Commitments and Contingencies - Retained Environmental Exposures - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended		12 Months Ended	1 Months Ended		12 Months Ended
	Jul. 31, 2010	Dec. 31, 2013	Dec. 31, 2013 Substation Injury Claim [Member]	Dec. 31, 2012 Pepco Energy Services [Member]	Jul. 31, 2010 Conectiv Energy [Member] Facility	Jul. 31, 2010 PHI [Member]	Dec. 31, 2013 Potomac Electric Power Co [Member]	Dec. 31, 2013 Potomac Electric Power Co [Member] Substation Injury Claim [Member]	Dec. 31, 2013 Delmarva Power & Light Co/De [Member]	Dec. 31, 2013 Atlantic City Electric Co [Member]
Commitment and Contingencies [Line Items]
Certain allegedly unauthorized charges				$ 7
Additional compounded interest				9
Loss contingency liabilities		30					19		2	9
Loss contingency, maximum claim amount			28.1					28.1
Minimum period for public comment		30 days					30 days
Environmental remediation expense minimum	7
Environmental remediation expense maximum	18
Third party maximum and seller floor for environmental remediation costs					$ 10	$ 10
Number of facility locations					9