Debt (Long-term Debt) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term debt		$ 4,902.5	$ 4,357.1
Current maturities		(855.7)	(4.6)
Unamortized debt issuance costs		(25.4)	(23.4)
Unamortized debt (discount) and premium, net		(10.8)	(13.5)
Long-term debt, net	[1]	4,010.6	4,315.6
Senior Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	2,425.0	2,175.0
Senior Debentures [Member] | 5.875% senior debenture, due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 100.0	$ 100.0
Interest rate		5.875%	5.875%
Senior Debentures [Member] | 7.25% senior debenture, due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250.0	$ 250.0
Interest rate		7.25%	7.25%
Senior Debentures [Member] | 3.65% senior debenture, due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 200.0	$ 200.0
Interest rate		3.65%	3.65%
Senior Debentures [Member] | 3.25% senior debenture, due 2024 [Member]
Debt Instrument [Line Items]
Long-term debt	[2],[3]	$ 500.0	$ 250.0
Interest rate		3.25%	3.25%
Senior Debentures [Member] | 3.4% senior debenture, due 2025 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250.0	$ 250.0
Interest rate		3.40%	3.40%
Senior Debentures [Member] | 5.5% senior debenture, due 2025 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 50.0	$ 50.0
Interest rate		5.50%	5.50%
Senior Debentures [Member] | 6.45% senior debenture, due 2033 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 100.0	$ 100.0
Interest rate		6.45%	6.45%
Senior Debentures [Member] | 6.3% senior debenture, due 2034 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 125.0	$ 125.0
Interest rate		6.30%	6.30%
Senior Debentures [Member] | 6.25% senior debenture, due 2039 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 300.0	$ 300.0
Interest rate		6.25%	6.25%
Senior Debentures [Member] | 4.7% senior debenture, due 2043 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250.0	$ 250.0
Interest rate		4.70%	4.70%
Senior Debentures [Member] | 3.7% senior debenture, due 2046 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 300.0	$ 300.0
Interest rate		3.70%	3.70%
Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 1,850.0	$ 1,550.0
Debentures [Member] | 5% debenture, due 2019 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250.0	$ 250.0
Interest rate		5.00%	5.00%
Debentures [Member] | 4.6% debenture, due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 150.0	$ 150.0
Interest rate		4.60%	4.60%
Debentures [Member] | 2.25% debenture, due 2022 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250.0	$ 250.0
Interest rate		2.25%	2.25%
Debentures [Member] | 3.05% debenture, due 2027 [Member]
Debt Instrument [Line Items]
Long-term debt	[2],[4]	$ 300.0
Interest rate		3.05%
Debentures [Member] | 6.25% debenture, due 2034 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 100.0	$ 100.0
Interest rate		6.25%	6.25%
Debentures [Member] | 6.375% debenture, due 2037 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 300.0	$ 300.0
Interest rate		6.375%	6.375%
Debentures [Member] | 7.6% debenture, due 2038 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250.0	$ 250.0
Interest rate		7.60%	7.60%
Debentures [Member] | 4.1% debenture, due 2044 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250.0	$ 250.0
Interest rate		4.10%	4.10%
Other Long Term Debt [Member]
Debt Instrument [Line Items]
Long-term debt		$ 627.5	$ 632.1
Other [Member] | Other, 1% at December 31, 2017, due 2018 to 2025 [Member]
Debt Instrument [Line Items]
Long-term debt		$ 2.9	3.3
Interest rate		1.00%
IPL [Member]
Debt Instrument [Line Items]
Long-term debt		$ 2,425.0	2,175.0
Current maturities		(350.0)	0.0
Unamortized debt issuance costs		(14.3)	(13.7)
Unamortized debt (discount) and premium, net		(4.7)	(7.8)
Long-term debt, net	[1]	2,056.0	2,153.5
IPL [Member] | Senior Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	2,425.0	2,175.0
IPL [Member] | Senior Debentures [Member] | 5.875% senior debenture, due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 100.0	$ 100.0
Interest rate		5.875%	5.875%
IPL [Member] | Senior Debentures [Member] | 7.25% senior debenture, due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250.0	$ 250.0
Interest rate		7.25%	7.25%
IPL [Member] | Senior Debentures [Member] | 3.65% senior debenture, due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 200.0	$ 200.0
Interest rate		3.65%	3.65%
IPL [Member] | Senior Debentures [Member] | 3.25% senior debenture, due 2024 [Member]
Debt Instrument [Line Items]
