Income Taxes - Accounting for Uncertainty in Income Taxes (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	$ 125	$ 507
Exelon Generation Co L L C [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	49	441
Pepco Holdings LLC [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	53	48
Atlantic City Electric Company [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	$ 16	$ 14