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Receivables (Receivables Sold Under The Agreement) (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Receivables [Line Items] |  |
Fair value of deferred proceeds | $ 243.7 | $ 222.1
IPL [Member] |  |
Receivables [Line Items] |  |
Fair value of deferred proceeds | 243.7 | 222.1
Receivables Sold [Member] | IPL [Member] |  |
Receivables [Line Items] |  |
Customer accounts receivable | 179.0 | 133.8
Unbilled utility revenues | 78.2 | 112.7
Other receivables | 0.1 | 0.3
Receivables sold to third party | 257.3 | 246.8
Less: cash proceeds | 1.0 | 12.0
Deferred proceeds | 256.3 | 234.8
Less: allowance for doubtful accounts | 12.6 | 12.7
Fair value of deferred proceeds | $ 243.7 | $ 222.1

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- Definition Allowance for Doubtful Accounts Receivable Sold + References No definition available. + Details
Name: | lnt_AllowanceForDoubtfulAccountsReceivableSold
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Cash Proceeds From Receivables Sold + References No definition available. + Details
Name: | lnt_CashProceedsFromReceivablesSold
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Customer Accounts Receivable Sold, Amount + References No definition available. + Details
Name: | lnt_CustomerAccountsReceivableSoldAmount
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Deferred proceeds of receivables sold + References No definition available. + Details
Name: | lnt_DeferredProceedsOfReceivablesSold
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Fair value of deferred proceeds of receivables sold + References No definition available. + Details
Name: | lnt_FairValueOfDeferredProceedsOfReceivablesSold
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Other Receivables Sold Amount + References No definition available. + Details
Name: | lnt_OtherReceivablesSoldAmount
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Receivables sold + References No definition available. + Details
Name: | lnt_ReceivablesSold
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Unbilled Utility Revenues Sold, Amount + References No definition available. + Details
Name: | lnt_UnbilledUtilityRevenuesSoldAmount
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: | dei_LegalEntityAxis=lnt_IplMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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