Consolidated Balance Sheets - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents		$ 16.3	$ 20.9
Accounts receivable, less allowance for doubtful accounts		402.1	350.4
Production fuel, at weighted average cost		77.7	61.4
Gas stored underground, at weighted average cost		49.1	49.0
Materials and supplies, at weighted average cost		100.5	101.4
Regulatory assets		86.4	79.8
Prepaid gross receipts tax		41.7	42.2
Other		101.7	80.0
Total current assets		875.5	785.1
Property, plant and equipment, net		13,527.1	12,462.4
Investments:
ATC Holdings		320.1	293.6
Other		147.7	137.7
Total investments		467.8	431.3
Other assets:
Regulatory assets		1,758.3	1,657.5
Deferred charges and other		72.0	89.7
Total other assets		1,830.3	1,747.2
Total assets		16,700.7	15,426.0
Current liabilities:
Current maturities of long-term debt		657.2	256.5
Commercial paper		337.4	441.2
Accounts payable		422.3	543.3
Regulatory liabilities		212.0	142.7
Other		425.2	260.4
Total current liabilities		2,054.1	1,644.1
Long-term debt, net (excluding current portion)	[1]	5,533.0	5,246.3
Other liabilities:
Deferred tax liabilities		1,714.0	1,603.1
Regulatory liabilities		1,211.6	1,350.5
Pension and other benefit obligations		484.0	509.1
Other		298.9	287.2
Total other liabilities		3,708.5	3,749.9
Commitments and contingencies (Note 17)
Common equity:
Common stock		2.5	2.4
Additional paid-in capital		2,445.9	2,045.5
Retained earnings		2,765.4	2,545.9
Accumulated other comprehensive income		1.3	1.7
Shares in deferred compensation trust - 381,232 and 384,580 shares at a weighted average cost of $26.24 and $25.60 per share		(10.0)	(9.8)
Total common equity		5,205.1	4,585.7
Cumulative preferred stock of Interstate Power and Light Company		200.0	200.0
Total equity		5,405.1	4,785.7
Total liabilities and equity		16,700.7	15,426.0
IPL [Member]
Current assets:
Cash and cash equivalents		9.3	9.7
Accounts receivable, less allowance for doubtful accounts		202.8	153.5
Production fuel, at weighted average cost		47.1	44.8
Gas stored underground, at weighted average cost		21.7	26.1
Materials and supplies, at weighted average cost		55.0	55.4
Regulatory assets		43.5	39.2
Other		30.0	43.1
Total current assets		409.4	371.8
Property, plant and equipment, net		7,480.7	6,781.5
Other assets:
Regulatory assets		1,355.8	1,239.8
Deferred charges and other		31.6	18.3
Total other assets		1,387.4	1,258.1
Total assets		9,277.5	8,411.4
Current liabilities:
Current maturities of long-term debt		200.0	0.0
Commercial paper		0.0	50.4
Accounts payable		207.0	304.9
Regulatory liabilities		115.9	90.0
Accrued taxes		63.3	45.8
Accrued interest		36.6	31.2
Other		207.7	84.8
Total current liabilities		830.5	607.1
Long-term debt, net (excluding current portion)	[1]	2,947.3	2,552.3
Other liabilities:
Deferred tax liabilities		1,008.0	957.3
Regulatory liabilities		598.8	664.9
Pension and other benefit obligations		167.7	178.4
Other		253.4	220.7
Total other liabilities		2,027.9	2,021.3
Commitments and contingencies (Note 17)
Common equity:
Common stock		33.4	33.4
Additional paid-in capital		2,347.8	2,222.8
Retained earnings		890.6	774.5
Total common equity		3,271.8	3,030.7
Cumulative preferred stock of Interstate Power and Light Company		200.0	200.0
Total equity		3,471.8	3,230.7
Total liabilities and equity		9,277.5	8,411.4
WPL [Member]
Current assets:
Cash and cash equivalents		4.4	8.7
Accounts receivable, less allowance for doubtful accounts		190.9	190.1
Production fuel, at weighted average cost		30.6	16.6
Gas stored underground, at weighted average cost		27.4	22.9
Materials and supplies, at weighted average cost		43.1	42.9
Regulatory assets		42.9	40.6
Prepaid gross receipts tax		41.7	42.2
Other		61.7	20.6
Total current assets		442.7	384.6
Property, plant and equipment, net		5,638.3	5,287.3
Other assets:
Regulatory assets		402.5	417.7
Deferred charges and other		23.0	62.9
Total other assets		425.5	480.6
Total assets		6,506.5	6,152.5
Current liabilities:
Current maturities of long-term debt		150.0	250.0
Commercial paper		168.2	105.5
Accounts payable		159.9	180.9
Accounts payable to associated companies		41.8	31.8
Regulatory liabilities		96.1	52.7
Other		74.3	73.7
Total current liabilities		690.3	694.6
Long-term debt, net (excluding current portion)	[1]	1,782.7	1,584.9
Other liabilities:
Deferred tax liabilities		626.2	582.0
Regulatory liabilities		612.8	685.6
Finance lease obligations - Sheboygan Falls Energy Facility		51.4	60.0
Pension and other benefit obligations		210.8	217.7
Other		168.7	178.2
Total other liabilities		1,669.9	1,723.5
Commitments and contingencies (Note 17)
Common equity:
Common stock		66.2	66.2
Additional paid-in capital		1,434.0	1,309.0
Retained earnings		863.4	774.3
Total common equity		2,363.6	2,149.5
Total equity		2,363.6	2,149.5
Total liabilities and equity		$ 6,506.5	$ 6,152.5

[1] There were no significant sinking fund requirements related to the outstanding long-term debt.