Redeemable Convertible Preferred Stock and Stockholders' Deficit - Summary of Common Stock Reserved for Future Issuance (Details) - shares	Sep. 30, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Common stock reserved for future issuance	15,262,054	10,738,626
Conversion of Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Common stock reserved for future issuance	13,200,115	9,131,999
Redeemable Convertible Preferred Stock Warrants
Class Of Stock [Line Items]
Common stock reserved for future issuance	708,484	486,356
Stock Options Issued and Outstanding
Class Of Stock [Line Items]
Common stock reserved for future issuance	1,315,625	1,075,209
Authorized for Future Options
Class Of Stock [Line Items]
Common stock reserved for future issuance	37,830	45,062