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
	Sep. 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%
Multi-Family | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	80.40%
Office | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	6.80%
Retail | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	5.60%
Student Housing | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	3.20%
Mixed-Use | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	3.00%
Self-Storage | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	1.00%
Southwest | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	41.30%
South | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	20.90%
Midwest | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	20.00%
Mid-Atlantic | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	9.90%
West | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	7.90%