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Benefit Plans (Summary Of Performance Shares Activity) (Details) (Performance Shares [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) 144,424us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber [1] 139,940us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber [1]
Granted (in shares/awards) 45,403us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod [1] 51,221us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod [1]
Vested (in shares/awards) (45,612)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod [1] (45,235)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod [1]
Nonvested, March 31 (in shares/awards) 144,215us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber [1] 145,926us-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) (45,612)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardDateAxis
= lnt_TwoThousandTwelveGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
 
Vested percentage of the target 167.50%lnt_VestedPercentageOfTarget
/ us-gaap_AwardDateAxis
= lnt_TwoThousandTwelveGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
 
Aggregate payout value 5.1us-gaap_EmployeeBenefitsAndShareBasedCompensation
/ us-gaap_AwardDateAxis
= lnt_TwoThousandTwelveGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
 
Payout - cash 3.2us-gaap_EmployeeServiceShareBasedCompensationCashFlowEffectCashUsedToSettleAwards
/ us-gaap_AwardDateAxis
= lnt_TwoThousandTwelveGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
 
Payout - common stock shares issued (in shares) 10,975us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod
/ us-gaap_AwardDateAxis
= lnt_TwoThousandTwelveGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
 
2011 Grant [Member]    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]    
Vested (in shares/awards)   (45,235)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardDateAxis
= lnt_TwoThousandElevenGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
Vested percentage of the target   147.50%lnt_VestedPercentageOfTarget
/ us-gaap_AwardDateAxis
= lnt_TwoThousandElevenGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
Aggregate payout value   3.4us-gaap_EmployeeBenefitsAndShareBasedCompensation
/ us-gaap_AwardDateAxis
= lnt_TwoThousandElevenGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
Payout - cash   2.9us-gaap_EmployeeServiceShareBasedCompensationCashFlowEffectCashUsedToSettleAwards
/ us-gaap_AwardDateAxis
= lnt_TwoThousandElevenGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
Payout - common stock shares issued (in shares)   4,810us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod
/ us-gaap_AwardDateAxis
= lnt_TwoThousandElevenGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
[1] Share amounts represent the target number of performance shares.