Retirement Benefits - Summary of Changes in Benefit Obligations (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 23, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Net benefit obligation at beginning of year		$ 17,753	$ 17,753	$ 18,256
Service cost			354	326	$ 293
Interest cost			830	710	749
Plan participants’ contributions			0	0
Actuarial (gain) loss			567	(582)
Plan amendments			(60)	0
Acquisitions/divestitures(a)			2,667	0
Settlements	[1]		0	(34)
Gross benefits paid			(1,051)	(923)
Net benefit obligation at end of year			21,060	17,753	18,256
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Net benefit obligation at beginning of year		2,490	2,490	2,638
Service cost		12		57	44
Interest cost		26		109	109
Actuarial (gain) loss		(30)		(151)
Gross benefits paid		(2)		(163)
Net benefit obligation at end of year		2,496		2,490	2,638
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Net benefit obligation at beginning of year		3,938	3,938	4,197
Service cost			107	119	117
Interest cost			185	167	186
Plan participants’ contributions			54	42
Actuarial (gain) loss			(136)	(341)
Plan amendments			0	(23)
Acquisitions/divestitures(a)			589	0
Settlements	[1]		0	0
Gross benefits paid			(280)	(223)
Net benefit obligation at end of year			4,457	3,938	4,197
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Net benefit obligation at beginning of year		563	$ 563	632
Service cost		1		7	7
Interest cost		6		24	26
Actuarial (gain) loss		(5)		(61)
Gross benefits paid		(1)		(39)
Net benefit obligation at end of year		$ 564		$ 563	$ 632

[1] ____________________ (a)