Commitments and Contingencies - Retained Environmental Exposures - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	0 Months Ended	1 Months Ended			1 Months Ended	12 Months Ended	1 Months Ended					0 Months Ended
	Sep. 30, 2014	Jul. 31, 2010	Sep. 30, 2014	Oct. 22, 2014 Subsequent Event [Member]	Jul. 31, 2014 Pepco Energy Services [Member]	Dec. 31, 2012 Pepco Energy Services [Member]	Jul. 31, 2010 Conectiv Energy [Member] Facility	Jul. 31, 2010 PHI [Member]	Sep. 30, 2014 Potomac Electric Power Co [Member]	Oct. 22, 2014 Potomac Electric Power Co [Member] Subsequent Event [Member]	Sep. 30, 2014 Delmarva Power & Light Co/De [Member]	Sep. 30, 2014 Atlantic City Electric Co [Member]	Sep. 30, 2014 Atlantic City Electric Co [Member]
Commitments and Contingencies [Line Items]
Loss contingency liabilities			$ 54.0	$ 21.7					$ 25.0	$ 21.7	$ 2.0		$ 27.0
Self insured retention amount	2											2
Certain allegedly unauthorized charges						7
Additional compounded interest						9
Litigation claim value, sought					3
Litigation claim value, interest disallowed					4
Third party maximum and seller floor for environmental remediation costs							10	10
Number of facility locations							9
Environmental remediation expense minimum		7
Environmental remediation expense maximum		$ 18