License and Collaboration Agreements - Additional Information (Details)			1 Months Ended				12 Months Ended
	Jul. 13, 2020 USD ($) $ / shares shares	Apr. 30, 2019 USD ($) shares	Dec. 31, 2020 USD ($) shares	Oct. 31, 2017 USD ($)	Sep. 30, 2017 USD ($) Program	Aug. 31, 2017 USD ($)	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
License And Collaboration Agreements [Line Items]
Upfront cash payment							$ 175,000,000
Issuance of common stock							326,246,000		$ 124,735,000
Cost sharing receivable, current			$ 1,049,000				1,049,000	$ 132,000
Total collaboration and license revenues							77,517,000	15,000,000	8,353,000
Deferred revenue recognized							7,000,000	7,000,000	5,353,000
Deferred revenue, current			74,571,000				74,571,000	7,000,000
Deferred revenue, noncurrent			$ 122,830,000				122,830,000	12,022,000
Development expense							$ 159,348,000	78,481,000	49,646,000
Shares probable of vesting | shares			9,892,697				9,892,697
Stock Purchase Agreement
License And Collaboration Agreements [Line Items]
Direct offering cost	$ 1,900,000
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
Premium on stock purchased	$ 90,600,000
Collaboration term for current and future clinical programs	10 years
Contingent milestone payments receivable	$ 400,000,000
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
Research and development information access rights payment related to year 2022	100,000,000.0
Initial transaction price	365,600,000
Option payment upon achievement of certain development milestones	250,000,000.0
Deferred revenue related to etrumadenant option			127,000,000.0				$ 127,000,000.0
Etrumadenant option initial term of agreement							4 years
Option payment upon achievement of certain development milestones	$ 275,000,000.0
Deferred revenue related to domvanalimab option			36,700,000				$ 36,700,000
Domvanalimab option recognized term							1 year
Current and future programs exclusive access period	10 years
Additional payment due at each of the fourth, sixth, and eighth anniversaries of the agreement	$ 100,000,000.0
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
Deferred revenue related to research and development pipeline			121,700,000				$ 121,700,000
Future development and promotion costs contract liability			9,700,000				9,700,000
Consultant and legal fees	7,300,000
Amortization expense							200,000
Cost sharing receivable			4,200,000				4,200,000
Gilead | Gilead Collaboration Agreement | Prepaid Expenses and Other Current Assets
License And Collaboration Agreements [Line Items]
Cost sharing receivable, current			1,000,000.0				1,000,000.0
Gilead | Gilead Collaboration Agreement | Other Noncurrent Assets
License And Collaboration Agreements [Line Items]
Cost sharing receivable, noncurrent			3,200,000				3,200,000
Gilead | Gilead Collaboration Agreement | Maximum
License And Collaboration Agreements [Line Items]
Contingent milestone payments receivable	400,000,000
Option fee per program for current clinical programs to exercise option	275,000,000
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 | 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
Total collaboration and license revenues							7,000,000.0	15,000,000.0
Deferred revenue recognized									8,300,000
Deferred revenue, current			7,000,000.0				7,000,000.0	7,000,000.0
Deferred revenue, noncurrent			5,000,000.0				$ 5,000,000.0	12,000,000.0
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
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
WuXi Biologics License Agreement
License And Collaboration Agreements [Line Items]
Clinical and regulatory milestones achieved			16,500,000
Range of tiered royalty payments on net sales							high single-digits to low teens
WuXi Biologics License Agreement | Research and Development
License And Collaboration Agreements [Line Items]
Upfront cash payment			$ 500,000
Milestone payments							$ 5,000,000.0	7,500,000	0
Sub-license fees incurred							10,100,000	1,200,000	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]
Development expense								0
Abmuno License Agreement | Research and Development
License And Collaboration Agreements [Line Items]
Milestone payments							3.0		$ 2,800,000
Abmuno License Agreement | Maximum
License And Collaboration Agreements [Line Items]
Clinical, regulatory and commercialization remaining milestone payments							98,000,000.0
Genentech
License And Collaboration Agreements [Line Items]
Development expense							500,000	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							1,700,000	1,000,000.0
Development cost reimbursed							$ 300,000	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 | Co-Development and Collaboration Agreement | Non-vested Restricted Stock
License And Collaboration Agreements [Line Items]
Shares probable of vesting | shares			0				0
Compensation expense recognized							$ 0
Strata Oncology Inc | Co-Development and Collaboration Agreement | Research and Development
License And Collaboration Agreements [Line Items]
Milestone payments							$ 0	$ 2,500,000
Strata Oncology Inc | Co-Development and Collaboration Agreement | Maximum
License And Collaboration Agreements [Line Items]
Regulatory and commercial milestone payable		$ 125,000,000.0