Derivative Financial Instruments - Summary of Credit Risk Exposure (Details) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Cash Collateral Posted		$ 41
Letters Of Credit Posted		23
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Financial institutions		189
Investor-owned utilities, marketers and power producers		656
Energy Cooperatives And Municipalities		438
Other		74
Cash Collateral Posted		742	$ 497
Letters Of Credit Posted		493	$ 293
Exelon Generation Co L L C [Member] | Total Exposure Before Credit Collateral [Member]
Derivative [Line Items]
Investment grade		986
Non-investment grade		112
No external ratings - internally rated - investment grade		223
No external ratings - internally rated - non-investment grade		100
Total		1,421
Exelon Generation Co L L C [Member] | Credit Collateral [Member]
Derivative [Line Items]
Investment grade		1
Non-investment grade		46
No external ratings - internally rated - investment grade		0
No external ratings - internally rated - non-investment grade		17
Total	[1]	64
Exelon Generation Co L L C [Member] | Net Exposure [Member]
Derivative [Line Items]
Investment grade		985
Non-investment grade		66
No external ratings - internally rated - investment grade		223
No external ratings - internally rated - non-investment grade		83
Total		1,357
Exelon Generation Co L L C [Member] | Number Of Counterparties Greater Than Ten Percent Of Net Exposure [Member]
Derivative [Line Items]
Investment grade		2
Non-investment grade
No external ratings - internally rated - investment grade
No external ratings - internally rated - non-investment grade
Total		2
Exelon Generation Co L L C [Member] | Net Exposure Of Counterparties Greater Than Ten Percent Of Net Exposure [Member]
Derivative [Line Items]
Investment grade		412
Non-investment grade
No external ratings - internally rated - investment grade
No external ratings - internally rated - non-investment grade
Total		$ 412

[1] As of March 31, 2018, credit collateral held from counterparties where Generation had credit exposure included $41 million of cash and $23 million of letters of credit. The credit collateral does not include non-liquid collateral.