COMMERCIAL MORTGAGE LOANS HELD-FOR-INVESTMENT - Summary of Concentration of Credit Risk (Details) - Commercial loans held-for-investment - Commercial real estate portfolio segment	3 Months Ended	12 Months Ended
	Mar. 31, 2022	Dec. 31, 2021
Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percent	100.00%	100.00%
Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percent	100.00%	100.00%
Multifamily | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percent	94.20%	92.00%
Self-Storage | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percent	3.20%	5.20%
Retail | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percent	1.50%	1.70%
Office | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percent	1.10%	1.10%
South | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percent	47.40%	46.20%
Southwest | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percent	26.50%	27.50%
West | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percent	9.40%	13.90%
Mid-Atlantic | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percent	11.90%	7.90%
Midwest | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percent	4.20%	4.50%
Northeast | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percent	0.60%	0.00%