Retirement Benefits - Allocated Portion of Pension and Postretirement Benefit Plan Costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs			$ 196	$ 203
Pension and other post retirement benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs	$ 101	$ 105	196	203
Exelon Generation Co L L C [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs			56	58
Exelon Generation Co L L C [Member] | Pension and other post retirement benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs	30	32	56	59
Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs			64	57
Commonwealth Edison Co [Member] | Pension and other post retirement benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs	32	28	64	57
PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs			4	3
PECO Energy Co [Member] | Pension and other post retirement benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs	2	1	4	3
Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs			29	31
Baltimore Gas and Electric Company [Member] | Pension and other post retirement benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs	16	16	31	31
Pepco Holdings LLC [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs			24	35
Pepco Holdings LLC [Member] | Pension and other post retirement benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs	12	18	24	35
Potomac Electric Power Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs			3	7
Potomac Electric Power Company [Member] | Pension and other post retirement benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs	2	4	3	7
Delmarva Power and Light Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs			1	4
Delmarva Power and Light Company [Member] | Pension and other post retirement benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs	1	2	1	4
Atlantic City Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs			5	7
Atlantic City Electric Company [Member] | Pension and other post retirement benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs	$ 3	$ 3	$ 5	$ 7