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
	Mar. 31, 2015	Mar. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized (loss) gain arising during the period	$ (1)	$ 2
Net realized (loss) gain recognized during the period	(3)	2
Delmarva Power & Light Co/De [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized (loss) gain arising during the period	(1)	2
Net realized (loss) gain recognized during the period	$ (3)	$ 2