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Option Activity under Stock-Based Compensation Plans (Detail)
$ / shares in Units, shares in Thousands, $ in Thousands
 9 Months Ended
Sep. 30, 2018
USD ($)
$ / shares
shares
Shares Underlying Outstanding Options  
Beginning balance | shares 3,555
Options granted | shares 2,386
Options exercised | shares (40)
Ending balance | shares 5,683
Vested and expected to vest at September 30, 2018 | shares 5,422
Exercisable at September 30, 2018 | shares 2,766
Weighted-Average Exercise Price Per Share  
Beginning balance | $ / shares $ 19.56
Options granted | $ / shares 16.56
Options exercised | $ / shares 11.95
Ending balance | $ / shares 18.33
Vested and expected to vest at September 30, 2018 | $ / shares 18.41
Exercisable at September 30, 2018 | $ / shares $ 20.01
Weighted-Average Remaining Contractual Term (years)  
Balance at September 30, 2018 5 years 9 months 3 days
Vested and expected to vest at September 30, 2018 5 years 8 months 19 days
Exercisable at September 30, 2018 5 years 1 month 20 days
Aggregate Intrinsic Value  
Balance at September 30, 2018 | $ $ 1,448
Vested and expected to vest at September 30, 2018 | $ 1,419
Exercisable at September 30, 2018 | $ $ 927
Unvested  
Shares Underlying Outstanding Options  
Options cancelled | shares (139)
Weighted-Average Exercise Price Per Share  
Options cancelled | $ / shares $ 13.89
Vested  
Shares Underlying Outstanding Options  
Options expired | shares (79)
Weighted-Average Exercise Price Per Share  
Options expired | $ / shares $ 30.61