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Income Taxes - Reconciliation of Consolidated Income Tax Expense from Continuing Operations (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Income Tax Rate Reconciliation [Line Items]    
Income tax at Federal statutory rate $ 29us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate $ 42us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate
State income taxes, net of federal effect 6us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes 7us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes
Asset removal costs (3)pom_IncomeTaxReconciliationAssetRemovalCosts (2)pom_IncomeTaxReconciliationAssetRemovalCosts
Change in estimates and interest related to uncertain and effectively settled tax positions   (1)pom_IncomeTaxChangesInEstimatesAndInterestRelatedToUncertainTaxPositions
Energy efficiency-related tax deductions (4)us-gaap_IncomeTaxReconciliationDeductions  
Merger-related costs 2pom_IncomeTaxReconciliationNonDeductibleMergerRelatedTransactionCosts  
Consolidated income tax expense 30us-gaap_IncomeTaxExpenseBenefit 46us-gaap_IncomeTaxExpenseBenefit
Income tax at Federal statutory rate, percentage 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
State income taxes, net of federal effect, percentage 7.20%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes 5.80%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes
Asset removal costs, percentage (3.60%)pom_EffectiveIncomeTaxRateReconciliationAssetRemovalCosts (1.70%)pom_EffectiveIncomeTaxRateReconciliationAssetRemovalCosts
Change in estimates and interest related to uncertain and effectively settled tax positions, percentage   (0.80%)pom_IncomeTaxRateChangesInEstimatesAndInterestRelatedToUncertainTaxPositions
Energy efficiency-related tax deductions (4.80%)us-gaap_EffectiveIncomeTaxRateReconciliationDeductions  
Merger-related costs, percentage 2.40%pom_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseMergerTransactionCosts  
Other, net, percentage (0.10%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments (0.30%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments
Consolidated income tax expense (benefit), percentage 36.10%us-gaap_EffectiveIncomeTaxRateContinuingOperations 38.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations
Potomac Electric Power Co [Member]    
Income Tax Rate Reconciliation [Line Items]    
Income tax at Federal statutory rate 13us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
17us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
State income taxes, net of federal effect 2us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
3us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
Asset removal costs (3)pom_IncomeTaxReconciliationAssetRemovalCosts
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
(2)pom_IncomeTaxReconciliationAssetRemovalCosts
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
Change in estimates and interest related to uncertain and effectively settled tax positions   (1)pom_IncomeTaxChangesInEstimatesAndInterestRelatedToUncertainTaxPositions
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
Other, net   (1)us-gaap_IncomeTaxReconciliationOtherAdjustments
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
Consolidated income tax expense 12us-gaap_IncomeTaxExpenseBenefit
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
16us-gaap_IncomeTaxExpenseBenefit
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
Income tax at Federal statutory rate, percentage 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
State income taxes, net of federal effect, percentage 5.30%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
6.30%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
Asset removal costs, percentage (7.90%)pom_EffectiveIncomeTaxRateReconciliationAssetRemovalCosts
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
(4.20%)pom_EffectiveIncomeTaxRateReconciliationAssetRemovalCosts
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
Change in estimates and interest related to uncertain and effectively settled tax positions, percentage   (2.10%)pom_IncomeTaxRateChangesInEstimatesAndInterestRelatedToUncertainTaxPositions
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
Other, net, percentage (0.80%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
(1.70%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
Consolidated income tax expense (benefit), percentage 31.60%us-gaap_EffectiveIncomeTaxRateContinuingOperations
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
33.30%us-gaap_EffectiveIncomeTaxRateContinuingOperations
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
Delmarva Power & Light Co/De [Member]    
Income Tax Rate Reconciliation [Line Items]    
Income tax at Federal statutory rate 19us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
22us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
State income taxes, net of federal effect 3us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
3us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
Other, net (1)us-gaap_IncomeTaxReconciliationOtherAdjustments
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
 
Consolidated income tax expense 21us-gaap_IncomeTaxExpenseBenefit
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
25us-gaap_IncomeTaxExpenseBenefit
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
Income tax at Federal statutory rate, percentage 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
State income taxes, net of federal effect, percentage 5.70%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
4.80%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
Other, net, percentage (1.10%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
0.50%us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
Consolidated income tax expense (benefit), percentage 39.60%us-gaap_EffectiveIncomeTaxRateContinuingOperations
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
40.30%us-gaap_EffectiveIncomeTaxRateContinuingOperations
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
Atlantic City Electric Co [Member]    
Income Tax Rate Reconciliation [Line Items]    
Income tax at Federal statutory rate 2us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
6us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
Consolidated income tax expense $ 2us-gaap_IncomeTaxExpenseBenefit
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
$ 6us-gaap_IncomeTaxExpenseBenefit
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
Income tax at Federal statutory rate, percentage 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
Other, net, percentage (1.70%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
2.50%us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
Consolidated income tax expense (benefit), percentage 33.30%us-gaap_EffectiveIncomeTaxRateContinuingOperations
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
37.50%us-gaap_EffectiveIncomeTaxRateContinuingOperations
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember