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RELATED PARTY TRANSACTIONS - Additional Information (Details) - USD ($)
12 Months Ended
Mar. 03, 2017
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Mar. 05, 2019
Dec. 12, 2018
Mar. 13, 2017
Oct. 18, 2016
Affiliated Entity                
RELATED PARTY TRANSACTIONS                
Investment in mutual fund   $ 800,000           $ 10,000,000.0
Fee earned on assets under management   0.75%            
Fund's cap expense   0.95%            
Affiliated Entity | B Participation Interest | Related Reserve IV LLC                
RELATED PARTY TRANSACTIONS                
Subordinate participation interest             $ 4,000,000.0  
Participating mortgage loan amount (up to)             $ 136,500,000  
Percentage of loans receivable with fixed rates of interest             17.00%  
Affiliated Entity | B Participation Interest | Related Reserve IV LLC | Participation Financing - Mortgage Loan Receivable                
RELATED PARTY TRANSACTIONS                
Interest expense, debt   $ 200,000 $ 500,000 $ 500,000        
Affiliated Entity | First mortgage loan | Brickell Heights Commercial LLC                
RELATED PARTY TRANSACTIONS                
First mortgage interest-only loan         $ 14,300,000 $ 6,400,000    
Related | Class A Common Stock                
RELATED PARTY TRANSACTIONS                
Related party purchases of shares from shareholders $ 80,000,000.0              
Corporate Joint Venture | Related Special Assets LLC | Brickell Heights Commercial LLC                
RELATED PARTY TRANSACTIONS                
Ownership Interest         0.70% 12.00%    
Corporate Joint Venture | First mortgage loan | Brickell Heights Commercial LLC | Woodbury, New York                
RELATED PARTY TRANSACTIONS                
Interest earned related to loan   100,000 $ 19,300          
Corporate Joint Venture | First mortgage loan | Brickell Heights Commercial LLC | Bloomfield Heights, Michigan                
RELATED PARTY TRANSACTIONS                
Interest earned related to loan   $ 300,000