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Stock compensation plans- summary of stock option activity (Detail)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2016
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 1,442,912
Number of Options, Granted (in shares) | shares 338,902
Number of Options, Exercised (in shares) | shares (152,491)
Number of Options, Forfeited or Expired (in shares) | shares (21,578)
Number of Options, Outstanding, ending of year (in shares) | shares 1,607,745
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 $ 86.98
Weighted Average Exercise Price, Granted (in dollars per share) | $ / shares 145.99
Weighted Average Exercise Price, Exercised (in dollars per share) | $ / shares 80.56
Weighted Average Exercise Price, Forfeited or Expired (in dollars per share) | $ / shares 125.71
Weighted Average Exercise Price, Outstanding, beginning of year (in dollars per share) | $ / shares $ 99.51
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]  
Number of Options, Exercisable, end of year (in shares) | shares 1,003,895
Weighted Average Exercise Price, Exercisable, end of year (in dollars per share) | $ / shares $ 80.64
Weighted Average Remaining Contractual Life in Years, Outstanding, end of year 6 years 9 months 18 days
Weighted Average Remaining Contractual Life in Years, Exercisable, end of year 5 years 8 months 12 days
Aggregate Intrinsic Value, Outstanding, end of year | $ $ 99,180
Aggregate Intrinsic Value, Exercisable, end of year | $ $ 80,823