Debt - Components of Short-Term Debt (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Short-term Debt [Line Items]
Short-term debt	$ 729	$ 565
Commercial Paper [Member]
Short-term Debt [Line Items]
Short-term debt	624	442
Variable Rate Demand Bonds [Member]
Short-term Debt [Line Items]
Short-term debt	105	123
Potomac Electric Power Co [Member]
Short-term Debt [Line Items]
Short-term debt	104	151
Potomac Electric Power Co [Member] | Commercial Paper [Member]
Short-term Debt [Line Items]
Short-term debt	104	151
Atlantic City Electric Co [Member]
Short-term Debt [Line Items]
Short-term debt	127	138
Atlantic City Electric Co [Member] | Commercial Paper [Member]
Short-term Debt [Line Items]
Short-term debt	127	120
Atlantic City Electric Co [Member] | Variable Rate Demand Bonds [Member]
Short-term Debt [Line Items]
Short-term debt		18
Delmarva Power & Light Co/De [Member]
Short-term Debt [Line Items]
Short-term debt	211	252
Delmarva Power & Light Co/De [Member] | Commercial Paper [Member]
Short-term Debt [Line Items]
Short-term debt	106	147
Delmarva Power & Light Co/De [Member] | Variable Rate Demand Bonds [Member]
Short-term Debt [Line Items]
Short-term debt	$ 105	$ 105