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Shareholders' Equity (Details) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Aug. 26, 2016
Aug. 19, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Common Share Issuances          
Net proceeds from sale of common shares $ 372,217   $ 0 $ 371,956 $ 0
RMR LLC | Officers and employees of RMR          
Common Share Issuances          
Share grants (in shares)     85,000 79,100 76,250
Aggregate value of awards granted during the period     $ 2,387 $ 2,260 $ 1,957
Public offering          
Common Share Issuances          
Common stock, shares issued (in shares)   11,000,000      
Price per share (in dollars per share)   $ 30.75      
Over-allotment option          
Common Share Issuances          
Common stock, shares issued (in shares) 1,650,000        
Price per share (in dollars per share) $ 30.75        
Common shares          
Common Share Issuances          
Common stock, shares issued (in shares)       12,650,000 1,553,119
Share grants (in shares)     100,000 91,600 90,060
Common shares | Trustees          
Common Share Issuances          
Aggregate value of awards granted during the period     $ 452 $ 319 $ 445
Common shares granted under equity compensation plan (in shares)     3,000 2,500 2,500
Market value of common stock issued per trustee     $ 90 $ 64 $ 74
2012 Equity Compensation Plan          
Common Share Issuances          
Remaining shares of common shares reserved to be issued under share award plans (in shares)     2,504,585