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Common Stock - Summary of Nonvested Performance Share Awards Activity (Details) (Performance Shares [Member], USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Performance Shares [Member]
   
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]    
Nonvested at December 31, 2013 2,014,190us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
[1]  
Granted 1,712,085us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
 
Change in performance 98,227exc_SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherthanOptionsChangeinPerformance
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
 
Vested (497,714)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
 
Forfeited (29,476)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
 
Undistributed vested awards (601,215)exc_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodUndistributed
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
[2]  
Nonvested at December 31, 2014 2,696,097us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
[1]  
Nonvested, Weighted Average Exercise Price (in usd per share) $ 30.62us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
[1] $ 32.74us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
[1]
Granted, Weighted Average Grant Date Fair Value (in usd per share) $ 28.75us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
 
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Change in Performance, Weighted Average Grant Date Fair Value $ 31.85exc_SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherthanOptionsChangeinPerformanceWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
 
Vested, Weighted Average Exercise Price (in usd per share) $ 35.05us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
 
Forfeited, Weighted Average Exercise Price (in usd per share) $ 30.16us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
 
Undistributed Vested Awards, Weighted Average Grant Date Fair Value (in usd per share) $ 28.96exc_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodUndistributedWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
[2]  
Fully Vested Stock Based Compensation Issued To Retirement Eligible Employees 1,535,791exc_FullyVestedStockBasedCompensationIssuedToRetirementEligibleEmployees
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
1,411,824exc_FullyVestedStockBasedCompensationIssuedToRetirementEligibleEmployees
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
[1] Excludes 1,535,791 and 1,411,824 of performance share awards issued to retirement-eligible employees as of December 31, 2014 and December 31, 2013, respectively, as they are fully vested.
[2] Represents performance share awards that vested but were not distributed to retirement-eligible employees during 2014.