Derivative Financial Instruments Derivative Financial Instruments - Summary of Cash Collateral and Letters of Credit (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Collateral, Right to Reclaim Cash	$ 299	$ 511
Derivative, Collateral, Right to Reclaim Securities	477	226
Derivative, Collateral, Obligation to Return Cash	1,872	110
Derivative, Collateral, Obligation to Return Securities	130	40
Incremental Collateral For Loss Of Investment Grade Credit Rating	3,001	1,432
Exelon Generation Co L L C [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Collateral, Right to Reclaim Cash	299	511
Derivative, Collateral, Right to Reclaim Securities	477	226
Derivative, Collateral, Obligation to Return Cash	1,872	110
Derivative, Collateral, Obligation to Return Securities	130	40
Incremental Collateral For Loss Of Investment Grade Credit Rating	$ 3,001	$ 1,432

The aggregate fair value of additional assets that would be required to be posted as collateral for derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period.

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Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets.

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