Retirement Benefits - Calculation of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Mar. 23, 2016	Jun. 30, 2017	[1]	Jun. 30, 2016	[2]
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 97	$ 91		$ 191		$ 170
Interest cost	211	212		422		403
Expected return on assets	(299)	(292)		(598)		(555)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement	2	0		2		0
Amortization of:
Prior service cost (benefit)	1	4		1		7
Actuarial loss	150	142		302		269
Net periodic benefit cost	162	157		320		294
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	28	28		54		54
Interest cost	46	48		91		90
Expected return on assets	(41)	(42)		(82)		(80)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement	0	0		0		0
Amortization of:
Prior service cost (benefit)	(47)	(47)		(94)		(91)
Actuarial loss	15	16		31		30
Net periodic benefit cost	$ 1	$ 3		$ 0		$ 3
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 cost (benefit)			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 cost (benefit)			(3)
Actuarial loss			2
Net periodic benefit cost			$ 1

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

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