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Summary of Stock Option Activity (Detail)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Options, Outstanding beginning balance | shares 294,250
Options, Granted | shares 179,750
Options, Exercised | shares (39,751)
Options, Forfeited | shares (34,166)
Options, Expired | shares (1,250)
Options, Outstanding ending balance | shares 398,833
Options, Exercisable | shares 33,919
Weighted-Average Exercise Price, Outstanding beginning balance | $ / shares $ 122.80
Weighted-Average Exercise Price, Granted | $ / shares 150.35
Weighted-Average Exercise Price, Exercised | $ / shares 98.05
Weighted-Average Exercise Price, Forfeited | $ / shares 139.40
Weighted-Average Exercise Price, Expired | $ / shares 67.25
Weighted-Average Exercise Price, Outstanding ending balance | $ / shares 136.44
Weighted-Average Exercise Price, Exercisable | $ / shares $ 121.65
Weighted-Average Remaining Contractual Term (in years), Outstanding 3 years 5 months 20 days
Weighted-Average Remaining Contractual Term (in years), Exercisable 2 years 9 months 18 days
Aggregate Intrinsic Value, Outstanding | $ $ 13,401
Aggregate Intrinsic Value, Exercisable | $ $ 1,641