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Stock-based Compensation - Restricted Stock Units (Details) - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Restricted Stock Units (RSUs) [Member]      
Non-vested RSU's, beginning balance (in shares) 428,295    
Non-vested RSU's, weighted-average grant-date fair value, beginning balance (in dollars per share) $ 24.42    
Granted RSU's (in shares) 173,528 263,574 280,590
Granted RSU's, weighted-average grant-date fair value (in dollars per share) $ 19.63    
Vested RSU's (in shares) (258,745)    
Vested RSU's, weighted-average grant-date fair value (in dollars per share) $ 22.43    
Forfeited RSU's (in shares) (6,180)    
Forfeited RSU's, weighted-average grant-date fair value (in dollars per share) $ 24.61    
Non-vested RSU's, ending balance (in shares) 336,898 428,295  
Non-vested RSU's, weighted-average grant-date fair value, ending balance (in dollars per share) $ 23.48 $ 24.42  
Vested RSU's but unissued (in shares) 234,481    
Vested RSU's but unissued, weighted-average grant-date fair value (in dollars per share) $ 23.25    
Performance-based Restricted Stock Units [Member]      
Non-vested RSU's, beginning balance (in shares) 34,186    
Non-vested RSU's, weighted-average grant-date fair value, beginning balance (in dollars per share) $ 22.38    
Granted RSU's (in shares) 72,143 67,352  
Granted RSU's, weighted-average grant-date fair value (in dollars per share) $ 20.38    
Vested RSU's (in shares) (39,749)    
Vested RSU's, weighted-average grant-date fair value (in dollars per share) $ 20.64    
Non-vested RSU's, ending balance (in shares) 66,580 34,186  
Non-vested RSU's, weighted-average grant-date fair value, ending balance (in dollars per share) $ 21.26 $ 22.38  
Vested RSU's but unissued (in shares) 67,115    
Vested RSU's but unissued, weighted-average grant-date fair value (in dollars per share) $ 21.35