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
	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share Reconciliation [Abstract]
Income available to common stockholders	$ 29,620	$ 22,050
Income available to common stockholders plus assumed exercises of stock options	$ 29,620	$ 22,050
Income available to common stockholders - Shares	32,574,251	31,729,614
Dilutive effect of stock options - Shares	743,493	746,382
Income available to common stockholders plus assumed exercises of stock options - Shares	33,317,744	32,475,996
Income available to common stockholders - Per Share Amount	$ 0.91	$ 0.70
Income available to common stockholders plus assumed exercises of stock options - Per Share Amount	$ 0.89	$ 0.68