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Income Taxes (Production Tax Credits (Net Of State Tax Impacts)) (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2015 USD ($) MW | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($)
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | $ 33.1 | $ 31.3 | $ 31.0
IPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 14.5 | 13.8 | 14.1
WPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 18.6 | 17.5 | 16.9
Cedar Ridge Wind Farm [Member] | WPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 4.5 | 4.1 | 4.2
Bent Tree Wind Farm [Member] | WPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 14.1 | 13.4 | 12.7
Whispering Willow - East Wind Farm [Member] | IPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | $ 14.5 | $ 13.8 | $ 14.1
Cedar Ridge Wind Farm [Member] | WPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Nameplate capacity (in megawatts) | MW | 68 |  |
Bent Tree Wind Farm [Member] | WPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Nameplate capacity (in megawatts) | MW | 201 |  |
Whispering Willow - East Wind Farm [Member] | IPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Nameplate capacity (in megawatts) | MW | 200 |  |

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