Income Taxes (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012	Mar. 31, 2012	Sep. 30, 2011	Jun. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Income Taxes [Line Items]
Consolidated effective tax rates	45.40%		40.70%		37.00%	37.60%
Reversal of permanent tax benefits	16			$ 22	16	$ 22
Income Tax Settlement					10
Tax Benefit From Internal Revenue Services Interest Settlement						11
Potomac Electric Power Co [Member]
Income Taxes [Line Items]
Consolidated effective tax rates	41.20%		37.70%		27.30%	26.70%
Income Tax Settlement		10		5
Income tax refunds received				5
State tax benefit related to prior years' asset dispositions				4		(4)
Delmarva Power & Light Co/De [Member]
Income Taxes [Line Items]
Consolidated effective tax rates	33.30%		47.60%		37.80%	36.40%
Income Tax Settlement				4		1
Deferred tax adjustment			1	1
Adjustments to prior year taxes	(2)		(1)		(2)	(1)
Atlantic City Electric Co [Member]
Income Taxes [Line Items]
Consolidated effective tax rates	39.40%		50.00%		36.80%	46.80%
Income Tax Settlement				1
Deferred tax adjustment			3	$ 1