COMMERCIAL MORTGAGE LOANS HELD-FOR-INVESTMENT - Summary of Concentration of Credit Risk (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
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 | Multifamily | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	88.20%	89.50%
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	7.20%	6.40%
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	3.70%	3.30%
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	0.90%	0.80%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Southwest | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	39.00%	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	30.40%	36.50%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Midwest | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	19.00%	16.80%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Mid-Atlantic | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	7.00%	6.20%
Commercial loans held-for-investment | Commercial real estate portfolio segment | West | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	4.60%	1.80%