Pension And Other Postretirement Benefits (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012	Mar. 31, 2012	Sep. 30, 2011	Mar. 31, 2011	Sep. 30, 2012	Sep. 30, 2011
Defined Benefit Plan Disclosure [Line Items]
Pension contributions					$ 200	$ 110
Pension and other postretirement net periodic benefit cost	28		24		84	70
Potomac Electric Power Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension contributions		85		40	85	40
Pension and other postretirement net periodic benefit cost	10		15		30	32
Delmarva Power & Light Co/De [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension contributions		85		40	85	40
Pension and other postretirement net periodic benefit cost	6		6		18	18
Atlantic City Electric Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension contributions		30		30	30	30
Pension and other postretirement net periodic benefit cost	$ 6		$ 5		$ 18	$ 15