Derivative Instruments and Hedging Activities - Net Unrealized and Realized Derivative Gain (Loss) Deferred as Regulatory Liabilities and Assets (Detail) (Other Derivative Activity [Member], USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized (losses) gains arising during the period		$ (9)	$ 2	$ (7)
Net realized gains (losses) recognized during the period	1	1	3	(3)
Delmarva Power & Light Co/De [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized (losses) gains arising during the period		(2)	2
Net realized gains (losses) recognized during the period	$ 1	$ 1	$ 3	$ (3)