Retirement Benefits - Calculation of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions			3 Months Ended
		Mar. 31, 2016	Mar. 31, 2017		Mar. 31, 2016		Mar. 23, 2016
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense			$ 157		$ 136
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost			95	[1]	78
Interest cost			210	[1]	190
Expected return on assets			(299)	[1]	(263)
Amortization of:
Prior service cost (benefit)			0	[1]	3
Defined Benefit Plan, Actuarial Gain (Loss)			152	[1]	127
Net periodic benefit cost			158	[1]	135
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost			26	[1]	26
Interest cost			45	[1]	43
Expected return on assets			(41)	[1]	(38)
Amortization of:
Prior service cost (benefit)			(47)	[1]	(44)
Defined Benefit Plan, Actuarial Gain (Loss)			16	[1]	14
Net periodic benefit cost			(1)	[1]	1
Potomac Electric Power Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense			7	[2]	8	[2]	$ 7
Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense							23
Pepco Holdings LLC [Member] | Successor [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense		$ 3	24
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
Defined Benefit Plan, Actuarial Gain (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)
Defined Benefit Plan, Actuarial Gain (Loss)							2
Net periodic benefit cost							1
Delmarva Power and Light Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense			3	[2]	5	[2]	4
Atlantic City Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense			3	[2]	4	[2]	3
Corporate, Non-Segment [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense	[3]		12		14
Corporate, Non-Segment [Member] | Pepco Holdings LLC [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense			$ 11	[2],[3]	$ 9	[2],[3]	$ 9

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

[2] Pepco's, DPL's, ACE's and PHISCO's pension and postretirement benefit costs for the three months ended March 31, 2016 include $7 million, $4 million, $3 million and $9 million, respectively, of costs incurred prior to the closing of Exelon’s merger with PHI on March 23, 2016.

[3] These amounts primarily represent amounts billed to Exelon’s subsidiaries through intercompany allocations. These amounts are not included in the Generation, ComEd, PECO, BGE, PHI, Pepco, DPL or ACE amounts above.