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Derivative Financial Instruments Derivative Financial Instruments - Summary of Cash Collateral and Letters of Credit (Details) - USD ($)
$ in Millions
Mar. 31, 2021
Dec. 31, 2020
Derivative Instruments, Gain (Loss) [Line Items]    
Derivative, Collateral, Right to Reclaim Cash $ 232 $ 511
Derivative, Collateral, Right to Reclaim Securities 242 226
Derivative, Collateral, Obligation to Return Cash 101 110
Derivative, Collateral, Obligation to Return Securities 41 40
Incremental Collateral For Loss Of Investment Grade Credit Rating 1,379 1,432
Exelon Generation Co L L C [Member]    
Derivative Instruments, Gain (Loss) [Line Items]    
Derivative, Collateral, Right to Reclaim Cash 232 511
Derivative, Collateral, Right to Reclaim Securities 242 226
Derivative, Collateral, Obligation to Return Cash 101 110
Derivative, Collateral, Obligation to Return Securities 41 40
Incremental Collateral For Loss Of Investment Grade Credit Rating $ 1,379 $ 1,432