Retirement Benefits - Calculation of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017		Jun. 30, 2018	Jun. 30, 2017
Amortization of:
Net periodic benefit cost	$ 11	$ 28		$ 21	$ 54
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	102	97		202	191
Interest cost	200	211		401	422
Expected return on assets	(313)	(299)		(626)	(598)
Amortization of:
Prior service cost (benefit)	0	1		1	1
Actuarial loss	157	150		314	302
Settlement charges	(1)	(2)		(1)	(2)
Net periodic benefit cost	147	162	[1]	293	320	[1]
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	28	28		56	54
Interest cost	44	46		88	91
Expected return on assets	(43)	(41)		(87)	(82)
Amortization of:
Prior service cost (benefit)	(47)	(47)		(93)	(94)
Actuarial loss	16	15		33	31
Settlement charges	0	0		0	0
Net periodic benefit cost	(2)	1	[1]	(3)	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.