Description of the Business - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands				36 Months Ended
	May 21, 2020	Feb. 12, 2019	Jan. 04, 2018	Sep. 30, 2020	Dec. 31, 2019
Description Of Business [Line Items]
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs (in shares)	9,433,963
Offering price (in dollars per share)	$ 13.25
Accumulated deficit				$ (512,926)	$ (334,170)
Funds raised through Series A and Series B Convertible Preferred Stock, convertible note financings and completed IPO				$ 942,000
AA Biopharma Inc.
Description Of Business [Line Items]
Cash received in connection with acquisition			$ 12,800
2027 Notes
Description Of Business [Line Items]
Debt instrument interest rate percentage	5.00%			5.00%
Initial Public Offering ("IPO")
Description Of Business [Line Items]
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs (in shares)		19,837,500
Offering price (in dollars per share)		$ 16.00
Net proceeds, after deducting underwriting discounts, commissions, and offering expenses		$ 291,300
Initial Public Offering ("IPO") | Conversion of Convertible Preferred Stock into Common Stock
Description Of Business [Line Items]
Common stock issued in conversion of convertible preferred stock (in shares)		30,493,460