Investments (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 MW	Dec. 31, 2016 USD ($)	Dec. 31, 2013
WPL [Member]
Schedule of Equity Method Investments [Line Items]
WPL's ownership interest in WPL Transco			100.00%
Net reduction in total equity due to transfer of ATC investment to ATI | $		$ 163.6
Alliant Energy Finance, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	50.00%
Electric capacity of wind farm (in megawatts) | MW	225