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Beneficial Interest - Additional Information (Details) - $ / shares
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Beneficial Interest      
Common shares, outstanding 8,608,000   8,488,000
Preferred shares, outstanding 0   0
Dividends paid $ 0.254375 $ 0.2475  
Total Beneficial Interest      
Beneficial Interest      
Common shares, authorized 100,000,000    
Common shares, par value $ 0.01    
Preferred shares, authorized 50,000,000    
Preferred shares, par value $ 0.01