Derivative Financial Instruments Derivative Financial Instruments - Summary of Cash Collateral and Letters of Credit (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Collateral, Right to Reclaim Cash	$ 977	$ 982
Derivative, Collateral, Right to Reclaim Securities	256	264
Derivative, Collateral, Obligation to Return Cash	105	103
Derivative, Collateral, Obligation to Return Securities	115	112
Incremental Collateral For Loss Of Investment Grade Credit Rating	1,468	1,509
Exelon Generation Co L L C [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Collateral, Right to Reclaim Cash	977	982
Derivative, Collateral, Right to Reclaim Securities	256	264
Derivative, Collateral, Obligation to Return Cash	105	103
Derivative, Collateral, Obligation to Return Securities	115	112
Incremental Collateral For Loss Of Investment Grade Credit Rating	$ 1,468	$ 1,509