Property, Plant and Equipment (Narrative) (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) MW	Mar. 31, 2017 USD ($)	Dec. 31, 2020 MW	Apr. 30, 2018 USD ($)
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 14.9	$ 17.0
WPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	7.5	2.7
IPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 7.4	$ 14.3
West Riverside Energy Center [Member] | WPL [Member]
Property, Plant and Equipment [Line Items]
Fossil-fueled EGU capacity (in megawatts) | MW	730
Fossil-fueled EGU capacity acquired by electric cooperatives (in megawatts) | MW	60
Wholesale power supply agreement, term of extension	4 years
Wholesale power supply agreement, term of notice requirement	5 years
West Riverside Energy Center [Member] | Alliant Energy and WPL [Member]
Property, Plant and Equipment [Line Items]
Construction work in progress	$ 347.0
Allowance for funds used during construction	$ 20.0
Expansion of Wind Generation [Member]
Property, Plant and Equipment [Line Items]
Electric capacity of wind farm (in megawatts) | MW	129
Expansion of Wind Generation [Member] | WPL [Member]
Property, Plant and Equipment [Line Items]
Electric capacity of wind farm (in megawatts) | MW	55
Expansion of Wind Generation [Member] | Alliant Energy and IPL [Member]
Property, Plant and Equipment [Line Items]
Construction work in progress	$ 385.0
Allowance for funds used during construction	$ 16.0
Scenario, Forecast [Member] | Expansion of Wind Generation [Member] | IPL [Member]
Property, Plant and Equipment [Line Items]
Electric capacity of wind farm (in megawatts) | MW			1,000
Subsequent Event [Member] | Expansion of Wind Generation [Member] | WPL [Member]
Property, Plant and Equipment [Line Items]
Estimated fair value of net assets acquired				$ 74.0