Stockholders' Equity - Schedule of Common Stock Reserved for Issuance (Details) - shares	Jun. 30, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	5,792,947	4,277,531
Options and Awards Issued and Outstanding
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	3,904,668	2,964,549
2015 Stock Option and Incentive Plan
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	1,093,555	863,538
2015 Employee Stock Purchase Plan
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	794,724	449,444