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Common Stock - Summary of Nonvested Stock Option Activity (Details) (Nonvested Stock Option [Member], USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Nonvested Stock Option [Member]
   
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward]    
Nonvested at December 31, 2013 847,118us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= exc_NonvestedStockOptionMember
[1]  
Vested (406,197)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= exc_NonvestedStockOptionMember
 
Forfeited (8,886)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ us-gaap_AwardTypeAxis
= exc_NonvestedStockOptionMember
 
Nonvested at December 31, 2014 432,035.000000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= exc_NonvestedStockOptionMember
[1]  
Nonvested, Weighted Average Exercise Price (in usd per share) $ 39.91us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= exc_NonvestedStockOptionMember
[1] $ 40.22us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= exc_NonvestedStockOptionMember
[1]
Vested, Weighted Average Exercise Price (in usd per share) $ 40.21us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= exc_NonvestedStockOptionMember
 
Forfeited, Weighted Average Exercise Price (in usd per share) $ 55.78us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= exc_NonvestedStockOptionMember
 
Fully Vested Stock Based Compensation Issued To Retirement Eligible Employees 746,140exc_FullyVestedStockBasedCompensationIssuedToRetirementEligibleEmployees
/ us-gaap_AwardTypeAxis
= exc_NonvestedStockOptionMember
1,348,913exc_FullyVestedStockBasedCompensationIssuedToRetirementEligibleEmployees
/ us-gaap_AwardTypeAxis
= exc_NonvestedStockOptionMember
[1] Excludes 746,140 and 1,348,913 of stock options issued to retirement-eligible employees as of December 31, 2014 and December 31, 2013, respectively, as they are fully vested.