Segment Information - Segment Financial Information for Continuing Operations (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Goodwill	$ 1,406				$ 1,407				$ 1,406	$ 1,407
Operating Revenue	1,150	$ 1,362	$ 1,140	$ 1,371	1,118	$ 1,313	$ 1,117	$ 1,330	5,023	4,878	$ 4,666
Operating Expenses	941	1,179	1,001	1,229	1,004	1,147	966	1,157	4,350	4,274	3,998
Interest Expense									280	268	273
Depreciation and amortization									651	549	473
Gains on sales of land	46								46
Gains on sales of land, after tax	27
Increase in fair value of preferred stock derivative		15							15
Increase in fair value of preferred stock derivative, after-tax		10
Income Tax Benefit	(23)	$ (49)	$ (27)	$ (30)	(13)	$ (34)	$ (45)	$ (46)	(129)	(138)	(319)
Impairment losses										81	4
Tax Settlement [Member]
Segment Reporting Information [Line Items]
Income Tax Benefit	47
Power Delivery [Member]
Segment Reporting Information [Line Items]
Operating Revenue									4,805	4,607	4,472
Operating Expenses									4,124	3,916	3,828
Interest Expense									238	226	228
Depreciation and amortization									606	511	439
Gains on sales of land									46
Gains on sales of land, after tax									27
Income Tax Benefit									(177)	(185)	(155)
Interest benefit on uncertain tax positions											12
Power Delivery [Member] | Tax Settlement [Member]
Segment Reporting Information [Line Items]
Income Tax Benefit									3
Power Delivery [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Goodwill	$ 1,400				$ 1,400				1,400	1,400	1,400
Inter Company Amounts [Member] | Corporate and Other [Member]
Segment Reporting Information [Line Items]
Operating Revenue									(5)	(7)	(10)
Operating Expenses									(4)	(7)	(9)
Interest Expense									(5)	(4)	(5)
Pepco Energy Services [Member]
Segment Reporting Information [Line Items]
Operating Revenue									223	278	203
Operating Expenses									224	354	201
Interest Expense										1	1
Depreciation and amortization									4	7	6
Income Tax Benefit									4	36	(1)
Impairment losses										81
Impairment losses, after-tax										48
Pepco Energy Services [Member] | Tax Settlement [Member]
Segment Reporting Information [Line Items]
Income Tax Benefit									1
Pepco Energy Services [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Impairment losses										81	4
Impairment losses, after-tax										48	3
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Operating Revenue									(5)	(7)	(9)
Operating Expenses									2	4	(31)
Interest Expense									42	41	44
Depreciation and amortization									41	31	28
Increase in fair value of preferred stock derivative									15
Increase in fair value of preferred stock derivative, after-tax									10
Income Tax Benefit									44	$ 11	(163)
Interest expense on uncertain tax positions											66
Valuation allowances against deferred tax assets											$ 101
Corporate and Other [Member] | Tax Settlement [Member]
Segment Reporting Information [Line Items]
Income Tax Benefit									$ 43