Commitments and Contingencies - Retained Environmental Exposures - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended		12 Months Ended
	Jul. 31, 2010	Jul. 31, 2014	Dec. 31, 2012	Mar. 31, 2015	Oct. 22, 2014
Commitments and Contingencies [Line Items]
Loss contingency liabilities				$ 22.0	$ 21.7
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				14.0	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