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	Dec. 31, 2012 Pepco Energy Services [Member]	Jul. 31, 2010 Conectiv Energy [Member] Facility	Jul. 31, 2010 PHI [Member]
Commitment And Contingencies [Line Items]
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