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Investments (Narrative) (Details) $ in Millions | 12 Months Ended | 38 Months Ended |
--- | --- | --- | ---
Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2013
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Number of legal complaints (in number of complaints) | 2 |  |  |  |
Proceeds from liquidation of company-owned life insurance policies | $ 31.0 |  |  |  |
Alliant Energy and WPL [Member] |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Reduction in equity income from ATC | 9.0 | $ 12.0 | $ 3.0 | $ 24.0 |
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 to ATI | 163.6 |  |  |  |
IPL [Member] |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Proceeds from liquidation of company-owned life insurance policies | $ 19.0 |  |  |  |

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