Derivative Financial Instruments Derivative Financial Instruments - Summary of Cash Collateral and Letters of Credit (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Cash Collateral Posted	$ 25
Letters Of Credit Posted	9
Exelon Generation Co L L C [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Cash Collateral Posted	781	$ 418
Letters Of Credit Posted	228	367
Cash Collateral Held	64	47
Letters Of Credit Held	21	44
Incremental Collateral For Loss Of Investment Grade Credit Rating	$ 1,513	$ 2,104