Net Income Per Common Share - Basic and Diluted Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Earnings Per Share Reconciliation [Abstract]
Income available to common stockholders	$ 15,630	$ 18,793	$ 50,442	$ 48,138
Income available to common stockholders plus assumed exercises of stock options	$ 15,630	$ 18,793	$ 50,442	$ 48,138
Income available to common stockholders - Shares	15,581,593	15,425,920	15,542,027	15,412,611
Effect of stock options - Shares	324,531	369,923	329,964	361,686
Income available to common stockholders plus assumed exercises of stock options - Shares	15,906,124	15,795,843	15,871,991	15,774,297
Income available to common stockholders - Per Share Amount	$ 1.01	$ 1.22	$ 3.25	$ 3.12
Income available to common stockholders plus assumed exercises of stock options - Per Share Amount	$ 0.98	$ 1.19	$ 3.18	$ 3.05