Stockholders' Equity - Schedule of Common Stock Reserved for Issuance (Details) - shares	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	5,549,908		4,277,531
Options and Awards Issued and Outstanding
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	3,864,407		2,964,549
2015 Stock Option and Incentive Plan
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	904,785	1,432,511	863,538
2015 Employee Stock Purchase Plan
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	780,716		449,444