Investments (Narrative) (Details) $ in Millions	12 Months Ended			38 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2013
Schedule of Equity Method Investments [Line Items]
Number of legal complaints (in number of complaints)	2
Proceeds from liquidation of company-owned life insurance policies	$ 31.0
Alliant Energy and WPL [Member]
Schedule of Equity Method Investments [Line Items]
Reduction in equity income from ATC	9.0	$ 12.0	$ 3.0	$ 24.0
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 to ATI	163.6
IPL [Member]
Schedule of Equity Method Investments [Line Items]
Proceeds from liquidation of company-owned life insurance policies	$ 19.0