying value	$ 476	$ 458	$ 421
Equity (income) / loss	(61)	(53)	(55)
ATC Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value	331	320
Equity (income) / loss	$ (47)	(45)	(38)
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	$ 103	105
Equity (income) / loss	(8)	(5)	(16)
Other [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value	42	33
Equity (income) / loss	(6)	(3)	(1)
Total [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value	476	458
Equity (income) / loss	$ (61)	$ (53)	$ (55)

Investments (Summary Financial

Investments (Summary Financial Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues	$ 3,416	$ 3,648	$ 3,534
Current assets	887	876
Current liabilities	1,297	2,054
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Revenues	801	783	724
Operating income	376	359	325
Net income	343	340	$ 217
Current assets	121	139
Non-current assets	6,388	5,964
Current liabilities	317	507
Non-current liabilities	2,997	2,659
Noncontrolling interest	$ 192	$ 208

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
Schedule of Common Equity [Line Items]
Shares available for issuance under the 2020 OIP, Shareowner Direct Plan and 401(k) Savings Plan (in shares)	15,000,000
Proceeds from issuance of common stock, net	$ 247	$ 390	$ 197
Equity Forward Agreements [Member]
Schedule of Common Equity [Line Items]
Common stock issued during the period (in shares)	4,275,127	8,358,973	0
Proceeds from issuance of common stock, net	$ 222	$ 366
Initial forward sale price (in dollars per share)	$ 51.98	$ 43.75
At-the-market Offering Program [Member]
Schedule of Common Equity [Line Items]
Common stock issued during the period (in shares)	0	0	4,171,013
Proceeds from issuance of common stock, net			$ 173
Maximum aggregate gross sales price of common stock that can be offered and sold			175
Fees and commissions from issuance of common stock			$ 2

Common Equity (Common Share Act

Common Equity (Common Share Activity) (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Common Stock Oustanding [Roll Forward]
Shares outstanding, January 1 (in shares)	245,022,800	236,063,279	231,348,646
Common stock issued during the period, Shareowner Direct Plan (in shares)	472,283	501,808	576,965
Common stock issued during the period, Equity-based compensation plans (in shares)	98,205	101,478	5,078
Common stock issued during the period, Other (in shares)	0	(2,738)	(38,423)
Shares outstanding, December 31 (in shares)	249,868,415	245,022,800	236,063,279
Equity Forward Agreements [Member]
Common Stock Oustanding [Roll Forward]
Common stock issued during the period (in shares)	4,275,127	8,358,973	0
At-the-market Offering Program [Member]
Common Stock Oustanding [Roll Forward]
Common stock issued during the period (in shares)	0	0	4,171,013

Preferred Stock (Narrative) (De

Preferred Stock (Narrative) (Details) - IPL [Member]	12 Months Ended
	Dec. 31, 2020Qboard_member$ / sharesshares	Dec. 31, 2019shares
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

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, 2020	Dec. 31, 2019
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, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity		$ 1,000
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, 2020	Dec. 31, 2019
Commercial paper and borrowings under the single credit facility:
Amount outstanding	$ 389	$ 337
Weighted average interest rates	0.20%	1.90%
Available credit facility capacity	$ 611	$ 663
IPL [Member]
Commercial paper and borrowings under the single credit facility:
Amount outstanding	0	0
Available credit facility capacity	250	250
WPL [Member]
Commercial paper and borrowings under the single credit facility:
Amount outstanding	$ 257	$ 168
Weighted average interest rates	0.10%	1.80%
Available credit facility capacity	$ 43	$ 132

Debt (Other Short-Term Borrowin

Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 495	$ 601
Average amount outstanding (based on daily outstanding balances)	$ 292	$ 453
Weighted average interest rates	0.70%	2.50%
IPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 8	$ 50
Average amount outstanding (based on daily outstanding balances)	$ 0	$ 0
Weighted average interest rates	0.50%	2.80%
WPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 266	$ 195
Average amount outstanding (based on daily outstanding balances)	$ 117	$ 93
Weighted average interest rates	0.70%	2.40%

Debt (Long-term Debt) (Details)

Debt (Long-term Debt) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Long-term debt	$ 6,833	$ 6,240
Current maturities	(8)	(657)
Unamortized debt issuance costs	(41)	(37)
Unamortized debt (discount) and premium, net	(15)	(13)
Long-term debt, net	6,769	5,533
Senior Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	3,375	3,175
Senior Debentures [Member] | 3.25% senior debenture, due 2024 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 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	$ 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	$ 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	$ 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	$ 300	$ 300
Interest rate	3.60%	3.60%
Senior Debentures [Member] | 2.3% senior debenture, due 2030
Debt Instrument [Line Items]
Long-term debt	$ 400	$ 0
Interest rate	2.30%
Senior Debentures [Member] | 6.45% senior debenture, due 2033 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 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	$ 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	$ 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	$ 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	$ 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	$ 300	$ 300
Interest rate	3.50%	3.50%
Senior Debentures [Member] | 3.65% senior debenture, retired in 2020 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 0	$ 200
Interest rate		3.65%
Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	2,150	$ 1,950
Debentures [Member] | 2.25% debenture, due 2022 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 250	$ 250
Interest rate	2.25%	2.25%
Debentures [Member] | 3.05% debenture, due 2027 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 300	$ 300
Interest rate	3.05%	3.05%
Debentures [Member] | 3% debenture, due 2029 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 350	$ 350
Interest rate	3.00%	3.00%
Debentures [Member] | 6.25% debenture, due 2034 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 100	$ 100
Interest rate	6.25%	6.25%
Debentures [Member] | 6.375% debenture, due 2037 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 300	$ 300
Interest rate	6.375%	6.375%
Debentures [Member] | 7.6% debenture, due 2038 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 250	$ 250
Interest rate	7.60%	7.60%
Debentures [Member] | 4.1% debenture, due 2044 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 250	$ 250
Interest rate	4.10%	4.10%
Debentures [Member] | 3.65% debenture, due 2050
Debt Instrument [Line Items]
Long-term debt	$ 350	$ 0
Interest rate	3.65%
Debentures [Member] | 4.6% debenture, retired in 2020 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 0	$ 150
Interest rate		4.60%
Other Long Term Debt [Member]
Debt Instrument [Line Items]
Long-term debt	1,308	$ 1,115
Other [Member] | Other, 1% at December 31, 2020, due 2021 to 2025 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 2	2
Interest rate	1.00%
IPL [Member]
Debt Instrument [Line Items]
Long-term debt	$ 3,375	3,175
Current maturities	0	(200)
Unamortized debt issuance costs	(22)	(21)
Unamortized debt (discount) and premium, net	(8)	(7)
Long-term debt, net	3,345	2,947
IPL [Member] | Senior Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	3,375	3,175
IPL [Member] | Senior Debentures [Member] | 3.25% senior debenture, due 2024 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 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	$ 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	$ 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	$ 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	$ 300	$ 300
Interest rate	3.60%	3.60%
IPL [Member] | Senior Debentures [Member] | 2.3% senior debenture, due 2030
Debt Instrument [Line Items]
Long-term debt	$ 400	$ 0
Interest rate	2.30%
IPL [Member] | Senior Debentures [Member] | 6.45% senior debenture, due 2033 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 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	$ 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	$ 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	$ 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	$ 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	$ 300	$ 300
Interest rate	3.50%	3.50%
IPL [Member] | Senior Debentures [Member] | 3.65% senior debenture, retired in 2020 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 0	$ 200
Interest rate		3.65%
WPL [Member]
Debt Instrument [Line Items]
Long-term debt	2,150	$ 1,950
Current maturities	0	(150)
Unamortized debt issuance costs	(14)	(11)
Unamortized debt (discount) and premium, net	(6)	(6)
Long-term debt, net	2,130	1,783
WPL [Member] | Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	2,150	1,950
WPL [Member] | Debentures [Member] | 2.25% debenture, due 2022 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 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	$ 300	$ 300
Interest rate	3.05%	3.05%
WPL [Member] | Debentures [Member] | 3% debenture, due 2029 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 350	$ 350
Interest rate	3.00%	3.00%
WPL [Member] | Debentures [Member] | 6.25% debenture, due 2034 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 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	$ 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	$ 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	$ 250	$ 250
Interest rate	4.10%	4.10%
WPL [Member] | Debentures [Member] | 3.65% debenture, due 2050
Debt Instrument [Line Items]
Long-term debt	$ 350	$ 0
Interest rate	3.65%
WPL [Member] | Debentures [Member] | 4.6% debenture, retired in 2020 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 0	$ 150
Interest rate		4.60%
Alliant Energy Finance, LLC [Member] | Term Loan Credit Agreement [Member] | Term loan credit agreement through March 2022
Debt Instrument [Line Items]
Long-term debt	$ 300	$ 0
Interest rate	1.00%
Alliant Energy Finance, LLC [Member] | Term Loan Credit Agreement [Member] | Term loan credit agreement, retired in 2020 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 0	$ 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	$ 400	$ 400
Interest rate	3.75%	3.75%
Alliant Energy Finance, LLC [Member] | Senior Notes [Member] | 1.4% senior notes, due 2026
Debt Instrument [Line Items]
Long-term debt	$ 200	$ 0
Interest rate	1.40%
Alliant Energy Finance, LLC [Member] | Senior Notes [Member] | 4.25% senior notes, due 2028 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 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	$ 75	$ 75
Interest rate	3.45%	3.45%
Sheboygan Power, LLC [Member] | Senior Secured Notes [Member] | 5.06% senior secured notes, due 2021 to 2024 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 31	$ 38
Interest rate	5.06%	5.06%

