Condensed Consolidated Balance Sheets - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 14	$ 54
Accounts receivable, less allowance for expected credit losses	325	412
Production fuel, at weighted average cost	57	66
Gas stored underground, at weighted average cost	25	46
Materials and supplies, at weighted average cost	122	105
Regulatory assets	95	81
Other	117	123
Total current assets	755	887
Property, plant and equipment, net	14,353	14,336
Investments:
ATC Holdings	334	331
Other	160	154
Total investments	494	485
Other assets:
Regulatory assets	1,912	1,929
Deferred charges and other	80	73
Total other assets	1,992	2,002
Total assets	17,594	17,710
Current liabilities:
Current maturities of long-term debt	308	8
Commercial paper	336	389
Accounts payable	307	377
Regulatory liabilities	208	249
Other	249	274
Total current liabilities	1,408	1,297
Long-term debt, net (excluding current portion)	6,471	6,769
Other liabilities:
Deferred tax liabilities	1,843	1,814
Regulatory liabilities	1,047	1,057
Pension and other benefit obligations	495	511
Other	364	374
Total other liabilities	3,749	3,756
Commitments and contingencies (Note 12)
Common equity:
Common stock	3	2
Additional paid-in capital	2,712	2,704
Retained earnings	3,063	2,994
Accumulated other comprehensive loss	(1)	(1)
Shares in deferred compensation trust - 365,523 and 380,542 shares at a weighted average cost of $29.17 and $28.73 per share	(11)	(11)
Total common equity	5,766	5,688
Cumulative preferred stock of Interstate Power and Light Company	200	200
Total equity	5,966	5,888
Total liabilities and equity	17,594	17,710
IPL [Member]
Current assets:
Cash and cash equivalents	11	50
Accounts receivable, less allowance for expected credit losses	130	210
Production fuel, at weighted average cost	45	48
Gas stored underground, at weighted average cost	8	20
Materials and supplies, at weighted average cost	73	63
Regulatory assets	73	52
Other	53	53
Total current assets	393	496
Property, plant and equipment, net	7,872	7,889
Other assets:
Regulatory assets	1,421	1,431
Deferred charges and other	40	33
Total other assets	1,461	1,464
Total assets	9,726	9,849
Current liabilities:
Accounts payable	119	162
Accounts payable to associated companies	0	45
Regulatory liabilities	87	103
Other	131	137
Total current liabilities	337	447
Long-term debt, net (excluding current portion)	3,346	3,345
Other liabilities:
Deferred tax liabilities	1,047	1,035
Regulatory liabilities	574	573
Pension and other benefit obligations	180	186
Other	300	299
Total other liabilities	2,101	2,093
Commitments and contingencies (Note 12)
Common equity:
Common stock	33	33
Additional paid-in capital	2,752	2,752
Retained earnings	957	979
Total common equity	3,742	3,764
Cumulative preferred stock of Interstate Power and Light Company	200	200
Total equity	3,942	3,964
Total liabilities and equity	9,726	9,849
WPL [Member]
Current assets:
Cash and cash equivalents	2	3
Accounts receivable, less allowance for expected credit losses	183	192
Production fuel, at weighted average cost	12	18
Gas stored underground, at weighted average cost	17	26
Materials and supplies, at weighted average cost	47	40
Regulatory assets	22	29
Other	47	54
Total current assets	330	362
Property, plant and equipment, net	6,053	6,022
Other assets:
Regulatory assets	491	498
Deferred charges and other	36	30
Total other assets	527	528
Total assets	6,910	6,912
Current liabilities:
Commercial paper	148	257
Accounts payable	125	154
Accounts payable to associated companies	17	35
Regulatory liabilities	121	146
Other	94	82
Total current liabilities	505	674
Long-term debt, net (excluding current portion)	2,131	2,130
Other liabilities:
Deferred tax liabilities	719	702
Regulatory liabilities	473	484
Finance lease obligations - Sheboygan Falls Energy Facility	39	42
Pension and other benefit obligations	215	222
Other	183	180
Total other liabilities	1,629	1,630
Commitments and contingencies (Note 12)
Common equity:
Common stock	66	66
Additional paid-in capital	1,584	1,459
Retained earnings	995	953
Total common equity	2,645	2,478
Total equity	2,645	2,478
Total liabilities and equity	$ 6,910	$ 6,912