Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 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	667
Number of Options, Forfeited	(477)
Number of Options, Outstanding, Ending balance	1,806	1,616
Number of Options, Vested and expected to vest	1,806
Number of Options, Exercisable	966
Weighted-Average Exercise Price, Outstanding, Beginning balance	$ 22.07
Weighted-Average Exercise Price, Granted	10.31
Weighted-Average Exercise Price, Forfeited	21.58
Weighted-Average Exercise Price, Outstanding, Ending balance	17.86	$ 22.07
Weighted-Average Exercise Price, Vested and expected to vest	17.86
Weighted-Average Exercise Price, Exercisable	$ 23.01
Weighted-Average Remaining Contractual Term	7 years 2 months 26 days	6 years 7 months 24 days
Weighted-Average Remaining Contractual Term, Vested and expected to vest	7 years 2 months 26 days
Weighted-Average Remaining Contractual Term, Exercisable	5 years 9 months 21 days
Aggregate Intrinsic Value, Outstanding		$ 60,031