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, 2019	Mar. 31, 2018
Earnings Per Share Reconciliation [Abstract]
Income available to common stockholders	$ 31,837	$ 29,620
Income available to common stockholders plus assumed exercises of stock options	$ 31,837	$ 29,620
Income available to common stockholders - Shares	32,612,399	32,574,251
Dilutive effect of stock options - Shares	680,453	743,493
Income available to common stockholders plus assumed exercises of stock options - Shares	33,292,852	33,317,744
Income available to common stockholders - Per Share Amount	$ 0.98	$ 0.91
Income available to common stockholders plus assumed exercises of stock options - Per Share Amount	$ 0.96	$ 0.89