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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share Reconciliation [Abstract]
Income available to common stockholders	$ 32,883	$ 21,710	$ 93,089	$ 66,942
Income available to common stockholders plus assumed exercises of stock options	$ 32,883	$ 21,710	$ 93,089	$ 66,942
Income available to common stockholders - Shares	32,742,480	31,838,392	32,678,310	31,792,270
Dilutive effect of stock options - Shares	761,663	753,885	752,451	741,949
Income available to common stockholders plus assumed exercises of stock options - Shares	33,504,143	32,592,277	33,430,761	32,534,219
Income available to common stockholders - Per Share Amount	$ 1.01	$ 0.68	$ 2.85	$ 2.11
Income available to common stockholders plus assumed exercises of stock options - Per Share Amount	$ 0.98	$ 0.67	$ 2.78	$ 2.06