Retirement Benefits - Summary of Changes in Plan Assets (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 23, 2016	Dec. 31, 2016	Dec. 31, 2015
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of net plan assets at beginning of year		$ 14,347	$ 14,347	$ 14,874
Actual return on plan assets			1,061	(32)
Employer contributions			347	462
Plan participants’ contributions			0	0
Gross benefits paid			(1,051)	(923)
Acquisitions/divestitures(a)			2,087	0
Settlements	[1]		0	(34)
Fair value of net plan assets at end of year			16,791	14,347
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of net plan assets at beginning of year		2,293	2,293	2,430
Actual return on plan assets			128	4
Employer contributions			50	40
Plan participants’ contributions			54	42
Gross benefits paid			(280)	(223)
Acquisitions/divestitures(a)			333	0
Settlements	[1]		0	0
Fair value of net plan assets at end of year			2,578	2,293
Predecessor [Member] | Pepco Holdings LLC [Member] | Pension Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of net plan assets at beginning of year		2,018	2,018	2,236
Actual return on plan assets		0		(61)
Employer contributions		4		6
Gross benefits paid		(2)		(163)
Fair value of net plan assets at end of year		2,020		2,018
Predecessor [Member] | Pepco Holdings LLC [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of net plan assets at beginning of year		348	$ 348	367
Actual return on plan assets		0		1
Employer contributions		1		5
Gross benefits paid		(1)		(39)
Defined Benefit Plan, Other Changes				25
Fair value of net plan assets at end of year		$ 348		$ 348

[1] ____________________ (a)