COMMERCIAL MORTGAGE LOANS HELD-FOR-INVESTMENT - Additional Portfolio Information (Details) - Commercial real estate portfolio segment - USD ($)	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 585,770,803	$ 555,172,891
Las Vegas, NV
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 6,647,000
Max Remaining Term (Years)	3 years 9 months 18 days
LTV(2)	76.00%
Multi-Family | Winston-Salem, NC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 4,675,000
Unpaid Principal Balance	$ 4,675,000
Coupon	6.00%
Max Remaining Term (Years)	1 year 3 months 18 days
LTV(2)	77.20%
Multi-Family | Pascagoula, MS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 5,535,000
Unpaid Principal Balance	$ 5,535,000
Coupon	4.50%
Max Remaining Term (Years)	1 year 8 months 12 days
LTV(2)	72.90%
Multi-Family | New Orleans, LA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 6,370,000
Unpaid Principal Balance	$ 6,370,000
Coupon	4.10%
Max Remaining Term (Years)	3 years 8 months 12 days
LTV(2)	75.50%
Multi-Family | Hattiesburg, MS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 14,715,000
Unpaid Principal Balance	$ 14,715,000
Coupon	4.80%
Max Remaining Term (Years)	3 years 8 months 12 days
LTV(2)	78.40%
Multi-Family | North Highlands, CA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 10,317,000
Unpaid Principal Balance	$ 9,518,294
Coupon	4.00%
Max Remaining Term (Years)	3 years 10 months 24 days
LTV(2)	79.00%
Multi-Family | Dallas, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 5,634,482
Unpaid Principal Balance	$ 5,543,885
Coupon	4.80%
Max Remaining Term (Years)	3 years 3 months 18 days
LTV(2)	75.20%
Multi-Family | Phoenix, AZ
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 30,505,000
Unpaid Principal Balance	$ 30,505,000
Coupon	3.80%
Max Remaining Term (Years)	3 years 9 months 18 days
LTV(2)	74.30%
Multi-Family | Austell, GA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 12,364,000
Unpaid Principal Balance	$ 11,950,194
Coupon	4.20%
Max Remaining Term (Years)	3 years 7 months 6 days
LTV(2)	80.40%
Multi-Family | Various, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 8,882,738
Unpaid Principal Balance	$ 8,882,738
Coupon	5.50%
Max Remaining Term (Years)	3 months 18 days
LTV(2)	69.20%
Multi-Family | Tuscon, AZ
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 19,110,000
Unpaid Principal Balance	$ 19,110,000
Coupon	4.50%
Max Remaining Term (Years)	3 years 9 months 18 days
LTV(2)	80.30%
Multi-Family | Dallas, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 35,000,000
Unpaid Principal Balance	$ 31,939,667
Coupon	3.70%
Max Remaining Term (Years)	4 years 4 months 24 days
LTV(2)	81.20%
Multi-Family | Philadelphia, PA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 7,600,000
Unpaid Principal Balance	$ 7,600,000
Coupon	4.10%
Max Remaining Term (Years)	3 years 9 months 18 days
LTV(2)	79.80%
Multi-Family | Baltimore, MD
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 33,830,000
Unpaid Principal Balance	$ 29,338,307
Coupon	3.10%
Max Remaining Term (Years)	4 years 4 months 24 days
LTV(2)	77.60%
Multi-Family | Dallas, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 9,250,000
Unpaid Principal Balance	$ 8,511,430
Coupon	3.70%
Max Remaining Term (Years)	4 years 8 months 12 days
LTV(2)	78.40%
Multi-Family | Philadelphia, PA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 3,569,150
Unpaid Principal Balance	$ 2,788,015
Coupon	4.60%
Max Remaining Term (Years)	4 years 8 months 12 days
LTV(2)	79.60%
Multi-Family | Various
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 72,000,000
Unpaid Principal Balance	$ 33,000,000
Coupon	4.10%
Max Remaining Term (Years)	4 years 9 months 18 days
LTV(2)	70.40%
Multi-Family | Greensboro, NC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 21,000,000
