Common Stock Reserved for Issuance - Schedule of Common Stock Reserved for Issuance (Detail) - shares	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jul. 15, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Outstanding stock options	7,470,601	6,543,654
Total common stock reserved for future issuance	9,674,371	8,636,175
2008 Plan and 2015 Plan [Member]
Class of Stock [Line Items]
Outstanding stock options	7,470,601	6,543,654	3,522,813		2,138,096
Total common stock reserved for future issuance	1,503,770	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