Condensed Consolidated Balance Sheets - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 8.4	$ 8.2
Accounts receivable, less allowance for doubtful accounts	424.9	493.3
Production fuel, at weighted average cost	86.7	98.1
Gas stored underground, at weighted average cost	12.2	37.6
Materials and supplies, at weighted average cost	88.8	86.6
Regulatory assets	58.4	57.8
Other	70.4	95.5
Total current assets	749.8	877.1
Property, plant and equipment, net	10,448.8	10,279.2
Investments:
Investment in American Transmission Company LLC	327.7	317.6
Other	19.1	20.0
Total investments	346.8	337.6
Other assets:
Regulatory assets	1,904.6	1,857.3
Deferred charges and other	14.9	22.6
Total other assets	1,919.5	1,879.9
Total assets	13,464.9	13,373.8
Current liabilities:
Current maturities of long-term debt	4.6	4.6
Commercial paper	302.8	244.1
Accounts payable	369.5	445.3
Regulatory liabilities	215.2	186.2
Other	272.9	281.8
Total current liabilities	1,165.0	1,162.0
Long-term debt, net (excluding current portion)	4,316.1	4,315.6
Other liabilities:
Deferred tax liabilities	2,625.9	2,570.2
Regulatory liabilities	481.4	494.8
Pension and other benefit obligations	481.7	489.9
Other	297.9	279.3
Total other liabilities	3,886.9	3,834.2
Commitments and contingencies (Note 12)
Common equity:
Common stock	2.3	2.3
Additional paid-in capital	1,699.2	1,693.1
Retained earnings	2,205.9	2,177.0
Accumulated other comprehensive loss	(0.4)	(0.4)
Shares in deferred compensation trust - 440,704 and 441,695 shares at a weighted average cost of $22.89 and $22.71 per share	(10.1)	(10.0)
Total common equity	3,896.9	3,862.0
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Total equity	4,096.9	4,062.0
Total liabilities and equity	13,464.9	13,373.8
IPL [Member]
Current assets:
Cash and cash equivalents	4.1	3.3
Accounts receivable, less allowance for doubtful accounts	177.2	240.7
Production fuel, at weighted average cost	64.1	70.3
Gas stored underground, at weighted average cost	2.8	16.3
Materials and supplies, at weighted average cost	48.8	46.5
Regulatory assets	18.4	17.7
Other	14.7	27.7
Total current assets	330.1	422.5
Property, plant and equipment, net	5,503.9	5,435.6
Other assets:
Regulatory assets	1,482.4	1,441.1
Deferred charges and other	6.6	5.5
Total other assets	1,489.0	1,446.6
Total assets	7,323.0	7,304.7
Current liabilities:
Commercial paper	9.2	0.0
Accounts payable	149.2	186.3
Regulatory liabilities	166.6	149.6
Accrued taxes	37.2	53.8
Other	131.5	132.1
Total current liabilities	493.7	521.8
Long-term debt, net (excluding current portion)	2,154.0	2,153.5
Other liabilities:
Deferred tax liabilities	1,553.2	1,511.8
Regulatory liabilities	281.7	281.2
Pension and other benefit obligations	172.2	173.2
Other	221.1	214.2
Total other liabilities	2,228.2	2,180.4
Commitments and contingencies (Note 12)
Common equity:
Common stock	33.4	33.4
Additional paid-in capital	1,597.8	1,597.8
Retained earnings	615.9	617.8
Total common equity	2,247.1	2,249.0
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Total equity	2,447.1	2,449.0
Total liabilities and equity	7,323.0	7,304.7
WPL [Member]
Current assets:
Cash and cash equivalents	2.1	4.2
Accounts receivable, less allowance for doubtful accounts	212.0	226.3
Production fuel, at weighted average cost	22.6	27.8
Gas stored underground, at weighted average cost	9.4	21.3
Materials and supplies, at weighted average cost	36.5	36.3
Regulatory assets	40.0	40.1
Other	45.5	60.5
Total current assets	368.1	416.5
Property, plant and equipment, net	4,523.9	4,426.7
Other assets:
Regulatory assets	422.2	416.2
Deferred charges and other	20.8	30.9
Total other assets	443.0	447.1
Total assets	5,335.0	5,290.3
Current liabilities:
Commercial paper	91.2	52.3
Accounts payable	157.2	192.9
Regulatory liabilities	48.6	36.6
Other	123.0	112.9
Total current liabilities	420.0	394.7
Long-term debt, net (excluding current portion)	1,535.5	1,535.2
Other liabilities:
Deferred tax liabilities	984.7	971.6
Regulatory liabilities	199.7	213.6
Capital lease obligations - Sheboygan Falls Energy Facility	75.5	77.2
Pension and other benefit obligations	205.9	207.8
Other	168.9	159.4
Total other liabilities	1,634.7	1,629.6
Commitments and contingencies (Note 12)
Common equity:
Common stock	66.2	66.2
Additional paid-in capital	1,019.0	1,019.0
Retained earnings	659.6	645.6
Total common equity	1,744.8	1,730.8
Total liabilities and equity	$ 5,335.0	$ 5,290.3