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, 2018		Dec. 31, 2017		Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost			$ 47		$ 109		$ 116
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	[1]		(1,561)		1,182
Service cost			405	[2]	387	[3]	354
Interest cost			802	[2]	842	[3]	830
Expected return on assets			(1,252)	[2]	(1,196)	[3]	(1,141)
Prior service cost (credit)			2	[2]	1	[3]	14
Actuarial loss			629	[2]	607	[3]	554
Settlement and other charges(c)			3	[2],[4]	3	[3],[4]	2
Net periodic benefit cost			589	[2]	644	[3]	613
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 12
Interest cost		26
Expected return on assets		30
Prior service cost (credit)		0
Actuarial loss		(14)
Net periodic benefit cost		22
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	[1]		(540)		350
Service cost			112	[2]	106	[3]	107
Interest cost			175	[2]	182	[3]	185
Expected return on assets			(173)	[2]	(162)	[3]	(162)
Prior service cost (credit)			(186)	[2]	(188)	[3]	(185)
Actuarial loss			66	[2]	61	[3]	63
Settlement and other charges(c)			1	[2],[4]	0	[3],[4]	0
Net periodic benefit cost			$ (5)	[2]	$ (1)	[3]	$ 8
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		1
Interest cost		6
Expected return on assets		5
Prior service cost (credit)		(3)
Actuarial loss		(2)
Net periodic benefit cost		$ 1

[1] (a)The pension actuarial gain in 2018 primarily reflects an increase in the discount rate. The OPEB actuarial gain in 2018 primarily reflects an increase in the discount rate and favorable health care claims experience. The pension and OPEB actuarial losses in 2017 primarily reflect a decrease in the discount rate.

[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.