Investments (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended		26 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2013
Alliant Energy and WPL [Member]
Schedule of Equity Method Investments [Line Items]
Reduction in equity income from ATC	$ 12	$ 3	$ 15
WPL [Member]
Schedule of Equity Method Investments [Line Items]
WPL's ownership interest in WPL Transco	93.00%		93.00%	100.00%
ATI [Member]
Schedule of Equity Method Investments [Line Items]
ATI's ownership interest in WPL Transco	7.00%		7.00%