Property, Plant and Equipment (Equity and Debt AFUDC) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 34.8	$ 30.8	$ 21.9
Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	11.7	10.5	7.8
Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	23.1	20.3	14.1
IPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	25.9	21.0	8.4
IPL [Member] | Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	8.8	7.2	3.2
IPL [Member] | Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	17.1	13.8	5.2
WPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	8.9	9.8	13.5
WPL [Member] | Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	2.9	3.3	4.6
WPL [Member] | Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 6.0	$ 6.5	$ 8.9