Debt (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended
	Oct. 31, 2017	Jul. 31, 2017	Dec. 31, 2017	Sep. 30, 2017
Debt [Line Items]
Line of credit facility, current borrowing capacity				$ 1,000.0
Parent Company [Member]
Debt [Line Items]
Line of credit facility, current borrowing capacity				300.0
IPL [Member]
Debt [Line Items]
Line of credit facility, current borrowing capacity				300.0
WPL [Member]
Debt [Line Items]
Line of credit facility, current borrowing capacity				400.0
Alliant Energy and IPL [Member]
Debt [Line Items]
Commercial paper, long-term				$ 40.0
Commercial paper, long-term, interest rate				1.40%
Term Loan Credit Agreement [Member] | Alliant Energy Finance LLC [Member]
Debt [Line Items]
Proceeds from issuance of debt		$ 95.0
Variable-rate term loan credit agreement, period		364 days
Interest rate				1.80%
Subsequent Event [Member] | 3.05% debenture, due 2027 [Member] | Debentures [Member] | WPL [Member]
Debt [Line Items]
Proceeds from issuance of debt	$ 300.0
Interest rate	3.05%
Scenario, Forecast [Member] | Parent Company [Member]
Debt [Line Items]
Line of credit facility, maximum borrowing capacity			$ 500.0
Scenario, Forecast [Member] | IPL [Member]
Debt [Line Items]
Line of credit facility, maximum borrowing capacity			400.0
Scenario, Forecast [Member] | WPL [Member]
Debt [Line Items]
Line of credit facility, maximum borrowing capacity			$ 500.0