Stock-Based Compensation - Summary of Stock-Based Compensation Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Options Outstanding, Number of Shares Outstanding
Number of Shares Outstanding, Beginning balance	3,522,813
Number of Shares Outstanding, Ending balance	6,629,163	3,522,813
2008 Plan and 2015 Plan [Member]
Options Outstanding, Shares Available for Grant
Shares Available for Grant, Beginning balance	3,523,879
Shares Available for Grant, Awards authorized	1,202,324
Shares Available for Grant, Granted	(3,702,880)
Shares Available for Grant, Exercised	0
Shares Available for Grant, forfeited/Cancelled	304,075
Shares Available for Grant, Ending balance	1,327,398	3,523,879
Options Outstanding, Number of Shares Outstanding
Number of Shares Outstanding, Beginning balance	3,522,813
Number of Shares Outstanding, Options granted	3,702,880
Number of Shares Outstanding, Options exercised	(292,455)
Number of Shares Outstanding, Options forfeited/cancelled	(304,075)
Number of Shares Outstanding, Ending balance	6,629,163	3,522,813
Number of Shares Outstanding, Exercisable	2,244,374
Number of Shares Outstanding, Vested and expected to vest	6,394,550
Options Outstanding, Weighted-Average Exercise Price Per Share
Weighted-Average Exercise Price Per Share, Beginning balance	$ 3.81
Weighted-Average Exercise Price Per Share, Options granted	3.22
Weighted-Average Exercise Price Per Share, Options exercised	0.66
Weighted-Average Exercise Price Per Share, Options forfeited/cancelled	7.89
Weighted-Average Exercise Price Per Share, Ending balance	3.43	$ 3.81
Weighted-Average Exercise Price Per Share, Exercisable	2.01
Weighted-Average Exercise Price Per Share, Vested and expected to vest	$ 3.42
Options Outstanding, Weighted-Average Remaining Contractual Term (Years)
Weighted-Average Remaining Contractual Term (Years)	9 years 1 month 2 days	8 years 9 months 11 days
Weighted-Average Remaining Contractual Term (Year), Exercisable	8 years 15 days
Weighted-Average Remaining Contractual Term (Year), Vested and expected to vest	9 years 29 days
Aggregate Intrinsic Value of Outstanding Options
Aggregate Intrinsic Value of Outstanding Options	$ 1,443	$ 40,425
Aggregate Intrinsic Value of Outstanding Options, Exercisable	1,443
Aggregate Intrinsic Value of Outstanding Options, Vested and expected to vest	$ 1,418