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