Derivative Financial Instruments - Summary of Credit Risk Exposure (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Cash collateral held		$ 17
Letters of credit held		35
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Cash collateral held		47	$ 35
Letters of credit held		44	$ 33
Total Exposure Before Credit Collateral [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Investment grade		795
Non-investment grade		133
Internally rated — investment grade		181
Internally rated — non-investment grade		92
Total		1,201
Credit Collateral [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Investment grade		0
Non-investment grade		45
Internally rated — investment grade		1
Internally rated — non-investment grade		6
Total	[1]	52
Net Exposure [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Investment grade		795
Non-investment grade		88
Internally rated — investment grade		180
Internally rated — non-investment grade		86
Total		1,149
Number Of Counterparties Greater Than Ten Percent Of Net Exposure [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Investment grade		1
Non-investment grade		0
Internally rated — investment grade		0
Internally rated — non-investment grade		0
Total		1
Net Exposure Of Counterparties Greater Than Ten Percent Of Net Exposure [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Investment grade		153
Non-investment grade		0
Internally rated — investment grade		0
Internally rated — non-investment grade		0
Total		$ 153

[1] As of December 31, 2018, credit collateral held from counterparties where Generation had credit exposure included $17 million of cash and $35 million of letters of credit. The credit collateral does not include non-liquid collateral.