Common Stock - Summary of Common Stock Reserved for Future Issuance (Detail) - shares	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	2,926,138	9,873,128
Series Seed Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance		806,711
Series A Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance		7,611,438
2018 Stock Option and Incentive Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	385,561
2018 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	223,200
Restricted Stock [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	153,276	276,301
Employee Stock Option [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	2,164,101	1,034,961
2015 Stock Option and Grant Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance		143,717