Retirement Benefits - Allocated Portion of Pension and Postretirement Benefit Plan Costs (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
Other Pension Plan, Postretirement or Supplemental Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense		$ 161			$ 159		458	476
Corporate, Non-Segment [Member] | Other Pension Plan, Postretirement or Supplemental Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense	[1]	13			14		37	43
Pepco Holdings LLC [Member] | Other Pension Plan, Postretirement or Supplemental Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense		28				$ 58
Pepco Holdings LLC [Member] | Corporate, Non-Segment [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense				$ 9
Pepco Holdings LLC [Member] | Corporate, Non-Segment [Member] | Other Pension Plan, Postretirement or Supplemental Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense	[1],[2]	12			12		33	29
Exelon Generation Co L L C [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense							163	200
Exelon Generation Co L L C [Member] | Other Pension Plan, Postretirement or Supplemental Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense		54			67		163	200
Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense							124	155
Commonwealth Edison Co [Member] | Other Pension Plan, Postretirement or Supplemental Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense		41			52		124	155
PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense							25	29
PECO Energy Co [Member] | Other Pension Plan, Postretirement or Supplemental Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense		8			10		25	29
Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense							50	49
Baltimore Gas and Electric Company [Member] | Other Pension Plan, Postretirement or Supplemental Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense		17			16		51	49
Potomac Electric Power Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense				7			24	22
Potomac Electric Power Company [Member] | Other Pension Plan, Postretirement or Supplemental Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense	[2]	8			7		24	22
Delmarva Power and Light Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense				4			13	11
Delmarva Power and Light Company [Member] | Other Pension Plan, Postretirement or Supplemental Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense	[2]	4			3		13	11
Atlantic City Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense				$ 3			11	11
Atlantic City Electric Company [Member] | Other Pension Plan, Postretirement or Supplemental Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense	[2]	$ 4			3		$ 11	11
Predecessor [Member] | Pepco Holdings LLC [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense			$ 23					73
Predecessor [Member] | Pepco Holdings LLC [Member] | Other Pension Plan, Postretirement or Supplemental Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense			$ 23		$ 25			$ 73
Successor [Member] | Pepco Holdings LLC [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense						$ 58

[1] 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.

[2] Pepco's, DPL's, ACE's and PHISCO's pension and postretirement benefit costs for the nine months ended September 30, 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.