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Derivative Financial Instruments - Summary of Cash Collateral and Letters of Credit (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]    
Derivative, Collateral, Right to Reclaim Cash $ 982 $ 418
Derivative, Collateral, Right to Reclaim Securities 264 367
Derivative, Collateral, Obligation to Return Cash 103 47
Derivative, Collateral, Obligation to Return Securities 112 44
Incremental Collateral For Loss Of Investment Grade Credit Rating 1,509 2,104
Exelon Generation Co L L C [Member]    
Derivative Instruments, Gain (Loss) [Line Items]    
Derivative, Collateral, Right to Reclaim Cash 982 418
Derivative, Collateral, Right to Reclaim Securities 264 367
Derivative, Collateral, Obligation to Return Cash 103 47
Derivative, Collateral, Obligation to Return Securities 112 44
Incremental Collateral For Loss Of Investment Grade Credit Rating $ 1,509 $ 2,104