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Fair Value Measurements (Fair Value Of Financial Instruments) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Assets:    
Derivative assets $ 33.0us-gaap_DerivativeAssets $ 23.5us-gaap_DerivativeAssets
Capitalization and liabilities:    
Long-term debt (including current maturities) 3,802.8us-gaap_LongTermDebt 3,348.4us-gaap_LongTermDebt
Long-term debt (including current maturities) 4,418.2us-gaap_DebtInstrumentFairValue 3,712.3us-gaap_DebtInstrumentFairValue
Cumulative preferred stock 200.2lnt_PreferredStockFairValue 167.0lnt_PreferredStockFairValue
Derivative liabilities 32.0us-gaap_DerivativeLiabilities 17.6us-gaap_DerivativeLiabilities
IPL [Member]    
Assets:    
Derivative assets 24.7us-gaap_DerivativeAssets
/ dei_LegalEntityAxis
= lnt_IplMember
19.5us-gaap_DerivativeAssets
/ dei_LegalEntityAxis
= lnt_IplMember
Capitalization and liabilities:    
Long-term debt (including current maturities) 1,775.0us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= lnt_IplMember
1,563.4us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= lnt_IplMember
Long-term debt (including current maturities) 2,053.0us-gaap_DebtInstrumentFairValue
/ dei_LegalEntityAxis
= lnt_IplMember
1,726.4us-gaap_DebtInstrumentFairValue
/ dei_LegalEntityAxis
= lnt_IplMember
Cumulative preferred stock 200.2lnt_PreferredStockFairValue
/ dei_LegalEntityAxis
= lnt_IplMember
167.0lnt_PreferredStockFairValue
/ dei_LegalEntityAxis
= lnt_IplMember
Derivative liabilities 16.2us-gaap_DerivativeLiabilities
/ dei_LegalEntityAxis
= lnt_IplMember
3.6us-gaap_DerivativeLiabilities
/ dei_LegalEntityAxis
= lnt_IplMember
WPL [Member]    
Assets:    
Derivative assets 8.3us-gaap_DerivativeAssets
/ dei_LegalEntityAxis
= lnt_WplMember
4.0us-gaap_DerivativeAssets
/ dei_LegalEntityAxis
= lnt_WplMember
Capitalization and liabilities:    
Long-term debt (including current maturities) 1,580.6us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= lnt_WplMember
1,339.1us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= lnt_WplMember
Long-term debt (including current maturities) 1,908.9us-gaap_DebtInstrumentFairValue
/ dei_LegalEntityAxis
= lnt_WplMember
1,532.9us-gaap_DebtInstrumentFairValue
/ dei_LegalEntityAxis
= lnt_WplMember
Derivative liabilities 15.8us-gaap_DerivativeLiabilities
/ dei_LegalEntityAxis
= lnt_WplMember
14.0us-gaap_DerivativeLiabilities
/ dei_LegalEntityAxis
= lnt_WplMember
Carrying Amount [Member]    
Assets:    
Derivative assets 38.6us-gaap_DerivativeAssets
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
26.7us-gaap_DerivativeAssets
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
Deferred proceeds (sales of receivables) 177.2lnt_DeferredProceedsOfReceivablesSold
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
203.5lnt_DeferredProceedsOfReceivablesSold
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
Capitalization and liabilities:    
Long-term debt (including current maturities) 3,789.7us-gaap_LongTermDebt
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
3,336.3us-gaap_LongTermDebt
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
Cumulative preferred stock 200.0us-gaap_PreferredStockValue
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
200.0us-gaap_PreferredStockValue
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
Derivative liabilities 37.6us-gaap_DerivativeLiabilities
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
20.8us-gaap_DerivativeLiabilities
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
Carrying Amount [Member] | IPL [Member]    
Assets:    
Derivative assets 28.0us-gaap_DerivativeAssets
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
/ dei_LegalEntityAxis
= lnt_IplMember
21.1us-gaap_DerivativeAssets
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
/ dei_LegalEntityAxis
= lnt_IplMember
Deferred proceeds (sales of receivables) 177.2lnt_DeferredProceedsOfReceivablesSold
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
/ dei_LegalEntityAxis
= lnt_IplMember
203.5lnt_DeferredProceedsOfReceivablesSold
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
/ dei_LegalEntityAxis
= lnt_IplMember
Capitalization and liabilities:    
Long-term debt (including current maturities) 1,768.7us-gaap_LongTermDebt
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
/ dei_LegalEntityAxis
= lnt_IplMember
1,558.4us-gaap_LongTermDebt
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
/ dei_LegalEntityAxis
= lnt_IplMember
Cumulative preferred stock 200.0us-gaap_PreferredStockValue
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
/ dei_LegalEntityAxis
= lnt_IplMember
200.0us-gaap_PreferredStockValue
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
/ dei_LegalEntityAxis
= lnt_IplMember
