Stockholders' Equity - Schedule of Common Stock Reserved for Issuance (Details) - shares	Mar. 31, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	7,518,528	5,549,908
Options and Awards Issued and Outstanding
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	5,257,140	3,864,407
2015 Stock Option and Incentive Plan
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	1,077,783	904,785
2015 Employee Stock Purchase Plan
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	1,183,605	780,716