License and Collaboration Agreements - Additional Information (Details)		1 Months Ended			12 Months Ended
	Apr. 30, 2019 USD ($) shares	Oct. 31, 2017 USD ($)	Sep. 30, 2017 USD ($) Program	Aug. 31, 2017 USD ($)	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
License And Collaboration Agreements [Line Items]
Revenue recognized					$ 7,000,000
Deferred revenue, current					7,000,000	$ 6,250,000
Deferred revenue, noncurrent					12,022,000	16,984,000
Research and development					$ 78,481,000	49,646,000
Shares probable of vesting | shares					4,738,004
Taiho Pharmaceutical Co Ltd | Taiho Agreement
License And Collaboration Agreements [Line Items]
Option period			5 years
Non refundable and non creditable cash payments							$ 35,000,000
Payment received for license agreement		$ 5,000,000			$ 5,000,000	$ 5,000,000	$ 25,000,000
Range of royalties receivable on net sales					high single-digits to mid-teens
Royalties payable description					Royalties will be payable on a licensed product-by-licensed product and country-by-country basis during the period of time commencing on the first commercial sale of a licensed product in a country and ending upon the later of: (a) ten (10) years from the date of first commercial sale of such licensed product in such country; and (b) expiration of the last-to-expire valid claim of the Company’s patents covering the manufacture, use or sale or exploitation of such licensed product in such country (the Royalty Term)
Upfront cash payment			$ 20,000,000
Non-refundable, non-creditable upfront cash payments					$ 35,000,000
Estimated performance period					5 years	4 years	5 years
Payment for first option exercise					$ 8,000,000	$ 3,000,000
Licensing revenue recognized					8,000,000
Clinical and regulatory milestones achieved					0
Sales milestone or royalty revenue recognized					0
Revenue recognized					15,000,000	8,300,000
Deferred revenue, current					7,000,000
Deferred revenue, noncurrent					$ 12,000,000
Taiho Pharmaceutical Co Ltd | Taiho Agreement | Minimum
License And Collaboration Agreements [Line Items]
Number of programs, IND enabling studies not initiated | Program			5
Payment for option exercise			$ 3,000,000
Taiho Pharmaceutical Co Ltd | Taiho Agreement | Maximum
License And Collaboration Agreements [Line Items]
Extended option agreement period			7 years
Payment for option exercise			$ 15,000,000
Additional clinical and regulatory milestone payments receivable			130,000,000
Contingent payments receivable			$ 145,000,000
WuXi Biologics License Agreement
License And Collaboration Agreements [Line Items]
Range of tiered royalty payments on net sales					high single-digits to low teens
WuXi Biologics License Agreement | Research and Development Expenses
License And Collaboration Agreements [Line Items]
Milestone payments					$ 7,500,000	0
Sub-license fees					1,200,000
WuXi Biologics License Agreement | Maximum
License And Collaboration Agreements [Line Items]
Clinical, regulatory and commercialization milestone payments				$ 375,000,000
Abmuno License Agreement
License And Collaboration Agreements [Line Items]
Milestone payments					0
Abmuno License Agreement | Research and Development Expenses
License And Collaboration Agreements [Line Items]
Milestone payments						$ 2,800,000
Abmuno License Agreement | Maximum
License And Collaboration Agreements [Line Items]
Clinical, regulatory and commercialization remaining milestone payments					101,000,000
Genentech
License And Collaboration Agreements [Line Items]
Research and development					0
Strata Oncology Inc | Co-Development and Collaboration Agreement
License And Collaboration Agreements [Line Items]
Development milestone payable	$ 2,500,000
Development cost recorded within research and development expenses					1,000,000
Development cost reimbursed					$ 200,000
Number of restricted shares of common stock issued | shares	1,257,651
Fair value of restricted shares of common stock issued	$ 15,000,000
Strata Oncology Inc | Co-Development and Collaboration Agreement | Non-vested Restricted Stock
License And Collaboration Agreements [Line Items]
Shares probable of vesting | shares					0
Compensation expense recognized					$ 0
Strata Oncology Inc | Co-Development and Collaboration Agreement | Research and Development Expenses
License And Collaboration Agreements [Line Items]
Milestone payments					$ 2,500,000
Strata Oncology Inc | Co-Development and Collaboration Agreement | Maximum
License And Collaboration Agreements [Line Items]
Regulatory and commercial milestone payable	$ 125,000,000