License and Collaboration Agreements - Additional Information (Details)			1 Months Ended			3 Months Ended			9 Months Ended		12 Months Ended				17 Months Ended	38 Months Ended
	Jul. 13, 2020 USD ($) $ / shares shares	Apr. 30, 2019 USD ($) shares	Oct. 31, 2017 USD ($)	Sep. 30, 2017 USD ($) Program	Aug. 31, 2017 USD ($)	Sep. 30, 2020 USD ($) shares	Jun. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)		Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2020 USD ($) shares	Sep. 30, 2020 USD ($) shares
License And Collaboration Agreements [Line Items]
Upfront cash payment									$ 175,000,000
Issuance of common stock							$ 326,246,000
Cost sharing receivable, current						$ 2,579,000			2,579,000		$ 132,000	[1]			$ 2,579,000	$ 2,579,000
Development expense						51,801,000		$ 17,241,000	110,636,000	$ 57,795,000
Compensation expense recognized						6,002,000		2,740,000	13,934,000	6,544,000
Research and Development Expenses
License And Collaboration Agreements [Line Items]
Compensation expense recognized						3,075,000		$ 978,000	7,179,000	$ 2,693,000
Stock Purchase Agreement
License And Collaboration Agreements [Line Items]
Direct offering cost	$ 1,900,000
Development And Commercialization Services
License And Collaboration Agreements [Line Items]
Reimbursed under cost-sharing arrangement						500,000
Gilead | Gilead Collaboration Agreement
License And Collaboration Agreements [Line Items]
Upfront cash payment	175,000,000.0								175,000,000
Unconstrained consideration to be received						100,000,000			100,000,000						100,000,000	100,000,000
Premium on stock purchased	$ 90,600,000
Present value of unconstrained consideration									99,100,000
Collaboration term for current and future clinical programs	10 years
Contingent milestone payments receivable	$ 400,000,000.0
Additional collaboration term for programs entering clinical development prior to end of collaboration term	3 years
Option fee per program for all other programs entering clinical development to exercise option	$ 150,000,000
Ongoing research and development support	400,000,000
Present value of research and development information access rights payment related to year 2022	100,000,000.0
Initial transaction price	364,700,000
Option payment upon achievement of certain development milestones	250,000,000.0
Option payment upon achievement of certain development milestones	$ 275,000,000.0
Current and future programs exclusive access period	10 years
Additional payment due at each of the fourth, sixth, and eighth anniversaries of the agreement	$ 100,000,000
Contingent milestone payment start period	2022
Contractual obligation for first payment	$ 100,000,000.0
Performance obligation period	4 years
Contractual obligation remaining amount not obligated to pay	$ 300,000,000.0
Consultant and legal fees	7,300,000
Amortization expense						1,100,000			1,100,000
Cost sharing receivable						4,300,000			4,300,000						4,300,000	4,300,000
Future development and promotion costs contract liability	9,200,000
Sub-license fees incurred	10,100,000
Gilead | Gilead Collaboration Agreement | Other Current Liabilities
License And Collaboration Agreements [Line Items]
Future development and promotion costs contract liability current						1,900,000			1,900,000						1,900,000	1,900,000
Gilead | Gilead Collaboration Agreement | Prepaid Expenses and Other Current Assets
License And Collaboration Agreements [Line Items]
Cost sharing receivable, current	2,900,000
Gilead | Gilead Collaboration Agreement | Other Noncurrent Liabilities
License And Collaboration Agreements [Line Items]
Future development and promotion costs contract liability noncurrent						7,300,000			7,300,000						7,300,000	7,300,000
Gilead | Gilead Collaboration Agreement | Other Noncurrent Assets
License And Collaboration Agreements [Line Items]
Cost sharing receivable, noncurrent	1,400,000
Gilead | Gilead Collaboration Agreement | Minimum
License And Collaboration Agreements [Line Items]
Option fee per program for current clinical programs to exercise option	200,000,000
Gilead | Gilead Collaboration Agreement | Maximum
License And Collaboration Agreements [Line Items]
Contingent milestone payments receivable	400,000,000.0
Option fee per program for current clinical programs to exercise option	275,000,000
Gilead | Stock Purchase Agreement
License And Collaboration Agreements [Line Items]
Funds received for purchase of common stock									200,000,000
Fair value of stock purchased									109,400,000
Premium on stock purchased									$ 90,600,000
Issuance of common stock	$ 200,000,000					107,468,000
Issuance of common stock, shares | shares	5,963,029
Shares Issued, Price Per Share | $ / shares	$ 33.54
Percentage of option to purchase maximum shares of common stock	35.00%
Period over common stock to be purchased	5 years
Percentage of premium purchase price of common stock	20.00%
Trailing days average closing price	5 days
Share Price | $ / shares	$ 33.54
Taiho Pharmaceutical Co., Ltd | Taiho Agreement
License And Collaboration Agreements [Line Items]
Upfront cash payment				$ 20,000,000.0
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).
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
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]
Contingent milestone payments receivable				$ 145,000,000.0
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
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						10,100,000			$ 15,100,000
Development milestone expense									5,000,000.0
WuXi Biologics License Agreement | Research and Development Expenses
License And Collaboration Agreements [Line Items]
Sub-license fees incurred																11,300,000
Upfront and milestone payments																$ 31,000,000.0
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									9,600,000
Development milestone expense						3,000,000.0			3,000,000.0
Clinical, regulatory and commercialization remaining milestone payments									98,000,000.0
Genentech
License And Collaboration Agreements [Line Items]
Development expense						100,000			100,000
Strata Oncology Inc | Strata Agreement
License And Collaboration Agreements [Line Items]
Reimbursed under cost-sharing arrangement						100,000			200,000						400,000
Development milestone payable		$ 2,500,000
Development cost recorded within research and development expenses						$ 300,000			$ 1,200,000						$ 2,200,000
Number of restricted shares of common stock issued | shares		1,257,651
Fair value of restricted shares of common stock issued		$ 15,000,000.0
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

[1] The Condensed Consolidated Balance Sheet as of December 31, 2019 has been derived from the audited financial statements as of that date.