Derivative Financial Instruments - Summary of Credit Risk Exposure (Details) - USD ($) $ in Millions		Jun. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Cash Collateral Posted		$ 22
Letters Of Credit Posted		21
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Financial institutions		97
Investor-owned utilities, marketers and power producers		627
Energy Cooperatives And Municipalities		392
Other		60
Cash Collateral Posted		420	$ 497
Letters Of Credit Posted		424	$ 293
Exelon Generation Co L L C [Member] | Total Exposure Before Credit Collateral [Member]
Derivative [Line Items]
Investment grade		823
Non-investment grade		90
No external ratings - internally rated - investment grade		228
No external ratings - internally rated - non-investment grade		78
Total		1,219
Exelon Generation Co L L C [Member] | Credit Collateral [Member]
Derivative [Line Items]
Investment grade		0
Non-investment grade		30
No external ratings - internally rated - investment grade		0
No external ratings - internally rated - non-investment grade		13
Total	[1]	43
Exelon Generation Co L L C [Member] | Net Exposure [Member]
Derivative [Line Items]
Investment grade		823
Non-investment grade		60
No external ratings - internally rated - investment grade		228
No external ratings - internally rated - non-investment grade		65
Total		1,176
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		206
Non-investment grade
No external ratings - internally rated - investment grade
No external ratings - internally rated - non-investment grade
Total		$ 206

[1] As of June 30, 2018, credit collateral held from counterparties where Generation had credit exposure included $22 million of cash and $21 million of letters of credit. The credit collateral does not include non-liquid collateral.