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Income Taxes (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Sep. 30, 2018
Income Tax Contingency [Line Items]      
Income taxes computed at the federal statutory rate 24.53% 35.00%  
Income tax expense $ 48,343 $ 15,673  
Federal income tax expense 37,730 12,744  
State income tax expense $ 10,613 $ 2,929  
Net income attributable to RMR Inc. per common share - basic (in dollars per share) $ 4.40 $ 1.46  
Tax receivable agreement remeasurement $ 24,710 $ 0  
Adjustment For Tax Cuts And Jobs Act [Member]      
Income Tax Contingency [Line Items]      
Income tax expense $ 19,817    
Net income attributable to RMR Inc. per common share - basic (in dollars per share) $ 1.23    
Forecast      
Income Tax Contingency [Line Items]      
Income taxes computed at the federal statutory rate     24.50%