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Commitments and Contingencies - Tax Legislation, Guarantees, Indemnifications, and Performance Contracts - Additional Information (Detail) (USD $)
In Millions, except Per Share data, unless otherwise specified
12 Months Ended 0 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Jan. 22, 2015
Commitment and Contingencies [Line Items]        
Dividend declared date Jan. 22, 2015      
Dividend declared, payment date Mar. 31, 2015      
Dividends payable, date of record Mar. 10, 2015      
Contractual obligations due in 2015 $ 276us-gaap_ContractualObligationDueInNextTwelveMonths      
Contractual obligations due in 2016 to 2017 509us-gaap_ContractualObligationDueInSecondAndThirdYear      
Contractual obligations due in 2018 to 2019 478us-gaap_ContractualObligationDueInFourthAndFifthYear      
Contractual obligations due, thereafter 1,177us-gaap_ContractualObligationDueAfterFifthYear      
Rental expense for operating leases 59us-gaap_OperatingLeasesRentExpenseNet 54us-gaap_OperatingLeasesRentExpenseNet 52us-gaap_OperatingLeasesRentExpenseNet  
Future minimum operating lease payments, 2015 45us-gaap_OperatingLeasesFutureMinimumPaymentsDueCurrent      
Future minimum operating lease payments, 2016 43us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears      
Future minimum operating lease payments, 2017 40us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears      
Future minimum operating lease payments, 2018 40us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears      
Future minimum operating lease payments, 2019 28us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears      
Future minimum operating lease payments, thereafter 329us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter      
Subsequent Event [Member]        
Commitment and Contingencies [Line Items]        
Dividend declared on common stock, per share       $ 0.27us-gaap_CommonStockDividendsPerShareDeclared
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Pepco Energy Services [Member] | Completed Performance Contracts Associated With Savings Guarantees [Member]        
Commitment and Contingencies [Line Items]        
Value of guarantees completed projects 356pom_RemainingNotionalAmountOfGuaranteeOnEnergySavingCompletedProjects
/ dei_LegalEntityAxis
= pom_PepcoEnergyServicesMember
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= pom_CompletedPerformanceContractsAssociatedWithSavingsGuaranteesMember
     
Contract life, maximum remaining term 23 years      
Pepco Energy Services [Member] | Uncompleted Performance Contracts Associated With Savings Guarantees [Member]        
Commitment and Contingencies [Line Items]        
Value of guarantees on projects under construction 61pom_RemainingNotionalAmountOfGuaranteeOnEnergySavingUncompletedProjects
/ dei_LegalEntityAxis
= pom_PepcoEnergyServicesMember
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= pom_UncompletedPerformanceContractsAssociatedWithSavingsGuaranteesMember
     
Maximum term of project under construction 13 years      
Pepco Energy Services [Member] | Energy Savings or Combined Heat and Power Performance [Member]        
Commitment and Contingencies [Line Items]        
Contract life, maximum remaining term 20 years      
Value of guarantees under construction projects 15pom_PerformanceGuarantees
/ dei_LegalEntityAxis
= pom_PepcoEnergyServicesMember
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= pom_PerformanceGuaranteeContractsAssociatedWithProductionAtHeatAndPowerFacilitiesMember
     
Accrued liability on contracts 1us-gaap_AccruedLiabilitiesCurrentAndNoncurrent
/ dei_LegalEntityAxis
= pom_PepcoEnergyServicesMember
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= pom_PerformanceGuaranteeContractsAssociatedWithProductionAtHeatAndPowerFacilitiesMember
     
Potomac Electric Power Co [Member]        
Commitment and Contingencies [Line Items]        
Rental expense for operating leases 8us-gaap_OperatingLeasesRentExpenseNet
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
7us-gaap_OperatingLeasesRentExpenseNet
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
6us-gaap_OperatingLeasesRentExpenseNet
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
 
Future minimum operating lease payments, 2015 7us-gaap_OperatingLeasesFutureMinimumPaymentsDueCurrent
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
     
Future minimum operating lease payments, 2016 6us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
     
Future minimum operating lease payments, 2017 5us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
     
Future minimum operating lease payments, 2018 4us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
     
Future minimum operating lease payments, 2019 3us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
     
Future minimum operating lease payments, thereafter 22us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
     
Delmarva Power & Light Co/De [Member]        
Commitment and Contingencies [Line Items]        
Contractual obligations due in 2015 64us-gaap_ContractualObligationDueInNextTwelveMonths
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
     
Contractual obligations due in 2016 to 2017 129us-gaap_ContractualObligationDueInSecondAndThirdYear
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
     
Contractual obligations due in 2018 to 2019 128us-gaap_ContractualObligationDueInFourthAndFifthYear
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
     
Contractual obligations due, thereafter 291us-gaap_ContractualObligationDueAfterFifthYear
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
     
Rental expense for operating leases 14us-gaap_OperatingLeasesRentExpenseNet
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
13us-gaap_OperatingLeasesRentExpenseNet
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
12us-gaap_OperatingLeasesRentExpenseNet
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
 
Future minimum operating lease payments, 2015 13us-gaap_OperatingLeasesFutureMinimumPaymentsDueCurrent
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
     
Future minimum operating lease payments, 2016 12us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
     
Future minimum operating lease payments, 2017 11us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
     
Future minimum operating lease payments, 2018 14us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
     
Future minimum operating lease payments, 2019 5us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
     
Future minimum operating lease payments, thereafter 114us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
     
Ownership interest 11.90%pom_OwnershipInterestInPropertyLeased
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
     
Present value of future minimum lease payments 79us-gaap_OperatingLeasesFutureMinimumPaymentsDue
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
     
Atlantic City Electric Co [Member]        
Commitment and Contingencies [Line Items]        
Contractual obligations due in 2015 210us-gaap_ContractualObligationDueInNextTwelveMonths
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
     
Contractual obligations due in 2016 to 2017 381us-gaap_ContractualObligationDueInSecondAndThirdYear
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
     
Contractual obligations due in 2018 to 2019 349us-gaap_ContractualObligationDueInFourthAndFifthYear
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
     
Contractual obligations due, thereafter 886us-gaap_ContractualObligationDueAfterFifthYear
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
     
Rental expense for operating leases 12us-gaap_OperatingLeasesRentExpenseNet
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
12us-gaap_OperatingLeasesRentExpenseNet
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
11us-gaap_OperatingLeasesRentExpenseNet
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
 
Future minimum operating lease payments, 2015 6us-gaap_OperatingLeasesFutureMinimumPaymentsDueCurrent
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
     
Future minimum operating lease payments, 2016 6us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
     
Future minimum operating lease payments, 2017 5us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
     
Future minimum operating lease payments, 2018 4us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
     
Future minimum operating lease payments, 2019 4us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
     
Future minimum operating lease payments, thereafter $ 30us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember