Stockholders' Equity (Deficit) - Schedule of Reserved Common Stock on an if-Converted Basis for Issuance (Details) - shares	Dec. 31, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	5,259,271	32,858,930
Convertible Preferred Stock
Class Of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance		30,459,574
Common Stock Options Issued and Outstanding
Class Of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	1,458,080	544,116
Remaining shares available for issuance under 2015 stock plan
Class Of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	3,801,191	1,855,240