Stock-Based Compensation - Summary of Activity Under Stock Option Plan (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Outstanding at beginning period - Options	1,414,900	2,036,298
Options granted	121,500	60,000
Options exercised	(262,775)	(621,398)
Options canceled, forfeited or expired - Options		(60,000)
Outstanding at ending period - Options	1,273,625	1,414,900
Exercisable at ending period - Options	566,625	650,400
Outstanding at beginning period - Wgtd. Avg. Exercise Price	$ 22.46	$ 20.35
Options granted - Wgtd. Avg. Exercise Price	48.26	29.04
Options exercised - Wgtd. Avg. Exercise Price	17.73	15.64
Options cancelled, forfeited, or expired - Wgtd. Avg. Exercise Price		27.94
Outstanding at ending period - Wgtd. Avg. Exercise Price	25.90	22.46
Exercisable at ending period - Wgtd. Avg. Exercise Price	$ 20.07	$ 18.60
Outstanding at ending period - Wgtd. Avg. Remaining Contractual Term, years	9 years 8 months 23 days	9 years 7 months 2 days
Exercisable at ending period - Wgtd. Avg. Remaining Contractual Term, years	6 years 11 months 1 day	6 years 9 months 11 days
Outstanding at ending period - Aggregate Intrinsic Value	$ 32,165	$ 34,049
Exercisable at ending period - Aggregate Intrinsic Value	$ 17,611	$ 18,167