Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Numerator:
Net income	$ 7,449	$ 5,309	$ 24,575	$ 19,078	$ 29,963
Denominator:
Weighted average shares outstanding for basic earnings per share	11,672	11,563	11,649	11,537
Effect of dilutive securities	461	249	452	215
Weighted average shares adjusted for dilutive securities	12,133	11,812	12,101	11,752
Basic	$ 0.64	$ 0.46	$ 2.11	$ 1.65
Diluted	$ 0.61	$ 0.45	$ 2.03	$ 1.62