Common Stock Reserved for Issuance - Schedule of Common Stock Reserved for Issuance (Detail) - shares	Jun. 30, 2017	Dec. 31, 2016	Jul. 15, 2015
Class of Stock [Line Items]
Outstanding and issued stock options	7,716,043	6,543,654
Total common stock reserved for future issuance	9,801,151	8,636,175
2008 Plan and 2015 Plan [Member]
Class of Stock [Line Items]
Outstanding and issued stock options	7,716,043	6,543,654
Total common stock reserved for future issuance	1,385,108	1,392,521
2015 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	700,000	700,000	700,000