Condensed Consolidated Balance Sheets - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 55.2	$ 16.3
Accounts receivable, less allowance for expected credit losses	396.4	402.1
Production fuel, at weighted average cost	80.2	77.7
Gas stored underground, at weighted average cost	35.4	49.1
Materials and supplies, at weighted average cost	105.0	100.5
Regulatory assets	102.5	86.4
Other	137.5	143.4
Total current assets	912.2	875.5
Property, plant and equipment, net	13,685.0	13,527.1
Investments:
ATC Holdings	319.1	320.1
Other	150.4	147.7
Total investments	469.5	467.8
Other assets:
Regulatory assets	1,752.4	1,758.3
Deferred charges and other	59.9	72.0
Total other assets	1,812.3	1,830.3
Total assets	16,879.0	16,700.7
Current liabilities:
Current maturities of long-term debt	357.2	657.2
Commercial paper	80.5	337.4
Other short-term borrowings	190.0	0.0
Accounts payable	375.4	422.3
Regulatory liabilities	239.8	212.0
Other	407.3	425.2
Total current liabilities	1,650.2	2,054.1
Long-term debt, net (excluding current portion)	5,833.9	5,533.0
Other liabilities:
Deferred tax liabilities	1,736.1	1,714.0
Regulatory liabilities	1,154.1	1,211.6
Pension and other benefit obligations	452.6	484.0
Other	349.7	298.9
Total other liabilities	3,692.5	3,708.5
Commitments and contingencies (Note 13)
Common equity:
Common stock	2.5	2.5
Additional paid-in capital	2,674.1	2,445.9
Retained earnings	2,833.7	2,765.4
Accumulated other comprehensive income	2.0	1.3
Shares in deferred compensation trust - 360,852 and 381,232 shares at a weighted average cost of $27.44 and $26.24 per share	(9.9)	(10.0)
Total common equity	5,502.4	5,205.1
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Total equity	5,702.4	5,405.1
Total liabilities and equity	16,879.0	16,700.7
IPL [Member]
Current assets:
Cash and cash equivalents	25.0	9.3
Accounts receivable, less allowance for expected credit losses	204.8	202.8
Production fuel, at weighted average cost	54.2	47.1
Gas stored underground, at weighted average cost	8.3	21.7
Materials and supplies, at weighted average cost	60.5	55.0
Regulatory assets	54.3	43.5
Other	28.5	30.0
Total current assets	435.6	409.4
Property, plant and equipment, net	7,589.4	7,480.7
Other assets:
Regulatory assets	1,350.6	1,355.8
Deferred charges and other	28.5	31.6
Total other assets	1,379.1	1,387.4
Total assets	9,404.1	9,277.5
Current liabilities:
Current maturities of long-term debt	200.0	200.0
Accounts payable	184.3	207.0
Accounts payable to associated companies	49.3	39.3
Regulatory liabilities	136.4	115.9
Other	265.1	268.3
Total current liabilities	835.1	830.5
Long-term debt, net (excluding current portion)	2,947.9	2,947.3
Other liabilities:
Deferred tax liabilities	1,010.4	1,008.0
Regulatory liabilities	584.4	598.8
Pension and other benefit obligations	161.5	167.7
Other	269.4	253.4
Total other liabilities	2,025.7	2,027.9
Commitments and contingencies (Note 13)
Common equity:
Common stock	33.4	33.4
Additional paid-in capital	2,447.8	2,347.8
Retained earnings	914.2	890.6
Total common equity	3,395.4	3,271.8
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Total equity	3,595.4	3,471.8
Total liabilities and equity	9,404.1	9,277.5
WPL [Member]
Current assets:
Cash and cash equivalents	22.7	4.4
Accounts receivable, less allowance for expected credit losses	181.1	190.9
Production fuel, at weighted average cost	26.0	30.6
Gas stored underground, at weighted average cost	27.1	27.4
Materials and supplies, at weighted average cost	42.2	43.1
Regulatory assets	48.2	42.9
Other	88.4	103.4
Total current assets	435.7	442.7
Property, plant and equipment, net	5,691.2	5,638.3
Other assets:
Regulatory assets	401.8	402.5
Deferred charges and other	23.2	23.0
Total other assets	425.0	425.5
Total assets	6,551.9	6,506.5
Current liabilities:
Current maturities of long-term debt	150.0	150.0
Commercial paper	0.0	168.2
Other short-term borrowings	182.0	0.0
Accounts payable	136.4	159.9
Regulatory liabilities	103.4	96.1
Other	128.4	116.1
Total current liabilities	700.2	690.3
Long-term debt, net (excluding current portion)	1,783.1	1,782.7
Other liabilities:
Deferred tax liabilities	647.5	626.2
Regulatory liabilities	569.7	612.8
Finance lease obligations - Sheboygan Falls Energy Facility	49.1	51.4
Pension and other benefit obligations	203.7	210.8
Other	162.5	168.7
Total other liabilities	1,632.5	1,669.9
Commitments and contingencies (Note 13)
Common equity:
Common stock	66.2	66.2
Additional paid-in capital	1,459.0	1,434.0
Retained earnings	910.9	863.4
Total common equity	2,436.1	2,363.6
Total equity	2,436.1	2,363.6
Total liabilities and equity	$ 6,551.9	$ 6,506.5