COMMERCIAL MORTGAGE LOANS HELD-FOR-INVESTMENT Additional Portfolio Information (Details) - Commercial Real Estate Portfolio Segment - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current Principal Amount	$ 555,172,891	$ 0
Multi-family | Winston-Salem, NC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	4,675,000
Current Principal Amount	$ 4,675,000
Max Remaining Term (Years)	1 year 8 months 12 days
Coupon Rate	6.00%
LTV	77.20%
Multi-family | Pascagoula, MS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 5,535,000
Current Principal Amount	$ 5,535,000
Max Remaining Term (Years)	2 years
Coupon Rate	4.50%
LTV	72.90%
Multi-family | New Orleans, LA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 6,370,000
Current Principal Amount	$ 6,370,000
Max Remaining Term (Years)	4 years
Coupon Rate	4.10%
LTV	75.50%
Multi-family | Hattiesburg, MS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 14,715,000
Current Principal Amount	$ 14,715,000
Max Remaining Term (Years)	4 years
Coupon Rate	4.80%
LTV	78.40%
Multi-family | North Highlands, CA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 10,317,000
Current Principal Amount	$ 9,407,298
Max Remaining Term (Years)	4 years 3 months 18 days
Coupon Rate	4.00%
LTV	79.00%
Multi-family | Dallas, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 5,810,000
Current Principal Amount	$ 5,721,511
Max Remaining Term (Years)	3 years 8 months 12 days
Coupon Rate	4.80%
LTV	75.20%
Multi-family | Phoenix, AZ
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 30,505,000
Current Principal Amount	$ 30,505,000
Max Remaining Term (Years)	4 years 1 month 6 days
Coupon Rate	3.80%
LTV	74.30%
Multi-family | Austell, GA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 12,364,000
Current Principal Amount	$ 11,813,177
Max Remaining Term (Years)	3 years 10 months 24 days
Coupon Rate	4.20%
LTV	80.40%
Multi-family | Seattle, WA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 15,250,000
Current Principal Amount	$ 15,250,000
Max Remaining Term (Years)	9 months 18 days
Coupon Rate	4.50%
LTV	54.10%
Multi-family | Various, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 8,882,738
Current Principal Amount	$ 8,882,738
Max Remaining Term (Years)	8 months 12 days
Coupon Rate	5.50%
LTV	69.20%
Multi-family | Tucson, AZ
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 19,110,000
Current Principal Amount	$ 19,110,000
Max Remaining Term (Years)	4 years 1 month 6 days
Coupon Rate	4.50%
LTV	80.30%
Multi-family | Dallas, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 35,000,000
Current Principal Amount	$ 31,772,256
Max Remaining Term (Years)	4 years 9 months 18 days
Coupon Rate	3.70%
LTV	81.20%
Multi-family | Philadelphia, PA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 7,600,000
Current Principal Amount	$ 7,600,000
Max Remaining Term (Years)	4 years 2 months 12 days
Coupon Rate	4.10%
LTV	79.80%
Multi-family | Tulsa, OK
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 6,360,000
Current Principal Amount	$ 6,360,000
Max Remaining Term (Years)	4 years
Coupon Rate	4.50%
LTV	70.10%
Multi-family | Baltimore, MD
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 33,830,000
Current Principal Amount	$ 28,759,119
Max Remaining Term (Years)	4 years 9 months 18 days
Coupon Rate	3.10%
LTV	77.60%
Multi-family | Dallas, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 9,250,000
Current Principal Amount	$ 8,305,000
Max Remaining Term (Years)	5 years
Coupon Rate	3.70%
LTV	78.40%
Multi-family | Philadelphia, PA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 3,569,150
Current Principal Amount	$ 2,684,000
Max Remaining Term (Years)	5 years
Coupon Rate	4.60%
LTV	79.60%
Multi-family | Greensboro, NC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 21,000,000
Current Principal Amount	$ 16,994,000
Max Remaining Term (Years)	5 years 1 month 6 days
Coupon Rate	3.40%
LTV	79.80%
Multi-family | Seattle, WA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 17,000,000
Current Principal Amount	$ 17,000,000
Max Remaining Term (Years)	3 years 2 months 12 days
Coupon Rate	3.80%
LTV	53.70%
Multi-family | Palatine, IL
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 50,858,145
Current Principal Amount	$ 33,534,144
Max Remaining Term (Years)	4 years 7 months 6 days
Coupon Rate	4.30%
LTV	68.50%
Multi-family | Woodridge, IL
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 28,000,000
Current Principal Amount	$ 25,355,116
Max Remaining Term (Years)	4 years 7 months 6 days
Coupon Rate	3.80%
LTV	76.40%
Multi-family | Richmond, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 22,500,000
