Related Party Transactions (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Potomac Electric Power Co [Member] | Phi Service Company [Member]
Costs directly charged or allocated	$ 55	$ 47	$ 158	$ 133
Potomac Electric Power Co [Member] | Pepco Energy Services [Member]
Maintenance services	4	6	14	14
Atlantic City Electric Co [Member] | Phi Service Company [Member]
Costs directly charged or allocated	31	27	86	75
Delmarva Power & Light Co/De [Member] | Phi Service Company [Member]
Costs directly charged or allocated	$ 40	$ 35	$ 114	$ 97