Condensed Consolidated Balance Sheets - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 20	$ 54
Accounts receivable, less allowance for expected credit losses	392	412
Production fuel, at weighted average cost	42	66
Gas stored underground, at weighted average cost	58	46
Materials and supplies, at weighted average cost	118	105
Regulatory assets	111	81
Other	274	123
Total current assets	1,015	887
Property, plant and equipment, net	14,738	14,336
Investments:
ATC Holdings	338	331
Other	176	154
Total investments	514	485
Other assets:
Regulatory assets	1,907	1,929
Deferred charges and other	155	73
Total other assets	2,062	2,002
Total assets	18,329	17,710
Current liabilities:
Current maturities of long-term debt	383	8
Commercial paper	316	389
Accounts payable	361	377
Regulatory liabilities	231	249
Other	266	274
Total current liabilities	1,557	1,297
Long-term debt, net (excluding current portion)	6,692	6,769
Other liabilities:
Deferred tax liabilities	1,906	1,814
Regulatory liabilities	1,130	1,057
Pension and other benefit obligations	452	511
Other	407	374
Total other liabilities	3,895	3,756
Commitments and contingencies (Note 14)
Common equity:
Common stock	3	2
Additional paid-in capital	2,733	2,704
Retained earnings	3,261	2,994
Accumulated other comprehensive loss	(1)	(1)
Shares in deferred compensation trust - 377,875 and 380,542 shares at a weighted average cost of $30.14 and $28.73 per share	(11)	(11)
Total common equity	5,985	5,688
Cumulative preferred stock of Interstate Power and Light Company	200	200
Total equity	6,185	5,888
Total liabilities and equity	18,329	17,710
IPL [Member]
Current assets:
Cash and cash equivalents	13	50
Accounts receivable, less allowance for expected credit losses	205	210
Production fuel, at weighted average cost	32	48
Gas stored underground, at weighted average cost	33	20
Materials and supplies, at weighted average cost	71	63
Regulatory assets	86	52
Other	108	53
Total current assets	548	496
Property, plant and equipment, net	7,931	7,889
Other assets:
Regulatory assets	1,421	1,431
Deferred charges and other	78	33
Total other assets	1,499	1,464
Total assets	9,978	9,849
Current liabilities:
Accounts payable	140	162
Regulatory liabilities	104	103
Other	171	182
Total current liabilities	415	447
Long-term debt, net (excluding current portion)	3,347	3,345
Other liabilities:
Deferred tax liabilities	1,079	1,035
Regulatory liabilities	616	573
Pension and other benefit obligations	161	186
Other	325	299
Total other liabilities	2,181	2,093
Commitments and contingencies (Note 14)
Common equity:
Common stock	33	33
Additional paid-in capital	2,802	2,752
Retained earnings	1,000	979
Total common equity	3,835	3,764
Cumulative preferred stock of Interstate Power and Light Company	200	200
Total equity	4,035	3,964
Total liabilities and equity	9,978	9,849
WPL [Member]
Current assets:
Cash and cash equivalents	4	3
Accounts receivable, less allowance for expected credit losses	175	192
Production fuel, at weighted average cost	10	18
Gas stored underground, at weighted average cost	25	26
Materials and supplies, at weighted average cost	45	40
Regulatory assets	25	29
Prepaid gross receipts tax	29	42
Other	99	12
Total current assets	412	362
Property, plant and equipment, net	6,367	6,022
Other assets:
Regulatory assets	486	498
Deferred charges and other	76	30
Total other assets	562	528
Total assets	7,341	6,912
Current liabilities:
Commercial paper	3	257
Accounts payable	160	154
Regulatory liabilities	127	146
Other	116	117
Total current liabilities	406	674
Long-term debt, net (excluding current portion)	2,429	2,130
Other liabilities:
Deferred tax liabilities	747	702
Regulatory liabilities	514	484
Finance lease obligations - Sheboygan Falls Energy Facility	34	42
Pension and other benefit obligations	193	222
Other	206	180
Total other liabilities	1,694	1,630
Commitments and contingencies (Note 14)
Common equity:
Common stock	66	66
Additional paid-in capital	1,704	1,459
Retained earnings	1,042	953
Total common equity	2,812	2,478
Total equity	2,812	2,478
Total liabilities and equity	$ 7,341	$ 6,912