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Income Taxes (Production Tax Credits (Net Of State Tax Impacts)) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2013 | Mar. 31, 2012
Production Tax Credits [Line Items] |  |
Production tax credits (net of state tax impacts) | $ 8.6 | $ 6.4
WPL [Member] |  |
Production Tax Credits [Line Items] |  |
Production tax credits (net of state tax impacts) | 4.7 | 2.8
Cedar Ridge Wind Project [Member] | WPL [Member] |  |
Production Tax Credits [Line Items] |  |
Production tax credits (net of state tax impacts) | 1.2 | 1.3
Bent Tree - Phase I Wind Project [Member] | WPL [Member] |  |
Production Tax Credits [Line Items] |  |
Production tax credits (net of state tax impacts) | 3.5 | 1.5
Whispering Willow - East Wind Project [Member] | IPL [Member] |  |
Production Tax Credits [Line Items] |  |
Production tax credits (net of state tax impacts) | $ 3.9 | $ 3.6

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- Details
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Namespace Prefix: | lnt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Production Tax Credits Net Of State Tax Impacts + References No definition available. + Details
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Balance Type: | credit
Period Type: | duration