Net Income Per Common Share - Basic and Diluted Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share Reconciliation [Abstract]
Income available to common stockholders	$ 99,586	$ 134,879	$ 125,814
Income available to common stockholders plus assumed exercises of stock options	$ 99,586	$ 134,879	$ 125,814
Income available to common stockholders - Shares	32,672,522	32,639,396	32,689,228
Dilutive effect of stock options - Shares	538,430	690,448	741,486
Income available to common stockholders plus assumed exercises of stock options - Shares	33,210,952	33,329,844	33,430,714
Income available to common stockholders - Per Share Amount	$ 3.05	$ 4.13	$ 3.85
Income available to common stockholders plus assumed exercises of stock options - Per Share Amount	$ 3.00	$ 4.05	$ 3.76