Property, Plant and Equipment (AFUDC By Project) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 62.5	$ 36.9	$ 34.8
IPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	52.0	28.2	25.9
IPL [Member] | Marshalltown Generating Station [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	43.8	20.7	3.7
IPL [Member] | Environmental controls - Ottumwa Unit 1 [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	0.0	0.0	10.6
IPL [Member] | Other [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	8.2	7.5	11.6
WPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	10.5	8.7	8.9
WPL [Member] | Environmental controls - Edgewater Unit 5 [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	4.3	5.1	2.7
WPL [Member] | Other [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 6.2	$ 3.6	$ 6.2