Retirement Benefits - Calculation of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Amortization of:
Net periodic benefit cost	$ (10)	$ (26)
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	101	95
Interest cost	201	210
Expected return on assets	(312)	(299)
Amortization of:
Prior service benefit	0	0
Actuarial loss	157	152
Net periodic benefit cost	147	158
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	28	26
Interest cost	43	45
Expected return on assets	(43)	(41)
Amortization of:
Prior service benefit	(46)	(47)
Actuarial loss	16	16
Net periodic benefit cost	(2)	(1)
Pepco Holdings LLC [Member]
Amortization of:
Net periodic benefit cost	$ 0	$ 0