Stockholders' Equity - Additional information (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended		9 Months Ended
	Jul. 13, 2020	Jun. 30, 2020	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2020
Class Of Stock [Line Items]
Net proceeds from public offering after deducting underwriting discounts, commissions and other offering expenses					$ 433,776
Common stock share value				$ 326,246
Stock Purchase Agreement | Gilead
Class Of Stock [Line Items]
Common stock shares issued	5,963,029
Shares issued, price per share	$ 33.54
Net proceeds from public offering after deducting underwriting discounts, commissions and other offering expenses	$ 107,500
Common stock share value	200,000		$ 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