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Shareholders' Equity (Details) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Sep. 29, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Nov. 04, 2016
Class of Stock [Line Items]          
Shares issued under dividend reinvestment plan (in shares)   17,911 15,619 16,524  
Issuance of preferred shares, net of costs   $ 144,991 $ 0 $ 0  
Preferred shares, par value   $ 0.01 $ 0.01    
Conversion price per share   $ 104.69      
Net cash proceeds of common stock   $ 118,583 $ 329,103 $ 110,855  
At The Market Equity Program          
Class of Stock [Line Items]          
Shares issued   826,517 1,156,571    
Net cash proceeds of common stock   $ 108,300 $ 174,800    
Aggregate offering price of common shares         $ 400,000
Weighted average price per common share   $ 132.56 $ 152.92    
Remaining capacity to issue   $ 261,300      
At The Market Equity Program | Commissions          
Class of Stock [Line Items]          
Commissions related to sales of common shares   1,100 $ 1,800    
At The Market Equity Program | Other Offering Costs          
Class of Stock [Line Items]          
Commissions related to sales of common shares   $ 200 $ 200    
5.0% Series C Cumulative Redeemable Preferred Shares          
Class of Stock [Line Items]          
Preferred shares, shares outstanding   6,000 0    
Preferred shares, percentage 5.00% 5.00% 5.00% 5.00%  
Preferred shares, liquidation preference per share $ 25,000 $ 25,000      
Issuance of preferred shares, net of costs $ 145,000        
Preferred shares, par value $ 0.01        
5.417% Series 1 Cumulative Convertible Preferred Shares          
Class of Stock [Line Items]          
Preferred shares, shares outstanding   399,896 399,896 399,896  
Preferred shares, percentage   5.417% 5.417% 5.417%  
Preferred shares, liquidation preference per share   $ 25 $ 25 $ 25  
Preferred shares, par value       $ 0.01  
Depository Shares | 5.0% Series C Cumulative Redeemable Preferred Shares          
Class of Stock [Line Items]          
Preferred shares, liquidation preference per share $ 25.00        
Preferred Shares          
Class of Stock [Line Items]          
Shares issued   6,000      
Preferred Shares | Depository Shares | 5.0% Series C Cumulative Redeemable Preferred Shares          
Class of Stock [Line Items]          
Shares issued 6,000,000