COMMERCIAL MORTGAGE LOANS HELD-FOR-INVESTMENT - Summary of Concentration of Credit Risk (Details) - Commercial loans held-for-investment - Commercial real estate portfolio segment	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage (as a percent)	100.00%	100.00%
Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage (as a percent)	100.00%	100.00%
Multifamily | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage (as a percent)	89.40%	89.50%
Self-Storage | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage (as a percent)	7.00%	0.80%
Office | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage (as a percent)	1.50%	3.30%
Retail | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage (as a percent)	2.10%	6.40%
South | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage (as a percent)	44.80%	36.50%
Southwest | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage (as a percent)	31.80%	38.70%
West | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage (as a percent)	16.00%	1.80%
Midwest | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage (as a percent)	3.10%	16.80%
Mid-Atlantic | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage (as a percent)	4.30%	6.20%