Derivative Instruments and Hedging Activities - Net Outstanding Commodity Forward Contracts That Did Not Qualify For Hedge Accounting (Detail) (Natural Gas [Member])	3 Months Ended
	Mar. 31, 2015 MMBTU	Dec. 31, 2014 MMBTU
Derivative [Line Items]
Quantity	2,997,500	3,892,500
Net Position	Long
Delmarva Power & Light Co/De [Member]
Derivative [Line Items]
Quantity	2,997,500	3,892,500
Net Position	Long
2014 [Member]
Derivative [Line Items]
Net Position	Long
2014 [Member] | Delmarva Power & Light Co/De [Member]
Derivative [Line Items]
Net Position	Long