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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share Reconciliation [Abstract]
Income available to common stockholders	$ 34,167	$ 30,586	$ 66,004	$ 60,206
Income available to common stockholders plus assumed exercises of stock options	$ 34,167	$ 30,586	$ 66,004	$ 60,206
Income available to common stockholders - Shares	32,629,146	32,716,350	32,620,819	32,645,693
Dilutive effect of stock options - Shares	688,047	742,207	685,610	744,269
Income available to common stockholders plus assumed exercises of stock options - Shares	33,317,193	33,458,557	33,306,429	33,389,962
Income available to common stockholders - Per Share Amount	$ 1.04	$ 0.93	$ 2.02	$ 1.84
Income available to common stockholders plus assumed exercises of stock options - Per Share Amount	$ 1.02	$ 0.91	$ 1.98	$ 1.80