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Performance Incentive Plan (Details) - USD ($)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Jun. 30, 2007
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of shares authorized, 2007 plan (in shares)       5,900,000
Stock options outstanding or exercisable 0      
Number of Shares        
Restricted shares granted (in shares) 33,690      
Weighted Average Share Price        
Restricted shares granted (in dollars per share) $ 45.44      
Shares available under the 2007 plan for grant, end of period 3,088,970      
Share-based compensation cost $ 10,758,000 $ 9,671,000 $ 9,224,000  
Unrecognized compensation cost $ 13,398,000      
Weighted average period for recognition of unrecognized compensation cost (in years) 2 years 4 months 26 days      
Restricted stock        
Number of Shares        
Non-vested restricted shares outstanding (in shares) 896,667      
Restricted shares granted (in shares) 269,448      
Restricted shares vested (in shares) (247,106)      
Restricted shares forfeited (in shares) (38,138)      
Restricted shares repurchased (in shares) (9,153)      
Non-vested restricted shares outstanding (in shares) 871,718 896,667    
Weighted Average Share Price        
Non-vested restricted shares, January 1 (in dollars per share) $ 35.13      
Restricted shares granted (in dollars per share) 44.70      
Restricted shares vested (in dollars per share) 33.33      
Restricted shares forfeited (in dollars per share) 33.93      
Restricted shares repurchased (in dollars per share) 28.94      
Non-vested restricted shares, December 31 (in dollars per share) $ 38.88 $ 35.13    
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) 142,199 145,916    
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) $ 34.60 $ 22.72    
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) $ 44.70 $ 41.00    
Directors’ fees        
Number of Shares        
Restricted shares granted (in shares) 16,125      
Weighted Average Share Price        
Restricted shares granted (in dollars per share) $ 45.27      
Deferred directors’ fees        
Number of Shares        
Restricted shares granted (in shares) 17,565      
Weighted Average Share Price        
Restricted shares granted (in dollars per share) $ 45.60