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Benefit Plans (Summary Of Performance Shares Activity) (Details) (Performance Shares [Member], USD $)
In Millions, except Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]      
Nonvested, January 1 (in shares/awards) 139,940us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber [1] 145,277us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber [1] 236,979us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber [1]
Granted (in shares) 51,221us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod [1] 49,093us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod [1] 45,612us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod [1]
Vested (in shares) (45,235)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod [1] (54,430)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod [1] (111,980)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod [1]
Forfeited (in shares) (1,502)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod [1],[2] 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod [1],[2] (25,334)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod [1],[2]
Nonvested, December 31 (in shares/awards) 144,424us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber [1] 139,940us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber [1] 145,277us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber [1]
Instrument valuation based on shares of common stock, number of shares 1lnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsInstrumentValuationBasedOnSharesOfCommonStockNumberOfShares    
Actual number of shares paid out upon vesting, minimum percentage of target shares 0.00%lnt_ActualNumberOfSharesPaidOutUponVestingMinimum    
Actual number of shares paid out upon vesting, maximum percentage of target shares 200.00%lnt_ActualNumberOfSharesPaidOutUponVestingMaximum    
2011 Grant [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]      
Vested (in shares) (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		    
Cash and stock payout value 3.4us-gaap_EmployeeBenefitsAndShareBasedCompensation
/ us-gaap_AwardDateAxis
= lnt_TwoThousandElevenGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		    
Cash payout value 2.9us-gaap_EmployeeServiceShareBasedCompensationCashFlowEffectCashUsedToSettleAwards
/ us-gaap_AwardDateAxis
= lnt_TwoThousandElevenGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		    
Common stock shares from vested performance shares (in shares) 4,810us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod
/ us-gaap_AwardDateAxis
= lnt_TwoThousandElevenGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		    
2010 Grant [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]      
Vested (in shares)   (54,430)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardDateAxis
= lnt_TwoThousandTenGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		  
Vested percentage of the target   197.50%lnt_VestedPercentageOfTarget
/ us-gaap_AwardDateAxis
= lnt_TwoThousandTenGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		  
Cash and stock payout value   4.8us-gaap_EmployeeBenefitsAndShareBasedCompensation
/ us-gaap_AwardDateAxis
= lnt_TwoThousandTenGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		  
Cash payout value   4.4us-gaap_EmployeeServiceShareBasedCompensationCashFlowEffectCashUsedToSettleAwards
/ us-gaap_AwardDateAxis
= lnt_TwoThousandTenGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		  
Common stock shares from vested performance shares (in shares)   4,177us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod
/ us-gaap_AwardDateAxis
= lnt_TwoThousandTenGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		  
2009 Grant [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]      
Vested (in shares)     (111,980)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardDateAxis
= lnt_TwoThousandNineGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
Vested percentage of the target     162.50%lnt_VestedPercentageOfTarget
/ us-gaap_AwardDateAxis
= lnt_TwoThousandNineGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
Cash and stock payout value     8.0us-gaap_EmployeeBenefitsAndShareBasedCompensation
/ us-gaap_AwardDateAxis
= lnt_TwoThousandNineGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
Cash payout value     7.8us-gaap_EmployeeServiceShareBasedCompensationCashFlowEffectCashUsedToSettleAwards
/ us-gaap_AwardDateAxis
= lnt_TwoThousandNineGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
Common stock shares from vested performance shares (in shares)     6,399us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod
/ us-gaap_AwardDateAxis
= lnt_TwoThousandNineGrantMember
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
[1] Share amounts represent the target number of performance shares. Each performance share’s value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the performance period. The actual number of shares that will be paid out upon vesting is dependent upon actual performance and may range from zero to 200% of the target number of shares.
[2] Forfeitures were primarily caused by retirements and voluntary terminations of participants.