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	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 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	$ (1)	$ 1	(4)	3
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	$ (1)	$ 1	$ (4)	$ 3