Derivative Financial Instruments - Summary of Credit Risk Exposure (Details) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Cash Collateral Posted		$ 4
Letters Of Credit Posted		34
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Financial institutions		19
Investor-owned utilities, marketers and power producers		572
Energy Cooperatives And Municipalities		357
Other		80
Cash Collateral Posted		255	$ 497
Letters Of Credit Posted		283	$ 293
Exelon Generation Co L L C [Member] | Total Exposure Before Credit Collateral [Member]
Derivative [Line Items]
Investment grade		647
Non-investment grade		101
No external ratings - internally rated - investment grade		179
No external ratings - internally rated - non-investment grade		139
Total		1,066
Exelon Generation Co L L C [Member] | Credit Collateral [Member]
Derivative [Line Items]
Investment grade		0
Non-investment grade		20
No external ratings - internally rated - investment grade		1
No external ratings - internally rated - non-investment grade		17
Total	[1]	38
Exelon Generation Co L L C [Member] | Net Exposure [Member]
Derivative [Line Items]
Investment grade		647
Non-investment grade		81
No external ratings - internally rated - investment grade		178
No external ratings - internally rated - non-investment grade		122
Total		1,028
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		176
Non-investment grade
No external ratings - internally rated - investment grade
No external ratings - internally rated - non-investment grade
Total		$ 176

[1] As of September 30, 2018, credit collateral held from counterparties where Generation had credit exposure included $4 million of cash and $34 million of letters of credit. The credit collateral does not include non-liquid collateral.