Common Stock Reserved for Issuance - Schedule of Common Stock Reserved for Issuance (Detail) - shares	Sep. 30, 2018	Dec. 31, 2017	Jul. 15, 2015
Class of Stock [Line Items]
Total common stock reserved for future issuance	13,371,825	9,674,371
2015 Plan [Member]
Class of Stock [Line Items]
Awards outstanding	10,719,737	7,470,601
Total common stock reserved for future issuance	1,731,500	1,503,770
2015 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	700,000	700,000	700,000
Warrants to Purchase Common Stock [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	147,059
Outstanding warrant	73,529