Property, Plant and Equipment (Equity and Debt AFUDC) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 92.7	$ 75.6	$ 49.7
Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	65.5	51.4	33.6
Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	27.2	24.2	16.1
IPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	49.4	42.2	31.4
IPL [Member] | Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	35.2	28.6	21.1
IPL [Member] | Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	14.2	13.6	10.3
WPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	43.3	33.4	18.3
WPL [Member] | Equity [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	30.3	22.8	12.5
WPL [Member] | Debt [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 13.0	$ 10.6	$ 5.8