Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 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	625
Number of Options, Forfeited	(298)
Number of Options, Outstanding, Ending balance	1,943	1,616
Number of Options, Vested and expected to vest	1,943
Number of Options, Exercisable	1,028
Weighted-Average Exercise Price, Outstanding, Beginning balance	$ 22.07
Weighted-Average Exercise Price, Granted	10.64
Weighted-Average Exercise Price, Forfeited	22.04
Weighted-Average Exercise Price, Outstanding, Ending balance	18.40	$ 22.07
Weighted-Average Exercise Price, Vested and expected to vest	18.40
Weighted-Average Exercise Price, Exercisable	$ 23.81
Weighted-Average Remaining Contractual Term	6 years 11 months 23 days	6 years 7 months 24 days
Weighted-Average Remaining Contractual Term, Vested and expected to vest	6 years 11 months 23 days
Weighted-Average Remaining Contractual Term, Exercisable	4 years 11 months 1 day
Aggregate Intrinsic Value, Outstanding		$ 60,031