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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 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Earnings Per Share Reconciliation [Abstract]                      
Income available to common stockholders                 $ 86,439 $ 70,674 $ 66,170
Income available to common stockholders plus assumed exercises of stock options                 $ 86,439 $ 70,674 $ 66,170
Income available to common stockholders - Shares                 31,813,572 31,230,340 31,118,118
Effect of stock options - Shares                 754,533 592,184 654,372
Income available to common stockholders plus assumed exercises of stock options - Shares                 32,568,105 31,822,524 31,772,490
Income available to common stockholders - Per Share Amount $ 0.61 $ 0.68 $ 0.73 $ 0.70 $ 0.60 $ 0.58 $ 0.56 $ 0.53 $ 2.72 $ 2.27 $ 2.13
Income available to common stockholders plus assumed exercises of stock options - Per Share Amount $ 0.59 $ 0.67 $ 0.71 $ 0.68 $ 0.58 $ 0.57 $ 0.55 $ 0.52 $ 2.65 $ 2.22 $ 2.09