Net Income Per Common Share - Basic and Diluted Net Income Per Common Share Based on Weighted-average Shares Outstanding (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Earnings Per Share Reconciliation [Abstract]
Income available to common stockholders	$ 17,982	$ 15,630	$ 52,054	$ 50,442
Income available to common stockholders plus assumed exercises of stock options	$ 17,982	$ 15,630	$ 52,054	$ 50,442
Income available to common stockholders - Shares	15,631,094	15,581,593	15,571,990	15,542,027
Dilutive effect of stock options - Shares	291,115	324,531	290,790	329,964
Income available to common stockholders plus assumed exercises of stock options - Shares	15,922,209	15,906,124	15,862,780	15,871,991
Income available to common stockholders - Per Share Amount	$ 1.15	$ 1.01	$ 3.34	$ 3.25
Income available to common stockholders plus assumed exercises of stock options - Per Share Amount	$ 1.13	$ 0.98	$ 3.28	$ 3.18