Stock-Based Compensation - Summary of Stock Options Activity (Detail) - 2005 Plan, 2008 Plan and 2015 Plan [Member] $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Available for Grant, Beginning balance	1,550,101
Shares Available for Grant, Awards authorized	3,400,000
Shares Available for Grant, Granted	(1,217,568)
Shares Available for Grant, Exercised	0
Shares Available for Grant, Cancelled	0
Shares Available for Grant, Ending balance	3,732,533	1,550,101
Number of Shares Outstanding, Beginning balance	2,138,096
Number of Shares Outstanding, Options granted	1,217,568
Number of Shares Outstanding, Options exercised	(41,505)
Number of Shares Outstanding, Options cancelled	0
Number of Shares Outstanding, Ending balance	3,314,159	2,138,096
Number of Shares Outstanding, Exercisable	2,127,353
Number of Shares Outstanding, Vested and expected to vest	3,217,100
Weighted-Average Exercise Price Per Share, Beginning balance | $ / shares	$ 0.91
Weighted-Average Exercise Price Per Share, Options granted | $ / shares	5.96
Weighted-Average Exercise Price Per Share, Options exercised | $ / shares	0.76
Weighted-Average Exercise Price Per Share, Options cancelled | $ / shares	0
Weighted-Average Exercise Price Per Share, Ending balance | $ / shares	2.77	$ 0.91
Weighted-Average Exercise Price Per Share, Exercisable | $ / shares	0.98
Weighted-Average Exercise Price Per Share, Vested and expected to vest | $ / shares	$ 2.72
Weighted-Average Remaining Contractual Term (Years)	8 years 11 months 16 days	9 years 3 months 15 days
Weighted-Average Remaining Contractual Term (Year), Exercisable	8 years 7 months 2 days
Weighted-Average Remaining Contractual Term (Year), Vested and expected to vest	8 years 11 months 12 days
Aggregate Intrinsic Value of Outstanding Options | $	$ 58,866	$ 3,153
Aggregate Intrinsic Value of Outstanding Options, Exercisable | $	41,564
Aggregate Intrinsic Value of Outstanding Options, Vested and expected to vest | $	$ 57,303