Description of the Business - Additional Information (Detail)				18 Months Ended
	May 02, 2019 USD ($) Tranche	Feb. 12, 2019 USD ($) $ / shares shares	Jan. 04, 2018 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2019 USD ($) Tranche	Dec. 31, 2018 USD ($)
Description Of Business [Line Items]
Accumulated deficit					$ (279,472,000)	$ (153,863,000)
Funds raised through Series A and Series B Convertible Preferred Stock, convertible note financings and completed IPO				$ 601,300,000
Term Loan
Description Of Business [Line Items]
Maximum borrowing capacity	$ 150,000,000
Amount funded	$ 30,000,000
Number of additional tranches executed | Tranche	3				0
Remaining borrowing capacity	$ 120,000,000
Term Loan | Tranche One
Description Of Business [Line Items]
Remaining borrowing capacity	40,000,000
Term Loan | Tranche Two
Description Of Business [Line Items]
Remaining borrowing capacity	30,000,000
Term Loan | Tranche Three
Description Of Business [Line Items]
Remaining borrowing capacity	$ 50,000,000
AA Biopharma Inc.
Description Of Business [Line Items]
Cash received in connection with acquisition			$ 12,800,000
Initial Public Offering ("IPO")
Description Of Business [Line Items]
Sale of shares of common stock | shares		19,837,500
Offering price, per share | $ / shares		$ 16.00
Net proceeds, after deducting underwriting discounts, commissions, and offering expenses		$ 291,300,000
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 | shares		30,493,460