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Fair Value Disclosures - Fair Value of Financial Liabilities Measured on Recurring Basis (Parenthetical) (Detail) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2014
Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, carrying amount $ 4,909 $ 4,456
Transition bonds, carrying amount 226 255
Potomac Electric Power Co [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, carrying amount 2,124 1,899
Delmarva Power & Light Co/De [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, carrying amount 1,171 967
Atlantic City Electric Co [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, carrying amount 903 860
Transition bonds, carrying amount $ 226 $ 255