Stockholders' Equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended			3 Months Ended		6 Months Ended
	Jul. 13, 2020	Feb. 28, 2021	Jan. 31, 2021	Jun. 30, 2020	Mar. 31, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Class Of Stock [Line Items]
Net proceeds from public offering after deducting underwriting discounts, commissions and other offering expenses							$ 220,235	$ 326,512
Common stock share value						$ 326,246
Stock Purchase Agreement | Gilead
Class Of Stock [Line Items]
Common stock shares issued	5,963,029	5,650,000	5,650,000
Shares issued, price per share	$ 33.54	$ 39.00	$ 39.00
Net proceeds from public offering after deducting underwriting discounts, commissions and other offering expenses	$ 107,500		$ 220,400
Common stock share value	200,000	$ 220,400			$ 220,296
Purchase price of common stock allocation to performance obligation	90,600
Deferred offering expenses	$ 1,900
May 2020 Public Offering
Class Of Stock [Line Items]
Common stock shares issued				12,650,000
Shares issued, price per share				$ 27.50		$ 27.50		$ 27.50
Net proceeds from public offering after deducting underwriting discounts, commissions and other offering expenses				$ 326,200
Base Shares
Class Of Stock [Line Items]
Common stock shares issued				11,000,000
Underwriters Option Exercise
Class Of Stock [Line Items]
Common stock shares issued				1,650,000