Debt (Schedule Of Debt Maturiti

Debt (Schedule Of Debt Maturities) (Details) $ in Millions	Dec. 31, 2020USD ($)
2021	$ 8
2022	633
2023	408
2024	509
2025	300
IPL [Member]
2021	0
2022	0
2023	0
2024	500
2025	300
WPL [Member]
2021	0
2022	250
2023	0
2024	0
2025	0
Corporate Services [Member]
2021	0
2022	75
2023	0
2024	0
2025	0
Alliant Energy Finance, LLC [Member]
2021	8
2022	308
2023	408
2024	9
2025	$ 0

Leases (Narrative) (Details)

Leases (Narrative) (Details)	12 Months Ended
	Dec. 31, 2020lease_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) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Leases [Line Items]
Operating leases assets, Property, plant and equipment, net	$ 15	$ 16
Operating leases liabilities, Other current liabilities	2	2
Operating leases liabilities, Other liabilities	13	14
Operating leases liabilities, Total	$ 15	$ 16
Operating leases, Weighted average remaining lease term	10 years	11 years
Operating leases, Weighted average discount rate, percent	4.00%	4.00%
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent	us-gaap:OtherLiabilitiesCurrent
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent	us-gaap:OtherLiabilitiesNoncurrent
IPL [Member]
Leases [Line Items]
Operating leases assets, Property, plant and equipment, net	$ 9	$ 10
Operating leases liabilities, Other current liabilities	1	1
Operating leases liabilities, Other liabilities	8	9
Operating leases liabilities, Total	$ 9	$ 10
Operating leases, Weighted average remaining lease term	11 years	12 years
Operating leases, Weighted average discount rate, percent	4.00%	4.00%
WPL [Member]
Leases [Line Items]
Operating leases assets, Property, plant and equipment, net	$ 6	$ 6
Operating leases liabilities, Other current liabilities	1	1
Operating leases liabilities, Other liabilities	5	5
Operating leases liabilities, Total	$ 6	$ 6
Operating leases, Weighted average remaining lease term	9 years	10 years
Operating leases, Weighted average discount rate, percent	4.00%	4.00%

Leases (Finance Lease) (Details

Leases (Finance Lease) (Details) - WPL [Member] - Sheboygan Falls Energy Facility [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Line Items]
Finance lease assets, Property, plant and equipment, net	$ 27	$ 32
Finance lease liabilities, Other current liabilities	9	9
Finance lease liabilities, Sheboygan Falls Energy Facility	42	51
Finance lease liabilities, Total	$ 51	$ 60
Finance lease, Remaining lease term	4 years	5 years
Finance lease, Discount rate, percent	11.00%	11.00%
Finance lease, Depreciation expense	$ 6	$ 6
Finance lease, Interest expense	6	7
Finance lease, Total expense	$ 12	$ 13

Leases (Expected Maturities of

Leases (Expected Maturities of Operating and Finance Lease Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Leases [Line Items]
Operating Leases Liability, 2021	$ 2
Operating Leases Liability, 2022	2
Operating Leases Liability, 2023	2
Operating Leases Liability, 2024	2
Operating Leases Liability, 2025	2
Operating Leases Liability, Thereafter	8
Operating Leases Liability, Total	18
Operating Leases Liability, Less: amount representing interest	3
Operating Leases Liability, Present value of minimum lease payments	15	$ 16
IPL [Member]
Leases [Line Items]
Operating Leases Liability, 2021	1
Operating Leases Liability, 2022	1
Operating Leases Liability, 2023	1
Operating Leases Liability, 2024	1
Operating Leases Liability, 2025	1
Operating Leases Liability, Thereafter	6
Operating Leases Liability, Total	11
Operating Leases Liability, Less: amount representing interest	2
Operating Leases Liability, Present value of minimum lease payments	9	10
WPL [Member]
Leases [Line Items]
Operating Leases Liability, 2021	1
Operating Leases Liability, 2022	1
Operating Leases Liability, 2023	1
Operating Leases Liability, 2024	1
Operating Leases Liability, 2025	1
Operating Leases Liability, Thereafter	2
Operating Leases Liability, Total	7
Operating Leases Liability, Less: amount representing interest	1
Operating Leases Liability, Present value of minimum lease payments	6	6
Sheboygan Falls Energy Facility [Member] | WPL [Member]
Leases [Line Items]
Finance Lease Liability, 2021	15
Finance Lease Liability, 2022	15
Finance Lease Liability, 2023	15
Finance Lease Liability, 2024	15
Finance Lease Liability, 2025	5
Finance Lease Liability, Thereafter	0
Finance Lease Liability, Total	65
Finance Lease Liability, Less: amount representing interest	14
Finance Lease Liability, Present value of minimum lease payments	$ 51	$ 60

Disaggregation of Revenues (Det

Disaggregation of Revenues (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 3,416	$ 3,648	$ 3,534
IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,947	2,089	2,042
WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,395	1,476	1,453
Electric [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,920	3,064	3,000
Electric [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,695	1,781	1,731
Electric [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,225	1,283	1,269
Electric [Member] | Retail - Residential [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,093	1,092	1,063
Electric [Member] | Retail - Residential [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	602	601	590
Electric [Member] | Retail - Residential [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	491	491	473
Electric [Member] | Retail - Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenues	718	770	736
Electric [Member] | Retail - Commercial [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	474	510	487
Electric [Member] | Retail - Commercial [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	244	260	249
Electric [Member] | Retail - Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenues	841	889	889
Electric [Member] | Retail - Industrial [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	488	504	501
Electric [Member] | Retail - Industrial [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	353	385	388
Electric [Member] | Wholesale [Member]
Disaggregation of Revenue [Line Items]
Revenues	168	177	188
Electric [Member] | Wholesale [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	57	64	71
Electric [Member] | Wholesale [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	111	113	117
Electric [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	100	136	124
Electric [Member] | Other [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	74	102	82
Electric [Member] | Other [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	26	34	42
Gas [Member]
Disaggregation of Revenue [Line Items]
Revenues	373	455	447
Gas [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	208	264	266
Gas [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	165	191	181
Gas [Member] | Retail - Residential [Member]
Disaggregation of Revenue [Line Items]
Revenues	214	259	254
Gas [Member] | Retail - Residential [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	116	149	152
Gas [Member] | Retail - Residential [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	98	110	102
Gas [Member] | Retail - Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenues	107	133	133
Gas [Member] | Retail - Commercial [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	59	75	76
Gas [Member] | Retail - Commercial [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	48	58	57
Gas [Member] | Retail - Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenues	12	16	15
Gas [Member] | Retail - Industrial [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	8	12	10
Gas [Member] | Retail - Industrial [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	4	4	5
Gas [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	40	47	45
Gas [Member] | Other [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	25	28	28
Gas [Member] | Other [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	15	19	17
Other Utility [Member]
Disaggregation of Revenue [Line Items]
Revenues	49	46	48
Other Utility [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	44	44	45
Other Utility [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	5	2	3
Other Utility [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	13	9	13
Other Utility [Member] | Other [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	8	7	10
Other Utility [Member] | Other [Member] | WPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	5	2	3
Other Utility [Member] | Steam [Member]
Disaggregation of Revenue [Line Items]
Revenues	36	37	35
Other Utility [Member] | Steam [Member] | IPL [Member]
Disaggregation of Revenue [Line Items]
Revenues	36	37	35
Non-Utility and Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	74	83	39
Non-Utility and Other [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 74	$ 83	$ 39

