Discontinued Operations - Schedule of Derivative Gain (Loss) Amounts Recognized in Income (Detail) (Discontinued Operations [Member], USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized mark-to-market gain (loss)			$ (4)
Pepco Energy Services [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Reclassification of mark-to-market to realized on settlement of contracts	5	7	10	17
Unrealized mark-to-market gain (loss)		5		(5)
Total net (loss) gain	$ 5	$ 12	$ 10	$ 12