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Income Taxes (Production Tax Credits (Net Of State Tax Impacts)) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
--- | ---
Mar. 31, 2014 | Mar. 31, 2013
Production Tax Credits [Line Items] |  |
Production Tax Credits | $ 10.0 | $ 8.6
WPL [Member] |  |
Production Tax Credits [Line Items] |  |
Production Tax Credits | 5.4 | 4.7
Cedar Ridge Wind Project [Member] | WPL [Member] |  |
Production Tax Credits [Line Items] |  |
Nameplate Capacity (in megawatts) | 68 |
Bent Tree Phase I Wind Project [Member] | WPL [Member] |  |
Production Tax Credits [Line Items] |  |
Nameplate Capacity (in megawatts) | 201 |
Whispering Willow East Wind Project [Member] | IPL [Member] |  |
Production Tax Credits [Line Items] |  |
Nameplate Capacity (in megawatts) | 200 |
Bent Tree Phase I Wind Project [Member] | WPL [Member] |  |
Production Tax Credits [Line Items] |  |
Production Tax Credits | 4.2 | 3.5
Cedar Ridge Wind Project [Member] | WPL [Member] |  |
Production Tax Credits [Line Items] |  |
Production Tax Credits | 1.2 | 1.2
Whispering Willow East Wind Project [Member] | IPL [Member] |  |
Production Tax Credits [Line Items] |  |
Production Tax Credits | $ 4.6 | $ 3.9

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- Definition Electric Capacity Of Wind Project + References No definition available. + Details
Name: | lnt_ElectricCapacityOfWindProject
Namespace Prefix: | lnt_
Data Type: | num:powerItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | lnt_ProductionTaxCreditsLineItems
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
Name: | lnt_ProductionTaxCreditsNetOfStateTaxImpacts
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration