Derivative Financial Instruments - Summary of Credit Risk Exposure (Details) - USD ($) $ in Millions		Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Cash Collateral Posted		$ 37
Letters Of Credit Posted		19
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Financial institutions		13
Investor-owned utilities, marketers and power producers		762
Energy Cooperatives And Municipalities		287
Other		35
Cash Collateral Posted		542	$ 418
Letters Of Credit Posted		289	$ 367
Exelon Generation Co L L C [Member] | Total Exposure Before Credit Collateral [Member]
Derivative [Line Items]
Investment grade		819
Non-investment grade		86
No external ratings - internally rated - investment grade		162
No external ratings - internally rated - non-investment grade		87
Total		1,154
Exelon Generation Co L L C [Member] | Credit Collateral [Member]
Derivative [Line Items]
Investment grade		11
Non-investment grade		39
No external ratings - internally rated - investment grade		0
No external ratings - internally rated - non-investment grade		7
Total	[1]	57
Exelon Generation Co L L C [Member] | Net Exposure [Member]
Derivative [Line Items]
Investment grade		808
Non-investment grade		47
No external ratings - internally rated - investment grade		162
No external ratings - internally rated - non-investment grade		80
Total		1,097
Exelon Generation Co L L C [Member] | Number Of Counterparties Greater Than Ten Percent Of Net Exposure [Member]
Derivative [Line Items]
Investment grade		1
Non-investment grade
No external ratings - internally rated - investment grade
No external ratings - internally rated - non-investment grade
Total		1
Exelon Generation Co L L C [Member] | Net Exposure Of Counterparties Greater Than Ten Percent Of Net Exposure [Member]
Derivative [Line Items]
Investment grade		135
Non-investment grade
No external ratings - internally rated - investment grade
No external ratings - internally rated - non-investment grade
Total		$ 135

[1] As of March 31, 2019, credit collateral held from counterparties where Generation had credit exposure included $37 million of cash and $19 million of letters of credit. The credit collateral does not include non-liquid collateral.