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Revenue Recognition - Advisory Agreements (Details) - USD ($)
$ in Thousands
3 Months Ended
Jan. 01, 2018
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Related Party Transaction [Line Items]        
Advisory fees     $ 1,382 $ 1,010
Management services revenue     $ 48,570 42,727
RIF        
Related Party Transaction [Line Items]        
Advisory fee percent     0.85%  
RMR Advisors and Tremont Advisors        
Related Party Transaction [Line Items]        
Advisory fees     $ 729 606
Tremont Advisors        
Related Party Transaction [Line Items]        
Advisory fees     653 404
Management services revenue     $ 238 $ 206
Tremont Advisors        
Related Party Transaction [Line Items]        
Advisory fee percentage based on weighted average outstanding balance of strategic investments     1.35%  
Advisory fee percentage based on outstanding principal balance of outstanding loans     0.50%  
Business management fee percent based on management agreements     1.50%  
Minimum | Tremont Advisors        
Related Party Transaction [Line Items]        
Advisory fee percentage based on aggregate principal amounts of loan originate     0.50%  
Maximum | Tremont Advisors        
Related Party Transaction [Line Items]        
Advisory fee percentage based on aggregate principal amounts of loan originate     1.00%  
Subsequent event | Tremont Advisors        
Related Party Transaction [Line Items]        
Advisory fee percentage based on weighted average outstanding balance of strategic investments 0.35%      
Forecast | Tremont Advisors        
Related Party Transaction [Line Items]        
Advisory fee percent   20.00%    
Advisory fee percentage   7.00%