Related Parties - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended
	Jul. 13, 2020	May 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Related Party Transaction [Line Items]
Cost sharing receivable			$ 1,049	$ 132
Deferred revenue, noncurrent ($117,808 and $0 to a related party)			122,830	12,022
Upfront cash payment			175,000
Research and Development
Related Party Transaction [Line Items]
Reimbursed under cost-sharing provisions of arrangement			3,446	$ 0	$ 0
Gilead Collaboration Agreement | Gilead
Related Party Transaction [Line Items]
Upfront cash payment	$ 175,000		175,000
Unconstrained consideration to be received			100,000
Premium on stock purchased	$ 90,600
Stock Purchase Agreement | Gilead
Related Party Transaction [Line Items]
Issuance of common stock, shares	5,963,029
Funds received for purchase of common stock			200,000
Fair value of stock purchased			109,400
Premium on stock purchased			$ 90,600
Gilead
Related Party Transaction [Line Items]
Percentage of outstanding common stock held			13.00%
Price per share		$ 27.50
Value of common stock issued		$ 56,700
Number of shares issued		2,200,000
Cost sharing receivable			$ 900
Deferred revenue			185,400
Deferred revenue, noncurrent ($117,808 and $0 to a related party)			117,800
Future development and commercialization services contract liability			9,700
Gilead | Research and Development
Related Party Transaction [Line Items]
Reimbursed under cost-sharing provisions of arrangement			3,400
Gilead | Gilead Collaboration Agreement
Related Party Transaction [Line Items]
Revenue recognized			$ 70,500
Gilead | Stock Purchase Agreement
Related Party Transaction [Line Items]
Issuance of common stock, shares	5,963,029
Gilead | Maximum
Related Party Transaction [Line Items]
Right to purchase additional outstanding voting common stock percentage			35.00%