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	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized (loss) gain arising during the period	$ (3)	$ 4	$ (6)
Net realized gain (loss) recognized during the period	2	(4)	(16)
Delmarva Power & Light Co/De [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized (loss) gain arising during the period	(3)	1	(3)
Net realized gain (loss) recognized during the period	$ 2	$ (4)	$ (16)