Schedule of Information about All Outstanding Options (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number, Options Outstanding	9,639,949	8,770,133	8,400,183	7,897,451
Weighted average exercise price, Options Outstanding	$ 112.40	$ 99.06	$ 80.50	$ 57.29
Aggregate intrinsic value, Options Outstanding	$ 65,133,533
Weighted average remaining contractual life, Options Outstanding	2 years 10 months 24 days
Number, Options Exercisable	2,869,346
Weighted Average Exercise Price, Options Exercisable	$ 100.51
Aggregate intrinsic value, Options Exercisable	$ 44,588,121
Weighted average remaining contractual life, Options Exercisable	2 years