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Option Activity under Stock-Based Compensation Plans (Detail)
$ / shares in Units, shares in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2018
USD ($)
$ / shares
shares
Shares Underlying Outstanding Options  
Beginning balance | shares 3,555
Options granted | shares 2,503
Options exercised | shares (42)
Ending balance | shares 5,750
Vested and expected to vest at December 31, 2018 | shares 5,534
Exercisable at December 31, 2018 | shares 2,975
Weighted-Average Exercise Price Per Share  
Beginning balance | $ / shares $ 19.56
Options granted | $ / shares 16.30
Options exercised | $ / shares 11.80
Ending balance | $ / shares 18.20
Vested and expected to vest at December 31, 2018 | $ / shares 18.28
Exercisable at December 31, 2018 | $ / shares $ 19.90
Weighted-Average Remaining Contractual Term (years)  
Balance at December 31, 2018 5 years 5 months 19 days
Vested and expected to vest at December 31, 2018 5 years 5 months 8 days
Exercisable at December 31, 2018 4 years 10 months 13 days
Aggregate Intrinsic Value  
Balance at December 31, 2018 | $ $ 651
Vested and expected to vest at December 31, 2018 | $ 644
Exercisable at December 31, 2018 | $ $ 466
Unvested  
Shares Underlying Outstanding Options  
Options cancelled | shares (178)
Weighted-Average Exercise Price Per Share  
Options cancelled | $ / shares $ 14.29
Vested  
Shares Underlying Outstanding Options  
Options cancelled | shares (88)
Weighted-Average Exercise Price Per Share  
Options cancelled | $ / shares $ 29.58