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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

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