License and Collaboration Agreements - Additional Information (Details)			1 Months Ended						3 Months Ended		12 Months Ended			23 Months Ended	44 Months Ended
	Jul. 13, 2020 USD ($) $ / shares shares	Apr. 30, 2019 USD ($) shares	Feb. 28, 2021 USD ($) $ / shares shares	Jan. 31, 2021 USD ($) $ / shares shares	Dec. 31, 2020 USD ($)		Sep. 30, 2017 USD ($) Program	Aug. 31, 2017 USD ($)	Mar. 31, 2021 USD ($) shares	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2021 USD ($) shares	Mar. 31, 2021 USD ($) shares
License And Collaboration Agreements [Line Items]
Cost sharing receivable, current					$ 1,049,000	[1]			$ 5,235,000					$ 5,235,000	$ 5,235,000
Development expense									66,387,000	$ 23,142,000
Compensation expense recognized									12,761,000	3,462,000
Research and Development
License And Collaboration Agreements [Line Items]
Compensation expense recognized									$ 6,170,000	1,736,000
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
Development milestone expense									$ 10,000,000.0
WuXi Biologics License Agreement | Research and Development
License And Collaboration Agreements [Line Items]
Upfront cash payment					$ 500,000
Sub-license fees incurred															11,300,000
Upfront and milestone payments															41,000,000.0
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									14,600,000
Development milestone expense									5,000,000.0
Clinical, regulatory and commercialization remaining milestone payments									93,000,000.0
Genentech
License And Collaboration Agreements [Line Items]
Net expenses of inception									800,000
Development expense									300,000	0
Gilead Collaboration Agreement | Gilead
License And Collaboration Agreements [Line Items]
Upfront cash payment	$ 175,000,000.0								175,000,000
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
Present value of research and development information access rights payment related to year 2022	100,000,000.0
Premium on stock purchased	90,600,000
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	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	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
Contingent milestone payment start period	2022
Contractual obligation for first payment	$ 100,000,000
Performance obligation period	4 years
Contractual obligation remaining amount not obligated to pay	$ 300,000,000
Deferred revenue related to research and development pipeline									$ 114,000,000.0					114,000,000.0	114,000,000.0
Future development and promotion costs contract liability									9,700,000					9,700,000	9,700,000
Revenue recognized from this performance obligation									0
Consultant and legal fees	7,300,000
Amortization expense									100,000
Cost sharing receivable									4,100,000					4,100,000	4,100,000
Gilead Collaboration Agreement | Gilead | Prepaid Expenses and Other Current Assets
License And Collaboration Agreements [Line Items]
Cost sharing receivable, current									1,000,000.0					1,000,000.0	1,000,000.0
Gilead Collaboration Agreement | Gilead | Other Noncurrent Assets
License And Collaboration Agreements [Line Items]
Cost sharing receivable, noncurrent									3,100,000					3,100,000	$ 3,100,000
Gilead Collaboration Agreement | Gilead | Minimum
License And Collaboration Agreements [Line Items]
Option fee per program for current clinical programs to exercise option	200,000,000
Gilead Collaboration Agreement | Gilead | 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
Purchase Agreement
License And Collaboration Agreements [Line Items]
Direct offering cost	1,900,000
Purchase Agreement | Gilead
License And Collaboration Agreements [Line Items]
Issuance of common stock	$ 200,000,000		$ 220,400,000						220,296,000
Issuance of common stock, shares | shares	5,963,029		5,650,000	5,650,000
Shares issued, price per share | $ / shares	$ 33.54		$ 39.00	$ 39.00
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
Issuance of common stock and rights to purchase additional shares in accordance with Amended and Restated Gilead Purchase Agreement, net of $55 offering costs, Shares | shares	5,963,029		5,650,000	5,650,000
Gross proceeds	$ 107,500,000			$ 220,400,000
Premium on stock purchased									$ 90,600,000
Taiho Agreement | Taiho Pharmaceutical Co., Ltd
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	$ 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
Clinical and regulatory milestones achieved									$ 0
Sales milestone or royalty revenue recognized									0
Taiho Agreement | Taiho Pharmaceutical Co., Ltd | 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 Agreement | Taiho Pharmaceutical Co., Ltd | 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
Strata Agreement | Strata Oncology Inc
License And Collaboration Agreements [Line Items]
Development milestone payable		$ 2,500,000
Development cost recorded within research and development expenses									600,000	400,000				3,300,000
Development cost reimbursed									$ 100,000	$ 100,000				$ 700,000
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 Agreement | Strata Oncology Inc | Non-vested Restricted Stock
License And Collaboration Agreements [Line Items]
Shares probable of vesting | shares									0					0	0
Compensation expense recognized									$ 0
Strata Agreement | Strata Oncology Inc | Research and Development
License And Collaboration Agreements [Line Items]
Milestone payments														$ 2,500,000
Strata Agreement | Strata Oncology Inc | 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, 2020 has been derived from the audited financial statements as of that date.