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Receivables (Receivables Sold Under The Agreement) (Details) (USD $) In Millions, unless otherwise specified | Sep. 30, 2013 | Dec. 31, 2012
--- | --- | ---
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Fair value of deferred proceeds | $ 86.9 |  | $ 66.8 |
IPL [Member] |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Customer accounts receivable | 158.8 |  | 118.2 |
Unbilled utility revenues | 70.9 |  | 77.4 |
Other receivables | 0.1 |  | 2.8 |
Receivables sold | 229.8 |  | 198.4 |
Less: cash proceeds | 140.0 | [1] | 130.0 | [1]
Deferred proceeds | 89.8 |  | 68.4 |
Less: allowance for doubtful accounts | 2.9 |  | 1.6 |
Fair value of deferred proceeds | 86.9 |  | 66.8 |
Outstanding receivables past due | $ 22.0 |  | $ 16.1 |
[1] | Changes in cash proceeds are presented in “Sales of accounts receivable” in operating activities in Alliant Energy’s and IPL’s Condensed Consolidated Statements of Cash Flows.

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