Segment Information - Segment Financial Information (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Goodwill	$ 1,406		$ 1,406		$ 1,407
Operating Revenue	1,362	$ 1,313	3,873	$ 3,760
Operating Expenses	1,179	1,147	3,409	3,270
Depreciation and amortization	178	145	494	410
Increase in fair value of preferred stock derivative	15		15
Interest Expense	71	68	210	200
Impairment loss		53		53
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Operating Revenue	(1)	(2)	(4)	(6)
Operating Expenses	(1)		(1)	2
Depreciation and amortization	12	8	33	23
Increase in fair value of preferred stock derivative	15		15
Increase in fair value of preferred stock derivative, after-tax	10		10
Interest Expense	12	9	33	30
Power Delivery [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Goodwill	1,400	1,400	1,400	1,400
Operating Revenue	1,316	1,242	3,707	3,554
Operating Expenses	1,131	1,021	3,238	3,005
Depreciation and amortization	165	135	459	381
Interest Expense	59	58	177	169
Inter Company Amounts [Member] | Corporate and Other [Member]
Segment Reporting Information [Line Items]
Operating Revenue	(1)	(2)	(4)	(6)
Operating Expenses	1	(1)	(4)	(5)
Interest and dividend income	(1)		(5)
Interest Expense		(2)	(2)	(2)
Interest Income		(2)		(3)
Pepco Energy Services [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating Revenue	47	73	170	212
Operating Expenses	49	126	172	263
Depreciation and amortization	$ 1	2	$ 2	6
Interest Expense		1		1
Impairment loss		53		53
Impairment loss, after-tax		$ 32		$ 32