Regulatory Matters (Revenue Requirement Adjustment) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Regulatory Liabilities [Line Items]
Electric revenues	$ 2,875.5	$ 2,770.5	$ 2,713.6
Alliant Energy and IPL [Member] | Revenue Requirement Adjustment [Member]
Regulatory Liabilities [Line Items]
Electric revenues	$ 14.0	$ 14.0	$ 15.0