Income Taxes - Other Taxes (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule of Income Tax [Line Items]
Gross Receipts/Delivery	$ 124	$ 123	$ 133
Property	92	84	77
County Fuel and Energy	143	143	153
Environmental, Use and Other	68	63	65
Total	427	413	428
Potomac Electric Power Co [Member]
Schedule of Income Tax [Line Items]
Gross Receipts/Delivery	107	107	108
Property	55	51	45
County Fuel and Energy	143	143	153
Environmental, Use and Other	64	62	62
Total	369	363	368
Delmarva Power & Light Co/De [Member]
Schedule of Income Tax [Line Items]
Gross Receipts/Delivery	17	16	15
Property	28	24	24
Environmental, Use and Other	2	2	1
Total	47	42	40
Atlantic City Electric Co [Member]
Schedule of Income Tax [Line Items]
Gross Receipts/Delivery			10
Property	4	3	3
Environmental, Use and Other	1	(1)	1
Total	$ 5	$ 2	$ 14