Retirement Benefits - Defined Contribution Savings Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Savings Plans Matching Contributions	$ 33	$ 33
Exelon Generation Co L L C [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plans Matching Contributions	13	13
Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plans Matching Contributions	8	7
PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plans Matching Contributions	3	3
Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plans Matching Contributions	2	2
Pepco Holdings LLC [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plans Matching Contributions	3	3
Potomac Electric Power Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plans Matching Contributions	1	1
Delmarva Power and Light Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plans Matching Contributions	$ 1	$ 1