Derivative Financial Instruments - Summary of Credit Risk Exposure (Details) $ in Millions	Dec. 31, 2019 USD ($)
Derivative [Line Items]
Cash collateral held	$ 25
Letters of credit held	81
Total Exposure Before Credit Collateral [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Investment grade	877
Non-investment grade	79
Internally rated — investment grade	218
Internally rated — non-investment grade	139
Total	1,313
Credit Collateral [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Investment grade	20
Non-investment grade	63
Internally rated — investment grade	0
Internally rated — non-investment grade	23
Total	106
Net Exposure [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Investment grade	857
Non-investment grade	16
Internally rated — investment grade	218
Internally rated — non-investment grade	116
Total	1,207
Financial institutions	9
Investor-owned utilities, marketers, power producers	930
Energy cooperatives and municipalities	235
Other	33
Total	1,207
Number Of Counterparties Greater Than Ten Percent Of Net Exposure [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Investment grade	0
Total	0
Net Exposure Of Counterparties Greater Than Ten Percent Of Net Exposure [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Investment grade	0
Total	$ 0