Consolidated Balance Sheets - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents		$ 20.9	$ 27.9
Accounts receivable, less allowance for doubtful accounts		350.4	482.8
Production fuel, at weighted average cost		61.4	72.3
Gas stored underground, at weighted average cost		49.0	44.5
Materials and supplies, at weighted average cost		101.4	105.6
Regulatory assets		79.8	84.3
Prepaid gross receipts tax		42.2	41.3
Other		80.0	46.4
Total current assets		785.1	905.1
Property, plant and equipment, net		12,462.4	11,234.5
Investments:
ATC Holdings		293.6	274.2
Other		137.7	121.9
Total investments		431.3	396.1
Other assets:
Regulatory assets		1,657.5	1,582.4
Deferred charges and other		89.7	69.7
Total other assets		1,747.2	1,652.1
Total assets		15,426.0	14,187.8
Current liabilities:
Current maturities of long-term debt		256.5	855.7
Commercial paper		441.2	320.2
Other short-term borrowings		0.0	95.0
Accounts payable		543.3	477.3
Regulatory liabilities		142.7	140.0
Other		260.4	260.8
Total current liabilities		1,644.1	2,149.0
Long-term debt, net (excluding current portion)	[1]	5,246.3	4,010.6
Other liabilities:
Deferred tax liabilities		1,603.1	1,478.4
Regulatory liabilities		1,350.5	1,357.2
Pension and other benefit obligations		509.1	504.0
Other		287.2	306.4
Total other liabilities		3,749.9	3,646.0
Commitments and contingencies (Note 17)
Common equity:
Common stock		2.4	2.3
Additional paid-in capital		2,045.5	1,845.5
Retained earnings		2,545.9	2,346.0
Accumulated other comprehensive income (loss)		1.7	(0.5)
Shares in deferred compensation trust - 384,580 and 463,365 shares at a weighted average cost of $25.60 and $23.91 per share		(9.8)	(11.1)
Total common equity		4,585.7	4,182.2
Cumulative preferred stock of Interstate Power and Light Company		200.0	200.0
Total equity		4,785.7	4,382.2
Total liabilities and equity		15,426.0	14,187.8
IPL [Member]
Current assets:
Cash and cash equivalents		9.7	3.6
Accounts receivable, less allowance for doubtful accounts		153.5	264.9
Production fuel, at weighted average cost		44.8	52.4
Gas stored underground, at weighted average cost		26.1	20.3
Materials and supplies, at weighted average cost		55.4	60.6
Regulatory assets		39.2	41.9
Other		43.1	32.3
Total current assets		371.8	476.0
Property, plant and equipment, net		6,781.5	5,926.2
Other assets:
Regulatory assets		1,239.8	1,189.7
Deferred charges and other		18.3	14.1
Total other assets		1,258.1	1,203.8
Total assets		8,411.4	7,606.0
Current liabilities:
Current maturities of long-term debt		0.0	350.0
Commercial paper		50.4	0.0
Accounts payable		304.9	220.3
Accounts payable to associated companies		28.8	50.1
Regulatory liabilities		90.0	69.7
Accrued taxes		45.8	47.1
Accrued interest		31.2	32.1
Other		56.0	58.4
Total current liabilities		607.1	827.7
Long-term debt, net (excluding current portion)	[1]	2,552.3	2,056.0
Other liabilities:
Deferred tax liabilities		957.3	910.7
Regulatory liabilities		664.9	685.7
Pension and other benefit obligations		178.4	173.8
Other		220.7	242.4
Total other liabilities		2,021.3	2,012.6
Commitments and contingencies (Note 17)
Common equity:
Common stock		33.4	33.4
Additional paid-in capital		2,222.8	1,797.8
Retained earnings		774.5	678.5
Total common equity		3,030.7	2,509.7
Cumulative preferred stock of Interstate Power and Light Company		200.0	200.0
Total equity		3,230.7	2,709.7
Total liabilities and equity		8,411.4	7,606.0
WPL [Member]
Current assets:
Cash and cash equivalents		8.7	23.1
Accounts receivable, less allowance for doubtful accounts		190.1	212.2
Production fuel, at weighted average cost		16.6	19.9
Gas stored underground, at weighted average cost		22.9	24.2
Materials and supplies, at weighted average cost		42.9	42.1
Regulatory assets		40.6	42.4
Prepaid gross receipts tax		42.2	41.3
Other		20.6	13.4
Total current assets		384.6	418.6
Property, plant and equipment, net		5,287.3	4,917.9
Other assets:
Regulatory assets		417.7	392.7
Deferred charges and other		62.9	27.3
Total other assets		480.6	420.0
Total assets		6,152.5	5,756.5
Current liabilities:
Current maturities of long-term debt		250.0	0.0
Commercial paper		105.5	25.0
Accounts payable		180.9	201.7
Accounts payable to associated companies		31.8	22.2
Regulatory liabilities		52.7	70.3
Accrued interest		25.5	25.6
Other		48.2	51.4
Total current liabilities		694.6	396.2
Long-term debt, net (excluding current portion)	[1]	1,584.9	1,833.4
Other liabilities:
Deferred tax liabilities		582.0	522.4
Regulatory liabilities		685.6	671.5
Capital lease obligations - Sheboygan Falls Energy Facility		60.0	70.2
Pension and other benefit obligations		217.7	213.7
Other		178.2	167.6
Total other liabilities		1,723.5	1,645.4
Commitments and contingencies (Note 17)
Common equity:
Common stock		66.2	66.2
Additional paid-in capital		1,309.0	1,109.0
Retained earnings		774.3	706.3
Total common equity		2,149.5	1,881.5
Total equity		2,149.5	1,881.5
Total liabilities and equity		$ 6,152.5	$ 5,756.5

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