Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Options Outstanding, Weighted Average Exercise Price, and Additional Disclosures [Abstract]
Outstanding, Number of Shares, Beginning Balance	1,104,172
Granted, Number of Shares	1,938,705
Exercised, Number of Shares	(12,572)
Cancelled or forfeited, Number of Shares	(8,981)
Outstanding, Number of Shares, Ending Balance	3,021,324	1,104,172
Options vested and expected to vest, Number of Shares	3,021,324
Options exercisable, Number of Shares	311,081
Outstanding, Weighted Average Exercise Price, Beginning Balance	$ 0.53
Granted, Weighted Average Exercise Price	4.06
Exercised, Weighted Average Exercise Price	0.38
Cancelled or forfeited, Weighted Average Exercise Price	0.38
Outstanding, Weighted Average Exercise Price, Ending Balance	2.80	$ 0.53
Options vested and expected to vest, Weighted Average Exercise Price	2.80
Options exercisable, Weighted Average Exercise Price	$ 1.24
Outstanding, Weighted Average Remaining Contractual Term	9 years 4 months 24 days	9 years 8 months 12 days
Options vested and expected to vest, Aggregate Intrinsic Value	9 years 4 months 24 days
Options exercisable, Weighted Average Remaining Contractual Term	9 years
Outstanding, Aggregate Intrinsic Value, Beginning Balance	$ 67
Outstanding, Aggregate Intrinsic Value, Ending Balance	22,359	$ 67
Options vested and expected to vest, Number of Shares	22,359
Options exercisable, Aggregate Intrinsic Value	$ 2,788