Retirement Benefits - Calculation of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended					9 Months Ended
	Sep. 30, 2017	[1]	Sep. 30, 2016	[2]	Mar. 23, 2016	Sep. 30, 2017	[1]	Sep. 30, 2016	[2]
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 98		$ 92			$ 290		$ 262
Interest cost	211		215			632		616
Expected return on assets	(300)		(293)			(898)		(847)
Amortization of:
Prior service (benefit) cost	(1)		3			0		10
Actuarial loss	152		142			455		411
Settlement charges	1		0			3		0
Net periodic benefit cost	161		159			482		452
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	26		27			79		80
Interest cost	45		47			136		138
Expected return on assets	(39)		(42)			(121)		(121)
Amortization of:
Prior service (benefit) cost	(47)		(48)			(140)		(138)
Actuarial loss	15		18			46		47
Settlement charges	0		0			0		0
Net periodic benefit cost	$ 0		$ 2			$ 0		$ 6
Pepco Holdings LLC [Member] | Pension Plan, Defined Benefit [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost					$ 12
Interest cost					26
Expected return on assets					(30)
Amortization of:
Prior service (benefit) cost					0
Actuarial loss					14
Net periodic benefit cost					22
Pepco Holdings LLC [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost					1
Interest cost					6
Expected return on assets					(5)
Amortization of:
Prior service (benefit) cost					(3)
Actuarial loss					2
Net periodic benefit cost					$ 1

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

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