COMMERCIAL MORTGAGE LOANS HELD-FOR-INVESTMENT Summary of Concentration of Credit Risk (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Collateral Property
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	100.00%	100.00%
Commercial Real Estate Portfolio Segment | Commercial Loans Held-For-Investment | Geographic Concentration Risk
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	100.00%	100.00%
Commercial Real Estate Portfolio Segment | Multi-Family | Commercial Loans Held-For-Investment | Collateral Property
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	93.90%	87.20%
Commercial Real Estate Portfolio Segment | Retail | Commercial Loans Held-For-Investment | Collateral Property
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	2.70%	1.20%
Commercial Real Estate Portfolio Segment | Office | Commercial Loans Held-For-Investment | Collateral Property
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	2.00%	7.60%
Commercial Real Estate Portfolio Segment | Mixed-Use | Commercial Loans Held-For-Investment | Collateral Property
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	0.70%	3.00%
Commercial Real Estate Portfolio Segment | Self-Storage | Commercial Loans Held-For-Investment | Collateral Property
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	0.70%	1.00%
Commercial Real Estate Portfolio Segment | Southwest | Commercial Loans Held-For-Investment | Geographic Concentration Risk
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	38.70%	30.20%
Commercial Real Estate Portfolio Segment | South | Commercial Loans Held-For-Investment | Geographic Concentration Risk
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	27.50%	22.60%
Commercial Real Estate Portfolio Segment | Midwest | Commercial Loans Held-For-Investment | Geographic Concentration Risk
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	16.90%	20.20%
Commercial Real Estate Portfolio Segment | Mid-Atlantic | Commercial Loans Held-For-Investment | Geographic Concentration Risk
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	8.40%	10.30%
Commercial Real Estate Portfolio Segment | Various | Commercial Loans Held-For-Investment | Geographic Concentration Risk
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	5.50%	5.90%
Commercial Real Estate Portfolio Segment | West | Commercial Loans Held-For-Investment | Geographic Concentration Risk
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (as a percentage)	3.00%	10.80%