Income Taxes (Narrative) (Detai

Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax [Line Items]
Valuation allowance, reversal		$ 4
Income tax expense (benefits) Federal Tax Reform				$ (6)
Overall income tax rate		(10.00%)	11.00%	8.00%
IPL [Member]
Income Tax [Line Items]
Income tax expense (benefits) Federal Tax Reform				$ (1)
Overall income tax rate		(16.00%)	8.00%	(1.00%)
WPL [Member]
Income Tax [Line Items]
Income tax expense (benefits) Federal Tax Reform				$ (5)
Overall income tax rate		(8.00%)	17.00%	15.00%
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, 2020	Dec. 31, 2019	Dec. 31, 2018
Current tax expense (benefit):
Federal	$ 1	$ (7)	$ (1)
State	8	24	(5)
Deferred tax expense (benefit):
Federal	22	70	68
State	8	42	30
Production tax credits	(95)	(55)	(30)
Investment tax credits	(1)	(1)	(1)
Income tax expense (benefit)	(57)	69	48
IPL's tax benefit riders [Member]
Current tax expense (benefit):
IPL's tax benefit riders	0	(4)	(13)
IPL [Member]
Current tax expense (benefit):
Federal	6	(11)	15
State	(1)	24	(7)
Deferred tax expense (benefit):
Federal	30	26	10
State	(2)	31	7
Production tax credits	(80)	(42)	(14)
Investment tax credits	0	0	(1)
Income tax expense (benefit)	(47)	24	(3)
IPL [Member] | IPL's tax benefit riders [Member]
Current tax expense (benefit):
IPL's tax benefit riders	0	(4)	(13)
WPL [Member]
Current tax expense (benefit):
Federal	(11)	12	(9)
State	7	13	(5)
Deferred tax expense (benefit):
Federal	(9)	31	44
State	10	6	22
Production tax credits	(15)	(13)	(15)
Investment tax credits	(1)	0	(1)
Income tax expense (benefit)	$ (19)	$ 49	$ 36

Income Taxes (Schedule Of Effec

Income Taxes (Schedule Of Effective Income Tax Rates) (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Effective Tax Rate [Line Items]
Statutory federal income tax rate	21.00%	21.00%	21.00%
State income taxes, net of federal benefits	2.00%	7.00%	7.00%
Production tax credits	(17.00%)	(9.00%)	(5.00%)
Amortization of excess deferred taxes	(13.00%)	(1.00%)	(1.00%)
Effect of rate-making on property-related differences	(3.00%)	(6.00%)	(8.00%)
Adjustment for prior period taxes	1.00%	1.00%	(2.00%)
Federal Tax Reform adjustments	0.00%	0.00%	(1.00%)
Other items, net	(1.00%)	(1.00%)	(1.00%)
Overall income tax rate	(10.00%)	11.00%	8.00%
IPL [Member]
Effective Tax Rate [Line Items]
Statutory federal income tax rate	21.00%	21.00%	21.00%
State income taxes, net of federal benefits	(1.00%)	8.00%	8.00%
Production tax credits	(28.00%)	(13.00%)	(5.00%)
Amortization of excess deferred taxes	(5.00%)	0.00%	0.00%
Effect of rate-making on property-related differences	(4.00%)	(10.00%)	(14.00%)
Adjustment for prior period taxes	1.00%	3.00%	(5.00%)
Federal Tax Reform adjustments	0.00%	0.00%	0.00%
Other items, net	0.00%	0.00%	(1.00%)
Overall income tax rate	(16.00%)	8.00%	(1.00%)
WPL [Member]
Effective Tax Rate [Line Items]
Statutory federal income tax rate	21.00%	21.00%	21.00%
State income taxes, net of federal benefits	6.00%	6.00%	6.00%
Production tax credits	(7.00%)	(5.00%)	(7.00%)
Amortization of excess deferred taxes	(26.00%)	(2.00%)	0.00%
Effect of rate-making on property-related differences	(2.00%)	(3.00%)	(2.00%)
Adjustment for prior period taxes	0.00%	0.00%	0.00%
Federal Tax Reform adjustments	0.00%	0.00%	(2.00%)
Other items, net	0.00%	0.00%	(1.00%)
Overall income tax rate	(8.00%)	17.00%	15.00%
IPL's tax benefit riders [Member]
Effective Tax Rate [Line Items]
IPL’s tax benefit riders	0.00%	(1.00%)	(2.00%)
IPL's tax benefit riders [Member] | IPL [Member]
Effective Tax Rate [Line Items]
IPL’s tax benefit riders	0.00%	(1.00%)	(5.00%)

Income Taxes (Schedule Of Defer

Income Taxes (Schedule Of Deferred Tax Assets And Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Deferred tax liabilities, property	$ 2,232	$ 2,022
Deferred tax liabilities, other	101	84
Total deferred tax liabilities	2,449	2,217
Deferred tax assets, federal credit carryforwards	454	355
Deferred tax assets, net operating losses carryforwards - federal	77	60
Deferred tax assets, net operating losses carryforwards - state	37	37
Deferred tax assets, other	73	61
Subtotal deferred tax assets	641	513
Deferred tax assets, valuation allowances	(6)	(10)
Total deferred tax assets	635	503
Total deferred tax liabilities, net	1,814	1,714
IPL [Member]
Deferred tax liabilities, property	1,312	1,184
Deferred tax liabilities, other	83	76
Total deferred tax liabilities	1,395	1,260
Deferred tax assets, federal credit carryforwards	258	175
Deferred tax assets, net operating losses carryforwards - federal	71	56
Deferred tax assets, net operating losses carryforwards - state	1	1
Deferred tax assets, other	30	20
Subtotal deferred tax assets	360	252
Deferred tax assets, valuation allowances	0	0
Total deferred tax assets	360	252
Total deferred tax liabilities, net	1,035	1,008
WPL [Member]
Deferred tax liabilities, property	854	770
Deferred tax liabilities, other	41	32
Total deferred tax liabilities	895	802
Deferred tax assets, federal credit carryforwards	175	160
Deferred tax assets, net operating losses carryforwards - federal	1	1
Deferred tax assets, net operating losses carryforwards - state	0	0
Deferred tax assets, other	18	16
Subtotal deferred tax assets	194	177
Deferred tax assets, valuation allowances	(1)	(1)
Total deferred tax assets	193	176
Total deferred tax liabilities, net	702	626
ATC Holdings [Member]
Deferred tax liabilities, ATC Holdings	$ 116	$ 111

Income Taxes (Summary Of Tax Cr

Income Taxes (Summary Of Tax Credit Carryforwards) (Details) $ in Millions	Dec. 31, 2020USD ($)
Federal [Member]
Tax carryforwards, net operating losses	$ 366
Tax carryforwards, tax credits	454
Federal [Member] | IPL [Member]
Tax carryforwards, net operating losses	339
Tax carryforwards, tax credits	258
Federal [Member] | WPL [Member]
Tax carryforwards, net operating losses	3
Tax carryforwards, tax credits	175
State [Member]
Tax carryforwards, net operating losses	622
State [Member] | IPL [Member]
Tax carryforwards, net operating losses	10
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, 2020
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, 2020	Dec. 31, 2019
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	9.00%	11.00%
Unrecognized compensation cost	$ 5
Alliant Energy common stock closing price on December 31, 2020 (in dollars per share)	$ 51.53
Percentage of base salary and short-term cash incentive compensation	100.00%
Carrying value of deferred compensation obligations	$ 22	$ 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
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 (in shares)	9,000,000
Shares included in diluted earnings per share (in shares)	537,485

Benefit Plans (Assumptions Used

Benefit Plans (Assumptions Used To Measure Benefit Plans) (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate for benefit obligations	2.57%	3.48%	4.34%
Discount rate for net periodic cost	3.48%	4.34%	3.66%
Expected rate of return on plan assets	7.10%	7.60%	7.60%
Interest crediting rate for Alliant Energy Cash Balance Pension Plan	4.76%	5.52%	5.04%
Pension Plans, Defined Benefit [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate for benefit obligations	2.61%	3.51%	4.35%
Discount rate for net periodic cost	3.51%	4.35%	3.68%
Expected rate of return on plan assets	7.10%	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	2.64%	3.50%	4.35%
Discount rate for net periodic cost	3.50%	4.35%	3.69%
Expected rate of return on plan assets	7.10%	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	2.31%	3.40%	4.24%
Discount rate for net periodic cost	3.40%	4.24%	3.53%
Expected rate of return on plan assets	4.50%	5.44%	5.44%
Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate for benefit obligations	2.28%	3.39%	4.23%
Discount rate for net periodic cost	3.39%	4.23%	3.51%
Expected rate of return on plan assets	4.50%	5.60%	5.60%
Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate for benefit obligations	2.27%	3.39%	4.23%
Discount rate for net periodic cost	3.39%	4.23%	3.51%
Expected rate of return on plan assets	3.28%	3.81%	3.84%
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, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 11	$ 10	$ 12
Interest cost	43	50	47
Expected return on plan assets	(70)	(60)	(69)
Amortization of prior service cost (credit)	0	(1)	(1)
Amortization of actuarial loss	34	36	35
Settlement losses	12	0	0
Total	30	35	24
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	3	3	4
Interest cost	7	9	8
Expected return on plan assets	(5)	(5)	(6)
Amortization of prior service cost (credit)	0	0	0
Amortization of actuarial loss	3	3	3
Settlement losses	0	0	0
Total	8	10	9
IPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	7	6	7
Interest cost	20	23	21
Expected return on plan assets	(33)	(28)	(32)
Amortization of actuarial loss	15	15	15
Settlement losses	7	0	0
Total	16	16	11
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1	2
Interest cost	3	3	3
Expected return on plan assets	(4)	(4)	(4)
Amortization of actuarial loss	1	2	1
Settlement losses	0	0	0
Total	1	2	2
WPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	3	4
Interest cost	19	21	20
Expected return on plan assets	(31)	(26)	(30)
Amortization of prior service cost (credit)	0	0	0
Amortization of actuarial loss	16	18	17
Total	8	16	11
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	1	2
Interest cost	3	3	3
Expected return on plan assets	(1)	(1)	(1)
Amortization of prior service cost (credit)	(1)	0	0
Amortization of actuarial loss	2	2	2
Total	$ 5	$ 5	$ 6

