Derivative Instruments and Hedging Activities - Net Outstanding Commodity Forward Contracts that did not Qualify for Hedge Accounting (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2013 ForwardContracts	Dec. 31, 2012 ForwardContracts
Delmarva Power & Light Co/De [Member] | Natural Gas (MMBtu)) [Member]
Derivative [Line Items]
Commodity forward contracts	3,767,500	3,838,000
Net Position	Long	Long
Atlantic City Electric Co [Member] | Capacity (MWs) [Member]
Derivative [Line Items]
Commodity forward contracts	180	180
Net Position	Long	Long