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Stock compensation plans- summary of stock option activity (Detail)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2018
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 1,708,928
Number of Options, Granted (in shares) | shares 155,498
Number of Options, Exercised (in shares) | shares (383,198)
Number of Options, Forfeited or Expired (in shares) | shares (9,779)
Number of Options, Outstanding, ending of year (in shares) | shares 1,471,449
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 $ 113.49
Weighted Average Exercise Price, Granted (in dollars per share) | $ / shares 254.60
Weighted Average Exercise Price, Exercised (in dollars per share) | $ / shares 80.51
Weighted Average Exercise Price, Forfeited or Expired (in dollars per share) | $ / shares 170.78
Weighted Average Exercise Price, Outstanding, beginning of year (in dollars per share) | $ / shares $ 136.62
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]  
Number of Options, Exercisable, end of year (in shares) | shares 1,073,198
Weighted Average Exercise Price, Exercisable, end of year (in dollars per share) | $ / shares $ 112.13
Weighted Average Remaining Contractual Life in Years, Outstanding, end of year 5 years 8 months 6 days
Weighted Average Remaining Contractual Life in Years, Exercisable, end of year 4 years 11 months 19 days
Aggregate Intrinsic Value, Outstanding, end of year | $ $ 179,396
Aggregate Intrinsic Value, Exercisable, end of year | $ $ 157,070