Income Taxes (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended			12 Months Ended						3 Months Ended				12 Months Ended			3 Months Ended		12 Months Ended				3 Months Ended			12 Months Ended
	Sep. 30, 2012	Jun. 30, 2011	Dec. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2010	May 31, 2011	Mar. 31, 2010	Jun. 30, 2012 Potomac Electric Power Co [Member]	Jun. 30, 2011 Potomac Electric Power Co [Member]	Dec. 31, 2010 Potomac Electric Power Co [Member]	Sep. 30, 2010 Potomac Electric Power Co [Member]	Dec. 31, 2012 Potomac Electric Power Co [Member]	Dec. 31, 2011 Potomac Electric Power Co [Member]	Dec. 31, 2010 Potomac Electric Power Co [Member]	Sep. 30, 2012 Delmarva Power & Light Co/De [Member]	Sep. 30, 2011 Delmarva Power & Light Co/De [Member]	Dec. 31, 2012 Delmarva Power & Light Co/De [Member]	Dec. 31, 2011 Delmarva Power & Light Co/De [Member]	Dec. 31, 2010 Delmarva Power & Light Co/De [Member]	Dec. 31, 2011 Delmarva Power & Light Co/De [Member] Deposit Made in 2006 [Member]	Sep. 30, 2012 Atlantic City Electric Co [Member]	Jun. 30, 2011 Atlantic City Electric Co [Member]	Dec. 31, 2010 Atlantic City Electric Co [Member]	Dec. 31, 2012 Atlantic City Electric Co [Member]	Dec. 31, 2011 Atlantic City Electric Co [Member]	Dec. 31, 2010 Atlantic City Electric Co [Member]
Schedule Of Income Tax [Line Items]
Charge related to tax consequences	$ 16					$ 22							$ 2		$ 23
Income tax settlement				10										10
Additional income tax (expense) benefit after-tax		17		(3)						4	5				(1)		1	1		4		1	3	1	1		3
Additional state income tax expense					2		6
Federal and state net operating losses, years					20									20					20							20
Federal and state net operating losses, expiration period					2029 to 2032									2029 to 2032					2029 to 2032							2029 to 2032
Reversal of valuation allowance							8
Adjustment to accrued interest on uncertain tax positions			15				15					24				24					3
Reversal of accrued state interest									6			8			1	8					2				6			6
Other adjustments to income tax benefit									2			5				5									1			1
Reduction to tax expenses			4									4
Unrecognized tax benefits that would impact effective tax rate				36										8												6
Interest and penalties related to uncertain tax positions after-tax				14	14		1							11	5	16				4	4					1	3	5
Interest and penalties related to uncertain tax positions pre-tax				23	23		2							18	8	27			1	6	6					2	5	8
Accrued interest payable			12		4		12								6						5
Accrued interest receivable				10								8		5		8			1	1					14	7	6	14
Adjustments relating to unresolved tax matters							1
Other taxes for continuing operations				426	445		427
Receipt of refunds								5
State tax benefit carryforward															4
Amended state returns														20
Refunds received														4
Decrease in income taxes and depreciation				(1)			(3)							1	(1)	3			(1)	1	1					1
Amended state returns																										$ 1