Calculation of Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Calculation of Basic Earnings Per Share:
Net Income	$ 6,387	$ 6,046	$ 33,241	$ 25,190	$ 58,124
Weighted shares outstanding (shares)	16,369,641	16,340,665	16,360,540	16,329,009
Basic Earnings Per Share (in dollars per share)	$ 0.39	$ 0.37	$ 2.03	$ 1.54
Reconciliation of Numerator:
Net Income	$ 6,387	$ 6,046	$ 33,241	$ 25,190	$ 58,124
Reconciliation of Denominator:
Weighted shares outstanding - Basic (shares)	16,369,641	16,340,665	16,360,540	16,329,009
Effect of dilutive securities:
Share-based compensation (shares)	47,441	41,542	49,521	44,029
Adjusted denominator-Diluted (shares)	16,417,082	16,382,207	16,410,061	16,373,038
Diluted Earnings Per Share (in dollars per share)	$ 0.39	$ 0.37	$ 2.03	$ 1.54