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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Earnings Per Share Reconciliation [Abstract]
Income available to common stockholders	$ 17,493	$ 18,553	$ 34,072	$ 34,812
Income available to common stockholders plus assumed exercises of stock options	$ 17,493	$ 18,553	$ 34,072	$ 34,812
Income available to common stockholders - Shares	15,549,811	15,536,325	15,542,114	15,521,916
Dilutive effect of stock options - Shares	292,674	328,599	288,563	330,616
Income available to common stockholders plus assumed exercises of stock options - Shares	15,842,485	15,864,924	15,830,677	15,852,532
Income available to common stockholders - Per Share Amount	$ 1.12	$ 1.19	$ 2.19	$ 2.24
Income available to common stockholders plus assumed exercises of stock options - Per Share Amount	$ 1.10	$ 1.17	$ 2.15	$ 2.20