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Earnings Per Share (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Earnings Per Share [Abstract]    
Net income $ 41,635 $ 32,645
Denominator (in thousands):    
Weighted-average common shares for calculating basic earnings per share (shares) 46,426 48,389
Stock options and nonvested stock awards (shares) 53 13
Weighted-average common shares for calculating diluted earnings per share (shares) 46,479 48,402
Net income per share:    
Basic (in dollars per share) $ 0.90 $ 0.67
Diluted (in dollars per share) $ 0.90 $ 0.67