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Performance Incentive Plan (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
May 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of shares authorized, 2017 plan (in shares)       1,800,000
Stock options exercisable (in shares) 0      
Stock options outstanding (in shares) 0      
Number of Shares        
Restricted shares granted (in shares) 35,336      
Weighted Average Share Price        
Restricted shares granted (in dollars per share) $ 40.22      
Shares available under the 2017 Plan for grant, end of period (in shares) 1,733,296      
Share-based compensation cost $ 12,971,000 $ 10,758,000 $ 9,671,000  
Unrecognized compensation cost $ 10,542,000      
Weighted average period for recognition of unrecognized compensation cost (in years) 2 years 4 months 24 days      
Restricted stock        
Number of Shares        
Non-vested restricted shares, January 1 (in shares) 871,718      
Restricted shares granted (in shares) 292,968      
Restricted shares vested (in shares) (410,497)      
Restricted shares forfeited (in shares) (11,356)      
Restricted shares repurchased (in shares) (7,510)      
Non-vested restricted shares, December 31 (in shares) 735,323 871,718    
Weighted Average Share Price        
Non-vested restricted shares, January 1 (in dollars per share) $ 38.88      
Restricted shares granted (in dollars per share) 43.61      
Restricted shares vested (in dollars per share) 35.80      
Restricted shares forfeited (in dollars per share) 33.42      
Restricted shares repurchased (in dollars per share) 30.80      
Non-vested restricted shares, December 31 (in dollars per share) $ 42.65 $ 38.88    
Restricted stock | Minimum        
Weighted Average Share Price        
Vesting period (in years) 3 years      
Restricted stock | Maximum        
Weighted Average Share Price        
Vesting period (in years) 5 years      
Performance shares        
Number of Shares        
Restricted shares granted (in shares) 169,495 142,199    
Weighted Average Share Price        
Performance period used In calculation (in years) 3 years      
Weighted average grant price, grants to certain executive officers (in USD per share) $ 25.77 $ 34.60    
Share-based compensation cost $ 0 $ 0 $ 0  
Performance shares | Executive officers        
Weighted Average Share Price        
Weighted average grant price, grants to certain executive officers (in USD per share) $ 43.73 $ 44.70    
Directors’ fees        
Number of Shares        
Restricted shares granted (in shares) 14,007      
Weighted Average Share Price        
Restricted shares granted (in dollars per share) $ 40.09      
Deferred directors’ fees        
Number of Shares        
Restricted shares granted (in shares) 21,329      
Weighted Average Share Price        
Restricted shares granted (in dollars per share) $ 40.31