Derivative Instruments and Hedging Activities (Fair Value of Derivative Instruments by Balance Sheet Location) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Derivative [Line Items]
Derivative assets (current assets)	$ 2		$ 5
Derivative assets (non-current assets)	8
Total Derivative assets	216		192
Derivative liabilities (current liabilities)	(13)		(26)
Derivative liabilities (non-current liabilities)	(10)		(6)
Delmarva Power & Light Co/De [Member]
Derivative [Line Items]
Total Derivative assets	44		3
Derivative liabilities (current liabilities)	(7)		(12)
Derivative liabilities (non-current liabilities)			(3)
Cash Flow Hedging [Member]
Derivative [Line Items]
Derivative assets (current assets)			17	[1]
Total Derivative assets			17	[1]
Derivative liabilities (current liabilities)	(16)	[1]	(55)	[1]
Derivative liabilities (non-current liabilities)	(2)	[1]	(11)	[1]
Total Derivative liabilities	(18)	[1]	(66)	[1]
Net Derivative (liability) asset	(18)	[1]	(49)	[1]
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets (current assets)	3		6
Derivative assets (non-current assets)	8		1
Total Derivative assets	11		7
Derivative liabilities (current liabilities)	(21)		(48)
Derivative liabilities (non-current liabilities)	(11)		(10)
Total Derivative liabilities	(32)		(58)
Net Derivative (liability) asset	(21)		(51)
Not Designated as Hedging Instrument [Member] | Delmarva Power & Light Co/De [Member]
Derivative [Line Items]
Derivative assets (current assets)	1
Total Derivative assets	1
Derivative liabilities (current liabilities)	(7)		(14)
Derivative liabilities (non-current liabilities)			(3)
Total Derivative liabilities	(7)		(17)
Net Derivative (liability) asset	(6)		(17)
Gross Derivative Instruments [Member]
Derivative [Line Items]
Derivative assets (current assets)	3		23
Derivative assets (non-current assets)	8		1
Total Derivative assets	11		24
Derivative liabilities (current liabilities)	(37)		(103)
Derivative liabilities (non-current liabilities)	(13)		(21)
Total Derivative liabilities	(50)		(124)
Net Derivative (liability) asset	(39)		(100)
Gross Derivative Instruments [Member] | Delmarva Power & Light Co/De [Member]
Derivative [Line Items]
Derivative assets (current assets)	1
Total Derivative assets	1
Derivative liabilities (current liabilities)	(7)		(14)
Derivative liabilities (non-current liabilities)			(3)
Total Derivative liabilities	(7)		(17)
Net Derivative (liability) asset	(6)		(17)
Netting [Member]
Derivative [Line Items]
Derivative assets (current assets)	(1)		(18)
Derivative assets (non-current assets)			(1)
Total Derivative assets	(1)		(19)
Derivative liabilities (current liabilities)	24		77
Derivative liabilities (non-current liabilities)	3		15
Total Derivative liabilities	27		92
Net Derivative (liability) asset	26		73
Netting [Member] | Delmarva Power & Light Co/De [Member]
Derivative [Line Items]
Derivative assets (current assets)	(1)
Total Derivative assets	(1)
Derivative liabilities (current liabilities)			2
Total Derivative liabilities			2
Net Derivative (liability) asset	(1)		2
Net Investment Hedging [Member]
Derivative [Line Items]
Derivative assets (current assets)	2		5
Derivative assets (non-current assets)	8
Total Derivative assets	10		5
Derivative liabilities (current liabilities)	(13)		(26)
Derivative liabilities (non-current liabilities)	(10)		(6)
Total Derivative liabilities	(23)		(32)
Net Derivative (liability) asset	(13)		(27)
Net Investment Hedging [Member] | Delmarva Power & Light Co/De [Member]
Derivative [Line Items]
Derivative liabilities (current liabilities)	(7)		(12)
Derivative liabilities (non-current liabilities)			(3)
Total Derivative liabilities	(7)		(15)
Net Derivative (liability) asset	$ (7)		$ (15)

[1] Amounts included in Derivatives Designated as Hedging Instruments primarily consist of derivatives that were designated as cash flow hedges prior to Pepco Energy Services' election to discontinue cash flow hedge accounting for these derivatives.