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Derivative Instruments (Credit Risk-related Contingent Features) (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Derivative Instruments [Line Items]    
Aggregate fair value $ 42.7us-gaap_DerivativeNetLiabilityPositionAggregateFairValue $ 37.6us-gaap_DerivativeNetLiabilityPositionAggregateFairValue
Credit support to be posted if triggered 42.2us-gaap_AdditionalCollateralAggregateFairValue 37.4us-gaap_AdditionalCollateralAggregateFairValue
IPL [Member]    
Derivative Instruments [Line Items]    
Aggregate fair value 20.0us-gaap_DerivativeNetLiabilityPositionAggregateFairValue
/ dei_LegalEntityAxis
= lnt_IplMember		 19.5us-gaap_DerivativeNetLiabilityPositionAggregateFairValue
/ dei_LegalEntityAxis
= lnt_IplMember
Credit support to be posted if triggered 20.0us-gaap_AdditionalCollateralAggregateFairValue
/ dei_LegalEntityAxis
= lnt_IplMember		 19.5us-gaap_AdditionalCollateralAggregateFairValue
/ dei_LegalEntityAxis
= lnt_IplMember
WPL [Member]    
Derivative Instruments [Line Items]    
Aggregate fair value 22.7us-gaap_DerivativeNetLiabilityPositionAggregateFairValue
/ dei_LegalEntityAxis
= lnt_WplMember		 18.1us-gaap_DerivativeNetLiabilityPositionAggregateFairValue
/ dei_LegalEntityAxis
= lnt_WplMember
Credit support to be posted if triggered $ 22.2us-gaap_AdditionalCollateralAggregateFairValue
/ dei_LegalEntityAxis
= lnt_WplMember		 $ 17.9us-gaap_AdditionalCollateralAggregateFairValue
/ dei_LegalEntityAxis
= lnt_WplMember