COMMERCIAL MORTGAGE LOANS HELD-FOR-INVESTMENT - Summary of Concentration of Credit Risk (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	100.00%	100.00%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	100.00%	100.00%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Multi-Family | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	90.30%	93.90%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Retail | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	5.50%	2.70%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Office | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	2.80%	2.00%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Mixed-Use | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	0.70%	0.70%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Self-Storage | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	0.70%	0.70%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Southwest | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	34.40%	38.70%
Commercial loans held-for-investment | Commercial real estate portfolio segment | South | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	32.40%	27.50%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Midwest | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	16.20%	16.90%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Mid-Atlantic | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	8.50%	8.40%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Various | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	5.50%	5.50%
Commercial loans held-for-investment | Commercial real estate portfolio segment | West | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	3.00%	3.00%