Benefit Plans (Funded Status of

Benefit Plans (Funded Status of Benefits Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations	$ 1,351	$ 1,280
Change in benefit obligation:
Net benefit obligation at January 1	1,280	1,175
Service cost	11	10	$ 12
Interest cost	43	50	47
Plan participants' contributions	0	0
Actuarial (gain) loss	132	124
Gross benefits paid	(115)	(79)
Net benefit obligation at December 31	1,351	1,280	1,175
Change in plan assets:
Fair value of plan assets at January 1	931	809
Actual return on plan assets	108	168
Employer contributions	60	33
Plan participants' contributions	0	0
Gross benefits paid	(115)	(79)
Fair value of plan assets at December 31	984	931	809
Under funded status at December 31	(367)	(349)
Fair value of plan assets	984	931
Pension Plans, Defined Benefit [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations	615	581
Change in benefit obligation:
Net benefit obligation at January 1	581	534
Service cost	7	6	7
Interest cost	20	23	21
Plan participants' contributions	0	0
Actuarial (gain) loss	60	56
Gross benefits paid	(53)	(38)
Net benefit obligation at December 31	615	581	534
Change in plan assets:
Fair value of plan assets at January 1	436	378
Actual return on plan assets	51	79
Employer contributions	19	17
Plan participants' contributions	0	0
Gross benefits paid	(53)	(38)
Fair value of plan assets at December 31	453	436	378
Under funded status at December 31	(162)	(145)
Fair value of plan assets	453	436
Pension Plans, Defined Benefit [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations	588	550
Change in benefit obligation:
Net benefit obligation at January 1	550	507
Service cost	4	3	4
Interest cost	19	21	20
Plan participants' contributions	0	0
Actuarial (gain) loss	55	54
Gross benefits paid	(40)	(35)
Net benefit obligation at December 31	588	550	507
Change in plan assets:
Fair value of plan assets at January 1	406	352
Actual return on plan assets	48	73
Employer contributions	22	16
Plan participants' contributions	0	0
Gross benefits paid	(40)	(35)
Fair value of plan assets at December 31	436	406	352
Under funded status at December 31	(152)	(144)
Fair value of plan assets	436	406
Other Postretirement Benefits Plans [Member]
Change in benefit obligation:
Net benefit obligation at January 1	214	206
Service cost	3	3	4
Interest cost	7	9	8
Plan participants' contributions	4	3
Actuarial (gain) loss	18	14
Gross benefits paid	(19)	(21)
Net benefit obligation at December 31	227	214	206
Change in plan assets:
Fair value of plan assets at January 1	105	99
Actual return on plan assets	11	15
Employer contributions	7	9
Plan participants' contributions	4	3
Gross benefits paid	(19)	(21)
Fair value of plan assets at December 31	108	105	99
Under funded status at December 31	(119)	(109)
Other Postretirement Benefits Plans [Member] | IPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	86	83
Service cost	1	1	2
Interest cost	3	3	3
Plan participants' contributions	1	1
Actuarial (gain) loss	8	6
Gross benefits paid	(8)	(8)
Net benefit obligation at December 31	91	86	83
Change in plan assets:
Fair value of plan assets at January 1	72	67
Actual return on plan assets	7	10
Employer contributions	2	2
Plan participants' contributions	1	1
Gross benefits paid	(8)	(8)
Fair value of plan assets at December 31	74	72	67
Under funded status at December 31	(17)	(14)
Other Postretirement Benefits Plans [Member] | WPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	85	83
Service cost	2	1	2
Interest cost	3	3	3
Plan participants' contributions	2	2
Actuarial (gain) loss	5	5
Gross benefits paid	(8)	(9)
Net benefit obligation at December 31	89	85	83
Change in plan assets:
Fair value of plan assets at January 1	17	17
Actual return on plan assets	2	1
Employer contributions	5	6
Plan participants' contributions	2	2
Gross benefits paid	(8)	(9)
Fair value of plan assets at December 31	18	17	$ 17
Under funded status at December 31	$ (71)	$ (68)

Benefit Plans (Amounts Recogniz

Benefit Plans (Amounts Recognized On The Consolidated Balance Sheets) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Amounts recognized on the balance sheets consist of:
Pension and other benefit obligations	$ (511)	$ (484)
Pension Plans, Defined Benefit [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	0	0
Current liabilities	(2)	(14)
Pension and other benefit obligations	(365)	(335)
Net amounts recognized at December 31	(367)	(349)
Amounts recognized in Regulatory Assets consist of:
Net actuarial loss	533	485
Prior service credit	(5)	(5)
Net amount recognized at December 31	528	480
Other Postretirement Benefits Plans [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	8	10
Current liabilities	(8)	(8)
Pension and other benefit obligations	(119)	(111)
Net amounts recognized at December 31	(119)	(109)
Amounts recognized in Regulatory Assets consist of:
Net actuarial loss	54	45
Prior service credit	(1)	(1)
Net amount recognized at December 31	53	44
IPL [Member]
Amounts recognized on the balance sheets consist of:
Pension and other benefit obligations	(186)	(168)
IPL [Member] | Pension Plans, Defined Benefit [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	0	0
Current liabilities	(1)	(1)
Pension and other benefit obligations	(161)	(144)
Net amounts recognized at December 31	(162)	(145)
Amounts recognized in Regulatory Assets consist of:
Net actuarial loss	228	208
Prior service credit	(1)	(1)
Net amount recognized at December 31	227	207
IPL [Member] | Other Postretirement Benefits Plans [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	4	7
Current liabilities	(2)	(2)
Pension and other benefit obligations	(19)	(19)
Net amounts recognized at December 31	(17)	(14)
Amounts recognized in Regulatory Assets consist of:
Net actuarial loss	20	17
Prior service credit	0	0
Net amount recognized at December 31	20	17
WPL [Member]
Amounts recognized on the balance sheets consist of:
Pension and other benefit obligations	(222)	(211)
WPL [Member] | Pension Plans, Defined Benefit [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	0	0
Current liabilities	0	0
Pension and other benefit obligations	(152)	(144)
Net amounts recognized at December 31	(152)	(144)
Amounts recognized in Regulatory Assets consist of:
Net actuarial loss	233	212
Prior service credit	(1)	(1)
Net amount recognized at December 31	232	211
WPL [Member] | Other Postretirement Benefits Plans [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	3	3
Current liabilities	(6)	(6)
Pension and other benefit obligations	(68)	(65)
Net amounts recognized at December 31	(71)	(68)
Amounts recognized in Regulatory Assets consist of:
Net actuarial loss	25	22
Prior service credit	(1)	(1)
Net amount recognized at December 31	$ 24	$ 21

Benefit Plans (Accumulated Bene

Benefit Plans (Accumulated Benefit Obligations) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	$ 1,305	$ 1,234
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	1,305	1,234
Fair value of plan assets	984	931
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	1,351	1,280
Fair value of plan assets	984	931
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	227	214
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	227	214
Fair value of plan assets	108	105
IPL [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	589	558
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	589	558
Fair value of plan assets	453	436
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	615	581
Fair value of plan assets	453	436
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	91	86
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	91	86
Fair value of plan assets	74	72
WPL [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	573	536
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	573	536
Fair value of plan assets	436	406
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	588	550
Fair value of plan assets	436	406
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	89	85
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	89	85
Fair value of plan assets	$ 18	$ 17

Benefit Plans (Regulatory Asset

Benefit Plans (Regulatory Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	$ 44	$ 39
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	$ 34	$ 29

Benefit Plans (Estimated Future

Benefit Plans (Estimated Future Funding) (Details) - Forecast [Member] $ in Millions	Dec. 31, 2021USD ($)
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2021	$ 39
Defined Benefit Pension Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2021	17
Defined Benefit Pension Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2021	18
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2021	8
Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2021	2
Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2021	$ 6

