Derivative Instruments and Hedging Activities - Net Outstanding Commodity Forward Contracts That Did Not Qualify For Hedge Accounting (Detail) - Natural Gas [Member] - Long [Member] - Other Derivative Instruments [Member] - MMBTU	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Quantity	4,190,000	3,892,500
Delmarva Power & Light Co/De [Member]
Derivative [Line Items]
Quantity	4,190,000	3,892,500