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Property, Plant and Equipment (Narrative) (Details) $ / shares in Units, $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Jun. 30, 2018 USD ($) MW | Sep. 30, 2017 USD ($) | Sep. 30, 2016 USD ($) $ / shares | Sep. 30, 2017 USD ($) MW | Sep. 30, 2016 USD ($) | Dec. 31, 2020 MW | Apr. 30, 2017 USD ($)
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Allowance for funds used during construction |  | $ 9.6 | $ 15.8 | $ 36.7 | $ 44.3 |  |
Asset valuation charges for Franklin County wind farm, pre-tax |  | 0.0 | 86.4 | 0.0 | 86.4 |  |
Asset valuation charge for Franklin County wind farm, after tax |  |  | $ 51.0 |  |  |  |
Asset valuation charges for Franklin County wind farm, after tax (in dollars per share) | $ / shares |  |  | $ 0.23 |  |  |  |
IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Allowance for funds used during construction |  | 4.7 | $ 13.8 | 25.1 | 36.2 |  |
Estimated fair value of net assets acquired |  |  |  |  |  |  | $ 32.0
WPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Allowance for funds used during construction |  | 4.9 | $ 2.0 | $ 11.6 | $ 8.1 |  |
Marshalltown Generating Station [Member] | IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Fossil-fueled EGU capacity (in megawatts) | MW |  |  |  | 660 |  |  |
Marshalltown Generating Station [Member] | Alliant Energy and IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Total project costs |  | 643.0 |  | $ 643.0 |  |  |
Allowance for funds used during construction |  |  |  | $ 81.0 |  |  |
West Riverside Energy Center [Member] | WPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Fossil-fueled EGU capacity (in megawatts) | MW |  |  |  | 730 |  |  |
West Riverside Energy Center [Member] | Alliant Energy and WPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Construction work in progress |  | 278.0 |  | $ 278.0 |  |  |
Allowance for funds used during construction |  |  |  | $ 9.0 |  |  |
Expansion of Wind Generation [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Electric capacity of wind farm | MW |  |  |  | 129 |  |  |
Expansion of Wind Generation [Member] | Alliant Energy and IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Construction work in progress |  | $ 184.0 |  | $ 184.0 |  |  |
Allowance for funds used during construction |  |  |  | $ 7.0 |  |  |
Scenario, Forecast [Member] | Expansion of Wind Generation [Member] | IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Electric capacity of wind farm | MW |  |  |  |  |  | 1,000 |
Scenario, Forecast [Member] | Expansion of Wind Generation [Member] | WPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Electric capacity of wind farm | MW | 55 |  |  |  |  |  |
Estimated fair value of net assets acquired | $ 74.0 |  |  |  |  |  |

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