Preferred Stock and Common Stock -Schedule of Number of Shares of Common Stock Reserved for Future Issuance (Details) - shares	Jun. 30, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	4,162,746	2,773,249
Options Issued and Outstanding
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	2,087,272	1,122,677
Unvested Restricted Stock Units
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	18,720	24,960
Options Available for Future Grants
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	1,501,171	1,350,582
Shares Available for Issuance under the 2018 ESPP
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	555,583	275,030