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Benefit Plans (Schedule Of Qualified and Non-qualified Pension And Other Postretirement Benefits Costs) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2013 | Mar. 31, 2012
Defined benefit pension plans [Member] | IPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Benefits costs associated with Corporate Services employees | $ 0.5 | $ 0.5
Defined benefit pension plans [Member] | WPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Benefits costs associated with Corporate Services employees | 0.3 | 0.3
Other Postretirement Benefits Costs [Member] | IPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Benefits costs associated with Corporate Services employees | (0.1) | 0
Other Postretirement Benefits Costs [Member] | WPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Benefits costs associated with Corporate Services employees | $ 0 | $ 0

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- Definition Pension and other postretirement benefits plans costs allocated from Corporate Services. + References No definition available. + Details
Name: | lnt_DefinedBenefitPlanPensionAndOtherPostretirementBenefitsPlansCostsAllocatedFromCorporateServices
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration