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
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Calculation of Basic Earnings Per Share:
Net Income	$ 8,304	$ 28,664	$ 6,387	$ 26,854	$ 36,968	$ 33,241
Weighted shares outstanding (shares)	16,401,028		16,369,641		16,393,022	16,360,540
Basic Earnings Per Share (in dollars per share)	$ 0.51		$ 0.39		$ 2.26	$ 2.03
Reconciliation of Numerator:
Net Income	$ 8,304	$ 28,664	$ 6,387	$ 26,854	$ 36,968	$ 33,241
Reconciliation of Denominator:
Weighted shares outstanding - Basic (shares)	16,401,028		16,369,641		16,393,022	16,360,540
Effect of dilutive securities:
Share-based compensation (shares)	44,715		47,441		46,311	49,521
Adjusted denominator-Diluted (shares)	16,445,743		16,417,082		16,439,333	16,410,061
Diluted Earnings Per Share (in dollars per share)	$ 0.50		$ 0.39		$ 2.25	$ 2.03