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		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
PHI [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized loss arising during the period	$ (9)	$ (1)	$ (7)	$ (5)
Net realized gain (loss) recognized during the period	1	(4)	(3)	(11)
Delmarva Power & Light Co/De [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized loss arising during the period	(2)			(4)
Net realized gain (loss) recognized during the period	$ 1	$ (4)	$ (3)	$ (11)