Current Principal Amount	$ 22,500,000
Max Remaining Term (Years)	2 years 1 month 6 days
Coupon Rate	3.90%
LTV	73.50%
Multi-family | Omaha, NE
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 24,700,000
Current Principal Amount	$ 19,430,000
Max Remaining Term (Years)	4 years 7 months 6 days
Coupon Rate	3.70%
LTV	77.30%
Multi-family | Greenville, SC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 17,000,000
Current Principal Amount	$ 14,800,000
Max Remaining Term (Years)	4 years 8 months 12 days
Coupon Rate	3.90%
LTV	76.30%
Multi-family | Fresno, CA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 14,320,000
Current Principal Amount	$ 12,920,000
Max Remaining Term (Years)	4 years 3 months 18 days
Coupon Rate	3.90%
LTV	82.40%
Multi-family | Austin, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 12,720,000
Current Principal Amount	$ 11,159,150
Max Remaining Term (Years)	4 years 7 months 6 days
Coupon Rate	3.60%
LTV	80.20%
Multi-family | Phoenix, AZ
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 11,000,000
Current Principal Amount	$ 9,440,000
Max Remaining Term (Years)	4 years 7 months 6 days
Coupon Rate	3.90%
LTV	69.40%
Multi-family | Atlanta, GA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 10,500,000
Current Principal Amount	$ 8,590,138
Max Remaining Term (Years)	4 years 3 months 18 days
Coupon Rate	4.30%
LTV	80.20%
Multi-family | Little Rock, AR
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 8,070,000
Current Principal Amount	$ 8,070,000
Max Remaining Term (Years)	4 years 3 months 18 days
Coupon Rate	4.30%
LTV	81.30%
Multi-family | Birmingham, AL
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 9,000,000
Current Principal Amount	$ 7,782,850
Max Remaining Term (Years)	4 years 9 months 18 days
Coupon Rate	3.50%
LTV	78.00%
Multi-family | Chicago, IL
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 6,200,000
Current Principal Amount	$ 5,667,487
Max Remaining Term (Years)	4 years 8 months 12 days
Coupon Rate	4.10%
LTV	80.50%
Multi-family | Wichita, KS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 4,080,000
Current Principal Amount	$ 3,580,000
Max Remaining Term (Years)	4 years 6 months
Coupon Rate	5.00%
LTV	69.00%
Retail | Akron, OH
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 13,500,000
Current Principal Amount	$ 12,226,810
Max Remaining Term (Years)	2 years 3 months 18 days
Coupon Rate	5.30%
LTV	56.70%
Retail | Washington, DC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 4,525,000
Current Principal Amount	$ 4,375,805
Max Remaining Term (Years)	4 years 8 months 12 days
Coupon Rate	4.70%
LTV	73.30%
Office | Stafford, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 5,000,000
Current Principal Amount	$ 4,618,553
Max Remaining Term (Years)	3 years 1 month 6 days
Coupon Rate	5.50%
LTV	56.40%
Office | Rochelle Park, NJ
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 19,235,000
Current Principal Amount	$ 13,841,399
Max Remaining Term (Years)	4 years 4 months 24 days
Coupon Rate	4.00%
LTV	76.80%
Office | Chicago, IL
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 16,200,000
Current Principal Amount	$ 12,075,000
Max Remaining Term (Years)	4 years 9 months 18 days
Coupon Rate	3.80%
LTV	72.70%
Office | Little Rock, AR
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 7,332,000
Current Principal Amount	$ 6,874,000
Max Remaining Term (Years)	4 years 4 months 24 days
Coupon Rate	4.90%
LTV	72.40%
Office | Decatur, GA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 8,250,000
Current Principal Amount	$ 4,714,340
Max Remaining Term (Years)	5 years
Coupon Rate	4.10%
LTV	56.80%
Retail | Goldsboro, NC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 10,000,000
Current Principal Amount	$ 6,500,000
Max Remaining Term (Years)	4 years 9 months 18 days
Coupon Rate	4.00%
LTV	56.50%
Self-Storage | Las Vegas, NV
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 6,647,000
Current Principal Amount	$ 5,547,000
Max Remaining Term (Years)	4 years 1 month 6 days
Coupon Rate	4.30%
LTV	76.00%
Office | Various
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 72,000,000
Current Principal Amount	$ 33,000,000
Max Remaining Term (Years)	5 years 1 month 6 days
Coupon Rate	4.10%
LTV	70.40%
Office | Waco, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 9,112,000
Current Principal Amount	$ 9,112,000
Max Remaining Term (Years)	4 years 4 months 24 days
Coupon Rate	4.80%
LTV	72.90%
Office | Blacksburg, VA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment	$ 9,034,000
Current Principal Amount	$ 8,000,000
Max Remaining Term (Years)	4 years 9 months 18 days
Coupon Rate	3.90%
LTV	66.60%