Property, Plant and Equipment (Narrative) (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) MW	Sep. 30, 2017 USD ($)	Dec. 31, 2020 MW	Apr. 30, 2018 USD ($)
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ 18.8	$ 9.6	$ 51.8	$ 36.7
WPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	7.8	4.9	23.5	11.6
IPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	11.0	$ 4.7	$ 28.3	$ 25.1
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	438.0		$ 438.0
Allowance for funds used during construction			$ 35.0
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	$ 645.0		$ 645.0
Allowance for funds used during construction			$ 33.0
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