Related Party Transactions - Schedule of Related Party Transactions Included in Balance Sheet (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Potomac Electric Power Co [Member]
Related Party Transaction [Line Items]
Payable to Related Party, Total	$ (32)	$ (30)
Potomac Electric Power Co [Member] | PHI Service Company [Member]
Related Party Transaction [Line Items]
Payable to Related Party, Total	(22)	(27)
Potomac Electric Power Co [Member] | Pepco Energy Services [Member]
Related Party Transaction [Line Items]
Payable to Related Party, Total	(5)	(2)
Potomac Electric Power Co [Member] | Other [Member]
Related Party Transaction [Line Items]
Payable to Related Party, Total	(5)	(1)
Delmarva Power & Light Co/De [Member]
Related Party Transaction [Line Items]
Payable to Related Party, Total	(22)	(17)
Delmarva Power & Light Co/De [Member] | PHI Service Company [Member]
Related Party Transaction [Line Items]
Payable to Related Party, Total	(20)	(18)
Delmarva Power & Light Co/De [Member] | Other [Member]
Related Party Transaction [Line Items]
Payable to Related Party, Total	(2)	1
Atlantic City Electric Co [Member]
Related Party Transaction [Line Items]
Payable to Related Party, Total	(15)	(15)
Atlantic City Electric Co [Member] | PHI Service Company [Member]
Related Party Transaction [Line Items]
Payable to Related Party, Total	(14)	(14)
Atlantic City Electric Co [Member] | Other [Member]
Related Party Transaction [Line Items]
Payable to Related Party, Total	$ (1)	$ (1)