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Derivative Financial Instruments - Summary of AOCI related to Cash Flow Hedges (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Dec. 31, 2013
Derivative [Line Items]        
Income taxes $ (363)us-gaap_IncomeTaxExpenseBenefit $ 54us-gaap_IncomeTaxExpenseBenefit    
Exelon Generation Co L L C [Member]        
Derivative [Line Items]        
Income taxes (226)us-gaap_IncomeTaxExpenseBenefit
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember		 199us-gaap_IncomeTaxExpenseBenefit
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember		    
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]        
Derivative [Line Items]        
Income taxes 10us-gaap_IncomeTaxExpenseBenefit
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		 [1] (15)us-gaap_IncomeTaxExpenseBenefit
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		 [1]    
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] | Exelon Generation Co L L C [Member]        
Derivative [Line Items]        
Income taxes 0us-gaap_IncomeTaxExpenseBenefit
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		 [1] (15)us-gaap_IncomeTaxExpenseBenefit
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		 [1]    
Energy Related Hedges [Member]        
Derivative [Line Items]        
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax (6)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax
/ us-gaap_DerivativeInstrumentRiskAxis
= exc_EnergyRelatedHedgesMember		 (4)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax
/ us-gaap_DerivativeInstrumentRiskAxis
= exc_EnergyRelatedHedgesMember		    
Energy Related Hedges [Member] | Exelon Generation Co L L C [Member]        
Derivative [Line Items]        
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss) (23)us-gaap_UnrealizedGainLossOnInterestRateCashFlowHedgesPretaxAccumulatedOtherComprehensiveIncomeLoss
/ us-gaap_DerivativeInstrumentRiskAxis
= exc_EnergyRelatedHedgesMember
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember		 88us-gaap_UnrealizedGainLossOnInterestRateCashFlowHedgesPretaxAccumulatedOtherComprehensiveIncomeLoss
/ us-gaap_DerivativeInstrumentRiskAxis
= exc_EnergyRelatedHedgesMember
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember		 (18)us-gaap_UnrealizedGainLossOnInterestRateCashFlowHedgesPretaxAccumulatedOtherComprehensiveIncomeLoss
/ us-gaap_DerivativeInstrumentRiskAxis
= exc_EnergyRelatedHedgesMember
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember		 116us-gaap_UnrealizedGainLossOnInterestRateCashFlowHedgesPretaxAccumulatedOtherComprehensiveIncomeLoss
/ us-gaap_DerivativeInstrumentRiskAxis
= exc_EnergyRelatedHedgesMember
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
Energy Related Hedges [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] | Exelon Generation Co L L C [Member] | Operating Revenue One [Member]        
Derivative [Line Items]        
Derivative Instruments, Gain Reclassified from Accumulated OCI into Income, Effective Portion (2)us-gaap_DerivativeInstrumentsGainReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortion
/ us-gaap_DerivativeInstrumentRiskAxis
= exc_EnergyRelatedHedgesMember
/ us-gaap_IncomeStatementLocationAxis
= exc_OperatingRevenue1Member
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		 (24)us-gaap_DerivativeInstrumentsGainReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortion
/ us-gaap_DerivativeInstrumentRiskAxis
= exc_EnergyRelatedHedgesMember
/ us-gaap_IncomeStatementLocationAxis
= exc_OperatingRevenue1Member
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		    
Energy Related Hedges [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] | Exelon Generation Co L L C [Member] | Interest Expense [Member]        
Derivative [Line Items]        
Derivative Instruments, Gain Reclassified from Accumulated OCI into Income, Effective Portion 3us-gaap_DerivativeInstrumentsGainReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortion
/ us-gaap_DerivativeInstrumentRiskAxis
= exc_EnergyRelatedHedgesMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_InterestExpenseMember
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		      
Energy Related Hedges [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] | Exelon Generation Co L L C [Member] | Other, net [Member]        
Derivative [Line Items]        
Derivative Instruments, Gain Reclassified from Accumulated OCI into Income, Effective Portion 0us-gaap_DerivativeInstrumentsGainReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortion
/ us-gaap_DerivativeInstrumentRiskAxis
= exc_EnergyRelatedHedgesMember
/ us-gaap_IncomeStatementLocationAxis
= exc_OtherNetMember
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		      
Cash Flow Hedging [Member]        
Derivative [Line Items]        
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax (11)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CashFlowHedgingMember		 (1)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CashFlowHedgingMember		    
Cash Flow Hedging [Member] | Exelon Generation Co L L C [Member]        
Derivative [Line Items]        
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss) (22)us-gaap_UnrealizedGainLossOnInterestRateCashFlowHedgesPretaxAccumulatedOtherComprehensiveIncomeLoss
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CashFlowHedgingMember
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember		 95us-gaap_UnrealizedGainLossOnInterestRateCashFlowHedgesPretaxAccumulatedOtherComprehensiveIncomeLoss
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CashFlowHedgingMember
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember		 (28)us-gaap_UnrealizedGainLossOnInterestRateCashFlowHedgesPretaxAccumulatedOtherComprehensiveIncomeLoss
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CashFlowHedgingMember
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember		 120us-gaap_UnrealizedGainLossOnInterestRateCashFlowHedgesPretaxAccumulatedOtherComprehensiveIncomeLoss
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CashFlowHedgingMember
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
Cash Flow Hedging [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] | Operating Revenue One [Member]        
Derivative [Line Items]        
Derivative Instruments, Gain Reclassified from Accumulated OCI into Income, Effective Portion (2)us-gaap_DerivativeInstrumentsGainReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortion
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= exc_OperatingRevenue1Member
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		 (24)us-gaap_DerivativeInstrumentsGainReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortion
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= exc_OperatingRevenue1Member
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		    
Cash Flow Hedging [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] | Interest Expense [Member]        
Derivative [Line Items]        
Derivative Instruments, Gain Reclassified from Accumulated OCI into Income, Effective Portion 3us-gaap_DerivativeInstrumentsGainReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortion
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_InterestExpenseMember
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		      
Cash Flow Hedging [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] | Other, net [Member]        
Derivative [Line Items]        
Derivative Instruments, Gain Reclassified from Accumulated OCI into Income, Effective Portion $ 16us-gaap_DerivativeInstrumentsGainReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortion
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= exc_OtherNetMember
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		      
[1] All amounts are net of tax. Amounts in parentheses represent a decrease in net income.