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Derivative Financial Instruments Derivative Financial Instruments - Summary of Cash Collateral and Letters of Credit (Details) - USD ($)
$ in Millions
Sep. 30, 2020
Dec. 31, 2019
Derivative Instruments, Gain (Loss) [Line Items]    
Derivative, Collateral, Right to Reclaim Cash $ 288 $ 982
Derivative, Collateral, Right to Reclaim Securities 212 264
Derivative, Collateral, Obligation to Return Cash 68 103
Derivative, Collateral, Obligation to Return Securities 74 112
Incremental Collateral For Loss Of Investment Grade Credit Rating 1,287 1,509
Exelon Generation Co L L C [Member]    
Derivative Instruments, Gain (Loss) [Line Items]    
Derivative, Collateral, Right to Reclaim Cash 288 982
Derivative, Collateral, Right to Reclaim Securities 212 264
Derivative, Collateral, Obligation to Return Cash 68 103
Derivative, Collateral, Obligation to Return Securities 74 112
Incremental Collateral For Loss Of Investment Grade Credit Rating $ 1,287 $ 1,509