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Option Activity under Stock-Based Compensation Plans (Detail)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended
Mar. 31, 2018
USD ($)
$ / shares
shares
Shares Underlying Outstanding Options  
Beginning balance | shares 3,555
Options granted | shares 1,807
Options exercised | shares (25)
Ending balance | shares 5,307
Vested and expected to vest at March 31, 2018 | shares 5,030
Exercisable at March 31, 2018 | shares 2,462
Weighted-Average Exercise Price Per Share  
Beginning balance | $ / shares $ 19.56
Options granted | $ / shares 16.81
Options exercised | $ / shares 14.67
Ending balance | $ / shares 18.47
Vested and expected to vest at March 31, 2018 | $ / shares 18.56
Exercisable at March 31, 2018 | $ / shares $ 20.55
Weighted-Average Remaining Contractual Term (years)  
Balance at March 31, 2018 6 years 2 months 4 days
Vested and expected to vest at March 31, 2018 6 years 1 month 13 days
Exercisable at March 31, 2018 5 years 7 months 9 days
Aggregate Intrinsic Value  
Balance at March 31, 2018 | $ $ 15,064
Vested and expected to vest at March 31, 2018 | $ 14,156
Exercisable at March 31, 2018 | $ $ 4,985
Unvested  
Shares Underlying Outstanding Options  
Options cancelled | shares (2)
Weighted-Average Exercise Price Per Share  
Options cancelled | $ / shares $ 21.00
Vested  
Shares Underlying Outstanding Options  
Options cancelled | shares (28)
Weighted-Average Exercise Price Per Share  
Options cancelled | $ / shares $ 50.70