Retirement Benefits - Calculation of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017		Sep. 30, 2018	Sep. 30, 2017
Amortization of:
Net periodic benefit cost	$ 12	$ 27		$ 33	$ 82
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	100	98		303	290
Interest cost	201	211		602	632
Expected return on assets	(312)	(300)		(939)	(898)
Amortization of:
Prior service cost (benefit)	0	(1)		1	0
Actuarial loss	158	152		472	455
Settlement charges	0	(1)		(1)	(3)
Net periodic benefit cost	147	161	[1]	440	482	[1]
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	28	26		84	79
Interest cost	43	45		131	136
Expected return on assets	(43)	(39)		(130)	(121)
Amortization of:
Prior service cost (benefit)	(47)	(47)		(140)	(140)
Actuarial loss	18	15		50	46
Settlement charges	0	0		0	0
Net periodic benefit cost	(1)	0	[1]	(5)	0	[1]
Pepco Holdings LLC [Member]
Amortization of:
Net periodic benefit cost	$ 0	$ 0		$ 0	$ 0

[1] FitzPatrick net benefit costs are included for the period after the acquisition date of March 31, 2017.