Share-Based Compensation - Share-Based Payment Award Activity (Detail)	6 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares
Options
Outstanding, beginning of period, in shares | shares	808,049.000
Granted, in shares | shares	87,352
Exercised, in shares | shares	(156,580)
Forfeited, in shares | shares	(1,926)
Expired, in shares | shares	(2,337)
Outstanding, end of period, in shares | shares	734,558
Vested and expected to vest, end of period, in shares | shares	710,108
Exercisable, end of period, in shares | shares	449,728
Weighted Average Exercise Price
Outstanding, beginning of period, in usd per share | $ / shares	$ 105.68
Granted, in usd per share | $ / shares	305.22
Exercised, in usd per share | $ / shares	84.83
Forfeited, in usd per share | $ / shares	130.69
Expired, in usd per share | $ / shares	60.37
Outstanding, end of period, in usd per share | $ / shares	133.93
Vested and expected to vest, end of period, in usd per share | $ / shares	131.27
Exercisable, end of period, in usd per share | $ / shares	$ 97.80
Weighted Average Remaining Contractual Life - in Years
Outstanding, end of period	3 years 11 months 1 day
Vested and expected to vest, end of period	3 years 10 months 6 days
Exercisable, end of period	2 years 9 months 26 days
Aggregate Intrinsic Value of In-The-Money Options
Outstanding, end of period | $	$ 115,185,279
Vested and expected to vest, end of period | $	113,119,954
Exercisable, end of period | $	$ 85,881,104