Stock-Based Compensation Plans - Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares, Beginning balance	855,740	939,036	1,014,420
Number of Shares, Granted	54,838	112,988	120,650
Number of Shares, Canceled		(874)
Number of Shares, Exercised	(380,967)	(195,410)	(196,034)
Number of Shares, Ending Balance	529,611	855,740	939,036
Number of Shares, Exercisable	254,079
Weighted Average Exercise Price, Beginning Balance	$ 36.80	$ 23.83	$ 13.81
Weighted Average Exercise Price, Granted	156.85	102.40	88.15
Weighted Average Exercise Price, Canceled		101.77
Weighted Average Exercise Price, Exercised	11.26	12.35	11.60
Weighted Average Exercise Price, Ending Balance	67.60	$ 36.80	$ 23.83
Weighted Average Exercise Price, Exercisable	$ 24.61
Remaining Contractual Term, Outstanding at December 31, 2016	3 years 3 months 18 days
Remaining Contractual Term, Exercisable at December 31, 2016	3 years
Intrinsic Value, Exercised	$ 68,068
Intrinsic Value, Ending Balance	71,046
Intrinsic Value, Exercisable	$ 45,009