COMMERCIAL MORTGAGE LOANS HELD-FOR-INVESTMENT - Summary of Concentration of Credit Risk (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
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	92.00%	87.20%
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	4.10%	7.60%
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	3.10%	1.20%
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	0.80%	3.00%
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	0.00%	1.00%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Southwest | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	39.30%	30.20%
Commercial loans held-for-investment | Commercial real estate portfolio segment | South | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	22.80%	22.60%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Midwest | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	18.40%	20.20%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Mid-Atlantic | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	9.40%	10.30%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Various | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	6.00%	5.90%
Commercial loans held-for-investment | Commercial real estate portfolio segment | West | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	4.10%	10.80%