Stockholders' Equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended			3 Months Ended			9 Months Ended
	Jul. 13, 2020	Feb. 28, 2021	Jan. 31, 2021	Jun. 30, 2020	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Class Of Stock [Line Items]
Net proceeds from public offering after deducting underwriting discounts, commissions and other offering expenses								$ 220,235	$ 433,776
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	$ 107,468
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
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