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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 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Earnings Per Share Reconciliation [Abstract]        
Income available to common stockholders $ 14,491 $ 13,860 $ 40,456 $ 39,594
Income available to common stockholders plus assumed exercises of stock options $ 14,491 $ 13,860 $ 40,456 $ 39,594
Income available to common stockholders - Shares 15,287,535 15,174,755 15,252,967 15,155,035
Effect of stock options - Shares 307,346 272,639 263,387 276,063
Income available to common stockholders plus assumed exercises of stock options - Shares 15,594,881 15,447,394 15,516,354 15,431,098
Income available to common stockholders - Per Share Amount $ 0.94 $ 0.91 $ 2.65 $ 2.61
Income available to common stockholders plus assumed exercises of stock options - Per Share Amount $ 0.93 $ 0.90 $ 2.61 $ 2.57