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Receivables (Receivables Sold Under The Agreement) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Fair value of deferred proceeds | $ 177.2 lnt_FairValueOfDeferredProceedsOfReceivablesSold |  | $ 203.5 lnt_FairValueOfDeferredProceedsOfReceivablesSold |
IPL [Member] |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Receivables sold to third party | 204.6 lnt_ReceivablesSold / dei_LegalEntityAxis = lnt_IplMember |  |  |
Fair value of deferred proceeds | 177.2 lnt_FairValueOfDeferredProceedsOfReceivablesSold / dei_LegalEntityAxis = lnt_IplMember |  | 203.5 lnt_FairValueOfDeferredProceedsOfReceivablesSold / dei_LegalEntityAxis = lnt_IplMember |
Receivables Sold [Member] | IPL [Member] |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Customer accounts receivable | 134.8 lnt_CustomerAccountsReceivableSoldAmount / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = lnt_ReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |  | 151.6 lnt_CustomerAccountsReceivableSoldAmount / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = lnt_ReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |
Unbilled utility revenues | 69.7 lnt_UnbilledUtilityRevenuesSoldAmount / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = lnt_ReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |  | 86.2 lnt_UnbilledUtilityRevenuesSoldAmount / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = lnt_ReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |
Other receivables | 0.1 lnt_OtherReceivablesSoldAmount / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = lnt_ReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |  | 0.2 lnt_OtherReceivablesSoldAmount / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = lnt_ReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |
Receivables sold to third party | 204.6 lnt_ReceivablesSold / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = lnt_ReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |  | 238.0 lnt_ReceivablesSold / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = lnt_ReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |
Less: cash proceeds | 22.0 lnt_CashProceedsFromReceivablesSold / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = lnt_ReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember | [1] | 29.0 lnt_CashProceedsFromReceivablesSold / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = lnt_ReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember | [1]
Deferred proceeds | 182.6 lnt_DeferredProceedsOfReceivablesSold / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = lnt_ReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |  | 209.0 lnt_DeferredProceedsOfReceivablesSold / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = lnt_ReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |
Less: allowance for doubtful accounts | 5.4 lnt_AllowanceForDoubtfulAccountsReceivableSold / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = lnt_ReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |  | 5.5 lnt_AllowanceForDoubtfulAccountsReceivableSold / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = lnt_ReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |
Fair value of deferred proceeds | 177.2 lnt_FairValueOfDeferredProceedsOfReceivablesSold / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = lnt_ReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |  | 203.5 lnt_FairValueOfDeferredProceedsOfReceivablesSold / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = lnt_ReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |
Outstanding receivables past due | $ 19.9 lnt_OutstandingReceivablesPastDue / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = lnt_ReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |  | $ 21.5 lnt_OutstandingReceivablesPastDue / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = lnt_ReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |
[1] | Changes in cash proceeds are presented in “Sales of accounts receivable” in operating activities in Alliant Energy’s and IPL’s cash flows statements.

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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

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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: |