Stock-Based Compensation Expense - Summary of Stock Option Activity (Detail) - $ / shares	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Options, Outstanding, Beginning Balance	1,110,744
Number of Options, Granted	761,382
Number of Options, Assumed	551,363
Number of Options, Exercised	(32,874)
Number of Options, Canceled	(16,619)
Number of Options, Outstanding, Ending Balance	2,373,996	1,110,744
Number of Options, Vested and expected to vest	2,310,312
Number of Options, Vested and exercisable	1,090,000
Weighted Average Exercise Price Per Share, Outstanding, Beginning Balance	$ 4.59
Weighted Average Exercise Price Per Share, Granted	15.08
Weighted Average Exercise Price Per Share, Assumed	30.11
Weighted Average Exercise Price Per Share, Exercised	3.51
Weighted Average Exercise Price Per Share, Canceled	20.30
Weighted Average Exercise Price Per Share, Outstanding, Ending Balance	14.00	$ 4.59
Weighted Average Exercise Price Per Share, Vested and expected to vest	14.08
Weighted Average Exercise Price Per Share, Vested and Exercisable	$ 17.77
Weighted Average Contractual Term, Outstanding	8 years 6 months 11 days	8 years 8 months 19 days
Weighted-average grant date fair value of Stock Options Outstanding, Granted	$ 10.95