Stock-Based Compensation - Summary of Company's Stock Option Activity and Related Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Shares Available for Grant
Shares Available for Grant, Beginning balance	295,101
Shares Available for Grant, Additional authorized (unaudited)	4,708,447
Shares Available for Grant, Granted (unaudited)	(1,911,137)
Shares Available for Grant, Cancelled (unaudited)	121,941
Shares Available for Grant, Ending balance (unaudited)	3,214,352	295,101
Number of Stock Options Outstanding
Number of Stock Options Outstanding, Beginning balance	4,248,519
Number of Stock Options Outstanding, Granted (unaudited)	1,911,137
Number of Stock Options Outstanding, Exercised (unaudited)	(26,555)
Number of Stock Options Outstanding, Cancelled (unaudited)	(121,941)
Number of Stock Options Outstanding, Ending balance (unaudited)	6,011,160	4,248,519
Number of Stock Options Outstanding, Vested and exercisable	2,911,734
Number of Stock Options Outstanding, Vested and expected to vest	4,576,826
Weighted- Average Exercise Price
Weighted- Average Exercise Price, Beginning balance	$ 0.76
Weighted- Average Exercise Price, Granted (unaudited)	5.04
Weighted- Average Exercise Price, Exercised (unaudited)	0.73
Weighted- Average Exercise Price, Cancelled (unaudited)	1.30
Weighted- Average Exercise Price, Ending balance (unaudited)	2.11	$ 0.76
Weighted- Average Exercise Price, Vested and exercisable (unaudited)	0.82
Weighted- Average Exercise Price, Vested and expected to vest (unaudited)	$ 1.88
Weighted- Average Remaining Contractual Life (Years)
Options Outstanding, Weighted- Average Remaining Contractual Life (Years)	7 years 9 months 18 days	7 years 8 months 12 days
Weighted- Average Remaining Contractual Life (Years), Vested and exercisable (unaudited)	6 years 6 months
Weighted- Average Remaining Contractual Life (Years), Vested and expected to vest (unaudited)	7 years 7 months 6 days
Aggregate Intrinsic Value
Aggregate Intrinsic Value, Beginning balance	$ 8,343
Aggregate Intrinsic Value, Ending balance (unaudited)	18,755	$ 8,343
Aggregate Intrinsic Value, Vested and exercisable (unaudited)	12,702
Aggregate Intrinsic Value, Vested and expected to vest (unaudited)	$ 15,329