Retirement Benefits - Calculation of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended	9 Months Ended
	Sep. 30, 2016	Mar. 23, 2016	Mar. 22, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2016		Sep. 30, 2015
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense						$ 458		$ 476
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 92			$ 82		262	[1]	245
Interest cost	215			178		616	[1]	533
Expected return on assets	(293)			(257)		(847)	[1]	(770)
Amortization of:
Prior service cost (benefit)	3			3		10	[1]	10
Defined Benefit Plan, Actuarial Gain (Loss)	142			142		411	[1]	427
Net periodic benefit cost	159			148		452	[1]	445
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	27			30		80	[1]	89
Interest cost	47			42		138	[1]	125
Expected return on assets	(42)			(38)		(121)	[1]	(113)
Amortization of:
Prior service cost (benefit)	(48)			(43)		(138)	[1]	(130)
Defined Benefit Plan, Actuarial Gain (Loss)	18			20		47	[1]	60
Net periodic benefit cost	$ 2			11		6	[1]	31
Potomac Electric Power Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense			$ 7			24		22
Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense		$ 23						73
Pepco Holdings LLC [Member] | Successor [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense					$ 58
Pepco Holdings LLC [Member] | Pension Plan, Defined Benefit [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		12		15				43
Interest cost		26		28				82
Expected return on assets		(30)		(35)				(105)
Amortization of:
Prior service cost (benefit)		0		0				1
Defined Benefit Plan, Actuarial Gain (Loss)		14		16				49
Net periodic benefit cost		22		24				70
Pepco Holdings LLC [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		1		2				5
Interest cost		6		6				18
Expected return on assets		(5)		(6)				(17)
Amortization of:
Prior service cost (benefit)		(3)		(3)				(9)
Defined Benefit Plan, Actuarial Gain (Loss)		2		2				6
Net periodic benefit cost		$ 1		$ 1				3
Delmarva Power and Light Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense			4			13		11
Atlantic City Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense			3			$ 11		$ 11
Corporate, Non-Segment [Member] | Pepco Holdings LLC [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense			$ 9

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