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Earnings Per Share - Computation of Basic Earnings and Diluted Earnings Per Share Data (Detail) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Numerator of Basic and Diluted Earnings Per Share [Abstract]        
Net income $ 38,924 $ 38,836 $ 115,910 $ 103,375
Denominator:        
Weighted-average basic common shares outstanding (in shares) 38,761 37,391 38,533 37,238
Assumed conversion of dilutive securities:        
Stock options (in shares) 1,767 1,951 1,812 2,028
Denominator for diluted earnings per share - Adjusted weighted-average shares (in shares) 40,528 39,342 40,345 39,266
Earnings per common share:        
Basic (usd per share) $ 1.00 $ 1.04 $ 3.01 $ 2.78
Diluted (usd per share) $ 0.96 $ 0.99 $ 2.87 $ 2.63