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Derivative Financial Instruments - Summary of Credit Risk Related Contingent Features (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Derivative [Line Items]    
Derivative, Collateral, Obligation to Return Cash $ 62us-gaap_DerivativeCollateralObligationToReturnCash  
Derivative, Collateral, Obligation to Return Securities 14us-gaap_DerivativeCollateralObligationToReturnSecurities  
Energy Supply Procurement [Member]    
Derivative [Line Items]    
Derivative Asset, Fair Value of Collateral 2us-gaap_DerivativeAssetFairValueOfCollateral
/ us-gaap_DerivativeByNatureAxis
= exc_EnergySupplyProcurementMember
 
ReNewAble Energy Contract [Member]    
Derivative [Line Items]    
Derivative Asset, Fair Value of Collateral 19us-gaap_DerivativeAssetFairValueOfCollateral
/ us-gaap_DerivativeByNatureAxis
= exc_ReNewAbleEnergyContractMember
 
Exelon Generation Co L L C [Member]    
Derivative [Line Items]    
Aggregate fair value of derivatives with credit-risk-related contingent features (1,420)us-gaap_DerivativeFairValueOfDerivativeLiability
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
(1,433)us-gaap_DerivativeFairValueOfDerivativeLiability
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
Contractual right of offset related to derivative assets 1,138us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
1,140us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
Net liability position after contractual right of offset (282)exc_NetAssetLiabilityPositionAfterContractualRightOfOffset
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
[1] (293)exc_NetAssetLiabilityPositionAfterContractualRightOfOffset
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
[1]
Derivative, Collateral, Right to Reclaim Cash 1,428us-gaap_DerivativeCollateralRightToReclaimCash
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
1,497us-gaap_DerivativeCollateralRightToReclaimCash
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
Derivative, Collateral, Right to Reclaim Securities 626us-gaap_DerivativeCollateralRightToReclaimSecurities
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
672us-gaap_DerivativeCollateralRightToReclaimSecurities
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
Derivative, Collateral, Obligation to Return Cash 69us-gaap_DerivativeCollateralObligationToReturnCash
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
77us-gaap_DerivativeCollateralObligationToReturnCash
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
Derivative, Collateral, Obligation to Return Securities $ 22us-gaap_DerivativeCollateralObligationToReturnSecurities
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
$ 24us-gaap_DerivativeCollateralObligationToReturnSecurities
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
[1] Amount represents the net fair value of out-of-the-money derivative contracts containing credit-risk related contingent features after considering the mitigating effects of offsetting positions under master netting arrangements and reflects the actual net liability upon which any potential contingent collateral obligations would be based.