Property, Plant and Equipment (Narrative) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2016 USD ($) MW	Mar. 31, 2015 USD ($)
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 13.2	$ 6.8
Edgewater Unit 5 [Member] | Alliant Energy and WPL [Member]
Property, Plant and Equipment [Line Items]
Construction work in progress	207.0
Allowance for funds used during construction	$ 10.0
Marshalltown [Member]
Property, Plant and Equipment [Line Items]
Fossil-fueled EGU capacity (in megawatts) | MW	650
Marshalltown [Member] | Alliant Energy and IPL [Member]
Property, Plant and Equipment [Line Items]
Construction work in progress	$ 492.0
Allowance for funds used during construction	$ 33.0