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Related Person Transactions - Tax Related Payments (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Related Party Transaction [Line Items]      
Tax receivable agreement remeasurement   $ 24,710 $ 0
Tax receivable agreement remeasurement (in dollars per share)   $ 1.53  
RMR LLC      
Related Party Transaction [Line Items]      
Tax distributions   $ 16,333 8,123
ABP Trust | RMR LLC      
Related Party Transaction [Line Items]      
Tax distributions   $ 15,155 7,577
ABP Trust | Up C Transaction      
Related Party Transaction [Line Items]      
Tax receivable agreement, percent of payment   85.00%  
Liability related to Tax Receivable Agreement   $ 37,288  
ABP Trust | Up C Transaction | Forecast      
Related Party Transaction [Line Items]      
Payments related to Tax Receivable Agreement $ 2,935    
ABP Trust and Managed REIT | RMR LLC      
Related Party Transaction [Line Items]      
Tax distributions   $ 31,488 $ 15,700