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	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized (loss) gain arising during the period	$ (3)	$ (3)	$ 4
Net realized (loss) gain recognized during the period	(5)	2	(4)
Delmarva Power & Light Co/De [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized (loss) gain arising during the period	(3)	(3)	1
Net realized (loss) gain recognized during the period	$ (5)	$ 2	$ (4)