Discontinued Operations - Schedule of Derivative Gain (Loss) Amounts Recognized in Income (Detail) (Discontinued Operations [Member], Pepco Energy Services [Member], USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Discontinued Operations [Member] | Pepco Energy Services [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Reclassification of mark-to-market to realized on settlement of contracts		$ 5	$ 10	$ 22
Unrealized mark-to-market gain (loss)		3		(2)
Total net gain		$ 8	$ 10	$ 20