Derivative Financial Instruments - Summary of Credit Risk Exposure (Details) $ in Millions	Sep. 30, 2020 USD ($)
Derivative [Line Items]
Cash Collateral Held	$ 31
Letters Of Credit Held	26
Exelon Generation Co L L C [Member] | Total Exposure Before Credit Collateral [Member]
Derivative [Line Items]
Investment grade	638
Non-investment grade	4
Internally rated - investment grade	168
Internally rated - non-investment grade	110
Total	920
Exelon Generation Co L L C [Member] | Credit Collateral [Member]
Derivative [Line Items]
Investment grade	27
Non-investment grade	0
Internally rated - investment grade	1
Internally rated - non-investment grade	29
Total	57
Exelon Generation Co L L C [Member] | Net Exposure [Member]
Derivative [Line Items]
Investment grade	611
Non-investment grade	4
Internally rated - investment grade	167
Internally rated - non-investment grade	81
Total	863
Financial institutions	26
Investor-owned utilities, marketers and power producers	650
Energy Cooperatives And Municipalities	142
Other	45
Exelon Generation Co L L C [Member] | Number Of Counterparties Greater Than Ten Percent Of Net Exposure [Member]
Derivative [Line Items]
Investment grade	0
Total	0
Exelon Generation Co L L C [Member] | Net Exposure Of Counterparties Greater Than Ten Percent Of Net Exposure [Member]
Derivative [Line Items]
Investment grade	0
Total	$ 0