License and Collaboration Agreements - Additional Information (Details)		1 Months Ended			3 Months Ended		6 Months Ended		12 Months Ended			14 Months Ended	35 Months Ended
	Apr. 30, 2019 USD ($) shares	Oct. 31, 2017 USD ($)	Sep. 30, 2017 USD ($) Program	Aug. 31, 2017 USD ($)	Jun. 30, 2020 USD ($) shares	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) shares	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2020 USD ($) shares	Jun. 30, 2020 USD ($) shares
License And Collaboration Agreements [Line Items]
Revenue recognized					$ 1,750,000	$ 1,750,000	$ 3,500,000	$ 3,500,000
Development expense					35,693,000	24,999,000	58,835,000	40,553,000
Compensation expense recognized					4,470,000	2,130,000	7,932,000	3,804,000
Research and Development Expenses
License And Collaboration Agreements [Line Items]
Compensation expense recognized					2,368,000	868,000	$ 4,104,000	1,715,000
Taiho Pharmaceutical Co., Ltd | Taiho Agreement
License And Collaboration Agreements [Line Items]
Option period			5 years
Non refundable and non creditable cash payments			$ 35,000,000.0
Payment received for license agreement		$ 5,000,000.0							$ 5,000,000.0	$ 5,000,000.0	$ 25,000,000.0
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			$ 20,000,000.0
Non-refundable, non-creditable upfront cash payments							$ 35,000,000.0
Estimated performance period							5 years
Payment for first option exercise									8,000,000.0	$ 3,000,000.0
Licensing revenue recognized									8,000,000.0
Clinical and regulatory milestones achieved							$ 0
Sales milestone or royalty revenue recognized							0
Revenue recognized					1,800,000	1,800,000	3,500,000	3,500,000
Deferred revenue, current					7,000,000.0		7,000,000.0					$ 7,000,000.0	$ 7,000,000.0
Deferred revenue, noncurrent					8,500,000		8,500,000					8,500,000	8,500,000
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			$ 3,000,000.0
Taiho Pharmaceutical Co., Ltd | Taiho Agreement | Maximum
License And Collaboration Agreements [Line Items]
Extended option agreement period			7 years
Payment for option exercise			$ 15,000,000.0
Additional clinical and regulatory milestone payments receivable			130,000,000.0
Contingent payments receivable			$ 145,000,000.0
WuXi Biologics License Agreement
License And Collaboration Agreements [Line Items]
Development milestone payable					5,000,000.0		$ 5,000,000.0					5,000,000.0	5,000,000.0
Range of tiered royalty payments on net sales							high single-digits to low teens
Development milestone expense					5,000,000.0		$ 5,000,000.0
WuXi Biologics License Agreement | Research and Development Expenses
License And Collaboration Agreements [Line Items]
Sub-license fees incurred													1,200,000
Upfront and milestone payments													$ 26,000,000.0
WuXi Biologics License Agreement | Maximum
License And Collaboration Agreements [Line Items]
Clinical, regulatory and commercialization milestone payments				$ 375,000,000.0
Abmuno License Agreement
License And Collaboration Agreements [Line Items]
Upfront and milestone payments					0	$ 0	0	$ 0	$ 6,600,000
Abmuno License Agreement | Maximum
License And Collaboration Agreements [Line Items]
Clinical, regulatory and commercialization remaining milestone payments							101,000,000.0
Genentech
License And Collaboration Agreements [Line Items]
Development expense					0		0
Strata Oncology Inc | Strata Agreement
License And Collaboration Agreements [Line Items]
Development milestone payable	$ 2,500,000
Development cost recorded within research and development expenses					600,000		1,000,000.0					2,000,000.0
Development cost reimbursed					$ 100,000		$ 200,000					$ 400,000
Number of restricted shares of common stock issued | shares	1,257,651
Fair value of restricted shares of common stock issued	$ 15,000,000.0
Strata Oncology Inc | Strata Agreement | Non-vested Restricted Stock
License And Collaboration Agreements [Line Items]
Shares probable of vesting | shares					0		0					0	0
Compensation expense recognized							$ 0
Strata Oncology Inc | Strata Agreement | Research and Development Expenses
License And Collaboration Agreements [Line Items]
Milestone payments												$ 2,500,000
Strata Oncology Inc | Strata Agreement | Maximum
License And Collaboration Agreements [Line Items]
Regulatory and commercial milestone payable	$ 125,000,000.0