Property, Plant and Equipment (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Electric plant	$ 9,803.9		$ 9,803.9		$ 9,415.7
Allowance for funds used during construction	8.4	7.0	17.5	12.6
George Neal Unit 3 [Member] | Alliant Energy and IPL [Member]
Property, Plant and Equipment [Line Items]
Electric plant	58.0		58.0
Allowance for funds used during construction			4.0
Columbia Unit 2 [Member] | Alliant Energy and WPL [Member]
Property, Plant and Equipment [Line Items]
Electric plant	152.0		152.0
Allowance for funds used during construction			8.0
Columbia Unit 1 [Member] | Alliant Energy and WPL [Member]
Property, Plant and Equipment [Line Items]
Electric plant	116.0		116.0
Allowance for funds used during construction			$ 7.0