Consolidated K-Series - Geographic Concentrations of Credit Risk (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Texas
Concentration Risk [Line Items]
Geographic concentration, percent	13.80%	24.30%
New York
Concentration Risk [Line Items]
Geographic concentration, percent	22.10%	24.10%
Geographic Concentration Risk | Multi-family loans held in securitization trusts | California
Concentration Risk [Line Items]
Geographic concentration, percent	13.80%	14.70%
Geographic Concentration Risk | Multi-family loans held in securitization trusts | Texas
Concentration Risk [Line Items]
Geographic concentration, percent	11.80%	12.70%
Geographic Concentration Risk | Multi-family loans held in securitization trusts | New York
Concentration Risk [Line Items]
Geographic concentration, percent	6.10%	6.50%
Geographic Concentration Risk | Multi-family loans held in securitization trusts | Maryland
Concentration Risk [Line Items]
Geographic concentration, percent	5.10%	5.50%