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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, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Earnings Per Share Reconciliation [Abstract]                      
Income available to common stockholders                 $ 125,814 $ 86,439 $ 70,674
Income available to common stockholders plus assumed exercises of stock options                 $ 125,814 $ 86,439 $ 70,674
Income available to common stockholders - Shares                 32,689,228 31,813,572 31,230,340
Effect of stock options - Shares                 741,486 754,533 592,184
Income available to common stockholders plus assumed exercises of stock options - Shares                 33,430,714 32,568,105 31,822,524
Income available to common stockholders - Per Share Amount $ 1.00 $ 1.01 $ 0.93 $ 0.91 $ 0.61 $ 0.68 $ 0.73 $ 0.70 $ 3.85 $ 2.72 $ 2.27
Income available to common stockholders plus assumed exercises of stock options - Per Share Amount $ 0.98 $ 0.98 $ 0.91 $ 0.89 $ 0.59 $ 0.67 $ 0.71 $ 0.68 $ 3.76 $ 2.65 $ 2.22