Commitments and Contingencies - Retained Environmental Exposures - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended			12 Months Ended	1 Months Ended
	Jul. 31, 2010	Mar. 31, 2014	Aug. 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]	Mar. 31, 2014 Potomac Electric Power Co [Member]	Aug. 31, 2013 Potomac Electric Power Co [Member] Substation Injury Claim [Member]	Mar. 31, 2014 Delmarva Power & Light Co/De [Member]	Mar. 31, 2014 Atlantic City Electric Co [Member]
Commitments and Contingencies [Line Items]
Loss contingency liabilities		$ 32					$ 18		$ 2	$ 12
Loss contingency, maximum claim amount			28.1					28.1
Certain allegedly unauthorized charges				7
Additional compounded interest				9
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