Commitments and Contingencies - Retained Environmental Exposures - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	0 Months Ended	1 Months Ended		12 Months Ended
	Nov. 05, 2014	Jul. 31, 2010	Jul. 31, 2014	Dec. 31, 2012	Dec. 31, 2014	Oct. 22, 2014
Commitments and Contingencies [Line Items]
Loss contingency liabilities					$ 33.0	$ 21.7
Self insured retention amount	2
Environmental remediation expense minimum		7
Environmental remediation expense maximum		18
Pepco Energy Services [Member]
Commitments and Contingencies [Line Items]
Certain allegedly unauthorized charges				7
Additional compounded interest				9
Litigation claim value, sought			3
Litigation claim value, interest disallowed			4
Conectiv Energy [Member]
Commitments and Contingencies [Line Items]
Third party maximum and seller floor for environmental remediation costs		10
Number of facility locations		9
PHI [Member]
Commitments and Contingencies [Line Items]
Third party maximum and seller floor for environmental remediation costs		10
Potomac Electric Power Co [Member]
Commitments and Contingencies [Line Items]
Loss contingency liabilities					25.0	21.7
Delmarva Power & Light Co/De [Member]
Commitments and Contingencies [Line Items]
Loss contingency liabilities					2.0
Atlantic City Electric Co [Member]
Commitments and Contingencies [Line Items]
Loss contingency liabilities					6.0
Self insured retention amount	$ 2