Commitments And Contingencies (Retained Environmental Exposures) (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended
	Jul. 31, 2010	Dec. 31, 2012 Plaintiff
Commitment And Contingencies [Line Items]
Number of plaintiffs		448
Compensatory damages sought per plaintiff		$ 2
Punitive damages sought per plaintiff		4
Environmental remediation expense minimum	7
Environmental remediation expense maximum	18
Mirant Corporation [Member]
Commitment And Contingencies [Line Items]
Compensatory damages sought per plaintiff		360
Cases still pending against Pepco		90
Pepco Energy Services [Member]
Commitment And Contingencies [Line Items]
Certain allegedly unauthorized charges		7
Additional interest		9
Conectiv Energy [Member]
Commitment And Contingencies [Line Items]
Third party maximum and seller floor for environmental remediation costs	10
PHI [Member]
Commitment And Contingencies [Line Items]
Third party maximum and seller floor for environmental remediation costs	10
Potomac Electric Power Co [Member]
Commitment And Contingencies [Line Items]
Number of plaintiffs		448
Compensatory damages sought per plaintiff		2
Punitive damages sought per plaintiff		4
Potomac Electric Power Co [Member] | Mirant Corporation [Member]
Commitment And Contingencies [Line Items]
Compensatory damages sought per plaintiff		$ 360
Cases still pending against Pepco		90