Property, Plant and Equipment (Equity and Debt AFUDC) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 49.7	$ 62.5	$ 36.9
Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	16.1	20.2	12.5
Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	33.6	42.3	24.4
IPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	31.4	52.0	28.2
IPL [Member] | Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	10.3	16.8	9.6
IPL [Member] | Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	21.1	35.2	18.6
WPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	18.3	10.5	8.7
WPL [Member] | Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	5.8	3.4	2.9
WPL [Member] | Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 12.5	$ 7.1	$ 5.8