Preferred Stock and Common Stock -Schedule of Number of Shares of Common Stock Reserved for Future Issuance (Details) - shares	Mar. 31, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	4,176,015	2,773,249
Options Issued and Outstanding
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	1,706,317	1,122,677
Unvested Restricted Stock Units
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	24,960	24,960
Options Available for Future Grants
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	1,889,155	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