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Benefit Plans (Carrying Value And Fair Market Value Of The Deferred Compensation Obligation for Company Stock Accounts) (Details) (Company Stock Accounts [Member], USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2013 | Dec. 31, 2012
Company Stock Accounts [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Carrying value of deferred compensation obligation | $ 8.0 | $ 7.3
Fair market value of deferred compensation obligation | $ 11.7 | $ 9.5

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- Definition Carrying value of deferred compensation + References No definition available. + Details
Name: | lnt_CarryingValueOfDeferredCompensation
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Fair market value of deferred compensation + References No definition available. + Details
Name: | lnt_FairMarketValueOfDeferredCompensation
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