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Shareholders' Equity (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
 0 Months Ended 1 Months Ended 9 Months Ended
May 09, 2012
Feb. 09, 2012
May 12, 2011
Feb. 10, 2011
Aug. 31, 2012
Jul. 31, 2012
Mar. 31, 2012
Nov. 30, 2011
Aug. 31, 2011
Sep. 30, 2012
Sep. 30, 2011
Oct. 09, 2012
Sep. 14, 2012
May 17, 2012
item
Dec. 31, 2011
Distributions                              
Distribution to common shareholders (in dollars per share) $ 0.38 $ 0.38 $ 0.37 $ 0.37 $ 0.38     $ 0.38 $ 0.37            
Distribution to common shareholders $ 61,813 $ 61,806 $ 52,490 $ 52,486 $ 67,061     $ 61,805 $ 56,748 $ 190,680 $ 161,724        
Distribution declared to common shareholders (in dollars per share)                       $ 0.39      
Distribution declared to common shareholders                       68,856      
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                         78,492 2,000  
Common shares of beneficial interest, par value (in dollars per share)                   $ 0.01     $ 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)                         $ 22.62 $ 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           $ 287,052       $ 287,052 $ 247,499