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Benefit Plans (Summary Of Performance Contingent Cash Awards Activity) (Details) (Performance Contingent Cash Awards [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) 78,930us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber 96,977us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
Granted (in shares/awards) 41,105us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod 42,446us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Vested (in shares/awards) (37,332)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod [1] (55,517)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod [1]
Forfeited (in shares/awards) (469)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod (3,406)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
Nonvested, March 31 (in shares/awards) 82,234us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber 80,500us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
2013 Grant [Member]    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]    
Vested (in shares/awards) (37,332)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardDateAxis
= lnt_TwoThousandThirteenGrantMember
/ us-gaap_AwardTypeAxis
= lnt_PerformanceContingentCashAwardsMember		 [1]  
Cash payout value 2.4us-gaap_EmployeeBenefitsAndShareBasedCompensation
/ us-gaap_AwardDateAxis
= lnt_TwoThousandThirteenGrantMember
/ us-gaap_AwardTypeAxis
= lnt_PerformanceContingentCashAwardsMember		 [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)   (34,766)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardDateAxis
= lnt_TwoThousandTwelveGrantMember
/ us-gaap_AwardTypeAxis
= lnt_PerformanceContingentCashAwardsMember		 [1]
Cash payout value   1.9us-gaap_EmployeeBenefitsAndShareBasedCompensation
/ us-gaap_AwardDateAxis
= lnt_TwoThousandTwelveGrantMember
/ us-gaap_AwardTypeAxis
= lnt_PerformanceContingentCashAwardsMember		 [1]
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_PerformanceContingentCashAwardsMember		 [1]
Cash payout value   1.1us-gaap_EmployeeBenefitsAndShareBasedCompensation
/ us-gaap_AwardDateAxis
= lnt_TwoThousandElevenGrantMember
/ us-gaap_AwardTypeAxis
= lnt_PerformanceContingentCashAwardsMember		 [1]
[1] In 2015, 37,332 performance contingent cash awards granted in 2013 vested, resulting in cash payouts valued at $2.4 million. In 2014, 34,766 and 20,751 performance contingent cash awards granted in 2012 and 2011 vested, resulting in cash payouts valued at $1.9 million and $1.1 million, respectively.