Investments (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended	35 Months Ended
	Sep. 30, 2016	Sep. 30, 2016
Schedule of Investments [Line Items]
Proceeds from liquidation of company-owned life insurance policies	$ 31
IPL [Member]
Schedule of Investments [Line Items]
Proceeds from liquidation of company-owned life insurance policies	19
ATC [Member] | Alliant Energy and WPL [Member]
Schedule of Investments [Line Items]
Reduction in equity income from ATC	$ 9	$ 24