COMMERCIAL MORTGAGE LOANS HELD-FOR-INVESTMENT - Summary of Concentration of Credit Risk (Details) - numberOfMulti-familyLoans	3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of multi-family loans reclassified	2
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		90.40%	93.90%
Increase (decrease) in concentration risk percentage		(3.60%)
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		5.90%	2.70%
Increase (decrease) in concentration risk percentage		2.80%
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		3.00%	2.00%
Increase (decrease) in concentration risk percentage		0.80%
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		0.00%	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		37.00%	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		35.50%	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		18.80%	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		7.00%	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		0.00%	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		1.70%	3.00%