Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share Basic [Abstract]
Net income – basic	$ 18,806	$ 19,701	$ 16,956	$ 16,169	$ 649	$ 16,577	$ 15,481	$ 13,524	$ 71,632	$ 46,231	$ 77,865
Basic weighted-average common shares									19,208,978	19,165,489	19,105,806
Basic earnings per share	$ 0.98	$ 1.03	$ 0.88	$ 0.84	$ 0.03	$ 0.86	$ 0.81	$ 0.71	$ 3.73	$ 2.41	$ 4.08
Net income - diluted									$ 71,632	$ 46,231	$ 77,865
Weighted average common shares									19,208,978	19,165,489	19,105,806
Stock options and restricted stock									84,104	80,377	97,844
Diluted weighted average common shares									19,293,082	19,245,866	19,203,650
Diluted earnings per common share	$ 0.98	$ 1.02	$ 0.88	$ 0.84	$ 0.03	$ 0.86	$ 0.81	$ 0.70	$ 3.71	$ 2.40	$ 4.05