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Fair Value Disclosures - Fair Value of Financial Liabilities Measured on Recurring Basis (Parenthetical) (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, carrying amount $ 5,015us-gaap_LongTermNotesPayable $ 4,807us-gaap_LongTermNotesPayable
Transition bonds, carrying amount 203us-gaap_LongtermTransitionBondCurrentAndNoncurrent 215us-gaap_LongtermTransitionBondCurrentAndNoncurrent
Potomac Electric Power Co [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, carrying amount 2,332us-gaap_LongTermNotesPayable
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
2,124us-gaap_LongTermNotesPayable
/ dei_LegalEntityAxis
= pom_PotomacElectricPowerCoMember
Delmarva Power & Light Co/De [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, carrying amount 1,071us-gaap_LongTermNotesPayable
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
1,071us-gaap_LongTermNotesPayable
/ dei_LegalEntityAxis
= pom_DelmarvaPowerLightCoMember
Atlantic City Electric Co [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, carrying amount 903us-gaap_LongTermNotesPayable
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
903us-gaap_LongTermNotesPayable
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
Transition bonds, carrying amount $ 203us-gaap_LongtermTransitionBondCurrentAndNoncurrent
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember
$ 215us-gaap_LongtermTransitionBondCurrentAndNoncurrent
/ dei_LegalEntityAxis
= pom_AtlanticCityElectricCoMember