Derivative Financial Instruments Derivative Financial Instruments - Summary of Cash Collateral and Letters of Credit (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Collateral, Right to Reclaim Cash	$ 67	$ 511
Derivative, Collateral, Right to Reclaim Securities	344	226
Derivative, Collateral, Obligation to Return Cash	621	110
Derivative, Collateral, Obligation to Return Securities	50	40
Incremental Collateral For Loss Of Investment Grade Credit Rating	1,899	1,432
Exelon Generation Co L L C [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Collateral, Right to Reclaim Cash	67	511
Derivative, Collateral, Right to Reclaim Securities	344	226
Derivative, Collateral, Obligation to Return Cash	621	110
Derivative, Collateral, Obligation to Return Securities	50	40
Incremental Collateral For Loss Of Investment Grade Credit Rating	$ 1,899	$ 1,432