Description of the Business - Additional Information (Detail) $ / shares in Units, $ in Thousands			1 Months Ended	3 Months Ended	18 Months Ended
	Feb. 12, 2019 USD ($) $ / shares shares	Jan. 04, 2018 USD ($)	Jan. 31, 2019	Mar. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Description Of Business [Line Items]
Reverse stock split				one for every 4.5 shares
Reverse stock spilt conversion ratio for common stock			0.0022
Accumulated deficit				$ (186,474)	$ (186,474)	$ (153,863)
Funds raised through Series A and Series B Convertible Preferred Stock, convertible note financings and completed IPO					$ 601,300
AA Biopharma Inc.
Description Of Business [Line Items]
Cash received in connection with acquisition		$ 12,800
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
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