Property, Plant and Equipment (AFUDC By Project) (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
IPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	25.9	21.0	8.4
IPL [Member] | Ottumwa Unit 1 [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	10.6	8.0	2.0
IPL [Member] | George Neal Units 3 and 4 [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	1.4	5.1	0.9
IPL [Member] | Marshalltown Generating Station [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	3.7	0	0
IPL [Member] | Other [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	10.2	7.9	5.5
WPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	8.9	9.8	13.5
WPL [Member] | Columbia Units 1-2 [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	4.0	7.2	3.9
WPL [Member] | Edgewater Unit 5 [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	2.7	0	7.2
WPL [Member] | Other [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 2.2	$ 2.6	$ 2.4