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Net Income Per Common Share - Basic and Diluted Net Income Per Common Share (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Earnings Per Share Reconciliation [Abstract]        
Income available to common stockholders $ 18,553 $ 14,688 $ 34,812 $ 29,345
Income available to common stockholders plus assumed exercises of stock options $ 18,553 $ 14,688 $ 34,812 $ 29,345
Income available to common stockholders - Shares 15,536,325 15,468,511 15,521,916 15,405,847
Effect of stock options - Shares 328,599 363,669 330,616 353,942
Income available to common stockholders plus assumed exercises of stock options - Shares 15,864,924 15,832,180 15,852,532 15,759,789
Income available to common stockholders - Per Share Amount $ 1.19 $ 0.94 $ 2.24 $ 1.90
Income available to common stockholders plus assumed exercises of stock options - Per Share Amount $ 1.17 $ 0.92 $ 2.20 $ 1.86