Retirement Benefits Retirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 23, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)		$ 1,182		$ 567	[1]
Service cost		387	[2]	354	[1],[3]	$ 326
Interest cost		842	[2]	830	[1],[3]	710
Expected return on assets		(1,196)	[2]	(1,141)	[3]	(1,026)
Prior service cost (credit)		1	[2]	14	[3]	13
Actuarial loss		607	[2]	554	[3]	571
Settlement and other charges(c)		3	[2],[4]	2	[3],[4]	2
Net periodic benefit cost		644	[2]	613	[3]	596
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	$ (30)
Service cost	12			57
Interest cost	26			109
Expected return on assets	30			140
Prior service cost (credit)	0			2
Actuarial loss	(14)			(65)
Net periodic benefit cost	22			93
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)		350		(136)	[1]
Service cost		106	[2]	107	[1],[3]	119
Interest cost		182	[2]	185	[1],[3]	167
Expected return on assets		(162)	[2]	(162)	[3]	(151)
Prior service cost (credit)		(188)	[2]	(185)	[3]	(174)
Actuarial loss		61	[2]	63	[3]	80
Settlement and other charges(c)		0	[2],[4]	0	[3],[4]	0
Net periodic benefit cost		$ (1)	[2]	8	[3]	$ 41
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	(5)
Service cost	1			7
Interest cost	6			24
Expected return on assets	5			22
Prior service cost (credit)	(3)			(13)
Actuarial loss	(2)			(8)
Net periodic benefit cost	$ 1			$ 4

[1] 2016 amounts include PHI for the period of March 24, 2016 through December 31, 2016.

[2] FitzPatrick net benefit costs are included for the period after acquisition.

[3] PHI net periodic benefit costs for the period prior to the merger are not included in the table above.

[4] 2016 amount includes an additional termination benefit for PHI.