Benefit Plans (Expected Benefit

Benefit Plans (Expected Benefit Payments) (Details) $ in Millions	Dec. 31, 2020USD ($)
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2021	$ 94
Expected benefit payments, 2022	96
Expected benefit payments, 2023	96
Expected benefit payments, 2024	96
Expected benefit payments, 2025	97
Expected benefit payments, 2026 - 2030	472
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2021	76
Expected benefit payments, 2022	78
Expected benefit payments, 2023	79
Expected benefit payments, 2024	79
Expected benefit payments, 2025	80
Expected benefit payments, 2026 - 2030	398
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2021	18
Expected benefit payments, 2022	18
Expected benefit payments, 2023	17
Expected benefit payments, 2024	17
Expected benefit payments, 2025	17
Expected benefit payments, 2026 - 2030	74
IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2021	45
Expected benefit payments, 2022	45
Expected benefit payments, 2023	46
Expected benefit payments, 2024	45
Expected benefit payments, 2025	45
Expected benefit payments, 2026 - 2030	210
IPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2021	38
Expected benefit payments, 2022	38
Expected benefit payments, 2023	39
Expected benefit payments, 2024	38
Expected benefit payments, 2025	38
Expected benefit payments, 2026 - 2030	181
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2021	7
Expected benefit payments, 2022	7
Expected benefit payments, 2023	7
Expected benefit payments, 2024	7
Expected benefit payments, 2025	7
Expected benefit payments, 2026 - 2030	29
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2021	39
Expected benefit payments, 2022	39
Expected benefit payments, 2023	39
Expected benefit payments, 2024	40
Expected benefit payments, 2025	39
Expected benefit payments, 2026 - 2030	191
WPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2021	32
Expected benefit payments, 2022	32
Expected benefit payments, 2023	32
Expected benefit payments, 2024	33
Expected benefit payments, 2025	33
Expected benefit payments, 2026 - 2030	162
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2021	7
Expected benefit payments, 2022	7
Expected benefit payments, 2023	7
Expected benefit payments, 2024	7
Expected benefit payments, 2025	6
Expected benefit payments, 2026 - 2030	$ 29

Benefit Plans (Allocation Of Pl

Benefit Plans (Allocation Of Plan Assets) (Details)	Dec. 31, 2020
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	26.00%
Pension Plans, Defined Benefit [Member] | Equity securities - international [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation, percentage	23.00%
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	9.00%
Pension Plans, Defined Benefit [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation, percentage	28.00%
Other Postretirement Benefits Plans [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation, percentage	1.00%
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	1.00%
Other Postretirement Benefits Plans [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation, percentage	73.00%
Minimum [Member] | Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	0.00%
Minimum [Member] | Pension Plans, Defined Benefit [Member] | Equity securites - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	11.00%
Minimum [Member] | Pension Plans, Defined Benefit [Member] | Equity securities - international [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	14.00%
Minimum [Member] | Pension Plans, Defined Benefit [Member] | Global asset securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	5.00%
Minimum [Member] | Pension Plans, Defined Benefit [Member] | Risk parity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	5.00%
Minimum [Member] | Pension Plans, Defined Benefit [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	20.00%
Minimum [Member] | Other Postretirement Benefits Plans [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	0.00%
Minimum [Member] | Other Postretirement Benefits Plans [Member] | Equity securites - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	0.00%
Minimum [Member] | Other Postretirement Benefits Plans [Member] | Equity securities - international [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	0.00%
Minimum [Member] | Other Postretirement Benefits Plans [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	20.00%
Maximum [Member] | Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	5.00%
Maximum [Member] | Pension Plans, Defined Benefit [Member] | Equity securites - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	41.00%
Maximum [Member] | Pension Plans, Defined Benefit [Member] | Equity securities - international [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	34.00%
Maximum [Member] | Pension Plans, Defined Benefit [Member] | Global asset securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	15.00%
Maximum [Member] | Pension Plans, Defined Benefit [Member] | Risk parity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	15.00%
Maximum [Member] | Pension Plans, Defined Benefit [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	40.00%
Maximum [Member] | Other Postretirement Benefits Plans [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	5.00%
Maximum [Member] | Other Postretirement Benefits Plans [Member] | Equity securites - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	46.00%
Maximum [Member] | Other Postretirement Benefits Plans [Member] | Equity securities - international [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	34.00%
Maximum [Member] | Other Postretirement Benefits Plans [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target range allocation, percentage	100.00%

Benefit Plans (Fair Value Of Pl

Benefit Plans (Fair Value Of Plan Assets By Asset Category And Fair Value Hierarchy Level) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 984	$ 931	$ 809
Assets measured at net asset value	426	410
Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	46	46
Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	4
Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	46	42
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	182	161
Pension Plans, Defined Benefit [Member] | Equity securites - U.S. [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	182	161
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	151	136
Pension Plans, Defined Benefit [Member] | Equity securities - international [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	151	136
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	45	47
Pension Plans, Defined Benefit [Member] | Global asset securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	45	47
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	134	131
Pension Plans, Defined Benefit [Member] | Fixed income securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	56	58
Pension Plans, Defined Benefit [Member] | Fixed income securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	78	73
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	558	521
Pension Plans, Defined Benefit [Member] | Subtotal [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	434	406
Pension Plans, Defined Benefit [Member] | Subtotal [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	124	115
Pension Plans, Defined Benefit [Member] | Subtotal [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | Accrued investment income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	1
Pension Plans, Defined Benefit [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	453	436	378
Assets measured at net asset value	196	192
Pension Plans, Defined Benefit [Member] | IPL [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	21	22
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	0	2
Pension Plans, Defined Benefit [Member] | IPL [Member] | Cash and equivalents [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	21	20
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	84	75
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	84	75
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	69	64
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	69	64
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	21	22
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	21	22
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	62	61
Pension Plans, Defined Benefit [Member] | IPL [Member] | Fixed income securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	26	27
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	36	34
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	257	244
Pension Plans, Defined Benefit [Member] | IPL [Member] | Subtotal [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	200	190
Pension Plans, Defined Benefit [Member] | IPL [Member] | Subtotal [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	57	54
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] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	436	406	352
Assets measured at net asset value	189	179
Pension Plans, Defined Benefit [Member] | WPL [Member] | Cash and equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	20	20
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	0	2
Pension Plans, Defined Benefit [Member] | WPL [Member] | Cash and equivalents [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	20	18
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	81	70
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	81	70
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	67	59
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	67	59
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	21
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	21
Pension Plans, Defined Benefit [Member] | WPL [Member] | Global asset securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | WPL [Member] | Global asset securities [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans, Defined Benefit [Member] | WPL [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	59	57
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	25
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	34	32
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	247	227
Pension Plans, Defined Benefit [Member] | WPL [Member] | Subtotal [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	193	177
Pension Plans, Defined Benefit [Member] | WPL [Member] | Subtotal [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	54	50
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
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	108	105	99
Assets measured at net asset value	27	26
Total assets in fair value hierarchy	81	79
Other Postretirement Benefit Plans, Defined Benefit [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	78	78
Other Postretirement Benefit Plans, Defined Benefit [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	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	2	3
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash and equivalents [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	3
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	2	0
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	4	4
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	4	4
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	3	3
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	3	3
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] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	72	69
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	71	68
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	1	1
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	74	72	67
Assets measured at net asset value	22	22
Total assets in fair value hierarchy	52	50
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	51	50
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	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	2
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Cash and equivalents [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	2
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	1	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	51	48
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	51	48
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Fixed income securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Fixed income securities [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18	17	$ 17
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18	17
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] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18	17
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Fixed income securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18	17
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	$ (1)	$ (1)

Benefit Plans (Employees Partic

Benefit Plans (Employees Participate In Defined Contribution Retirement Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
401(k) costs	$ 25	$ 25	$ 25
IPL [Member]
401(k) costs	13	13	13
WPL [Member]
401(k) costs	$ 12	$ 12	$ 11

Benefit Plans (Recognized Compe

Benefit Plans (Recognized Compensation Expense And Income Tax Benefits) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Compensation expense	$ 16	$ 26	$ 17
Income tax benefits	4	7	5
IPL [Member]
Compensation expense	9	15	9
Income tax benefits	2	4	3
WPL [Member]
Compensation expense	6	10	7
Income tax benefits	$ 2	$ 3	$ 2

Benefit Plans (Summary Of Perfo

Benefit Plans (Summary Of Performance Shares and Performance Units Activity) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
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)	74,193	0
Nonvested, January 1, weighted average grant date fair value (in dollars per share)	$ 47.44	$ 0
Granted (in shares/awards)	56,204	91,816
Granted, weighted average grant date fair value (in dollars per share)	$ 64.04	$ 47.23
Forfeited (in shares/awards)	(1,241)	(17,623)
Forfeited, weighted average grant date fair value (in dollars per share)	$ 50.94	$ 46.35
Nonvested, December 31 (in shares/awards)	129,156	74,193	0
Nonvested, December 31, weighted average grant date fair value (in dollars per share)	$ 54.63	$ 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)	131,872	203,188	223,511
Granted (in shares/awards)	0	0	74,163
Vested (in shares/awards)	(63,565)	(66,322)	(90,806)
Forfeited (in shares/awards)	0	(4,994)	(3,680)
Nonvested, December 31 (in shares/awards)	68,307	131,872	203,188
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)	34,574	57,761	71,737
Granted (in shares/awards)	0	0	19,840
Vested (in shares/awards)	(16,661)	(20,131)	(31,910)
Forfeited (in shares/awards)	(1,112)	(3,056)	(1,906)
Nonvested, December 31 (in shares/awards)	16,801	34,574	57,761

