Net Income Per Common Share - Basic and Diluted Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share Reconciliation [Abstract]
Income available to common stockholders									$ 70,674	$ 66,170	$ 63,887
Income available to common stockholders plus assumed exercises of stock options									$ 70,674	$ 66,170	$ 63,887
Income available to common stockholders - Shares									15,615,170	15,559,059	15,430,773
Effect of stock options - Shares									296,092	327,186	363,630
Income available to common stockholders plus assumed exercises of stock options - Shares									15,911,262	15,886,245	15,794,403
Income available to common stockholders - Per Share Amount	$ 1.19	$ 1.15	$ 1.12	$ 1.07	$ 1.00	$ 1.01	$ 1.19	$ 1.05	$ 4.53	$ 4.25	$ 4.14
Income available to common stockholders plus assumed exercises of stock options - Per Share Amount	$ 1.16	$ 1.13	$ 1.10	$ 1.05	$ 0.99	$ 0.98	$ 1.17	$ 1.03	$ 4.44	$ 4.17	$ 4.04