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Summary of Stock Options under Employee Equity Incentive Plan and Stock Option Plan for Non-Employee Directors (Detail)
$ / shares in Units, $ in Thousands
3 Months Ended
Mar. 31, 2018
USD ($)
$ / shares
shares
Shares  
Outstanding at beginning of period | shares 1,156,997
Granted | shares 17,531
Forfeited | shares 0
Expired | shares 0
Exercised | shares (168,773)
Outstanding at end of period | shares 1,005,755
Exercisable at end of period | shares 178,692
Vested and expected to vest at end of period | shares 374,181
Weighted-Average Exercise Price  
Outstanding at beginning of period | $ / shares $ 158.53
Granted | $ / shares 191.10
Forfeited | $ / shares 0
Expired | $ / shares 0
Exercised | $ / shares 113.82
Outstanding at end of period | $ / shares 178.90
Exercisable at end of period | $ / shares 104.02
Vested and expected to vest at end of period | $ / shares $ 143.93
Weighted-Average Remaining Contractual Term  
Outstanding at end of period 6 years 11 months 19 days
Exercisable at end of period 3 years 6 months 14 days
Vested and expected to vest at end of period 5 years 6 months 29 days
Aggregate Intrinsic Value  
Outstanding at end of period | $ $ 19,949
Exercisable at end of period | $ 16,108
Vested and expected to vest at end of period | $ $ 19,015