Share-Based Compensation - Reconciliation of 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, Share-based Payments [Abstract]
Number of options, Outstanding, Beginning Balance	897,000
Number of Options, Granted	126,000	167,000	54,100
Number of Options, Exercised	(263,000)
Number of Options, Forfeited or expired	(14,000)
Number of Options, Outstanding, Ending Balance	746,000	897,000
Number of Options, Options Exercisable	573,000
Weighted-Average Exercise Price, Outstanding, Beginning Balance	$ 12.22
Weighted-Average Exercise Price, Granted	24.20
Weighted-Average Exercise Price, Exercised	11.84
Weighted-Average Exercise Price, Forfeited or expired	14.34
Weighted-Average Exercise Price, Outstanding, Ending Balance	14.33	$ 12.22
Weighted-Average Exercise Price, Options exercisable	$ 12.75
Weighted-Average Remaining Contractual Term, Outstanding	6 years 2 months 12 days
Weighted- Average Remaining Contractual Term, Options exercisable	5 years 6 months
Aggregate Intrinsic Value, Exercised	$ 3,784	$ 2,900	$ 2,600
Aggregate Intrinsic Value, Outstanding	9,900
Aggregate Intrinsic Value, Options exercisable	$ 8,506