License and Collaboration Agreements - Additional Information (Details)		1 Months Ended			3 Months Ended			11 Months Ended	12 Months Ended			32 Months Ended
	Apr. 30, 2019 USD ($) shares	Oct. 31, 2017 USD ($)	Sep. 30, 2017 USD ($) Program	Aug. 31, 2017 USD ($)	Mar. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2020 USD ($) shares
License And Collaboration Agreements [Line Items]
Revenue recognized					$ 1,750,000		$ 1,750,000
Development expense					23,142,000		15,554,000
Compensation expense recognized					3,462,000		1,674,000
Research and Development Expenses
License And Collaboration Agreements [Line Items]
Compensation expense recognized					$ 1,736,000		847,000
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
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
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					1,800,000		$ 1,800,000
Deferred revenue, current					7,000,000			$ 7,000,000				$ 7,000,000
Deferred revenue, noncurrent					$ 10,300,000			10,300,000				10,300,000
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
Development expense					$ 0
WuXi Biologics License Agreement | Research and Development Expenses
License And Collaboration Agreements [Line Items]
Upfront and milestone payments												$ 26,000,000
Sub-license fees incurred						$ 1,200,000
Sub-license fees paid					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]
Upfront and milestone payments									$ 6,600,000
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]
Development expense					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					400,000			1,400,000
Development cost reimbursed					$ 100,000			$ 300,000
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			0				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