Share-Based Compensation - Share-Based Payment Award Activity (Detail)	3 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares
Options
Outstanding, beginning of period, in shares | shares	808,049.000
Exercised, in shares | shares	(51,271)
Granted, in shares | shares	471
Forfeited, in shares | shares	(1,926)
Outstanding, end of period, in shares | shares	755,323
Vested and expected to vest, end of period, in shares | shares	738,585
Exercisable, end of period, in shares | shares	416,281
Weighted Average Exercise Price
Outstanding, beginning of period, in usd per share | $ / shares	$ 105.68
Granted, in usd per share | $ / shares	260.72
Exercised, in usd per share | $ / shares	68.05
Forfeited, in usd per share | $ / shares	130.69
Outstanding, end of period, in usd per share | $ / shares	108.26
Vested and expected to vest, end of period, in usd per share | $ / shares	107.43
Exercisable, end of period, in usd per share | $ / shares	$ 87.28
Weighted Average Remaining Contractual Life - in Years
Outstanding, end of period	3 years 7 months 11 days
Vested and expected to vest, end of period	3 years 6 months 25 days
Exercisable, end of period	2 years 5 months 30 days
Aggregate Intrinsic Value of In-The-Money Options
Outstanding, end of period | $	$ 99,521,692
Vested and expected to vest, end of period | $	97,934,520
Exercisable, end of period | $	$ 63,585,697