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Income Taxes (Production Tax Credits (Net Of State Tax Impacts)) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | $ 31.0 | $ 24.8 | $ 27.1
IPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 14.4 | 12.5 | 12.3
WPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 16.6 | 12.3 | 14.8
Whispering Willow - East Wind Project [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 14.4 | 12.5 | 12.3
Whispering Willow - East Wind Project [Member] | IPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 14.4 | 12.5 | 12.3
Whispering Willow - East Wind Project [Member] | WPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 0 | 0 | 0
Bent Tree - Phase I Wind Project [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 12.5 | 9.3 | 9.3
Bent Tree - Phase I Wind Project [Member] | IPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 0 | 0 | 0
Bent Tree - Phase I Wind Project [Member] | WPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 12.5 | 9.3 | 9.3
Cedar Ridge Wind Project [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 4.1 | 4.0 | 4.5
Cedar Ridge Wind Project [Member] | IPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 0 | 0 | 0
Cedar Ridge Wind Project [Member] | WPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 4.1 | 4.0 | 4.5
Subtotal [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 31.0 | 25.8 | 26.1
Subtotal [Member] | IPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 14.4 | 12.5 | 12.3
Subtotal [Member] | WPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 16.6 | 13.3 | 13.8
Deferral [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 0 | (1.0) | 1.0
Deferral [Member] | IPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | 0 | 0 | 0
Deferral [Member] | WPL [Member] |  |  |
Production Tax Credits [Line Items] |  |  |
Production tax credits, net of state tax impacts | $ 0 | $ (1.0) | $ 1.0

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- Details
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Namespace Prefix: | lnt_
Data Type: | xbrli:stringItemType
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- Definition Production Tax Credits Net Of State Tax Impacts + References No definition available. + Details
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Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration