Derivative Instruments and Hedging Activities (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Credit risk derivative liabilities, at fair value	$ 14		$ 14		$ 54
Credit risk derivative liabilities, at fair value net	14		14		53
Cash collateral posted					1
Net settlement amount in the event of a downgrade below investment grade	56		56		124
Additional collateral that would have to be posted in the event of an investment downgrade	56		56		123
Cash plus borrowing capacity to meet future liquidity needs	1,078		1,078		994
Derivative assets (non-current assets)	8		8
Derivative liabilities (non-current liabilities)	10		10		6
Pepco Energy Services [Member]
Debt instrument unused borrowing capacity amount for subsidiary	515		515		283
Unrealized loss on derivatives	3	(5)	(2)	(10)
Pepco Energy Services [Member] | Natural Gas Derivatives [Member]
Derivative losses included in AOCL			18
Delmarva Power & Light Co/De [Member]
Credit risk derivative liabilities, at fair value	7		7		15
Credit risk derivative liabilities, at fair value net	7		7		15
Net settlement amount in the event of a downgrade below investment grade	8		8		15
Additional collateral that would have to be posted in the event of an investment downgrade	8		8		15
Cash plus borrowing capacity to meet future liquidity needs	563		563		711
Derivative liabilities (non-current liabilities)					3
Atlantic City Electric Co [Member]
Estimated percentage of New Jersey load			15.00%
Derivative assets (non-current assets)	8		8
Derivative liabilities (non-current liabilities)	9		9
Unrealized loss on derivatives	$ 1		$ 1