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		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Earnings Per Share Reconciliation [Abstract]
Income available to common stockholders	$ 12,593	$ 11,729	$ 25,965	$ 25,734
Income available to common stockholders plus assumed exercises of stock options	$ 12,593	$ 11,729	$ 25,965	$ 25,734
Income available to common stockholders - Shares	15,232,129	15,155,525	15,235,397	15,145,066
Effect of stock options - Shares	247,620	271,271	246,473	276,608
Income available to common stockholders plus assumed exercises of stock options - Shares	15,479,749	15,426,796	15,481,870	15,421,674
Income available to common stockholders - Per Share Amount	$ 0.83	$ 0.77	$ 1.70	$ 1.70
Income available to common stockholders plus assumed exercises of stock options - Per Share Amount	$ 0.81	$ 0.76	$ 1.68	$ 1.67