Regulatory Matters (Revenue Requirement Adjustment) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Regulatory Liabilities [Line Items]
Electric revenues	$ 2,894.7	$ 2,875.5	$ 2,770.5
Alliant Energy and IPL [Member] | Revenue Requirement Adjustment [Member]
Regulatory Liabilities [Line Items]
Electric revenues		$ 14.0	$ 14.0