Income Taxes - Allocation of Tax Benefits (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Exelon Generation Co L L C [Member]
Allocation of Tax Benefits [Line Items]
Allocation Of Federal Tax Benefit Under Tax Sharing Agreement	$ 64	$ 64
Commonwealth Edison Co [Member]
Allocation of Tax Benefits [Line Items]
Allocation Of Federal Tax Benefit Under Tax Sharing Agreement	1	14
PECO Energy Co [Member]
Allocation of Tax Benefits [Line Items]
Allocation Of Federal Tax Benefit Under Tax Sharing Agreement	19	17
Baltimore Gas and Electric Company [Member]
Allocation of Tax Benefits [Line Items]
Allocation Of Federal Tax Benefit Under Tax Sharing Agreement	0	0
Pepco Holdings LLC [Member]
Allocation of Tax Benefits [Line Items]
Allocation Of Federal Tax Benefit Under Tax Sharing Agreement	17	17
Potomac Electric Power Company [Member]
Allocation of Tax Benefits [Line Items]
Allocation Of Federal Tax Benefit Under Tax Sharing Agreement	16	8
Delmarva Power and Light Company [Member]
Allocation of Tax Benefits [Line Items]
Allocation Of Federal Tax Benefit Under Tax Sharing Agreement	0	6
Atlantic City Electric Company [Member]
Allocation of Tax Benefits [Line Items]
Allocation Of Federal Tax Benefit Under Tax Sharing Agreement	$ 0	$ 1