Stock Based Compensation - Summary of Changes in Number of Shares of Common Stock Under Option (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Options, Outstanding [Roll Forward]
Shares, outstanding, beginning balance (in shares)	6,146	5,003	5,131
Shares, granted (in shares)	2,885	1,780	654
Shares, exercised (in share)	(633)	(500)	(586)
Shares, forfeited (in shares)	(367)	(137)	(196)
Shares, outstanding, ending balance (in shares)	8,031	6,146	5,003
Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted average exercise price, outstanding, beginning balance (in usd per share)	$ 91.20	$ 72.72	$ 58.71
Weighted average exercise price, granted (in usd per share)	145.35	133.33	154.56
Weighted average exercise price, exercised (in usd per share)	71.43	42.36	33.97
Weighted average exercise price, forfeited (in usd per share)	144.51	140.67	95.16
Weighted average exercise price, outstanding, ending balance (in usd per share)	$ 109.78	$ 91.20	$ 72.72
Options, Additional Disclosures [Abstract]
Shares, expected to vest (in shares)	8,031
Weighted average exercise price of options to vest (in usd per share)	$ 109.78
Options exercisable (in shares)	4,029	3,429	2,545	2,370
Weighted average exercise price of options (in usd per share)	$ 75.80	$ 55.00	$ 26.82	$ 21.75
Weighted average fair value of options granted in period (in usd per share)	$ 32.57	$ 28.61	$ 35.32
Aggregate intrinsic value, options outstanding	$ 663,815	$ 309,238	$ 351,277	$ 461,770
Aggregate intrinsic value of options exercised during period	$ 76,546	$ 49,592	$ 63,863