Condensed Consolidated Balance Sheets - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 19.1	$ 27.9
Accounts receivable, less allowance for doubtful accounts	343.2	482.8
Production fuel, at weighted average cost	76.7	72.3
Gas stored underground, at weighted average cost	16.3	44.5
Materials and supplies, at weighted average cost	109.8	105.6
Regulatory assets	74.9	84.3
Other	85.9	87.7
Total current assets	725.9	905.1
Property, plant and equipment, net	11,439.6	11,234.5
Investments:
ATC Holdings	275.5	274.2
Other	139.2	121.9
Total investments	414.7	396.1
Other assets:
Regulatory assets	1,589.2	1,582.4
Deferred charges and other	72.4	69.7
Total other assets	1,661.6	1,652.1
Total assets	14,241.8	14,187.8
Current liabilities:
Current maturities of long-term debt	855.7	855.7
Commercial paper	336.4	320.2
Other short-term borrowings	95.0	95.0
Accounts payable	363.1	477.3
Regulatory liabilities	172.2	140.0
Other	251.1	260.8
Total current liabilities	2,073.5	2,149.0
Long-term debt, net (excluding current portion)	4,056.8	4,010.6
Other liabilities:
Deferred tax liabilities	1,511.6	1,478.4
Regulatory liabilities	1,365.7	1,357.2
Pension and other benefit obligations	488.9	504.0
Other	313.8	306.4
Total other liabilities	3,680.0	3,646.0
Commitments and contingencies (Note 13)
Common equity:
Common stock	2.3	2.3
Additional paid-in capital	1,851.4	1,845.5
Retained earnings	2,389.4	2,346.0
Accumulated other comprehensive loss	(0.5)	(0.5)
Shares in deferred compensation trust - 458,639 and 463,365 shares at a weighted average cost of $24.17 and $23.91 per share	(11.1)	(11.1)
Total common equity	4,231.5	4,182.2
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Total equity	4,431.5	4,382.2
Total liabilities and equity	14,241.8	14,187.8
IPL [Member]
Current assets:
Cash and cash equivalents	2.9	3.6
Accounts receivable, less allowance for doubtful accounts	145.9	264.9
Production fuel, at weighted average cost	55.0	52.4
Gas stored underground, at weighted average cost	3.6	20.3
Materials and supplies, at weighted average cost	61.6	60.6
Regulatory assets	38.1	41.9
Other	23.2	32.3
Total current assets	330.3	476.0
Property, plant and equipment, net	6,088.5	5,926.2
Other assets:
Regulatory assets	1,192.8	1,189.7
Deferred charges and other	16.2	14.1
Total other assets	1,209.0	1,203.8
Total assets	7,627.8	7,606.0
Current liabilities:
Current maturities of long-term debt	350.0	350.0
Commercial paper	0.0
Accounts payable	183.2	220.3
Accounts payable to associated companies	35.9	50.1
Regulatory liabilities	102.4	69.7
Other	127.4	137.6
Total current liabilities	798.9	827.7
Long-term debt, net (excluding current portion)	2,102.2	2,056.0
Other liabilities:
Deferred tax liabilities	926.5	910.7
Regulatory liabilities	674.9	685.7
Pension and other benefit obligations	171.6	173.8
Other	239.2	242.4
Total other liabilities	2,012.2	2,012.6
Commitments and contingencies (Note 13)
Common equity:
Common stock	33.4	33.4
Additional paid-in capital	1,797.8	1,797.8
Retained earnings	683.3	678.5
Total common equity	2,514.5	2,509.7
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Total equity	2,714.5	2,709.7
Total liabilities and equity	7,627.8	7,606.0
WPL [Member]
Current assets:
Cash and cash equivalents	3.7	23.1
Accounts receivable, less allowance for doubtful accounts	192.0	212.2
Production fuel, at weighted average cost	21.7	19.9
Gas stored underground, at weighted average cost	12.7	24.2
Materials and supplies, at weighted average cost	44.9	42.1
Regulatory assets	36.8	42.4
Other	57.7	54.7
Total current assets	369.5	418.6
Property, plant and equipment, net	4,962.6	4,917.9
Other assets:
Regulatory assets	396.4	392.7
Deferred charges and other	29.4	27.3
Total other assets	425.8	420.0
Total assets	5,757.9	5,756.5
Current liabilities:
Commercial paper	30.6	25.0
Accounts payable	134.6	201.7
Accounts payable to associated companies	28.1	22.2
Regulatory liabilities	69.8	70.3
Other	89.8	77.0
Total current liabilities	352.9	396.2
Long-term debt, net (excluding current portion)	1,833.7	1,833.4
Other liabilities:
Deferred tax liabilities	530.9	522.4
Regulatory liabilities	690.8	671.5
Capital lease obligations - Sheboygan Falls Energy Facility	65.8	70.2
Pension and other benefit obligations	210.8	213.7
Other	172.5	167.6
Total other liabilities	1,670.8	1,645.4
Commitments and contingencies (Note 13)
Common equity:
Common stock	66.2	66.2
Additional paid-in capital	1,109.0	1,109.0
Retained earnings	725.3	706.3
Total common equity	1,900.5	1,881.5
Total liabilities and equity	$ 5,757.9	$ 5,756.5