Stock compensation plans- summary of stock option activity (Detail) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017 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	1,607,745
Number of Options, Granted (in shares) | shares	260,171
Number of Options, Exercised (in shares) | shares	(144,995)
Number of Options, Forfeited or Expired (in shares) | shares	(13,993)
Number of Options, Outstanding, ending of year (in shares) | shares	1,708,928
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	$ 99.51
Weighted Average Exercise Price, Granted (in dollars per share) | $ / shares	191.42
Weighted Average Exercise Price, Exercised (in dollars per share) | $ / shares	94.83
Weighted Average Exercise Price, Forfeited or Expired (in dollars per share) | $ / shares	148.94
Weighted Average Exercise Price, Outstanding, beginning of year (in dollars per share) | $ / shares	$ 113.49
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Number of Options, Exercisable, end of year (in shares) | shares	1,168,768
Weighted Average Exercise Price, Exercisable, end of year (in dollars per share) | $ / shares	$ 91.21
Weighted Average Remaining Contractual Life in Years, Outstanding, end of year	6 years 5 months
Weighted Average Remaining Contractual Life in Years, Exercisable, end of year	5 years 5 months
Aggregate Intrinsic Value, Outstanding, end of year | $	$ 231,261
Aggregate Intrinsic Value, Exercisable, end of year | $	$ 184,206