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Income Taxes (Production Tax Credits (Net Of State Tax Impacts)) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2013 | Sep. 30, 2012 | Sep. 30, 2013 | Sep. 30, 2012
Production Tax Credits [Line Items] |  |  |  |
Production tax credits (net of state tax impacts) | $ 5.1 | $ 4.5 | $ 22.4 | $ 17.7
WPL [Member] |  |  |  |
Production Tax Credits [Line Items] |  |  |  |
Production tax credits (net of state tax impacts) | 2.8 | 2.5 | 12.1 | 9.0
Cedar Ridge Wind Project [Member] | WPL [Member] |  |  |  |
Production Tax Credits [Line Items] |  |  |  |
Production tax credits (net of state tax impacts) | 0.6 | 0.7 | 2.9 | 3.0
Bent Tree - Phase I Wind Project [Member] | WPL [Member] |  |  |  |
Production Tax Credits [Line Items] |  |  |  |
Production tax credits (net of state tax impacts) | 2.2 | 1.8 | 9.2 | 6.0
Whispering Willow - East Wind Project [Member] | IPL [Member] |  |  |  |
Production Tax Credits [Line Items] |  |  |  |
Production tax credits (net of state tax impacts) | $ 2.3 | $ 2.0 | $ 10.3 | $ 8.7

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- Definition Production Tax Credits Net Of State Tax Impacts + References No definition available. + Details
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