COMMERCIAL MORTGAGE LOANS HELD-FOR-INVESTMENT - Additional Portfolio Information (Details) - Commercial real estate portfolio segment - USD ($)	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Face Amount	$ 326,883,547	$ 0
Multi-Family | Winston-Salem, NC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	4,675,000
Outstanding Face Amount	$ 4,675,000
Coupon	6.00%
Max Remaining Term (Years)	2 years 1 month 6 days
LTV	77.20%
Multi-Family | Tyler, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 3,743,000
Outstanding Face Amount	$ 3,743,000
Coupon	5.50%
Max Remaining Term (Years)	1 year 8 months 12 days
LTV	58.30%
Multi-Family | Pascagoula, MS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 6,035,000
Outstanding Face Amount	$ 6,035,000
Coupon	4.50%
Max Remaining Term (Years)	2 years 4 months 24 days
LTV	70.70%
Multi-Family | New Orleans, LA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 6,370,000
Outstanding Face Amount	$ 6,370,000
Coupon	4.10%
Max Remaining Term (Years)	4 years 4 months 24 days
LTV	75.50%
Multi-Family | Hattiesburg, MS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 14,715,000
Outstanding Face Amount	$ 14,715,000
Coupon	4.80%
Max Remaining Term (Years)	4 years 4 months 24 days
LTV	78.40%
Multi-Family | North Highlands, CA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 10,317,000
Outstanding Face Amount	$ 8,960,694
Coupon	4.00%
Max Remaining Term (Years)	4 years 8 months 12 days
LTV	79.00%
Multi-Family | Dallas, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 5,810,000
Outstanding Face Amount	$ 5,625,170
Coupon	4.80%
Max Remaining Term (Years)	4 years 1 month 6 days
LTV	75.20%
Multi-Family | Phoenix, AZ
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 30,505,000
Outstanding Face Amount	$ 30,505,000
Coupon	3.80%
Max Remaining Term (Years)	4 years 6 months
LTV	74.30%
Multi-Family | Stamford, CT
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 28,125,000
Outstanding Face Amount	$ 28,125,000
Coupon	4.80%
Max Remaining Term (Years)	3 years 7 months 6 days
LTV	64.50%
Multi-Family | Tampa, FL
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 26,800,000
Outstanding Face Amount	$ 26,800,000
Coupon	5.50%
Max Remaining Term (Years)	1 year 3 months 18 days
LTV	70.30%
Multi-Family | League City, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 24,000,000
Outstanding Face Amount	$ 24,000,000
Coupon	4.50%
Max Remaining Term (Years)	2 years 3 months 18 days
LTV	76.40%
Multi-Family | Austell, GA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 12,664,000
Outstanding Face Amount	$ 11,658,308
Coupon	4.20%
Max Remaining Term (Years)	4 years 3 months 18 days
LTV	81.60%
Multi-Family | Greensboro, NC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 10,250,000
Outstanding Face Amount	$ 10,250,000
Coupon	5.30%
Max Remaining Term (Years)	1 year 4 months 24 days
LTV	83.80%
Multi-Family | Seattle, WA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 15,250,000
Outstanding Face Amount	$ 15,250,000
Coupon	4.50%
Max Remaining Term (Years)	1 year 3 months 18 days
LTV	54.10%
Multi-Family | Stockton, CA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 16,600,000
Outstanding Face Amount	$ 15,871,183
Coupon	5.10%
Max Remaining Term (Years)	3 years 6 months
LTV	76.50%
Multi-Family | Brookhaven, GA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 23,488,000
Outstanding Face Amount	$ 23,488,000
Coupon	4.80%
Max Remaining Term (Years)	3 years 10 months 24 days
LTV	72.80%
Multi-Family | Various
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 32,820,000
Outstanding Face Amount	$ 32,820,000
Coupon	5.50%
Max Remaining Term (Years)	1 year 1 month 6 days
LTV	70.60%
Multi-Family | Tuscon, AZ
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 19,110,000
Outstanding Face Amount	$ 19,110,000
Coupon	4.50%
Max Remaining Term (Years)	4 years 6 months
LTV	80.30%
Mixed Use | Akron, OH
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 13,500,000
Outstanding Face Amount	$ 12,226,811
Coupon	5.30%
Max Remaining Term (Years)	2 years 8 months 12 days
LTV	68.70%
Office | Stafford, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 5,000,000
Outstanding Face Amount	$ 4,526,252
Coupon	5.50%
Max Remaining Term (Years)	3 years 6 months
LTV	56.40%
Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Coupon	5.20%
Retail | Austin, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 16,500,000
Outstanding Face Amount	$ 13,437,757
Coupon	7.30%
Max Remaining Term (Years)	4 years
LTV	67.50%
Retail | Austin, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 11,200,000
Outstanding Face Amount	$ 8,691,382
Max Remaining Term (Years)	3 years 10 months 24 days
LTV	31.70%