Benefit Plans (Fair Values Of N

Benefit Plans (Fair Values Of Nonvested Performance Shares And Performance Units) (Details)	12 Months Ended
	Dec. 31, 2020$ / sharesshares
Performance Shares Liability Awards [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards)	68,307
Performance Units Liability Awards [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards)	16,801
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	172.50%
Fair values of each nonvested award (in dollars per share) | $ / shares	$ 88.89
2018 Grant [Member] | Performance Units Liability Awards [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards)	16,801
Estimated payout percentage based on performance criteria	172.50%
Fair values of each nonvested award (in dollars per share) | $ / shares	$ 88.89

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, 2020	Dec. 31, 2019	Dec. 31, 2018
Performance Shares Liability Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares/awards)	63,565	66,322	90,806
Performance Units Liability Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares/awards)	16,661	20,131	31,910
2017 Grant [Member] | Performance Shares Liability Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares/awards)	63,565
Vested percentage of the target	155.00%
Cash and stock payout value	$ 6
Cash payout value	$ 5
Common stock shares from vested performance shares (in shares)	9,543
2017 Grant [Member] | Performance Units Liability Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares/awards)	16,661
Vested percentage of the target	155.00%
Cash and stock payout value	$ 2
Cash payout value	$ 2
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
Cash payout value		$ 4
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
Cash payout value		$ 1
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
Cash payout value			$ 5
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
Cash payout value			$ 1

Benefit Plans (Summary of Restr

Benefit Plans (Summary of Restricted Stock Units) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
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)	89,281	0
Nonvested, January 1, weighted average grant date fair value (in dollars per share)	$ 46.04	$ 0
Granted (in shares/awards)	61,056	105,348
Granted, weighted average grant date fair value (in dollars per share)	$ 59.42	$ 45.98
Forfeited (in shares/awards)	(3,788)	(16,067)
Forfeited, weighted average grant date fair value (in dollars per share)	$ 49.01	$ 45.63
Nonvested, December 31 (in shares/awards)	146,549	89,281	0
Nonvested, December 31, weighted average grant date fair value (in dollars per share)	$ 51.54	$ 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)	113,033	174,163	113,749
Granted (in shares/awards)	0	0	63,568
Vested (in shares/awards)	(54,483)	(56,849)	0
Forfeited (in shares/awards)	0	(4,281)	(3,154)
Nonvested, December 31 (in shares/awards)	58,550	113,033	174,163

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, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Nonvested, January 1 (in shares/awards)	206,065	203,188	132,705
Nonvested, January 1, weighted average grant date fair value (in dollars per share)	$ 41.50	$ 37.23	$ 36.50
Granted (in shares/awards)	56,204	91,816	74,163
Granted, weighted average grant date fair value (in dollars per share)	$ 59.37	$ 46.10	$ 38.60
Vested (in shares/awards)	(63,565)	(66,322)	0
Vested, weighted average grant date fair value (in dollars per share)	$ 39.12	$ 33.93	$ 0
Forfeited (in shares/awards)	(1,241)	(22,617)	(3,680)
Forfeited, weighted average grant date fair value (in dollars per share)	$ 49.25	$ 44	$ 38.60
Nonvested, December 31 (in shares/awards)	197,463	206,065	203,188
Nonvested, December 31, weighted average grant date fair value (in dollars per share)	$ 47.31	$ 41.50	$ 37.23

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, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Carrying value of deferred compensation obligation	$ 11	$ 10
Fair market value of deferred compensation obligation	$ 20	$ 21

Asset Retirement Obligations (R

Asset Retirement Obligations (Reconciliation Of Changes In Asset Retirement Obligations) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1	$ 196	$ 177
Revisions in estimated cash flows	13	(6)
Liabilities settled	(13)	(8)
Liabilities incurred	48	26
Accretion expense	7	7
Balance, December 31	251	196
IPL [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1	134	118
Revisions in estimated cash flows	9	(7)
Liabilities settled	(9)	(8)
Liabilities incurred	38	26
Accretion expense	5	5
Balance, December 31	177	134
WPL [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1	62	59
Revisions in estimated cash flows	4	1
Liabilities settled	(4)	0
Liabilities incurred	10	0
Accretion expense	2	2
Balance, December 31	$ 74	$ 62

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, 2020DekathermsMWhTgal
FTRs (MWhs) [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs) | MWh	9,285
FTRs (MWhs) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs) | MWh	3,398
FTRs (MWhs) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in MWhs) | MWh	5,887
Gas (Dths) [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in Dths) | Dekatherms	208,542
Gas (Dths) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in Dths) | Dekatherms	109,063
Gas (Dths) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in Dths) | Dekatherms	99,479
Coal (Tons) [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in Tons) | T	5,648
Coal (Tons) [Member] | IPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in Tons) | T	2,548
Coal (Tons) [Member] | WPL [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in Tons) | T	3,100
Diesel Fuel (gallons) [Member]
Notional Amount of Derivatives [Line Items]
Notional unit amount of derivatives (in Gallons) | gal	5,544
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	5,544

Derivative Instruments (Fair Va

Derivative Instruments (Fair Value Of Financial Instruments) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Current derivative assets	$ 24	$ 16
Non-current derivative assets	10	11
Current derivative liabilities	9	19
Non-current derivative liabilities	16	19
IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	20	12
Non-current derivative assets	9	10
Current derivative liabilities	3	9
Non-current derivative liabilities	9	9
WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	4	4
Non-current derivative assets	1	1
Current derivative liabilities	6	10
Non-current derivative liabilities	$ 7	$ 10

Fair Value Measurements (Recurr

Fair Value Measurements (Recurring Fair Value Measurements) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Assets:
Money market fund investments	$ 44	$ 5
Deferred proceeds	188	188
Liabilities and equity:
Long-term debt (including current maturities)	8,109	6,920
Commodity Contracts [Member]
Assets:
Derivatives	34	27
Liabilities and equity:
Derivatives	25	38
Level 1 [Member]
Assets:
Money market fund investments	44	5
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:
Money market fund investments	0	0
Deferred proceeds	0	0
Liabilities and equity:
Long-term debt (including current maturities)	8,107	6,918
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	5	5
Liabilities and equity:
Derivatives	25	37
Level 3 [Member]
Assets:
Money market fund investments	0	0
Deferred proceeds	188	188
Liabilities and equity:
Long-term debt (including current maturities)	2	2
Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	29	22
Liabilities and equity:
Derivatives	0	1
Carrying Amount [Member]
Assets:
Money market fund investments	44	5
Deferred proceeds	188	188
Liabilities and equity:
Long-term debt (including current maturities)	6,777	6,190
Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	34	27
Liabilities and equity:
Derivatives	25	38
IPL [Member]
Assets:
Money market fund investments	44	5
Deferred proceeds	188	188
Liabilities and equity:
Long-term debt (including current maturities)	4,021	3,489
IPL [Member] | Commodity Contracts [Member]
Assets:
Derivatives	29	22
Liabilities and equity:
Derivatives	12	18
IPL [Member] | Level 1 [Member]
Assets:
Money market fund investments	44	5
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:
Money market fund investments	0	0
Deferred proceeds	0	0
Liabilities and equity:
Long-term debt (including current maturities)	4,021	3,489
IPL [Member] | Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	3	3
Liabilities and equity:
Derivatives	12	17
IPL [Member] | Level 3 [Member]
Assets:
Money market fund investments	0	0
Deferred proceeds	188	188
Liabilities and equity:
Long-term debt (including current maturities)	0	0
IPL [Member] | Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	26	19
Liabilities and equity:
Derivatives	0	1
IPL [Member] | Carrying Amount [Member]
Assets:
Money market fund investments	44	5
Deferred proceeds	188	188
Liabilities and equity:
Long-term debt (including current maturities)	3,345	3,147
IPL [Member] | Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	29	22
Liabilities and equity:
Derivatives	12	18
WPL [Member]
Liabilities and equity:
Long-term debt (including current maturities)	2,690	2,268
WPL [Member] | Commodity Contracts [Member]
Assets:
Derivatives	5	5
Liabilities and equity:
Derivatives	13	20
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)	2,690	2,268
WPL [Member] | Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	2	2
Liabilities and equity:
Derivatives	13	20
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	3	3
Liabilities and equity:
Derivatives	0	0
WPL [Member] | Carrying Amount [Member]
Liabilities and equity:
Long-term debt (including current maturities)	2,130	1,933
WPL [Member] | Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	5	5
Liabilities and equity:
Derivatives	$ 13	$ 20

