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, 2017	Mar. 31, 2016
Earnings Per Share Reconciliation [Abstract]
Income available to common stockholders	$ 22,050	$ 16,579
Income available to common stockholders plus assumed exercises of stock options	$ 22,050	$ 16,579
Income available to common stockholders - Shares	15,864,807	15,534,416
Dilutive effect of stock options - Shares	373,191	281,955
Income available to common stockholders plus assumed exercises of stock options - Shares	16,237,998	15,816,371
Income available to common stockholders - Per Share Amount	$ 1.39	$ 1.07
Income available to common stockholders plus assumed exercises of stock options - Per Share Amount	$ 1.36	$ 1.05