Retirement Benefits Retirement Benefits - Contributions made to Pension and Other Postretirement Benefit Plans (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended	12 Months Ended
		Mar. 23, 2016		Dec. 31, 2016	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					$ 383		$ 405		$ 397
Expected qualified pension plan contributions					301
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months					25
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year					44
Corporate, Non-Segment [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected qualified pension plan contributions					41
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months					7
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year					2
Constellation Legacy Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	[1]				0		21		25
Exelon Generation Co L L C [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					139		148		152
Expected qualified pension plan contributions					135
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months					7
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year					13
Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					42		41		38
Expected qualified pension plan contributions					65
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months					1
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year					2
PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					28		24		30
Expected qualified pension plan contributions					25
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months					1
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year					0
Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					54		53		49
Expected qualified pension plan contributions					34
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months					1
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year					15
Pepco Holdings LLC [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					(74)		(99)
Expected qualified pension plan contributions					1
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months					8
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year					12
Pepco Holdings LLC [Member] | Corporate, Non-Segment [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected qualified pension plan contributions					1
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months					5
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year					1
Pepco Holdings LLC [Member] | Successor [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions				$ (86)
Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions		$ (4)
Potomac Electric Power Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					17		72		32
Expected qualified pension plan contributions					0
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months					2
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year					10
Delmarva Power & Light Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					0		2		22
Expected qualified pension plan contributions					0
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months					1
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year					0
Atlantic City Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					6		20		17
Expected qualified pension plan contributions					0
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months					0
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year					1
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					337	[2]	341	[2]	347
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months					1,196
Pension Plan, Defined Benefit [Member] | Corporate, Non-Segment [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	[1],[2]				41		38		39
Pension Plan, Defined Benefit [Member] | Exelon Generation Co L L C [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	[2]				128		137		140
Pension Plan, Defined Benefit [Member] | Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	[2]				38		36		33
Pension Plan, Defined Benefit [Member] | PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	[2]				28		24		30
Pension Plan, Defined Benefit [Member] | Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	[2]				40		39		31
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions				67	62
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Corporate, Non-Segment [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions		4	[1]		50	[2],[3]	5	[2],[3]	17	[2],[3]
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Successor [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions		74
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions							4
Pension Plan, Defined Benefit [Member] | Exelon Legacy Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					2	[1]	4		6
Pension Plan, Defined Benefit [Member] | Potomac Electric Power Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	[2]				6		62		24
Pension Plan, Defined Benefit [Member] | Delmarva Power & Light Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	[2]				0		0		22
Pension Plan, Defined Benefit [Member] | Atlantic City Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	[2]				6		0		15
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					46		64		50
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months					255
Other Postretirement Benefit Plan, Defined Benefit [Member] | Corporate, Non-Segment [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	[1]				5		2		3
Other Postretirement Benefit Plan, Defined Benefit [Member] | Exelon Generation Co L L C [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					11		11		12
Other Postretirement Benefit Plan, Defined Benefit [Member] | Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					4		5		5
Other Postretirement Benefit Plan, Defined Benefit [Member] | PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					0		0		0
Other Postretirement Benefit Plan, Defined Benefit [Member] | Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					14		14		18
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions				$ 32	12
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Corporate, Non-Segment [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	[3]				1		0		2
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Successor [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions		$ 12
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions							0
Other Postretirement Benefit Plan, Defined Benefit [Member] | Potomac Electric Power Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					11		10		8
Other Postretirement Benefit Plan, Defined Benefit [Member] | Delmarva Power & Light Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					0		2		0
Other Postretirement Benefit Plan, Defined Benefit [Member] | Atlantic City Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					$ 0		$ 20		$ 2

[1] )Includes $2 million, $4 million, and $6 million of pension contributions funded by Exelon Corporate, for the years ended December 31, 2018, 2017, and 2016, respectively.

[2] Exelon's and Generation's pension contributions include $21 million and $25 million related to the legacy CENG plans that was funded by CENG as provided in an Employee Matters Agreement (EMA) between Exelon and CENG for the years ended December 31, 2017 and 2016, respectively

[3] PHISCO’s pension contributions for the year ended December 31, 2016 include $4 million of contributions made prior to the closing of Exelon’s merger with PHI on March 23, 2016