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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 $ 177.2lnt_FairValueOfDeferredProceedsOfReceivablesSold $ 203.5lnt_FairValueOfDeferredProceedsOfReceivablesSold    
Long-term debt issued 3,802.8us-gaap_LongTermDebt 3,348.4us-gaap_LongTermDebt    
Proceeds from Franklin County wind project cash grant 0lnt_ProceedsFromCashGrant 62.4lnt_ProceedsFromCashGrant 0lnt_ProceedsFromCashGrant  
IPL [Member]        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Cash proceeds to be received from third-party 46.4us-gaap_ProceedsFromSaleAndCollectionOfReceivables
/ dei_LegalEntityAxis
= lnt_IplMember		 105.9us-gaap_ProceedsFromSaleAndCollectionOfReceivables
/ dei_LegalEntityAxis
= lnt_IplMember		 119.8us-gaap_ProceedsFromSaleAndCollectionOfReceivables
/ dei_LegalEntityAxis
= lnt_IplMember		  
Receivables sold to third party 204.6lnt_ReceivablesSold
/ dei_LegalEntityAxis
= lnt_IplMember		      
Fair value of deferred proceeds of receivables sold 177.2lnt_FairValueOfDeferredProceedsOfReceivablesSold
/ dei_LegalEntityAxis
= lnt_IplMember		 203.5lnt_FairValueOfDeferredProceedsOfReceivablesSold
/ dei_LegalEntityAxis
= lnt_IplMember		    
Long-term debt issued 1,775.0us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= lnt_IplMember		 1,563.4us-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 150.0us-gaap_ProceedsFromSaleAndCollectionOfReceivables
/ dei_LegalEntityAxis
= lnt_IplMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		 170.0us-gaap_ProceedsFromSaleAndCollectionOfReceivables
/ dei_LegalEntityAxis
= lnt_IplMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		 160.0us-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 26us-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   62.4lnt_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 1,775.0us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= lnt_SeniorDebenturesMember		 1,525.0us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= lnt_SeniorDebenturesMember		    
Senior Debentures [Member] | IPL [Member]        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Long-term debt issued 1,775.0us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= lnt_IplMember
/ us-gaap_LongtermDebtTypeAxis
= lnt_SeniorDebenturesMember		 1,525.0us-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 204.6lnt_ReceivablesSold
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= lnt_ReceivablesSoldMember
/ dei_LegalEntityAxis
= lnt_IplMember		 238.0lnt_ReceivablesSold
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= lnt_ReceivablesSoldMember
/ dei_LegalEntityAxis
= lnt_IplMember		    
Cash proceeds from receivables sold 22.0lnt_CashProceedsFromReceivablesSold
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= lnt_ReceivablesSoldMember
/ dei_LegalEntityAxis
= lnt_IplMember		 [1] 29.0lnt_CashProceedsFromReceivablesSold
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= lnt_ReceivablesSoldMember
/ dei_LegalEntityAxis
= lnt_IplMember		 [1]    
Fair value of deferred proceeds of receivables sold 177.2lnt_FairValueOfDeferredProceedsOfReceivablesSold
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= lnt_ReceivablesSoldMember
/ dei_LegalEntityAxis
= lnt_IplMember		 203.5lnt_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 250.0us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= lnt_ThreePointTwoFivePercentSeniorDebenturesDueTwoThousandTwentyFourMemberDomain
/ us-gaap_LongtermDebtTypeAxis
= lnt_SeniorDebenturesMember		 [2] 0us-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 250.0us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= lnt_ThreePointTwoFivePercentSeniorDebenturesDueTwoThousandTwentyFourMemberDomain
/ dei_LegalEntityAxis
= lnt_IplMember
/ us-gaap_LongtermDebtTypeAxis
= lnt_SeniorDebenturesMember		 [2] 0us-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       $ 180.0us-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.