Share-Based Compensation - Stock Option Activity (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Shares
Number of Shares, Outstanding Beginning Balance	5,156	5,164	5,537
Number of Shares, Granted	824	846	747
Number of Shares, Exercised	(1,113)	(827)	(1,118)
Number of Shares, Forfeited	(50)	(27)	(2)
Number of Shares, Outstanding Ending Balance	4,817	5,156	5,164
Number of Shares, Exercisable at December 31, 2015	2,355
Weighted Average Exercise Price
Weighted Average Exercise Price, Outstanding (in dollar per share)	$ 83.64	$ 64.43	$ 44.61
Weighted Average Exercise Price, Granted (in dollars per share)	176.26	147.25	145.71
Weighted Average Exercise Price, Exercised (in dollars per share)	44.80	28.43	20.71
Weighted Average Exercise Price, Forfeited (in dollars per share)	134.83	95.33	19.61
Weighted Average Exercise Price, Outstanding (in dollars per share)	107.91	$ 83.64	$ 64.43
Weighted Average Exercise Price, Exercisable at December 31, 2015 (in dollars per share)	$ 78.40
Weighted Average Remaining Contractual Life (Years), Outstanding at December 31, 2015	7 years
Weighted Average Remaining Contractual Life (Years), Exercisable at December 31, 2015	6 years
Aggregate Intrinsic Value, Outstanding at December 31, 2015	$ 334,940
Aggregate Intrinsic Value, Exercisable at December 31, 2015	$ 232,366