Debt (Long-term Debt) (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term debt		$ 5,546.7	$ 4,902.5
Current maturities		(256.5)	(855.7)
Unamortized debt issuance costs		(32.1)	(25.4)
Unamortized debt (discount) and premium, net		(11.8)	(10.8)
Long-term debt, net	[1]	5,246.3	4,010.6
Senior Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	2,575.0	2,425.0
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]	$ 500.0	$ 500.0
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] | 4.1% senior debenture, due 2028 [Member]
Debt Instrument [Line Items]
Long-term debt	[2],[3]	$ 500.0
Interest rate		4.10%
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%
Senior Debentures [Member] | 5.875% senior debenture, retired in 2018 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 0.0	$ 100.0
Interest rate		0.00%	5.875%
Senior Debentures [Member] | 7.25% senior debenture, retired in 2018 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 0.0	$ 250.0
Interest rate		0.00%	7.25%
Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 1,850.0	$ 1,850.0
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]	$ 300.0	$ 300.0
Interest rate		3.05%	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		$ 1,121.7	$ 627.5
Other [Member] | Other, 1% at December 31, 2018, due 2019 to 2025 [Member]
Debt Instrument [Line Items]
Long-term debt		$ 2.4	2.9
Interest rate		1.00%
IPL [Member]
Debt Instrument [Line Items]
Long-term debt		$ 2,575.0	2,425.0
Current maturities		0.0	(350.0)
Unamortized debt issuance costs		(17.2)	(14.3)
Unamortized debt (discount) and premium, net		(5.5)	(4.7)
Long-term debt, net	[1]	2,552.3	2,056.0
IPL [Member] | Senior Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	2,575.0	2,425.0
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]	$ 500.0	$ 500.0
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] | 4.1% senior debenture, due 2028 [Member]
Debt Instrument [Line Items]
Long-term debt	[2],[3]	$ 500.0
Interest rate		4.10%
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%
IPL [Member] | Senior Debentures [Member] | 5.875% senior debenture, retired in 2018 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 0.0	$ 100.0
Interest rate		0.00%	5.875%
IPL [Member] | Senior Debentures [Member] | 7.25% senior debenture, retired in 2018 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 0.0	$ 250.0
Interest rate		0.00%	7.25%
WPL [Member]
Debt Instrument [Line Items]
Long-term debt		$ 1,850.0	$ 1,850.0
Current maturities		(250.0)	0.0
Unamortized debt issuance costs		(9.6)	(10.5)
Unamortized debt (discount) and premium, net		(5.5)	(6.1)
Long-term debt, net	[1]	1,584.9	1,833.4
WPL [Member] | Debentures [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	1,850.0	1,850.0
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]	$ 300.0	$ 300.0
Interest rate		3.05%	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 April 2020 [Member]
Debt Instrument [Line Items]
Long-term debt	[4]	$ 300.0
Interest rate		3.00%
Alliant Energy Finance, LLC [Member] | Term Loan Credit Agreement [Member] | Term loan credit agreement, retired in 2018, 2% at December 31, 2017 [Member]
Debt Instrument [Line Items]
Long-term debt	[5]	$ 0.0	$ 500.0
Interest rate			2.00%
Alliant Energy Finance, LLC [Member] | Senior Notes [Member] | 3.75% senior notes, due 2023 [Member]
Debt Instrument [Line Items]
Long-term debt	[2],[5]	$ 400.0
Interest rate		3.75%
Alliant Energy Finance, LLC [Member] | Senior Notes [Member] | 4.25% senior notes, due 2028 [Member]
Debt Instrument [Line Items]
Long-term debt	[2],[5]	$ 300.0
Interest rate		4.25%
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 2019 to 2024 [Member]
Debt Instrument [Line Items]
Long-term debt	[2]	$ 44.3	$ 49.6
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 September 2018, IPL issued $500 million of 4.1% senior debentures due 2028. The senior debentures were issued as green bonds, and all of the net proceeds were allocated for the construction and development of wind and solar projects.

[4] In April 2018, AEF entered into a $300 million variable-rate term loan credit agreement and used the proceeds from borrowings under this agreement for general corporate purposes. Refer to Note 9(a) for discussion of a financial covenant contained in AEF’s term loan credit agreement.

[5] In June 2018, AEF issued $400 million of 3.75% senior notes due 2023 and $300 million of 4.25% senior notes due 2028. The proceeds from the issuances were used by AEF to retire its $500 million and $95 million variable-rate term loan credit agreements expiring in 2018, to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes.