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Summary of Restricted Stock Unit Activity (Detail) - Restricted Stock Units (RSUs)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
USD ($)
shares
Options Outstanding  
Restricted stock units outstanding at December 31, 2016 353,838
Granted 293,004
Vested (114,690)
Forfeited/cancelled (26,917)
Restricted stock units outstanding at December 31, 2017 505,235
Vested and expected to vest at December 31, 2017 466,201 [1]
Weighted-Average Remaining Contractual Term (in years)  
Restricted stock units outstanding at December 31, 2017 2 years 7 months 24 days
Vested and expected to vest at December 31, 2017 2 years 4 months 24 days [1]
Aggregate Intrinsic Value  
Restricted stock units outstanding at December 31, 2017 | $ $ 18,330
Vested and expected to vest at December 31, 2017 | $ $ 16,914 [1]
[1] Represents the number of vested restricted stock units as of December 31, 2017 plus the number of unvested restricted stock units expected to vest as of December 31, 2017 based on the unvested outstanding restricted stock units at December 31, 2017 adjusted for estimated forfeiture rates of 8% for awards granted to non-executive level employees and 3% for awards granted to executive level employees.