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Benefit Plans (Summary Of Performance Contingent Cash Awards Activity) (Details) (USD $)
In Millions, except Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Performance Contingent Cash Awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]      
Nonvested, January 1 (in shares/awards) 96,977us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= lnt_PerformanceContingentCashAwardsMember		 59,639us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= lnt_PerformanceContingentCashAwardsMember		 46,676us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= lnt_PerformanceContingentCashAwardsMember
Granted (in shares/awards) 42,446us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= lnt_PerformanceContingentCashAwardsMember		 39,530us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= lnt_PerformanceContingentCashAwardsMember		 36,936us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= lnt_PerformanceContingentCashAwardsMember
Vested (in shares/awards) (55,517)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= lnt_PerformanceContingentCashAwardsMember		 [1] 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= lnt_PerformanceContingentCashAwardsMember		 [1] (21,605)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= lnt_PerformanceContingentCashAwardsMember		 [1]
Forfeited (in shares/awards) (4,976)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= lnt_PerformanceContingentCashAwardsMember		 (2,192)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= lnt_PerformanceContingentCashAwardsMember		 (2,368)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= lnt_PerformanceContingentCashAwardsMember
Nonvested, December 31 (in shares/awards) 78,930us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= lnt_PerformanceContingentCashAwardsMember		 96,977us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= lnt_PerformanceContingentCashAwardsMember		 59,639us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= lnt_PerformanceContingentCashAwardsMember
2012 Grant [Member] | Performance Contingent Cash Awards [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		    
Cash payout value 1.9us-gaap_EmployeeBenefitsAndShareBasedCompensation
/ us-gaap_AwardDateAxis
= lnt_TwoThousandTwelveGrantMember
/ us-gaap_AwardTypeAxis
= lnt_PerformanceContingentCashAwardsMember		    
2011 Grant [Member] | Performance Contingent Cash Awards [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		    
Cash payout value 1.1us-gaap_EmployeeBenefitsAndShareBasedCompensation
/ us-gaap_AwardDateAxis
= lnt_TwoThousandElevenGrantMember
/ us-gaap_AwardTypeAxis
= lnt_PerformanceContingentCashAwardsMember		    
2010 Grant [Member] | Performance Contingent Cash Awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]      
Vested (in shares/awards)     (21,605)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardDateAxis
= lnt_TwoThousandTenGrantMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= lnt_PerformanceContingentCashAwardsMember
Cash payout value     0.9us-gaap_EmployeeBenefitsAndShareBasedCompensation
/ us-gaap_AwardDateAxis
= lnt_TwoThousandTenGrantMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= lnt_PerformanceContingentCashAwardsMember
[1] 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. In 2012, 21,605 performance contingent cash awards granted in 2010 vested, resulting in cash payouts valued at $0.9 million.