Related Parties - Additional Information (Details) - USD ($) $ in Thousands		3 Months Ended	9 Months Ended
	Jul. 13, 2020	Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019	[1]
Related Party Transaction [Line Items]
Cost sharing receivable		$ 2,579	$ 2,579	$ 132
Long-term portion of revenue allocated		91,298	91,298	$ 12,022
Upfront cash payment			175,000
Gilead Collaboration Agreement | Gilead
Related Party Transaction [Line Items]
Future development and promotion costs contract liability	$ 9,200
Upfront cash payment	175,000		175,000
Unconstrained consideration to be received		100,000	100,000
Premium on stock purchased	$ 90,600
Development and Commercialization Activities Performance Obligation
Related Party Transaction [Line Items]
Reimbursed under cost-sharing arrangement		500
Stock Purchase Agreement | Gilead
Related Party Transaction [Line Items]
Funds received for purchase of common stock			200,000
Fair value of stock purchased			109,400
Premium on stock purchased			90,600
Other Current Liabilities | Gilead Collaboration Agreement | Gilead
Related Party Transaction [Line Items]
Future development and promotion costs contract liability current		1,900	1,900
Other Noncurrent Liabilities | Gilead Collaboration Agreement | Gilead
Related Party Transaction [Line Items]
Future development and promotion costs contract liability noncurrent		$ 7,300	$ 7,300
Gilead
Related Party Transaction [Line Items]
Percentage of outstanding common stock held		13.00%	13.00%
Cost sharing receivable		$ 2,500	$ 2,500
Deferred revenue		193,100	193,100
Long-term portion of revenue allocated		84,500	84,500
Future development and promotion costs contract liability		9,200	9,200
Reimbursed under cost-sharing arrangement			3,000
Gilead | Gilead Collaboration Agreement
Related Party Transaction [Line Items]
Revenue recognized		62,800	62,800
Gilead | Ongoing Collaboration Over Zimberelimab
Related Party Transaction [Line Items]
Reimbursed under cost-sharing arrangement			2,500
Gilead | Development and Commercialization Activities Performance Obligation
Related Party Transaction [Line Items]
Reimbursed under cost-sharing arrangement			500
Gilead | Other Current Liabilities
Related Party Transaction [Line Items]
Future development and promotion costs contract liability current		1,900	1,900
Gilead | Other Noncurrent Liabilities
Related Party Transaction [Line Items]
Future development and promotion costs contract liability noncurrent		$ 7,300	$ 7,300
Gilead | Maximum
Related Party Transaction [Line Items]
Right to purchase additional outstanding voting common stock percentage		35.00%	35.00%

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