COMMERCIAL MORTGAGE LOANS HELD-FOR-INVESTMENT Summary of Concentration of Credit Risk (Details) - Commercial Real Estate Portfolio Segment - Commercial Loans Held-For-Investment	12 Months Ended
	Dec. 31, 2018
Geographic Concentration Risk
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	100.00%
Collateral Property
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	100.00%
Multi-Family | Collateral Property
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	87.20%
Mixed-Use | Collateral Property
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	7.60%
Retail | Collateral Property
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	3.00%
Self-Storage | Collateral Property
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	1.20%
Self-Storage | Collateral Property
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	1.00%
Southwest | Geographic Concentration Risk
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	30.20%
South | Geographic Concentration Risk
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	22.60%
Midwest | Geographic Concentration Risk
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	20.20%
West | Geographic Concentration Risk
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	10.80%
Mid-Atlantic | Geographic Concentration Risk
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	10.30%
Various | Geographic Concentration Risk
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	5.90%