Condensed Consolidated Balance Sheets - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 15	$ 54
Accounts receivable, less allowance for expected credit losses	377	412
Production fuel, at weighted average cost	57	66
Gas stored underground, at weighted average cost	32	46
Materials and supplies, at weighted average cost	119	105
Regulatory assets	95	81
Other	204	123
Total current assets	899	887
Property, plant and equipment, net	14,540	14,336
Investments:
ATC Holdings	336	331
Other	170	154
Total investments	506	485
Other assets:
Regulatory assets	1,900	1,929
Deferred charges and other	94	73
Total other assets	1,994	2,002
Total assets	17,939	17,710
Current liabilities:
Current maturities of long-term debt	308	8
Commercial paper	535	389
Accounts payable	392	377
Regulatory liabilities	174	249
Other	230	274
Total current liabilities	1,639	1,297
Long-term debt, net (excluding current portion)	6,468	6,769
Other liabilities:
Deferred tax liabilities	1,865	1,814
Regulatory liabilities	1,080	1,057
Pension and other benefit obligations	477	511
Other	391	374
Total other liabilities	3,813	3,756
Commitments and contingencies (Note 13)
Common equity:
Common stock	3	2
Additional paid-in capital	2,722	2,704
Retained earnings	3,106	2,994
Accumulated other comprehensive loss	(1)	(1)
Shares in deferred compensation trust - 372,337 and 380,542 shares at a weighted average cost of $29.65 and $28.73 per share	(11)	(11)
Total common equity	5,819	5,688
Cumulative preferred stock of Interstate Power and Light Company	200	200
Total equity	6,019	5,888
Total liabilities and equity	17,939	17,710
IPL [Member]
Current assets:
Cash and cash equivalents	11	50
Accounts receivable, less allowance for expected credit losses	182	210
Income tax refunds receivable	15	26
Production fuel, at weighted average cost	38	48
Gas stored underground, at weighted average cost	12	20
Materials and supplies, at weighted average cost	72	63
Regulatory assets	72	52
Other	53	27
Total current assets	455	496
Property, plant and equipment, net	7,907	7,889
Other assets:
Regulatory assets	1,416	1,431
Deferred charges and other	47	33
Total other assets	1,463	1,464
Total assets	9,825	9,849
Current liabilities:
Accounts payable	126	162
Regulatory liabilities	64	103
Other	180	182
Total current liabilities	370	447
Long-term debt, net (excluding current portion)	3,346	3,345
Other liabilities:
Deferred tax liabilities	1,054	1,035
Regulatory liabilities	583	573
Pension and other benefit obligations	171	186
Other	322	299
Total other liabilities	2,130	2,093
Commitments and contingencies (Note 13)
Common equity:
Common stock	33	33
Additional paid-in capital	2,802	2,752
Retained earnings	944	979
Total common equity	3,779	3,764
Cumulative preferred stock of Interstate Power and Light Company	200	200
Total equity	3,979	3,964
Total liabilities and equity	9,825	9,849
WPL [Member]
Current assets:
Cash and cash equivalents	3	3
Accounts receivable, less allowance for expected credit losses	182	192
Production fuel, at weighted average cost	19	18
Gas stored underground, at weighted average cost	20	26
Materials and supplies, at weighted average cost	44	40
Regulatory assets	23	29
Other	92	54
Total current assets	383	362
Property, plant and equipment, net	6,203	6,022
Other assets:
Regulatory assets	484	498
Deferred charges and other	47	30
Total other assets	531	528
Total assets	7,117	6,912
Current liabilities:
Commercial paper	192	257
Accounts payable	203	154
Regulatory liabilities	110	146
Other	94	117
Total current liabilities	599	674
Long-term debt, net (excluding current portion)	2,131	2,130
Other liabilities:
Deferred tax liabilities	730	702
Regulatory liabilities	497	484
Finance lease obligations - Sheboygan Falls Energy Facility	37	42
Pension and other benefit obligations	205	222
Other	193	180
Total other liabilities	1,662	1,630
Commitments and contingencies (Note 13)
Common equity:
Common stock	66	66
Additional paid-in capital	1,669	1,459
Retained earnings	990	953
Total common equity	2,725	2,478
Total equity	2,725	2,478
Total liabilities and equity	$ 7,117	$ 6,912