Condensed Consolidated Balance Sheets - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 239.7	$ 27.9
Accounts receivable, less allowance for doubtful accounts	465.3	482.8
Production fuel, at weighted average cost	61.3	72.3
Gas stored underground, at weighted average cost	43.0	44.5
Materials and supplies, at weighted average cost	106.8	105.6
Regulatory assets	82.2	84.3
Other	126.5	87.7
Total current assets	1,124.8	905.1
Property, plant and equipment, net	12,005.2	11,234.5
Investments:
ATC Holdings	285.9	274.2
Other	138.1	121.9
Total investments	424.0	396.1
Other assets:
Regulatory assets	1,612.5	1,582.4
Deferred charges and other	103.3	69.7
Total other assets	1,715.8	1,652.1
Total assets	15,269.8	14,187.8
Current liabilities:
Current maturities of long-term debt	506.1	855.7
Commercial paper	136.8	320.2
Other short-term borrowings	0.0	95.0
Accounts payable	498.1	477.3
Regulatory liabilities	151.7	140.0
Other	255.6	260.8
Total current liabilities	1,548.3	2,149.0
Long-term debt, net (excluding current portion)	5,248.2	4,010.6
Other liabilities:
Deferred tax liabilities	1,575.2	1,478.4
Regulatory liabilities	1,353.9	1,357.2
Pension and other benefit obligations	487.0	504.0
Other	286.8	306.4
Total other liabilities	3,702.9	3,646.0
Commitments and contingencies (Note 13)
Common equity:
Common stock	2.4	2.3
Additional paid-in capital	2,038.2	1,845.5
Retained earnings	2,539.5	2,346.0
Accumulated other comprehensive loss	(0.2)	(0.5)
Shares in deferred compensation trust - 377,723 and 463,365 shares at a weighted average cost of $25.27 and $23.91 per share	(9.5)	(11.1)
Total common equity	4,570.4	4,182.2
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Total equity	4,770.4	4,382.2
Total liabilities and equity	15,269.8	14,187.8
IPL [Member]
Current assets:
Cash and cash equivalents	232.7	3.6
Accounts receivable, less allowance for doubtful accounts	277.3	264.9
Production fuel, at weighted average cost	42.6	52.4
Gas stored underground, at weighted average cost	25.4	20.3
Materials and supplies, at weighted average cost	59.3	60.6
Regulatory assets	40.3	41.9
Other	38.0	32.3
Total current assets	715.6	476.0
Property, plant and equipment, net	6,432.9	5,926.2
Other assets:
Regulatory assets	1,209.6	1,189.7
Deferred charges and other	20.1	14.1
Total other assets	1,229.7	1,203.8
Total assets	8,378.2	7,606.0
Current liabilities:
Current maturities of long-term debt	250.0	350.0
Commercial paper	0.0
Accounts payable	279.0	220.3
Regulatory liabilities	95.9	69.7
Other	173.8	187.7
Total current liabilities	798.7	827.7
Long-term debt, net (excluding current portion)	2,551.8	2,056.0
Other liabilities:
Deferred tax liabilities	933.0	910.7
Regulatory liabilities	666.7	685.7
Pension and other benefit obligations	168.3	173.8
Other	221.0	242.4
Total other liabilities	1,989.0	2,012.6
Commitments and contingencies (Note 13)
Common equity:
Common stock	33.4	33.4
Additional paid-in capital	2,027.8	1,797.8
Retained earnings	777.5	678.5
Total common equity	2,838.7	2,509.7
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Total equity	3,038.7	2,709.7
Total liabilities and equity	8,378.2	7,606.0
WPL [Member]
Current assets:
Cash and cash equivalents	6.4	23.1
Accounts receivable, less allowance for doubtful accounts	180.6	212.2
Production fuel, at weighted average cost	18.7	19.9
Gas stored underground, at weighted average cost	17.6	24.2
Materials and supplies, at weighted average cost	44.5	42.1
Regulatory assets	41.9	42.4
Prepaid gross receipts tax	30.7	41.3
Other	60.5	13.4
Total current assets	400.9	418.6
Property, plant and equipment, net	5,183.3	4,917.9
Other assets:
Regulatory assets	402.9	392.7
Deferred charges and other	57.1	27.3
Total other assets	460.0	420.0
Total assets	6,044.2	5,756.5
Current liabilities:
Current maturities of long-term debt	250.0	0.0
Commercial paper	38.4	25.0
Accounts payable	162.4	201.7
Regulatory liabilities	55.8	70.3
Other	95.1	99.2
Total current liabilities	601.7	396.2
Long-term debt, net (excluding current portion)	1,584.5	1,833.4
Other liabilities:
Deferred tax liabilities	579.6	522.4
Regulatory liabilities	687.2	671.5
Capital lease obligations - Sheboygan Falls Energy Facility	62.0	70.2
Pension and other benefit obligations	207.0	213.7
Other	175.6	167.6
Total other liabilities	1,711.4	1,645.4
Commitments and contingencies (Note 13)
Common equity:
Common stock	66.2	66.2
Additional paid-in capital	1,309.0	1,109.0
Retained earnings	771.4	706.3
Total common equity	2,146.6	1,881.5
Total liabilities and equity	$ 6,044.2	$ 5,756.5