Property, Plant and Equipment (Equity and Debt AFUDC) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 30.8	$ 21.9	$ 12.0
Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	10.5	7.8	4.4
Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	20.3	14.1	7.6
IPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	21.0	8.4	5.8
IPL [Member] | Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	7.2	3.2	2.3
IPL [Member] | Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	13.8	5.2	3.5
WPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	9.8	13.5	6.2
WPL [Member] | Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	3.3	4.6	2.1
WPL [Member] | Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 6.5	$ 8.9	$ 4.1