Stock-Based Compensation - Option Activity (Detail) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Number of Shares
Options, Outstanding Beginning Balance (in shares) | shares	439,550
Options, Granted (in shares) | shares	129,400
Options, Exercised (in shares) | shares	(212,775)
Option, Expired (in shares) | shares	(5,800)
Options, Forfeited (in shares) | shares	(44,150)
Options, Outstanding Ending Balance (in shares) | shares	306,225
Options, Exercisable at end of period (in shares) | shares	86,425
Weighted-Average Exercise Price Per Share
Options, Beginning Balance (in dollars per share) | $ / shares	$ 20.07
Options, Granted (in dollars per share) | $ / shares	29.19
Options, Exercised (in dollars per share) | $ / shares	20.38
Options, Expired, (in dollars per share) | $ / shares	21.12
Options, Forfeited (in dollars per share) | $ / shares	22.19
Options, Ending Balance (in dollars per share) | $ / shares	23.38
Options, Exercisable at end of period (in dollars per share) | $ / shares	$ 20.78
Options, Outstanding at end of period, Weighted Average Remaining Contractual Life (Years)	5 years 2 months 12 days
Options, Exercisable at end of period, Weighted Average Remaining Contractual Life (Years)	2 years 10 months 24 days
Options, Outstanding at end of period, Aggregate Intrinsic Value | $	$ 1,654
Options, Exercisable at end of period, Aggregate Intrinsic Value | $	$ 663