Celgene Collaboration Agreement - Narrative (Details) $ in Thousands		1 Months Ended	12 Months Ended
	Feb. 01, 2017 shares	Jul. 31, 2016 USD ($) extension deliverable customer_program shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) shares
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue—related party			$ 65,201	$ 71,644
Deferred revenue			97,872	$ 163,073
Cumulative catch-up adjustment from changes in costs estimated to be incurred			7,100
Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of early-stage programs | customer_program		4
Non-refundable upfront payment received for research agreement		$ 225,000
Aggregate option-exercise fees from program options		$ 182,500
Number of program options | deliverable		6
Initial research term		4 years
Potential addition to research term		3 years
Aggregate consideration for additional years of research		$ 120,000
Potential milestone revenue		2,300,000
Aggregate development milestone revenue		290,000
Aggregate regulatory approval milestone revenue		700,000
Aggregate commercial milestone revenue		$ 1,270,000
Number of program extensions | extension		3
Program extension research term		1 year
Series B-1 Convertible Preferred Stock
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Issuances of convertible preferred stock (in shares) | shares				10,448,100
Series B-1 Convertible Preferred Stock | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Issuances of convertible preferred stock		$ 36,100
Issuances of convertible preferred stock (in shares) | shares		10,448,100
Minimum | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Option-exercise fee for program option		$ 10,000
Consideration for additional year of research		30,000
Development milestone revenue, per program		32,500
Regulatory approval milestone revenue, per program		7,500
Commercial milestone revenue, per program		$ 40,000
Program option term		45 days
Maximum | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Option-exercise fee for program option		$ 60,000
Consideration for additional year of research		45,000
Development milestone revenue, per program		105,000
Regulatory approval milestone revenue, per program		50,000
Commercial milestone revenue, per program		$ 200,000
Program option term		60 days
JTX-2011 | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Percent of operating profit or loss retained		60.00%
JTX-2011 | Celgene Corporation | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Percent of operating profit or loss retained		40.00%
Lead Program | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Percent of operating profit or loss retained		25.00%
Lead Program | Celgene Corporation | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Percent of operating profit or loss retained		75.00%
Other Programs | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Percent of operating profit or loss retained		50.00%
Other Programs | Celgene Corporation | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Percent of operating profit or loss retained		50.00%
Other Programs | Celgene Corporation | Maximum | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of program options | deliverable		3
All Programs, Excluding JTX-4014 | Celgene Corporation | Minimum | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Percent of development costs		67.00%
All Programs, Excluding JTX-4014 | Celgene Corporation | Maximum | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Percent of development costs		85.00%
IPO
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Stock converted upon completion of IPO (in shares) | shares	22,283,690
Celgene Corporation | Celgene Collaboration Agreement | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Non-refundable upfront payment received for research agreement		$ 225,000
Celgene Corporation | Celgene Collaboration Agreement | Series B-1 Convertible Preferred Stock | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Issuances of convertible preferred stock		$ 36,100
Issuances of convertible preferred stock (in shares) | shares		10,448,100
Celgene Corporation | Celgene Collaboration Agreement | IPO | Common Stock
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Stock converted upon completion of IPO (in shares) | shares	2,831,463
Accounting Standards Update 2014-09 | Calculated under Revenue Guidance in Effect before Topic 606
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue—related party			$ 51,142	$ 71,600
Deferred revenue				$ 116,200