Property, Plant and Equipment (Equity and Debt AFUDC) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 36.9	$ 34.8	$ 30.8
Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	24.4	23.1	20.3
Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	12.5	11.7	10.5
IPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	28.2	25.9	21.0
IPL [Member] | Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	18.6	17.1	13.8
IPL [Member] | Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	9.6	8.8	7.2
WPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	8.7	8.9	9.8
WPL [Member] | Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	5.8	6.0	6.5
WPL [Member] | Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 2.9	$ 2.9	$ 3.3