Pension And Other Postretirement Benefits (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012	Mar. 31, 2012	Jun. 30, 2011	Mar. 31, 2011	Jun. 30, 2012	Jun. 30, 2011
Defined Benefit Plan Disclosure [Line Items]
Pension contributions					$ 200	$ 110
Pension and other postretirement net periodic benefit cost	30		19		56	46
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	9		7		20	17
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		5		12	12
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		$ 4		$ 12	$ 10