Common Stock Reserved for Issuance - Schedule of Common Stock Reserved for Issuance (Detail) - shares	Dec. 31, 2016	Dec. 31, 2015	Jul. 15, 2015
Class of Stock [Line Items]
Outstanding stock options	6,543,654	3,522,813
Total common stock reserved for future issuance	8,636,175	7,746,692
2015 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	700,000	700,000	700,000
Shares Reserved For Future Option Grants [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	1,392,521	3,523,879