Income Taxes - Allocation of Tax Benefits (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Exelon Generation Co L L C [Member]
Income Taxes [Line Items]
Allocation Of Federal Tax Benefit Under Tax Sharing Agreement	$ 41	$ 155	$ 102
Commonwealth Edison Co [Member]
Income Taxes [Line Items]
Allocation Of Federal Tax Benefit Under Tax Sharing Agreement	0	1	0
PECO Energy Co [Member]
Income Taxes [Line Items]
Allocation Of Federal Tax Benefit Under Tax Sharing Agreement	14	48	16
Baltimore Gas and Electric Company [Member]
Income Taxes [Line Items]
Allocation Of Federal Tax Benefit Under Tax Sharing Agreement	3	26	10
Pepco Holdings LLC [Member]
Income Taxes [Line Items]
Allocation Of Federal Tax Benefit Under Tax Sharing Agreement	7	2	7
Potomac Electric Power Company [Member]
Income Taxes [Line Items]
Allocation Of Federal Tax Benefit Under Tax Sharing Agreement	6	0	0
Delmarva Power and Light Company [Member]
Income Taxes [Line Items]
Allocation Of Federal Tax Benefit Under Tax Sharing Agreement	1	0	0
Atlantic City Electric Company [Member]
Income Taxes [Line Items]
Allocation Of Federal Tax Benefit Under Tax Sharing Agreement	$ 0	$ 0	$ 0