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Loan Sales And Mortgage Servicing Rights - Additional Information (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Loan Sales And Mortgage Servicing Rights [Line Items]      
Gain on sale of securitization loans $ 7,029 $ 7,812 $ 2,935
Mortgage servicing rights 4,800 2,200  
Freddie Mac      
Loan Sales And Mortgage Servicing Rights [Line Items]      
Payments related to closing of swap position   2,400  
Increased pricing on the loan sale   2,200  
FFB      
Loan Sales And Mortgage Servicing Rights [Line Items]      
Sale of multifamily loans through securitization 453,000 41,000  
Gain on sale of securitization loans 7,000 600  
Fair value of beneficial interest     900
Mortgage servicing rights 4,800 2,200  
Loans serviced for others 745,000 412,000  
Servicing fees earned on loans $ 700 300 0
FFB | Freddie Mac      
Loan Sales And Mortgage Servicing Rights [Line Items]      
Sale of multifamily loans through securitization   265,000 102,000
Gain on sale of securitization loans   $ 7,200 $ 2,700