Redeemable Convertible Preferred Stock and Common Stock -Schedule of Number of Shares of Common Stock Reserved for Future Issuance (Details) - shares	Sep. 30, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	2,780,278	17,736,568
Redeemable Convertible Preferred Stock, on an as-converted Basis
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance		15,870,559
Options Issued and Outstanding
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	1,066,678	195,668
Unvested Restricted Stock Units
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	24,960
Options Available for Future Grants
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	1,413,610	1,670,341
Shares Available for Issuance under the 2018 ESPP
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	275,030