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Investments (Narrative) (Details) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 MW | Dec. 31, 2016 USD ($) | Dec. 31, 2013
WPL [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
WPL's ownership interest in WPL Transco |  |  | 100.00%
Net reduction in total equity due to transfer of ATC investment to ATI | $ |  | $ 163.6 |
Alliant Energy Finance, LLC [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest | 50.00% |  |
Electric capacity of wind farm (in megawatts) | MW | 225 |  |

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