Derivative Instruments and Hedging Activities - Net Outstanding Commodity Forward Contracts That Did Not Qualify For Hedge Accounting (Detail) (Natural Gas (One Million British Thermal Units (MMBtu)) [Member])	9 Months Ended	12 Months Ended
	Sep. 30, 2014 MMBTU	Dec. 31, 2013 MMBTU
Derivative [Line Items]
Quantity	3,805,000	3,977,500
Net Position	Long	Long
Delmarva Power & Light Co/De [Member]
Derivative [Line Items]
Quantity	3,805,000	3,977,500
Net Position	Long	Long