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Stock compensation plans- summary of stock option activity (Detail)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2019
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 1,471,449
Number of Options, Granted (in shares) | shares 162,087
Number of Options, Exercised (in shares) | shares (291,696)
Number of Options, Forfeited or Expired (in shares) | shares (16,308)
Number of Options, Outstanding, ending of year (in shares) | shares 1,325,532
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 $ 136.62
Weighted Average Exercise Price, Granted (in dollars per share) | $ / shares 288.45
Weighted Average Exercise Price, Exercised (in dollars per share) | $ / shares 99.96
Weighted Average Exercise Price, Forfeited or Expired (in dollars per share) | $ / shares 245.65
Weighted Average Exercise Price, Outstanding, beginning of year (in dollars per share) | $ / shares $ 161.91
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]  
Number of Options, Exercisable, end of year (in shares) | shares 1,018,703
Weighted Average Exercise Price, Exercisable, end of year (in dollars per share) | $ / shares $ 133.71
Weighted Average Remaining Contractual Life in Years, Outstanding, end of year 6 years
Weighted Average Remaining Contractual Life in Years, Exercisable, end of year 5 years 3 months 18 days
Aggregate Intrinsic Value, Outstanding, end of year | $ $ 284,371
Aggregate Intrinsic Value, Exercisable, end of year | $ $ 247,275