Convertible Preferred Stock and Stockholders' Equity (Deficit) - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended
	Jul. 13, 2020	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2017
Class Of Stock [Line Items]
Capital stock shares authorized			410,000,000
Common stock, shares authorized			400,000,000		400,000,000
Preferred stock, shares authorized			10,000,000		10,000,000
Common stock, par value			$ 0.0001		$ 0.0001
Preferred stock, par value			$ 0.0001		$ 0.0001
Net proceeds from public offering after deducting underwriting discounts, commissions and other offering expenses			$ 433,776
Common stock share value			$ 326,246	$ 124,735
Convertible Preferred Stock
Class Of Stock [Line Items]
Convertible preferred stock, shares outstanding			0		0	30,459,574
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
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