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Earnings per Common Share (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Jun. 30, 2021
Mar. 31, 2021
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Sep. 30, 2021
Sep. 30, 2020
Aug. 20, 2018
Earnings Per Common Share [Line Items]                  
Conversion price (in dollars per share) $ 101.38     $ 101.38     $ 101.38 $ 101.38  
Numerator — basic and diluted                  
Net income $ 69,480 $ 62,782 $ 64,496 $ 50,172 $ 48,174 $ 56,747 $ 196,758 $ 155,093  
Denominator                  
Weighted average number of common shares outstanding — basic (in shares) 33,495     35,639     33,478 36,073  
Effect of dilutive convertible shares (in shares) 662     708     697 774  
Effect of dilutive stock options (in shares) 363     402     369 436  
Effect of dilutive convertible notes (in shares) 842     337     721 425  
Weighted average number of common shares outstanding — diluted (in shares) 35,362     37,086     35,265 37,708  
Earnings per common share — basic (in dollars per share) $ 2.07     $ 1.41     $ 5.88 $ 4.30  
Earnings per common share — diluted (in dollars per share) $ 1.96     $ 1.35     $ 5.58 $ 4.11  
Stock Options and Restricted Shares                  
Denominator                  
Antidilutive stock options and restricted shares (in shares) 2     83     5 50  
Senior Notes | 2023 Convertible Notes                  
Earnings Per Common Share [Line Items]                  
Interest rate (as a percent) 2.00%           2.00%   2.00%
Conversion price (in dollars per share)                 $ 101.38