Retirement Benefits - Components of Net Periodic Benefit Cost (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 Disclosure [Line Items]
Service cost		$ 354	$ 326	$ 293
Interest cost		830	710	749
Expected return on assets		(1,141)	(1,026)	(994)
Prior service cost (credit)		14	13	14
Actuarial loss		554	571	420
Settlement and other charges(b)		2	2	2
Net periodic benefit cost		613	596	484
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		107	119	117
Interest cost		185	167	186
Expected return on assets		(162)	(151)	(154)
Prior service cost (credit)		(185)	(174)	(122)
Actuarial loss		63	80	50
Settlement and other charges(b)		0	0	0
Net periodic benefit cost		$ 8	41	77
Predecessor [Member] | Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 12		57	44
Interest cost	26		109	109
Expected return on assets	(30)		(140)	(141)
Prior service cost (credit)	0		2	2
Actuarial loss	14		65	45
Net periodic benefit cost	22		93	59
Predecessor [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1		7	7
Interest cost	6		24	26
Expected return on assets	(5)		(22)	(24)
Prior service cost (credit)	(3)		(13)	(13)
Actuarial loss	2		8	3
Net periodic benefit cost	$ 1		$ 4	$ (1)