Derivative Financial Instruments - Summary of Credit Risk Exposure (Details) - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Cash Collateral Posted		$ 19
Letters Of Credit Posted		12
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Financial institutions		48
Investor-owned utilities, marketers and power producers		538
Energy Cooperatives And Municipalities		525
Other		146
Cash Collateral Posted		460	$ 347
Letters Of Credit Posted		255	$ 284
Exelon Generation Co L L C [Member] | Total Exposure Before Credit Collateral [Member]
Derivative [Line Items]
Investment grade		828
Non-investment grade		44
No external ratings - internally rated - investment grade		316
No external ratings - internally rated - non-investment grade		100
Total		1,288
Exelon Generation Co L L C [Member] | Credit Collateral [Member]
Derivative [Line Items]
Investment grade		9
Non-investment grade		4
No external ratings - internally rated - investment grade		0
No external ratings - internally rated - non-investment grade		18
Total	[1]	31
Exelon Generation Co L L C [Member] | Net Exposure [Member]
Derivative [Line Items]
Investment grade		819
Non-investment grade		40
No external ratings - internally rated - investment grade		316
No external ratings - internally rated - non-investment grade		82
Total		1,257
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		278
Non-investment grade
No external ratings - internally rated - investment grade
No external ratings - internally rated - non-investment grade
Total		$ 278

[1] As of September 30, 2017, credit collateral held from counterparties where Generation had credit exposure included $19 million of cash and $12 million of letters of credit. The credit collateral does not include non-liquid collateral.