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Benefit Plans (Summary Of Performance Unit Activity) (Details) (Performance Units [Member], USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]    
Nonvested, January 1 (in shares/awards) 63,665us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber [1] 65,912us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber [1]
Granted (in shares/awards) 17,837us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod [1] 20,422us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod [1]
Vested (in shares/awards) (22,845)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod [1] (20,751)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod [1]
Forfeited (in shares/awards) (93)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod [1] (311)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod [1]
Nonvested, March 31 (in shares/awards) 58,564us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber [1] 65,272us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber [1]
2012 Grant [Member]    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]    
Vested (in shares/awards) (22,845)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardDateAxis
= lnt_TwoThousandTwelveGrantMember
/ us-gaap_AwardTypeAxis
= lnt_PerformanceUnitsMember
 
Vested percentage of the target 167.50%lnt_VestedPercentageOfTarget
/ us-gaap_AwardDateAxis
= lnt_TwoThousandTwelveGrantMember
/ us-gaap_AwardTypeAxis
= lnt_PerformanceUnitsMember
 
Aggregate payout value 1.6us-gaap_EmployeeBenefitsAndShareBasedCompensation
/ us-gaap_AwardDateAxis
= lnt_TwoThousandTwelveGrantMember
/ us-gaap_AwardTypeAxis
= lnt_PerformanceUnitsMember
 
2011 Grant [Member]    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]    
Vested (in shares/awards)   (20,751)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardDateAxis
= lnt_TwoThousandElevenGrantMember
/ us-gaap_AwardTypeAxis
= lnt_PerformanceUnitsMember
Vested percentage of the target   147.50%lnt_VestedPercentageOfTarget
/ us-gaap_AwardDateAxis
= lnt_TwoThousandElevenGrantMember
/ us-gaap_AwardTypeAxis
= lnt_PerformanceUnitsMember
Aggregate payout value   1.2us-gaap_EmployeeBenefitsAndShareBasedCompensation
/ us-gaap_AwardDateAxis
= lnt_TwoThousandElevenGrantMember
/ us-gaap_AwardTypeAxis
= lnt_PerformanceUnitsMember
[1] Unit amounts represent the target number of performance units.