Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Number of Options, Outstanding, Beginning balance	1,902
Number of Options, Granted	2,111
Number of Options, Exercised	(323)
Number of Options, Forfeited	(2,074)
Number of Options, Outstanding, Ending balance	1,616	1,902
Number of Options, Vested and expected to vest	1,615
Number of Options, Exercisable	996
Weighted-Average Exercise Price, Outstanding, Beginning balance	$ 57.70
Weighted-Average Exercise Price, Granted	23.69
Weighted-Average Exercise Price, Exercised	20.70
Weighted-Average Exercise Price, Forfeited	56.60
Weighted-Average Exercise Price, Outstanding, Ending balance	22.07	$ 57.70
Weighted-Average Exercise Price, Vested and expected to vest	22.07
Weighted-Average Exercise Price, Exercisable	$ 26.43
Weighted-Average Remaining Contractual Term	6 years 7 months 24 days	5 years 11 months 19 days
Weighted-Average Remaining Contractual Term, Vested and expected to vest	6 years 7 months 20 days
Weighted-Average Remaining Contractual Term, Exercisable	4 years 11 months 15 days
Aggregate Intrinsic Value, Outstanding	$ 60,031	$ 7,564
Aggregate Intrinsic Value, Vested and expected to vest	60,031
Aggregate Intrinsic Value, Exercisable	$ 59,689