COMMERCIAL MORTGAGE LOANS HELD-FOR-INVESTMENT - Summary of Concentration of Credit Risk (Details) - Commercial loans held-for-investment - Commercial real estate portfolio segment	6 Months Ended
	Jun. 30, 2018
Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	100.00%
Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	100.00%
Multifamily | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	88.10%
Retail | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	6.80%
Mixed Use | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	3.70%
Office | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	1.40%
Texas | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	28.40%
Arizona | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	15.20%
Georgia | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	10.80%
Connecticut | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	8.60%
Florida | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	8.20%
California | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	7.60%
Mississippi | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	6.30%
Washington | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	4.70%
North Carolina | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	4.60%
Ohio | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	3.70%
Louisiana | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	1.90%