Fair Value Measurements (Fair V

Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
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	$ 21	$ 12
Total net gains included in changes in net assets (realized/unrealized)	11	8
Transfers out of Level 3	0	4
Purchases	14	14
Sales	(1)	0
Settlements	(16)	(17)
Ending balance, December 31	29	21
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	11
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	18	9
Total net gains included in changes in net assets (realized/unrealized)	10	11
Transfers out of Level 3	0	2
Purchases	11	9
Sales	(1)	0
Settlements	(12)	(13)
Ending balance, December 31	26	18
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	10	12
Deferred Proceeds [Member]
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance, January 1	188	119
Total net gains included in changes in net assets (realized/unrealized)	0	0
Transfers out of Level 3	0	0
Purchases	0	0
Sales	0	0
Settlements	0	69
Ending balance, December 31	188	188
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	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	188	119
Total net gains included in changes in net assets (realized/unrealized)	0	0
Transfers out of Level 3	0	0
Purchases	0	0
Sales	0	0
Settlements	0	69
Ending balance, December 31	188	188
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	$ 0	$ 0

Fair Value Measurements (Fair_2

Fair Value Measurements (Fair Value Of Net Derivative Assets (Liabilities)) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	$ 29	$ 21	$ 12
Excluding FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	18	15
FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	11	7
IPL [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	26	18	$ 9
IPL [Member] | Excluding FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	17	14
IPL [Member] | FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	9	5
WPL [Member] | Excluding FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	1	1
WPL [Member] | FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	$ 2	$ 2

Commitments And Contingencies_2

Commitments And Contingencies (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2017
Cumulative Effect, Period of Adoption, Adjustment [Member]
Long-term Purchase Commitment [Line Items]
New accounting standard, cumulative effect adjustment to retained earnings, pre-tax		$ 12
Whiting Petroleum Affiliate [Member]
Long-term Purchase Commitment [Line Items]
Partnership share, percent	6.00%
Obligations, maximum	$ 68
Credit loss liability	5
Credit loss expense	7
Capital Purchase Commitment [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	8
WPL [Member] | Capital Purchase Commitment [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments	7
Indemnification Agreement [Member]
Long-term Purchase Commitment [Line Items]
Obligations, maximum	74
Indemnification Agreement [Member] | Purchased Power [Member]
Long-term Purchase Commitment [Line Items]
Obligations, maximum	17
FERC [Member]
Long-term Purchase Commitment [Line Items]
Base return on equity, percentage		9.88%	10.02%
FERC [Member] | IPL [Member]
Long-term Purchase Commitment [Line Items]
Refunds related to MISO base return on equity complaints	15
FERC [Member] | WPL [Member]
Long-term Purchase Commitment [Line Items]
Refunds related to MISO base return on equity complaints	$ 5

Commitments And Contingencies_3

Commitments And Contingencies (Other Purchase Commitments) (Details) $ in Millions	Dec. 31, 2020USD ($)
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2021	$ 383
Minimum future commitments, 2022	218
Minimum future commitments, 2023	165
Minimum future commitments, 2024	113
Minimum future commitments, 2025	71
Minimum future commitments, Thereafter	214
Minimum future commitments, Total	1,164
Individual commitments incurred	1
Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2021	252
Minimum future commitments, 2022	174
Minimum future commitments, 2023	137
Minimum future commitments, 2024	105
Minimum future commitments, 2025	68
Minimum future commitments, Thereafter	185
Minimum future commitments, Total	921
Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2021	70
Minimum future commitments, 2022	27
Minimum future commitments, 2023	17
Minimum future commitments, 2024	0
Minimum future commitments, 2025	0
Minimum future commitments, Thereafter	0
Minimum future commitments, Total	114
Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2021	61
Minimum future commitments, 2022	17
Minimum future commitments, 2023	11
Minimum future commitments, 2024	8
Minimum future commitments, 2025	3
Minimum future commitments, Thereafter	29
Minimum future commitments, Total	129
IPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2021	187
Minimum future commitments, 2022	109
Minimum future commitments, 2023	96
Minimum future commitments, 2024	70
Minimum future commitments, 2025	40
Minimum future commitments, Thereafter	98
Minimum future commitments, Total	600
IPL [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2021	130
Minimum future commitments, 2022	88
Minimum future commitments, 2023	79
Minimum future commitments, 2024	68
Minimum future commitments, 2025	38
Minimum future commitments, Thereafter	71
Minimum future commitments, Total	474
IPL [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2021	32
Minimum future commitments, 2022	16
Minimum future commitments, 2023	13
Minimum future commitments, 2024	0
Minimum future commitments, 2025	0
Minimum future commitments, Thereafter	0
Minimum future commitments, Total	61
IPL [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2021	25
Minimum future commitments, 2022	5
Minimum future commitments, 2023	4
Minimum future commitments, 2024	2
Minimum future commitments, 2025	2
Minimum future commitments, Thereafter	27
Minimum future commitments, Total	65
WPL [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2021	186
Minimum future commitments, 2022	99
Minimum future commitments, 2023	63
Minimum future commitments, 2024	38
Minimum future commitments, 2025	31
Minimum future commitments, Thereafter	115
Minimum future commitments, Total	532
WPL [Member] | Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2021	122
Minimum future commitments, 2022	86
Minimum future commitments, 2023	58
Minimum future commitments, 2024	37
Minimum future commitments, 2025	30
Minimum future commitments, Thereafter	114
Minimum future commitments, Total	447
WPL [Member] | Coal [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2021	38
Minimum future commitments, 2022	11
Minimum future commitments, 2023	4
Minimum future commitments, 2024	0
Minimum future commitments, 2025	0
Minimum future commitments, Thereafter	0
Minimum future commitments, Total	53
WPL [Member] | Other [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments, 2021	26
Minimum future commitments, 2022	2
Minimum future commitments, 2023	1
Minimum future commitments, 2024	1
Minimum future commitments, 2025	1
Minimum future commitments, Thereafter	1
Minimum future commitments, Total	$ 32

Commitments And Contingencies_4

Commitments And Contingencies (Schedule Of Environmental Liabilities) (Details) - Natural Gas Processing Plant [Member] $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)
Current and non-current environmental liabilities	$ 15
IPL [Member]
Current and non-current environmental liabilities	12
Minimum [Member]
Range of estimated future costs	11
Minimum [Member] | IPL [Member]
Range of estimated future costs	8
Maximum [Member]
Range of estimated future costs	28
Maximum [Member] | IPL [Member]
Range of estimated future costs	$ 22

