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Receivables (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Fair value of deferred proceeds of receivables sold | $ 177.2 lnt_FairValueOfDeferredProceedsOfReceivablesSold |  | $ 203.5 lnt_FairValueOfDeferredProceedsOfReceivablesSold |  |  |
Long-term debt issued | 3,802.8 us-gaap_LongTermDebt |  | 3,348.4 us-gaap_LongTermDebt |  |  |
Proceeds from Franklin County wind project cash grant | 0 lnt_ProceedsFromCashGrant |  | 62.4 lnt_ProceedsFromCashGrant |  | 0 lnt_ProceedsFromCashGrant |
IPL [Member] |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Cash proceeds to be received from third-party | 46.4 us-gaap_ProceedsFromSaleAndCollectionOfReceivables / dei_LegalEntityAxis = lnt_IplMember |  | 105.9 us-gaap_ProceedsFromSaleAndCollectionOfReceivables / dei_LegalEntityAxis = lnt_IplMember |  | 119.8 us-gaap_ProceedsFromSaleAndCollectionOfReceivables / dei_LegalEntityAxis = lnt_IplMember |
Receivables sold to third party | 204.6 lnt_ReceivablesSold / dei_LegalEntityAxis = lnt_IplMember |  |  |  |  |
Fair value of deferred proceeds of receivables sold | 177.2 lnt_FairValueOfDeferredProceedsOfReceivablesSold / dei_LegalEntityAxis = lnt_IplMember |  | 203.5 lnt_FairValueOfDeferredProceedsOfReceivablesSold / dei_LegalEntityAxis = lnt_IplMember |  |  |
Long-term debt issued | 1,775.0 us-gaap_LongTermDebt / dei_LegalEntityAxis = lnt_IplMember |  | 1,563.4 us-gaap_LongTermDebt / dei_LegalEntityAxis = lnt_IplMember |  |  |
IPL [Member] | Maximum [Member] |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Cash proceeds to be received from third-party | 150.0 us-gaap_ProceedsFromSaleAndCollectionOfReceivables / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  | 170.0 us-gaap_ProceedsFromSaleAndCollectionOfReceivables / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  | 160.0 us-gaap_ProceedsFromSaleAndCollectionOfReceivables / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RangeAxis = us-gaap_MaximumMember |
Whiting Petroleum Corporation [Member] |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Final payment of certain future tax benefits expected to be realized | 26 us-gaap_ProceedsFromCollectionOfOtherReceivables / dei_LegalEntityAxis = lnt_WhitingPetroleumCorporationMember |  |  |  |  |
Franklin County Wind Project [Member] | American Recovery and Reinvestment Act of 2009 [Member] |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Proceeds from Franklin County wind project cash grant |  |  | 62.4 lnt_ProceedsFromCashGrant / lnt_GovernmentActAxis = lnt_AmericanRecoveryAndReinvestmentActOf2009Member / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_FranklinCountyWindProjectMember |  |  |
Senior Debentures [Member] |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Long-term debt issued | 1,775.0 us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 1,525.0 us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |  |
Senior Debentures [Member] | IPL [Member] |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Long-term debt issued | 1,775.0 us-gaap_LongTermDebt / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 1,525.0 us-gaap_LongTermDebt / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |  |
Receivables Sold [Member] | IPL [Member] |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
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 |  |  |
Cash proceeds from receivables sold | 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] |  |
Fair value of deferred proceeds of receivables sold | 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 |  |  |
3.25% senior debenture, due 2024 [Member] | Senior Debentures [Member] |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Long-term debt issued | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_ThreePointTwoFivePercentSeniorDebenturesDueTwoThousandTwentyFourMemberDomain / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember | [2] | 0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_ThreePointTwoFivePercentSeniorDebenturesDueTwoThousandTwentyFourMemberDomain / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember | [2] |  |
3.25% senior debenture, due 2024 [Member] | Senior Debentures [Member] | IPL [Member] |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Long-term debt issued | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_ThreePointTwoFivePercentSeniorDebenturesDueTwoThousandTwentyFourMemberDomain / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember | [2] | 0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_ThreePointTwoFivePercentSeniorDebenturesDueTwoThousandTwentyFourMemberDomain / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember | [2] |  |
Scenario, Forecast [Member] | IPL [Member] |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Cash proceeds to be received from third-party |  |  |  |  |  | $ 180.0 us-gaap_ProceedsFromSaleAndCollectionOfReceivables / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
[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.
[2] | In 2014, IPL issued $250.0 million of 3.25% senior debentures due 2024. The proceeds from the issuance were used by IPL to reduce cash proceeds received from its sales of accounts receivable program, reduce commercial paper classified as long-term debt by $60 million and for general corporate purposes.

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