Derivative Instruments and Hedging Activities - Net Unrealized Derivative Gain (Loss) Deferred as Regulatory Asset or Liability (Detail) (Other Derivative Activity [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized gain arising during the period	$ 2	$ 2
Net realized gain (loss) recognized during the period	2	(4)
Delmarva Power & Light Co/De [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized gain arising during the period	2	2
Net realized gain (loss) recognized during the period	$ 2	$ (4)