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Net Income Per Common Share - Basic and Diluted Net Income Per Common Share Based on Weighted-average Shares Outstanding (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Earnings Per Share Reconciliation [Abstract]        
Income available to common stockholders $ 30,586 $ 23,182 $ 60,206 $ 45,232
Income available to common stockholders plus assumed exercises of stock options $ 30,586 $ 23,182 $ 60,206 $ 45,232
Income available to common stockholders - Shares 32,716,350 31,807,610 32,645,693 31,768,828
Dilutive effect of stock options - Shares 742,207 715,766 744,269 731,162
Income available to common stockholders plus assumed exercises of stock options - Shares 33,458,557 32,523,376 33,389,962 32,499,990
Income available to common stockholders - Per Share Amount $ 0.93 $ 0.73 $ 1.84 $ 1.43
Income available to common stockholders plus assumed exercises of stock options - Per Share Amount $ 0.91 $ 0.71 $ 1.80 $ 1.39