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Stock compensation plans- Summary of stock option activity (Detail)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2020
USD ($)
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]  
Number of Options, Outstanding, beginning of year (in shares) | shares 1,325,532
Number of Options, Granted (in shares) | shares 130,206
Number of Options, Exercised (in shares) | shares (289,324)
Number of Options, Forfeited or Expired (in shares) | shares (9,099)
Number of Options, Outstanding, ending of year (in shares) | shares 1,157,315
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]  
Weighted Average Exercise Price, Outstanding, beginning of year (in dollars per share) | $ / shares $ 161.91
Weighted Average Exercise Price, Granted (in dollars per share) | $ / shares 347.64
Weighted Average Exercise Price, Exercised (in dollars per share) | $ / shares 106.79
Weighted Average Exercise Price, Forfeited or Expired (in dollars per share) | $ / shares 290.49
Weighted Average Exercise Price, Outstanding, beginning of year (in dollars per share) | $ / shares $ 195.57
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]  
Number of Options, Exercisable, end of year (in shares) | shares 903,680
Weighted Average Exercise Price, Exercisable, end of year (in dollars per share) | $ / shares $ 163.27
Weighted Average Remaining Contractual Life in Years, Outstanding, end of year 5 years 8 months 15 days
Weighted Average Remaining Contractual Life in Years, Exercisable, end of year 4 years 11 months 8 days
Aggregate Intrinsic Value, Outstanding, end of year | $ $ 249,979
Aggregate Intrinsic Value, Exercisable, end of year | $ $ 224,388