Net Income Per Common Share - Basic and Diluted Net Income Per Common Share (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share Reconciliation [Abstract]
Income available to common stockholders	$ 15,749	$ 18,793	$ 14,688	$ 14,657	$ 13,861	$ 14,491	$ 12,593	$ 13,372	$ 63,887	$ 54,317	$ 51,900
Income available to common stockholders plus assumed exercises of stock options									$ 63,887	$ 54,317	$ 51,900
Income available to common stockholders - Shares									15,430,773	15,268,843	15,172,457
Effect of stock options - Shares									363,630	279,979	271,188
Income available to common stockholders plus assumed exercises of stock options - Shares									15,794,403	15,548,822	15,443,645
Income available to common stockholders - Per Share Amount	$ 1.02	$ 1.22	$ 0.94	$ 0.96	$ 0.91	$ 0.94	$ 0.83	$ 0.88	$ 4.14	$ 3.56	$ 3.42
Income available to common stockholders plus assumed exercises of stock options - Per Share Amount	$ 0.99	$ 1.19	$ 0.92	$ 0.94	$ 0.88	$ 0.93	$ 0.82	$ 0.86	$ 4.04	$ 3.49	$ 3.36