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Receivables (Additional Attributes Of Receivables Sold Under The Agreement) (Details) - IPL [Member] - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Receivables [Line Items] |  |  |  |
Collections | $ 549.5 | $ 347.9 | $ 1,550.2 | $ 1,283.2
Write-offs, net of recoveries | $ 4.9 | $ 3.5 | $ 12.9 | $ 10.4

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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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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_AccountsNotesAndLoansReceivableLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | dei_LegalEntityAxis=lnt_IplMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |