Related Parties - Additional Information (Details) - USD ($)		1 Months Ended			3 Months Ended				9 Months Ended
	Jul. 13, 2020	Feb. 28, 2021	Jan. 31, 2021	May 31, 2020	Sep. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Related Party Transaction [Line Items]
Issuance of common stock								$ 326,246,000
Cost sharing receivable					$ 871,000				$ 871,000		$ 1,049,000
Long-term portion of revenue allocated					94,675,000				94,675,000		$ 122,830,000
Research and Development
Related Party Transaction [Line Items]
Reimbursed under cost-sharing provisions of arrangement					$ (756,000)		$ (2,971,000)		(6,856,000)	$ (2,971,000)
Gilead Collaboration Agreement | Gilead
Related Party Transaction [Line Items]
Revenue recognized									0
Upfront cash payment	$ 175,000,000.0								175,000,000
Premium on stock purchased	$ 90,600,000
Stock Purchase Agreement | Gilead
Related Party Transaction [Line Items]
Issuance of common stock, shares	5,963,029	5,650,000	5,650,000
Issuance of common stock	$ 200,000,000	$ 220,400,000				$ 220,296,000	$ 107,468,000
Premium on stock purchased									$ 90,600,000
Shares issued, price per share	$ 33.54	$ 39.00	$ 39.00
Gilead
Related Party Transaction [Line Items]
Percentage of outstanding common stock held					19.70%				19.70%
Number of shares issued				2,200,000
Value of common stock issued				$ 56,700,000
Cost sharing receivable					$ 600,000				$ 600,000
Deferred revenue					162,200,000				162,200,000
Long-term portion of revenue allocated					94,700,000				94,700,000
Future development and commercialization services contract liability					9,700,000				9,700,000
Gilead | Research and Development
Related Party Transaction [Line Items]
Reimbursed under cost-sharing provisions of arrangement					$ 800,000				$ 6,900,000
Gilead | Maximum
Related Party Transaction [Line Items]
Right to purchase additional outstanding voting common stock percentage					35.00%				35.00%
Gilead | Gilead Collaboration Agreement
Related Party Transaction [Line Items]
Revenue recognized					$ 7,700,000				$ 23,100,000
Reimbursement of milestone expense of cost sharing provisions									4,000,000.0
Unconstrained consideration to be received					$ 100,000,000				100,000,000
Gilead | Stock Purchase Agreement
Related Party Transaction [Line Items]
Issuance of common stock, shares	5,963,029	5,650,000
Issuance of common stock		$ 220,400,000							327,800,000
Funds received for purchase of common stock									200,000,000
Fair value of stock purchased									$ 109,400,000
Shares issued, price per share		$ 39.00