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Shareholders' Equity (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
 1 Months Ended 6 Months Ended
Jul. 31, 2012
May 31, 2012
Mar. 31, 2012
Aug. 31, 2011
May 31, 2011
Jun. 30, 2012
Jun. 30, 2011
Jul. 09, 2012
May 17, 2012
item
Dec. 31, 2011
Distributions                    
Distribution to common shareholders (in dollars per share)   $ 0.38   $ 0.37 $ 0.37          
Distribution to common shareholders   $ 61,813   $ 56,748 $ 52,490 $ 123,619 $ 104,976      
Distribution declared to common shareholders (in dollars per share)               $ 0.38    
Distribution declared to common shareholders               67,061    
Common shares issued in payment of an incentive fee for services rendered from RMR     20,462              
Value of common shares issued in payment of an incentive fee for services rendered from RMR     453              
Common shares of beneficial interest                    
Common shares of beneficial interest granted                 2,000  
Common shares of beneficial interest, par value (in dollars per share)           $ 0.01     $ 0.01 $ 0.01
Common stock price per share which is the closing price at New York Stock Exchange (in dollars per share)                 $ 20.48  
Number of trustees                 5  
Common shares issued in public offering (in shares) 13,800,000                  
Net proceeds from issuance of common shares issued in public offering $ 287,100           $ (103)