Segments Of Business (Narrative

Segments Of Business (Narrative) (Details)	12 Months Ended
	Dec. 31, 2020segment
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	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	$ 3,416	$ 3,648	$ 3,534
Depreciation and amortization	615	567	507
Operating income (loss)	740	778	694
Interest expense	275	273	247
Equity income from unconsolidated investments, net	(61)	(53)	(55)
Income tax expense (benefit)	(57)	69	48
Net income attributable to common shareowners	614	557	512
Total assets	17,710	16,701	15,426
Investments in equity method subsidiaries	476	458	421
Construction and acquisition expenditures	1,366	1,640	1,634
IPL [Member]
Segment Reporting Information [Line Items]
Revenues	1,947	2,089	2,042
Operating income (loss)	410	403	351
Interest expense	139	127	119
Income tax expense (benefit)	(47)	24	(3)
Net income attributable to common shareowners	324	284	264
Total assets	9,849	9,277
WPL [Member]
Segment Reporting Information [Line Items]
Revenues	1,395	1,476	1,453
Operating income (loss)	306	347	313
Interest expense	104	102	98
Income tax expense (benefit)	(19)	49	36
Total assets	6,912	6,507
Electric [Member]
Segment Reporting Information [Line Items]
Revenues	2,920	3,064	3,000
Depreciation and amortization	556	513	457
Operating income (loss)	643	679	610
Equity income from unconsolidated investments, net	(2)	(1)	(1)
Total assets	14,358	13,659	12,486
Investments in equity method subsidiaries	11	9	8
Construction and acquisition expenditures	1,109	1,439	1,422
Electric [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Revenues	1,695	1,781	1,731
Depreciation and amortization	321	295	255
Operating income (loss)	358	360	318
Total assets	8,518	8,075	7,220
Construction and acquisition expenditures	626	964	891
Electric [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Revenues	1,225	1,283	1,269
Depreciation and amortization	235	218	202
Operating income (loss)	285	319	292
Total assets	5,840	5,584	5,266
Construction and acquisition expenditures	483	475	531
Gas [Member]
Segment Reporting Information [Line Items]
Revenues	373	455	447
Depreciation and amortization	49	47	42
Operating income (loss)	74	70	53
Equity income from unconsolidated investments, net	0	0	0
Total assets	1,413	1,269	1,184
Investments in equity method subsidiaries	0	0	0
Construction and acquisition expenditures	182	100	147
Gas [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Revenues	208	264	266
Depreciation and amortization	30	29	25
Operating income (loss)	50	39	28
Total assets	766	734	687
Construction and acquisition expenditures	59	56	100
Gas [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Revenues	165	191	181
Depreciation and amortization	19	18	17
Operating income (loss)	24	31	25
Total assets	647	535	497
Construction and acquisition expenditures	123	44	47
Other Utility [Member]
Segment Reporting Information [Line Items]
Revenues	49	46	48
Depreciation and amortization	5	3	3
Operating income (loss)	(1)	1	1
Equity income from unconsolidated investments, net	0	0	0
Total assets	990	856	893
Investments in equity method subsidiaries	0	0	0
Construction and acquisition expenditures	2	0	0
Other Utility [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Revenues	44	44	45
Depreciation and amortization	5	3	3
Operating income (loss)	2	4	5
Total assets	565	468	504
Construction and acquisition expenditures	2	0	0
Other Utility [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Revenues	5	2	3
Depreciation and amortization	0	0	0
Operating income (loss)	(3)	(3)	(4)
Total assets	425	388	389
Construction and acquisition expenditures	0	0	0
Utility Business [Member]
Segment Reporting Information [Line Items]
Revenues	3,342	3,565	3,495
Depreciation and amortization	610	563	502
Operating income (loss)	716	750	664
Interest expense	243	229	217
Equity income from unconsolidated investments, net	(2)	(1)	(1)
Income tax expense (benefit)	(66)	73	33
Net income attributable to common shareowners	573	517	472
Total assets	16,761	15,784	14,563
Investments in equity method subsidiaries	11	9	8
Construction and acquisition expenditures	1,293	1,539	1,569
Utility Business [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Revenues	1,947	2,089	2,042
Depreciation and amortization	356	327	283
Operating income (loss)	410	403	351
Interest expense	139	127	119
Income tax expense (benefit)	(47)	24	(3)
Net income attributable to common shareowners	324	284	264
Total assets	9,849	9,277	8,411
Construction and acquisition expenditures	687	1,020	991
Utility Business [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Revenues	1,395	1,476	1,453
Depreciation and amortization	254	236	219
Operating income (loss)	306	347	313
Interest expense	104	102	98
Income tax expense (benefit)	(19)	49	36
Net income attributable to common shareowners	249	233	208
Total assets	6,912	6,507	6,152
Construction and acquisition expenditures	606	519	578
Non-Utility [Member]
Segment Reporting Information [Line Items]
Revenues	74	83	39
Depreciation and amortization	5	4	5
Operating income (loss)	24	28	30
Interest expense	32	44	30
Equity income from unconsolidated investments, net	(59)	(52)	(54)
Income tax expense (benefit)	9	(4)	15
Net income attributable to common shareowners	41	40	40
Total assets	949	917	863
Investments in equity method subsidiaries	465	449	413
Construction and acquisition expenditures	$ 73	$ 101	$ 65

Related Parties (Narrative) (De

Related Parties (Narrative) (Details) - WPL [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Related Party Transaction [Line Items]
Receipt of construction deposits	$ 46
WPL Owed ATC [Member]
Related Party Transaction [Line Items]
Net amounts owed	$ 9	$ 9

Related Parties (Service Agreem

Related Parties (Service Agreements) (Details) - Corporate Services [Member] - Subsidiary of Common Parent [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Administrative and General Services Billings [Member] | IPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	$ 176	$ 185	$ 170
Administrative and General Services Billings [Member] | WPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	142	142	132
Transmission Sales Credited [Member] | IPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	35	68	48
Transmission Sales Credited [Member] | WPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	3	7	28
Transmission Purchases Billed [Member] | IPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	329	331	358
Transmission Purchases Billed [Member] | WPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	$ 108	$ 120	$ 81

Related Parties (Net Intercompa

Related Parties (Net Intercompany Payables) (Details) - Corporate Services [Member] - Subsidiary of Common Parent [Member] - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
IPL [Member]
Related Party Transaction [Line Items]
Intercompany payables to Corporate Services	$ 110	$ 112
WPL [Member]
Related Party Transaction [Line Items]
Intercompany payables to Corporate Services	$ 73	$ 85

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, 2020	Dec. 31, 2019	Dec. 31, 2018
ATC Billings To WPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	$ 108	$ 109	$ 106
WPL Billings To ATC [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	$ 10	$ 13	$ 11

Condensed Parent Company Fina_3

Condensed Parent Company Financial Statements (Condensed Statements Of Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating expenses	$ 2,676	$ 2,870	$ 2,840
Operating income (loss)	740	778	694
Other (income) and deductions:
Equity earnings from consolidated subsidiaries	(61)	(53)	(55)
Interest expense	275	273	247
Total other (income) and deductions	173	142	124
Income before income taxes	567	636	570
Income tax benefit	(57)	69	48
Net income	624	567	522
Parent Company [Member]
Operating expenses	7	2	5
Operating income (loss)	(7)	(2)	(5)
Other (income) and deductions:
Equity earnings from consolidated subsidiaries	(625)	(562)	(523)
Interest expense	2	9	4
Other	4	7	2
Total other (income) and deductions	(619)	(546)	(517)
Income before income taxes	612	544	512
Income tax benefit	(4)	(15)	(1)
Net income	$ 616	$ 559	$ 513

Condensed Parent Company Fina_4

Condensed Parent Company Financial Statements (Condensed Balance Sheets) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Other	$ 123	$ 144
Total current assets	887	876
Investments:
Other	154	148
Total investments	485	468
Other assets	2,002	1,830
Total assets	17,710	16,701	$ 15,426
Current liabilities:
Commercial paper	389	337
Other	207	356
Total current liabilities	1,297	2,054
Other liabilities	374	299
Common Equity:
Retained earnings	2,994	2,766
Accumulated other comprehensive income (loss)	(1)	1
Shares in deferred compensation trust	(11)	(10)
Total common equity	5,688	5,205
Total liabilities and equity	17,710	16,701
Parent Company [Member]
Current assets:
Notes receivable from affiliated companies	32	27
Other	5	6
Total current assets	37	33
Investments:
Investments in consolidated subsidiaries	6,664	6,017
Other	2	1
Total investments	6,666	6,018
Other assets	88	89
Total assets	6,791	6,140
Current liabilities:
Commercial paper	132	169
Notes payable to affiliated companies	937	731
Other	29	19
Total current liabilities	1,098	919
Other liabilities	2	15
Common Equity:
Common stock and additional paid-in capital	2,706	2,448
Retained earnings	2,996	2,766
Accumulated other comprehensive income (loss)	0	2
Shares in deferred compensation trust	(11)	(10)
Total common equity	5,691	5,206
Total liabilities and equity	$ 6,791	$ 6,140

Condensed Parent Company Fina_5

Condensed Parent Company Financial Statements (Condensed Statements of Cash Flows) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net cash flows from operating activities	$ 501	$ 660	$ 528
Cash flows used for investing activities:
Other	(43)	(60)	(38)
Net cash flows used for investing activities	(951)	(1,287)	(1,067)
Cash flows used for financing activities:
Common stock dividends	(377)	(337)	(312)
Proceeds from issuance of common stock, net	247	390	197
Net change in commercial paper	52	(104)	26
Other	(27)	(24)	(24)
Net cash flows from (used for) financing activities	488	619	531
Net increase (decrease) in cash, cash equivalents and restricted cash	38	(8)	(8)
Cash, cash equivalents and restricted cash at beginning of period	18	26	34
Cash, cash equivalents and restricted cash at end of period	56	18	26
Supplemental cash flows information:
Interest	(274)	(268)	(247)
Income taxes, net	5	21	(5)
Parent Company [Member]
Net cash flows from operating activities	396	305	311
Cash flows used for investing activities:
Capital contributions to consolidated subsidiaries	(429)	(250)	(625)
Net change in notes receivable from and payable to affiliates	201	8	441
Net cash flows used for investing activities	(228)	(242)	(184)
Cash flows used for financing activities:
Common stock dividends	(377)	(338)	(312)
Proceeds from issuance of common stock, net	247	390	197
Net change in commercial paper	(37)	(116)	(10)
Other	(1)	1	(2)
Net cash flows from (used for) financing activities	(168)	(63)	(127)
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	(2)	(9)	(4)
Income taxes, net	$ 10	$ 14	$ 5

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, 2020	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	$ 7	$ 10	$ 12
Charged to Expense	25	17	21
Charged to Other Accounts	9	2	1
Deductions	23	22	24
Ending Balance	18	7	10
IPL [Member]
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	1	3	1
Charged to Expense	18	16	21
Charged to Other Accounts	0	0	0
Deductions	18	18	19
Ending Balance	1	1	3
WPL [Member]
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	6	7	11
Charged to Expense	7	1	0
Charged to Other Accounts	9	2	1
Deductions	5	4	5
Ending Balance	$ 17	$ 6	$ 7