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Performance Incentive Plan (Details) - USD ($)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Jun. 30, 2007
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of shares authorized, 2007 plan       5,900,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]        
Restricted shares granted (in shares) 34,140      
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Weighted Average Exercise Price [Abstract]        
Weighted average share price, restricted shares granted (in dollars per share) $ 39.00      
Shares available under the 2007 plan for grant, end of period 3,344,817      
Share-based compensation cost $ 9,671,000 $ 9,224,000 $ 7,459,000  
Unrecognized compensation cost $ 12,458,000      
Weighted average period for recognition of unrecognized compensation cost 2 years 3 months 19 days      
Restricted stock        
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]        
Non-vested restricted shares outstanding (in shares) 1,005,644      
Restricted shares granted (in shares) 267,133      
Restricted shares vested (in shares) (318,261)      
Restricted shares forfeited (in shares) (46,478)      
Restricted shares repurchased (in shares) (11,371)      
Non-vested restricted shares outstanding (in shares) 896,667 1,005,644    
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Weighted Average Exercise Price [Abstract]        
Weighted average share price, non-vested restricted shares, outstanding (in dollars per share) $ 30.93      
Weighted average share price, restricted shares granted (in dollars per share) 41.00      
Weighted average share price, restricted shares vested (in dollars per share) 27.83      
Weighted average share price, restricted shares forfeited (in dollars per share) 26.85      
Weighed average share price, restricted shares repurchased (in dollars per share) 27.20      
Weighted average share price, non-vested restricted shares, outstanding (in dollars per share) $ 35.13 $ 30.93    
Restricted stock | Minimum        
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Weighted Average Exercise Price [Abstract]        
Vesting period 3 years      
Restricted stock | Maximum        
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Weighted Average Exercise Price [Abstract]        
Vesting period 5 years      
Performance shares        
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]        
Restricted shares granted (in shares) 145,916 177,433    
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Weighted Average Exercise Price [Abstract]        
Performance period used In calculation 3 years      
Weighted average grant price, grants to certain executive officers $ 22.72 $ 21.92    
Share-based compensation cost $ 0 $ 0 $ 729,000  
Performance shares | Executive officers        
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Weighted Average Exercise Price [Abstract]        
Weighted average grant price, grants to certain executive officers $ 41.00 $ 33.42    
Directors' fees        
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]        
Restricted shares granted (in shares) 18,130      
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Weighted Average Exercise Price [Abstract]        
Weighted average share price, restricted shares granted (in dollars per share) $ 38.96      
Deferred director's fees        
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]        
Restricted shares granted (in shares) 16,010      
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Weighted Average Exercise Price [Abstract]        
Weighted average share price, restricted shares granted (in dollars per share) $ 39.04