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, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share Basic [Abstract]
Net income – basic	$ 28,501	$ 23,353	$ 23,948	$ 10,800	$ 34,014	$ 21,386	$ 17,890	$ 19,400	$ 86,602	$ 92,690	$ 71,632
Basic weighted average common shares									19,288,996	19,248,657	19,208,978
Basic earnings per share	$ 1.48	$ 1.21	$ 1.24	$ 0.56	$ 1.77	$ 1.11	$ 0.93	$ 1.01	$ 4.49	$ 4.82	$ 3.73
Net income – diluted									$ 86,602	$ 92,690	$ 71,632
Weighted average common shares									19,288,996	19,248,657	19,208,978
Restricted stock and stock options									75,000	80,000	84,000
Diluted weighted average common shares									19,363,809	19,329,238	19,293,082
Decrease to diluted earnings per common share	$ 1.47	$ 1.21	$ 1.24	$ 0.56	$ 1.76	$ 1.11	$ 0.93	$ 1.01	$ 4.47	$ 4.80	$ 3.71