Derivative Instruments and Hedging Activities (Net Unrealized Derivative Gain (Loss) Deferred as Regulatory Asset or Liability) (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
Power Delivery [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized gain (loss) arising during the period	$ 2	$ (4)	$ (3)	$ (6)
Net realized loss recognized during the period	(2)	(3)	(13)	(14)
Delmarva Power & Light Co/De [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized gain (loss) arising during the period	2	(4)	(2)	(6)
Net realized loss recognized during the period	(2)	(3)	(13)	(14)
Power Delivery [Member] | Pepco Holdings Inc [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized gain (loss) arising during the period		(1)		(1)
Net realized loss recognized during the period		(2)		(5)
Power Delivery [Member] | Delmarva Power & Light Co/De [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized gain (loss) arising during the period		(1)		(1)
Net realized loss recognized during the period		$ (2)		$ (5)