Net Income Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Basic and diluted net income per common share
Income available to common stockholders - Income	$ 11,729	$ 10,115	$ 25,734	$ 21,470
Income available to common stockholders - Shares	15,155,525	15,364,738	15,145,066	15,362,764
Income available to common stockholders - Per Share Amount	$ 0.77	$ 0.66	$ 1.70	$ 1.40
Effect of stock options - Shares	271,271	287,215	276,608	295,723
Income available to common stockholders plus assumed exercises of stock options - Income	$ 11,729	$ 10,115	$ 25,734	$ 21,470
Income available to common stockholders plus assumed exercises of stock options - Shares	15,426,796	15,651,953	15,421,674	15,658,487
Income available to common stockholders plus assumed exercises of stock options - Per Share Amount	$ 0.76	$ 0.65	$ 1.67	$ 1.37