Consolidated Balance Sheets - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents		$ 27.9	$ 8.2
Accounts receivable, less allowance for doubtful accounts		482.8	493.3
Production fuel, at weighted average cost		72.3	98.1
Gas stored underground, at weighted average cost		44.5	37.6
Materials and supplies, at weighted average cost		105.6	86.6
Regulatory assets		84.3	57.8
Other		87.7	95.5
Total current assets		905.1	877.1
Property, plant and equipment, net		11,234.5	10,279.2
Investments:
ATC Investment		274.2	317.6
Other		121.9	20.0
Total investments		396.1	337.6
Other assets:
Regulatory assets		1,582.4	1,857.3
Deferred charges and other		69.7	22.6
Total other assets		1,652.1	1,879.9
Total assets		14,187.8	13,373.8
Current liabilities:
Current maturities of long-term debt		855.7	4.6
Commercial paper		320.2	244.1
Other short-term borrowings		95.0	0.0
Accounts payable		477.3	445.3
Regulatory liabilities		140.0	186.2
Other		260.8	281.8
Total current liabilities		2,149.0	1,162.0
Long-term debt, net (excluding current portion)	[1]	4,010.6	4,315.6
Other liabilities:
Deferred tax liabilities		1,478.4	2,570.2
Regulatory liabilities		1,357.2	494.8
Pension and other benefit obligations		504.0	489.9
Other		306.4	279.3
Total other liabilities		3,646.0	3,834.2
Commitments and contingencies (Note 16)
Common equity:
Common stock		2.3	2.3
Additional paid-in capital		1,845.5	1,693.1
Retained earnings		2,346.0	2,177.0
Accumulated other comprehensive loss		(0.5)	(0.4)
Shares in deferred compensation trust - 463,365 and 441,695 shares at a weighted average cost of $23.91 and $22.71 per share		(11.1)	(10.0)
Total common equity		4,182.2	3,862.0
Cumulative preferred stock of Interstate Power and Light Company		200.0	200.0
Total equity		4,382.2	4,062.0
Total liabilities and equity		14,187.8	13,373.8
IPL [Member]
Current assets:
Cash and cash equivalents		3.6	3.3
Accounts receivable, less allowance for doubtful accounts		264.9	240.7
Production fuel, at weighted average cost		52.4	70.3
Gas stored underground, at weighted average cost		20.3	16.3
Materials and supplies, at weighted average cost		60.6	46.5
Regulatory assets		41.9	17.7
Other		32.3	27.7
Total current assets		476.0	422.5
Property, plant and equipment, net		5,926.2	5,435.6
Other assets:
Regulatory assets		1,189.7	1,441.1
Deferred charges and other		14.1	5.5
Total other assets		1,203.8	1,446.6
Total assets		7,606.0	7,304.7
Current liabilities:
Current maturities of long-term debt		350.0	0.0
Commercial paper		0.0	0.0
Accounts payable		220.3	186.3
Accounts payable to associated companies		50.1	43.3
Regulatory liabilities		69.7	149.6
Accrued taxes		47.1	53.8
Other		90.5	88.8
Total current liabilities		827.7	521.8
Long-term debt, net (excluding current portion)	[1]	2,056.0	2,153.5
Other liabilities:
Deferred tax liabilities		910.7	1,511.8
Regulatory liabilities		685.7	281.2
Pension and other benefit obligations		173.8	173.2
Other		242.4	214.2
Total other liabilities		2,012.6	2,180.4
Commitments and contingencies (Note 16)
Common equity:
Common stock		33.4	33.4
Additional paid-in capital		1,797.8	1,597.8
Retained earnings		678.5	617.8
Total common equity		2,509.7	2,249.0
Cumulative preferred stock of Interstate Power and Light Company		200.0	200.0
Total equity		2,709.7	2,449.0
Total liabilities and equity		7,606.0	7,304.7
WPL [Member]
Current assets:
Cash and cash equivalents		23.1	4.2
Accounts receivable, less allowance for doubtful accounts		212.2	226.3
Production fuel, at weighted average cost		19.9	27.8
Gas stored underground, at weighted average cost		24.2	21.3
Materials and supplies, at weighted average cost		42.1	36.3
Regulatory assets		42.4	40.1
Prepaid gross receipts tax		41.3	39.8
Other		13.4	20.7
Total current assets		418.6	416.5
Property, plant and equipment, net		4,917.9	4,426.7
Other assets:
Regulatory assets		392.7	416.2
Deferred charges and other		27.3	30.9
Total other assets		420.0	447.1
Total assets		5,756.5	5,290.3
Current liabilities:
Current maturities of long-term debt		0.0	0.0
Commercial paper		25.0	52.3
Accounts payable		201.7	192.9
Accounts payable to associated companies		22.2	34.6
Regulatory liabilities		70.3	36.6
Accrued interest		25.6	23.6
Other		51.4	54.7
Total current liabilities		396.2	394.7
Long-term debt, net (excluding current portion)	[1]	1,833.4	1,535.2
Other liabilities:
Deferred tax liabilities		522.4	971.6
Regulatory liabilities		671.5	213.6
Capital lease obligations - Sheboygan Falls Energy Facility		70.2	77.2
Pension and other benefit obligations		213.7	207.8
Other		167.6	159.4
Total other liabilities		1,645.4	1,629.6
Commitments and contingencies (Note 16)
Common equity:
Common stock		66.2	66.2
Additional paid-in capital		1,109.0	1,019.0
Retained earnings		706.3	645.6
Total common equity		1,881.5	1,730.8
Total equity		1,881.5	1,730.8
Total liabilities and equity		$ 5,756.5	$ 5,290.3

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