Share-Based Compensation - Share-Based Payment Award Activity (Detail)	9 Months Ended
	Sep. 30, 2018 USD ($) $ / shares shares
Options
Outstanding, beginning of period, in shares | shares	808,049
Granted, in shares | shares	89,616
Exercised, in shares | shares	(191,585)
Forfeited, in shares | shares	(9,498)
Expired, in shares | shares	(2,337)
Outstanding, end of period, in shares | shares	694,245
Vested and expected to vest, end of period, in shares | shares	673,674
Exercisable, end of period, in shares | shares	415,993
Weighted Average Exercise Price
Outstanding, beginning of period, in usd per share | $ / shares	$ 105.68
Granted, in usd per share | $ / shares	305.71
Exercised, in usd per share | $ / shares	83.37
Forfeited, in usd per share | $ / shares	187.76
Expired, in usd per share | $ / shares	60.37
Outstanding, end of period, in usd per share | $ / shares	136.68
Vested and expected to vest, end of period, in usd per share | $ / shares	134.25
Exercisable, end of period, in usd per share | $ / shares	$ 99.63
Weighted Average Remaining Contractual Life - in Years
Outstanding, end of period	3 years 9 months 3 days
Vested and expected to vest, end of period	3 years 8 months 11 days
Exercisable, end of period	2 years 7 months 27 days
Aggregate Intrinsic Value of In-The-Money Options
Outstanding, end of period | $	$ 120,932,288
Vested and expected to vest, end of period | $	118,982,041
Exercisable, end of period | $	$ 87,856,349