Derivative liabilities 19.5us-gaap_DerivativeLiabilities
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
/ dei_LegalEntityAxis
= lnt_IplMember
5.2us-gaap_DerivativeLiabilities
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
/ dei_LegalEntityAxis
= lnt_IplMember
Carrying Amount [Member] | WPL [Member]    
Assets:    
Derivative assets 10.6us-gaap_DerivativeAssets
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
/ dei_LegalEntityAxis
= lnt_WplMember
5.6us-gaap_DerivativeAssets
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
/ dei_LegalEntityAxis
= lnt_WplMember
Capitalization and liabilities:    
Long-term debt (including current maturities) 1,573.9us-gaap_LongTermDebt
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
/ dei_LegalEntityAxis
= lnt_WplMember
1,332.1us-gaap_LongTermDebt
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
/ dei_LegalEntityAxis
= lnt_WplMember
Derivative liabilities 18.1us-gaap_DerivativeLiabilities
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
/ dei_LegalEntityAxis
= lnt_WplMember
15.6us-gaap_DerivativeLiabilities
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_CarryingValueMember
/ dei_LegalEntityAxis
= lnt_WplMember
Fair Value [Member]    
Assets:    
Derivative assets 38.6us-gaap_DerivativeAssets
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
26.7us-gaap_DerivativeAssets
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
Deferred proceeds (sales of receivables) 177.2lnt_DeferredProceedsOfReceivablesSold
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
203.5lnt_DeferredProceedsOfReceivablesSold
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
Capitalization and liabilities:    
Long-term debt (including current maturities) 4,418.2us-gaap_DebtInstrumentFairValue
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
3,712.3us-gaap_DebtInstrumentFairValue
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
Cumulative preferred stock 200.2lnt_PreferredStockFairValue
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
167.0lnt_PreferredStockFairValue
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
Derivative liabilities 37.6us-gaap_DerivativeLiabilities
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
20.8us-gaap_DerivativeLiabilities
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
Fair Value [Member] | IPL [Member]    
Assets:    
Derivative assets 28.0us-gaap_DerivativeAssets
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
/ dei_LegalEntityAxis
= lnt_IplMember
21.1us-gaap_DerivativeAssets
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
/ dei_LegalEntityAxis
= lnt_IplMember
Deferred proceeds (sales of receivables) 177.2lnt_DeferredProceedsOfReceivablesSold
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
/ dei_LegalEntityAxis
= lnt_IplMember
203.5lnt_DeferredProceedsOfReceivablesSold
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
/ dei_LegalEntityAxis
= lnt_IplMember
Capitalization and liabilities:    
Long-term debt (including current maturities) 2,053.0us-gaap_DebtInstrumentFairValue
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
/ dei_LegalEntityAxis
= lnt_IplMember
1,726.4us-gaap_DebtInstrumentFairValue
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
/ dei_LegalEntityAxis
= lnt_IplMember
Cumulative preferred stock 200.2lnt_PreferredStockFairValue
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
/ dei_LegalEntityAxis
= lnt_IplMember
167.0lnt_PreferredStockFairValue
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
/ dei_LegalEntityAxis
= lnt_IplMember
Derivative liabilities 19.5us-gaap_DerivativeLiabilities
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
/ dei_LegalEntityAxis
= lnt_IplMember
5.2us-gaap_DerivativeLiabilities
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
/ dei_LegalEntityAxis
= lnt_IplMember
Fair Value [Member] | WPL [Member]    
Assets:    
Derivative assets 10.6us-gaap_DerivativeAssets
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
/ dei_LegalEntityAxis
= lnt_WplMember
5.6us-gaap_DerivativeAssets
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
/ dei_LegalEntityAxis
= lnt_WplMember
Capitalization and liabilities:    
Long-term debt (including current maturities) 1,908.9us-gaap_DebtInstrumentFairValue
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
/ dei_LegalEntityAxis
= lnt_WplMember
1,532.9us-gaap_DebtInstrumentFairValue
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
/ dei_LegalEntityAxis
= lnt_WplMember
Derivative liabilities $ 18.1us-gaap_DerivativeLiabilities
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
/ dei_LegalEntityAxis
= lnt_WplMember
$ 15.6us-gaap_DerivativeLiabilities
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= lnt_FairValueMember
/ dei_LegalEntityAxis
= lnt_WplMember