Long-term debt	[2],[3]	$ 500.0	$ 250.0
Proceeds from the issuance of debt		$ 250.0
Interest rate		3.25%	3.25%
IPL [Member] | Senior Debentures [Member] | 3.4% senior debenture, due 2025 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250.0	$ 250.0
Interest rate		3.40%	3.40%
IPL [Member] | Senior Debentures [Member] | 5.5% senior debenture, due 2025 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 50.0	$ 50.0
Interest rate		5.50%	5.50%
IPL [Member] | Senior Debentures [Member] | 6.45% senior debenture, due 2033 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 100.0	$ 100.0
Interest rate		6.45%	6.45%
IPL [Member] | Senior Debentures [Member] | 6.3% senior debenture, due 2034 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 125.0	$ 125.0
Interest rate		6.30%	6.30%
IPL [Member] | Senior Debentures [Member] | 6.25% senior debenture, due 2039 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 300.0	$ 300.0
Interest rate		6.25%	6.25%
IPL [Member] | Senior Debentures [Member] | 4.7% senior debenture, due 2043 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250.0	$ 250.0
Interest rate		4.70%	4.70%
IPL [Member] | Senior Debentures [Member] | 3.7% senior debenture, due 2046 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 300.0	$ 300.0
Interest rate		3.70%	3.70%
WPL [Member]
Debt Instrument [Line Items]
Long-term debt		$ 1,850.0	$ 1,550.0
Current maturities		0.0	0.0
Unamortized debt issuance costs		(10.5)	(9.1)
Unamortized debt (discount) and premium, net		(6.1)	(5.7)
Long-term debt, net	[1]	1,833.4	1,535.2
WPL [Member] | Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	1,850.0	1,550.0
WPL [Member] | Debentures [Member] | 5% debenture, due 2019 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250.0	$ 250.0
Interest rate		5.00%	5.00%
WPL [Member] | Debentures [Member] | 4.6% debenture, due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 150.0	$ 150.0
Interest rate		4.60%	4.60%
WPL [Member] | Debentures [Member] | 2.25% debenture, due 2022 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250.0	$ 250.0
Interest rate		2.25%	2.25%
WPL [Member] | Debentures [Member] | 3.05% debenture, due 2027 [Member]
Debt Instrument [Line Items]
Long-term debt	[2],[4]	$ 300.0
Proceeds from the issuance of debt	[2]	$ 300.0
Interest rate		3.05%
WPL [Member] | Debentures [Member] | 6.25% debenture, due 2034 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 100.0	$ 100.0
Interest rate		6.25%	6.25%
WPL [Member] | Debentures [Member] | 6.375% debenture, due 2037 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 300.0	$ 300.0
Interest rate		6.375%	6.375%
WPL [Member] | Debentures [Member] | 7.6% debenture, due 2038 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250.0	$ 250.0
Interest rate		7.60%	7.60%
WPL [Member] | Debentures [Member] | 4.1% debenture, due 2044 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 250.0	$ 250.0
Interest rate		4.10%	4.10%
Alliant Energy Finance, LLC [Member] | Term Loan Credit Agreement [Member] | Term loan credit agreement through 2018, 2% at December 31, 2017 [Member]
Debt Instrument [Line Items]
Long-term debt	[5]	$ 500.0	$ 500.0
Interest rate		2.00%
Corporate Services [Member] | Senior Notes [Member] | 3.45% senior notes, due 2022 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 75.0	$ 75.0
Interest rate		3.45%	3.45%
Sheboygan Power, LLC [Member] | Senior Secured Notes [Member] | 5.06% senior secured notes, due 2018 to 2024 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 49.6	$ 53.8
Interest rate		5.06%	5.06%

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

[2] Contains optional redemption provisions which, if elected by the issuer at its sole discretion, could require material redemption premium payments by the issuer. The redemption premium payments under these optional redemption provisions are variable and dependent on applicable U.S. Treasury rates at the time of redemption.

[3] In 2017, IPL issued $250 million of 3.25% senior debentures due 2024. The proceeds from the issuance were used by IPL to reduce commercial paper classified as long-term debt, reduce cash amounts received from its sales of accounts receivable program and for general corporate purposes.

[4] In 2017, WPL issued $300 million of 3.05% debentures due 2027. The proceeds from the issuance were used by WPL to reduce commercial paper and for general corporate purposes.

[5] Refer to Note 9(a) for discussion of a financial covenant contained in AEF’s term loan credit agreement.