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Consolidated K-Series - Narrative (Details)
$ in Millions
Mar. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
securitization
Dec. 31, 2006
securitization
Dec. 31, 2005
securitization
Multi-family collateralized debt obligations, at fair value        
Mortgage Loans on Real Estate [Line Items]        
Loans, unpaid principal balance $ 9,300.0 $ 9,400.0    
Weighted average interest rate 3.94% 3.92%    
Multi-family loans held in securitization trusts        
Mortgage Loans on Real Estate [Line Items]        
Loans, unpaid principal balance $ 9,300.0 $ 9,400.0    
Consolidated K-Series        
Mortgage Loans on Real Estate [Line Items]        
K-Series, carrying value $ 484.8 $ 468.0    
Consolidated K-Series | K-Series        
Mortgage Loans on Real Estate [Line Items]        
Number of securitizations | securitization   7 4 4