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Receivables (Additional Attributes Of Receivables Sold Under The Agreement) (Details) (IPL [Member], USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
IPL [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Collections reinvested in receivables | $ 1,997.9 lnt_CollectionsReinvestedInReceivables / dei_LegalEntityAxis = lnt_IplMember | $ 1,880.8 lnt_CollectionsReinvestedInReceivables / dei_LegalEntityAxis = lnt_IplMember | $ 1,771.6 lnt_CollectionsReinvestedInReceivables / dei_LegalEntityAxis = lnt_IplMember
Credit losses, net of recoveries | $ 11.4 lnt_CreditLossesNetOfRecoveries / dei_LegalEntityAxis = lnt_IplMember | $ 11.9 lnt_CreditLossesNetOfRecoveries / dei_LegalEntityAxis = lnt_IplMember | $ 10.0 lnt_CreditLossesNetOfRecoveries / dei_LegalEntityAxis = lnt_IplMember

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- Definition Collections reinvested related to sales of accounts receivable program. + References No definition available. + Details
Name: | lnt_CollectionsReinvestedInReceivables
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Credit losses, net of recoveries related to sales of accounts receivable program. + References No definition available. + Details
Name: | lnt_CreditLossesNetOfRecoveries
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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