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Investments (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended | 26 Months Ended |
--- | --- | --- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2015 | Dec. 31, 2013
Alliant Energy and WPL [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Reduction in equity income from ATC | $ 12 | $ 3 | $ 15 |
WPL [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
WPL's ownership interest in WPL Transco | 93.00% |  | 93.00% | 100.00%
ATI [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
ATI's ownership interest in WPL Transco | 7.00% |  | 7.00% |

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