Income Taxes - Reconciliation of Beginning and Ending Balances of Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits, Beginning balance	$ 850	$ 831	$ 200
Tax positions related to current year, Additions		4	3
Tax positions related to current year, Reductions		(2)
Tax positions related to prior years, Additions	13	27	646
Tax positions related to prior years, Reductions	(201)	(10)	(12)
Settlements	(628)		(6)
Unrecognized Tax Benefits, Ending balance	34	850	831
Potomac Electric Power Co [Member]
Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits, Beginning balance	97	101	91
Tax positions related to current year, Additions		1	1
Tax positions related to current year, Reductions		(2)
Tax positions related to prior years, Additions	10	1	12
Tax positions related to prior years, Reductions	(55)	(4)	(3)
Settlements	(40)
Unrecognized Tax Benefits, Ending balance	12	97	101
Delmarva Power & Light Co/De [Member]
Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits, Beginning balance	22	9	9
Tax positions related to current year, Additions		1
Tax positions related to current year, Reductions			0
Tax positions related to prior years, Additions	3	13
Tax positions related to prior years, Reductions	(13)	(1)
Settlements	(9)
Unrecognized Tax Benefits, Ending balance	3	22	9
Atlantic City Electric Co [Member]
Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits, Beginning balance	13	9	17
Tax positions related to current year, Additions		1	2
Tax positions related to prior years, Additions		5	1
Tax positions related to prior years, Reductions	(10)	(2)	(5)
Settlements	$ (3)		(6)
Unrecognized Tax Benefits, Ending balance		$ 13	$ 9