Stock-Based Compensation Plans - Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares, Beginning balance	529,611	855,740	939,036
Number of Shares, Granted	168,217	54,838	112,988
Number of Shares, Canceled	(1,676)		(874)
Number of Shares, Exercised	(120,588)	(380,967)	(195,410)
Number of Shares, Ending Balance	575,564	529,611	855,740
Number of Shares, Exercisable	203,564
Weighted Average Exercise Price, Beginning Balance	$ 67.60	$ 36.80	$ 23.83
Weighted Average Exercise Price, Granted	261.19	156.85	102.40
Weighted Average Exercise Price, Canceled	142.69		101.77
Weighted Average Exercise Price, Exercised	24.66	11.26	12.35
Weighted Average Exercise Price, Ending Balance	132.93	$ 67.60	$ 36.80
Weighted Average Exercise Price, Exercisable	$ 53.49
Remaining Contractual Term, Outstanding at December 31, 2018	3 years 2 months 12 days
Remaining Contractual Term, Exercisable at December 31, 2018	2 years 1 month 6 days
Intrinsic Value, Exercised	$ 21,823
Intrinsic Value, Ending Balance	53,651
Intrinsic Value, Exercisable	$ 32,126