Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Number of Options, Outstanding, Beginning balance	1,616
Number of Options, Granted	1,190
Number of Options, Forfeited	(573)
Number of Options, Outstanding, Ending balance	2,233	1,616
Number of Options, Vested and expected to vest	2,233
Number of Options, Exercisable	1,049
Weighted-Average Exercise Price, Outstanding, Beginning balance	$ 22.07
Weighted-Average Exercise Price, Granted	7.60
Weighted-Average Exercise Price, Forfeited	19.72
Weighted-Average Exercise Price, Outstanding, Ending balance	14.97	$ 22.07
Weighted-Average Exercise Price, Vested and expected to vest	14.97
Weighted-Average Exercise Price, Exercisable	$ 22.10
Weighted-Average Remaining Contractual Term	7 years 7 months 28 days	6 years 7 months 24 days
Weighted-Average Remaining Contractual Term, Vested and expected to vest	7 years 7 months 28 days
Weighted-Average Remaining Contractual Term, Exercisable	5 years 9 months 7 days
Aggregate Intrinsic Value, Outstanding		$ 60,031