Consolidated K-Series - Geographic Concentrations of Credit Risk (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Texas
Concentration Risk [Line Items]
Geographic concentrations	16.60%	24.30%
Geographic Concentration Risk | Multi-family loans held in securitization trusts | California
Concentration Risk [Line Items]
Geographic concentrations	14.80%	14.70%
Geographic Concentration Risk | Multi-family loans held in securitization trusts | Texas
Concentration Risk [Line Items]
Geographic concentrations	13.00%	12.70%
Geographic Concentration Risk | Multi-family loans held in securitization trusts | New York
Concentration Risk [Line Items]
Geographic concentrations	6.40%	6.50%
Geographic Concentration Risk | Multi-family loans held in securitization trusts | Maryland
Concentration Risk [Line Items]
Geographic concentrations	5.00%	5.50%