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Restricted Stock Units Activity (Detail) - Restricted Stock Units (RSUs) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Number of RSUs    
Unvested, beginning balance 2,271,684  
Granted 1,080,260  
Vested (1,762,733)  
Forfeited (72,313)  
Unvested, ending balance 1,516,898 2,271,684
Weighted-average grant date fair value    
Unvested, beginning balance (in dollars per share) $ 13.25  
Granted (in dollars per share) 6.31  
Vested (in dollars per share) 10.35  
Forfeited (in dollars per share) 11.52  
Unvested, ending balance (in dollars per share) $ 11.76 $ 13.25
Weighted-average remaining contractual terms (Years)    
Unvested, December 31, 2017 7 years 10 months 24 days 8 years 6 months
Aggregated intrinsic value    
Unvested, December 31, 2017 (in dollars) $ 12,105