Retirement Benefits - Allocated Portion of Pension and Postretirement Benefit Plan Costs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs			$ 304	$ 310
Pension and other post retirement benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs	$ 109	$ 107	304	310
Exelon Generation Co L L C [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs			92	89
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	36	30	92	89
Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs			97	85
Commonwealth Edison Co [Member] | Pension and other post retirement benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs	32	29	97	85
PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs			5	4
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	5	4
Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs			45	46
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	47	47
Pepco Holdings LLC [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs			36	52
Pepco Holdings LLC [Member] | Pension and other post retirement benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs	12	17	36	52
Potomac Electric Power Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs			5	11
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	5	11
Delmarva Power and Light Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs			2	6
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	2	6
Atlantic City Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Costs			8	10
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	$ 8	$ 10