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Stock compensation plans- summary of stock option activity (Detail)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2015
USD ($)
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]  
Number of Options, Outstanding, beginning of year | shares 1,233,672
Number of Options, Granted | shares 353,688
Number of Options, Exercised | shares (112,941)
Number of Options, Forfeited or Expired | shares (31,507)
Number of Options, Outstanding, ending of year | shares 1,442,912
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 | $ / shares $ 75.93
Weighted Average Exercise Price, Granted | $ / shares 121.10
Weighted Average Exercise Price, Exercised | $ / shares 68.53
Weighted Average Exercise Price, Forfeited or Expired | $ / shares 103.42
Weighted Average Exercise Price, Outstanding, beginning of year | $ / shares $ 86.98
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]  
Number of Options, Exercisable, end of year | shares 839,149
Weighted Average Exercise Price, Exercisable, end of year | $ / shares $ 71.65
Weighted Average Remaining Contractual Life in Years, Exercisable, end of year 6 years
Aggregate Intrinsic Value, Exercisable, end of year | $ $ 50,180
Weighted Average Remaining Contractual Life in Years, Outstanding, end of year 7 years
Aggregate Intrinsic Value, Outstanding, end of year | $ $ 63,480