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Share-Based Compensation - Stock Option Activity (Details) - Stock options - 2012 Equity Incentive Plan - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Shares        
Options outstanding, beginning of period (in shares) 1,529,800 2,252,275 3,052,450  
Granted (in shares) 0 20,000 140,000  
Exercised (in shares) (426,150) (730,125) (769,875)  
Forfeited (in shares) (13,350) (12,350) (170,300)  
Options outstanding, end of period (in shares) 1,090,300 1,529,800 2,252,275 3,052,450
Options exercisable at period end (in shares) 956,700      
Weighted- Average Exercise Price        
Options outstanding, beginning balance (in dollars per share) $ 17.40 $ 16.92 $ 16.65  
Granted (in dollars per share) 0 20.48 19.40  
Exercised (in dollars per share) 16.55 15.94 16.07  
Forfeited (in dollars per share) 21.89 20.80 17.93  
Options outstanding, ending balance (in dollars per share) 17.68 $ 17.40 $ 16.92 $ 16.65
Options exercisable at period end (in dollars per share) $ 17.15      
Options outstanding, weighted average contractual life 4 years 6 months 5 years 3 months 18 days 6 years 1 month 6 days 6 years 10 months 24 days
Options exercisable at period end, weighted average contractual life 4 years 2 months 12 days      
Share-based Compensation Arrangement by Share-based Payment Award, Intrinsic Value [Roll Forward]        
Options outstanding, intrinsic value $ 13,436 $ 13,479 $ 7,713 $ 16,421
Exercised, intrinsic value 4,911 $ 6,088 $ 4,754  
Options exercisable at period end, intrinsic value $ 12,289