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Net Income Per Common Share (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Basic and diluted net income per common share                      
Income available to common stockholders - Income $ 12,306 $ 13,860 $ 11,729 $ 14,005 $ 11,598 $ 12,553 $ 10,115 $ 11,355 $ 51,900 $ 45,621 $ 42,309
Income available to common stockholders - Shares                 15,172,457 15,267,357 15,348,102
Income available to common stockholders - Per Share Amount $ 0.81 $ 0.91 $ 0.77 $ 0.93 $ 0.77 $ 0.82 $ 0.66 $ 0.74 $ 3.42 $ 2.99 $ 2.76
Effect of stock options - Shares                 271,188 282,774 303,210
Income available to common stockholders plus assumed exercises of stock options - Income                 $ 51,900 $ 45,621 $ 42,309
Income available to common stockholders plus assumed exercises of stock options - Shares                 15,443,645 15,550,131 15,651,312
Income available to common stockholders plus assumed exercises of stock options - Per Share Amount $ 0.79 $ 0.90 $ 0.76 $ 0.91 $ 0.75 $ 0.81 $ 0.65 $ 0.72 $ 3.36 $ 2.93 $ 2.70