Fair Value Disclosures - Fair Value of Financial Liabilities Measured on Recurring Basis (Parenthetical) (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, carrying amount	$ 5,115	$ 4,807
Transition bonds, carrying amount	193	215
Potomac Electric Power Co [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, carrying amount	2,332	2,124
Delmarva Power & Light Co/De [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, carrying amount	1,171	1,071
Atlantic City Electric Co [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, carrying amount	903	903
Transition bonds, carrying amount	$ 193	$ 215