Calculation of Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ 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
Calculation of Basic Earnings Per Share:
Net Income	$ 5,538	$ 6,833	$ 38,779	$ 32,023	$ 58,124
Weighted shares outstanding (shares)	16,378,545	16,344,442	16,366,608	16,334,210
Basic Earnings Per Share (in dollars per share)	$ 0.34	$ 0.42	$ 2.37	$ 1.96
Reconciliation of Numerator:
Net Income	$ 5,538	$ 6,833	$ 38,779	$ 32,023	$ 58,124
Reconciliation of Denominator:
Weighted shares outstanding - Basic (shares)	16,378,545	16,344,442	16,366,608	16,334,210
Effect of dilutive securities:
Share-based compensation (shares)	49,894	45,193	49,647	44,423
Adjusted denominator-Diluted (shares)	16,428,439	16,389,635	16,416,255	16,378,633
Diluted Earnings Per Share (in dollars per share)	$ 0.34	$ 0.42	$ 2.36	$ 1.96