Property, Plant and Equipment (Narrative) (Details) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) MW	Jun. 30, 2017 USD ($)	Dec. 31, 2020 MW	Apr. 30, 2018 USD ($)
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 18.1	$ 10.1	$ 33.0	$ 27.1
WPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	8.2	4.0	15.7	6.7
IPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	9.9	$ 6.1	$ 17.3	$ 20.4
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	399.0		$ 399.0
Allowance for funds used during construction			$ 27.0
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
Estimated fair value of net assets acquired						$ 74.0
Expansion of Wind Generation [Member] | Alliant Energy and IPL [Member]
Property, Plant and Equipment [Line Items]
Construction work in progress	$ 468.0		$ 468.0
Allowance for funds used during construction			$ 24.0
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