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, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Earnings Per Share Reconciliation [Abstract]
Income available to common stockholders	$ 13,861	$ 14,491	$ 12,593	$ 13,372	$ 12,306	$ 13,860	$ 11,729	$ 14,005	$ 54,317	$ 51,900	$ 45,621
Income available to common stockholders plus assumed exercises of stock options									$ 54,317	$ 51,900	$ 45,621
Income available to common stockholders - Shares									15,268,843	15,172,457	15,267,357
Effect of stock options - Shares									279,979	271,188	282,774
Income available to common stockholders plus assumed exercises of stock options - Shares									15,548,822	15,443,645	15,550,131
Income available to common stockholders - Per Share Amount	$ 0.91	$ 0.94	$ 0.83	$ 0.88	$ 0.81	$ 0.91	$ 0.77	$ 0.93	$ 3.56	$ 3.42	$ 2.99
Income available to common stockholders plus assumed exercises of stock options - Per Share Amount	$ 0.88	$ 0.93	$ 0.82	$ 0.86	$ 0.79	$ 0.90	$ 0.76	$ 0.91	$ 3.49	$ 3.36	$ 2.93