Property, Plant and Equipment (Equity and Debt AFUDC) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 75.6	$ 49.7	$ 62.5
Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	51.4	33.6	42.3
Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	24.2	16.1	20.2
IPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	42.2	31.4	52.0
IPL [Member] | Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	28.6	21.1	35.2
IPL [Member] | Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	13.6	10.3	16.8
WPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	33.4	18.3	10.5
WPL [Member] | Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	22.8	12.5	7.1
WPL [Member] | Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 10.6	$ 5.8	$ 3.4