Derivative Instruments and Hedging Activities - Net Unrealized Derivative Gain (Loss) Deferred as Regulatory Asset or Liability (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
PHI [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized gain (loss) arising during the period		$ 2	$ (7)	$ (3)
Net realized losses recognized during the period		(2)	(3)	(13)
Delmarva Power & Light Co/De [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized gain (loss) arising during the period		2		(2)
Net realized losses recognized during the period		$ (2)	$ (3)	$ (13)