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Receivables (Narrative) (Details) - Receivables Sold [Member] - IPL [Member] $ in Millions | Sep. 30, 2017 USD ($)
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Receivables [Line Items] |
Available capacity | $ 1.5
Outstanding receivables past due | $ 54.1

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- Definition Outstanding receivables past due + References No definition available. + Details
Name: | lnt_OutstandingReceivablesPastDue
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Sale of Accounts Receivable Program, Capacity Available + References No definition available. + Details
Name: | lnt_SaleofAccountsReceivableProgramCapacityAvailable
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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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: | us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=lnt_ReceivablesSoldMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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