Unpaid Principal Balance	$ 17,448,900
Coupon	3.40%
Max Remaining Term (Years)	4 years 9 months 18 days
LTV(2)	79.80%
Multi-Family | Seattle, WA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 17,000,000
Unpaid Principal Balance	$ 17,000,000
Coupon	3.80%
Max Remaining Term (Years)	2 years 9 months 18 days
LTV(2)	53.70%
Multi-Family | Philadelphia, PA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 10,750,000
Unpaid Principal Balance	$ 7,958,000
Coupon	4.00%
Max Remaining Term (Years)	4 years 10 months 24 days
LTV(2)	71.30%
Multi-Family | Barytown, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 19,360,000
Unpaid Principal Balance	$ 15,862,000
Coupon	3.10%
Max Remaining Term (Years)	5 years 1 month 6 days
LTV(2)	80.50%
Multi-Family | Palatine, IL
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 50,858,145
Unpaid Principal Balance	$ 35,625,000
Coupon	4.30%
Max Remaining Term (Years)	4 years 3 months 18 days
LTV(2)	68.50%
Multi-Family | Woodridge, IL
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 28,000,000
Unpaid Principal Balance	$ 25,355,116
Coupon	3.80%
Max Remaining Term (Years)	4 years 3 months 18 days
LTV(2)	76.40%
Multi-Family | Richmond, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 22,500,000
Unpaid Principal Balance	$ 22,500,000
Coupon	3.90%
Max Remaining Term (Years)	1 year 9 months 18 days
LTV(2)	73.50%
Multi-Family | Omaha, NE
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 24,700,000
Unpaid Principal Balance	$ 19,430,000
Coupon	3.70%
Max Remaining Term (Years)	4 years 3 months 18 days
LTV(2)	77.30%
Multi-Family | Greenville, SC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 17,000,000
Unpaid Principal Balance	$ 14,800,000
Coupon	3.90%
Max Remaining Term (Years)	4 years 3 months 18 days
LTV(2)	76.30%
Multi-Family | Fresno, CA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 14,320,000
Unpaid Principal Balance	$ 12,920,000
Coupon	3.90%
Max Remaining Term (Years)	3 years 10 months 24 days
LTV(2)	82.40%
Multi-Family | Austin, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 12,720,000
Unpaid Principal Balance	$ 11,323,290
Coupon	3.60%
Max Remaining Term (Years)	4 years 3 months 18 days
LTV(2)	80.20%
Multi-Family | Phoenix, AZ
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 11,000,000
Unpaid Principal Balance	$ 9,440,000
Coupon	3.90%
Max Remaining Term (Years)	4 years 3 months 18 days
LTV(2)	69.40%
Multi-Family | Waco, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 9,112,000
Unpaid Principal Balance	$ 9,112,000
Coupon	4.80%
Max Remaining Term (Years)	4 years 1 month 6 days
LTV(2)	78.30%
Multi-Family | Atlanta, GA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 10,500,000
Unpaid Principal Balance	$ 8,708,582
Coupon	4.30%
Max Remaining Term (Years)	4 years
LTV(2)	80.20%
Multi-Family | Little Rock, AR
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 8,070,000
Unpaid Principal Balance	$ 8,070,000
Coupon	4.30%
Max Remaining Term (Years)	4 years
LTV(2)	81.30%
Multi-Family | Blacksburg, VA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 9,034,000
Unpaid Principal Balance	$ 8,000,000
Coupon	3.90%
Max Remaining Term (Years)	4 years 6 months
LTV(2)	66.60%
Multi-Family | Birmingham, AL
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 9,000,000
Unpaid Principal Balance	$ 8,053,748
Coupon	3.50%
Max Remaining Term (Years)	4 years 6 months
LTV(2)	78.00%
Multi-Family | Chicago, IL
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 6,200,000
Unpaid Principal Balance	$ 5,667,487
Coupon	4.10%
Max Remaining Term (Years)	4 years 3 months 18 days
LTV(2)	80.50%
Multi-Family | Washington, DC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 4,525,000
Unpaid Principal Balance	$ 4,404,365
Max Remaining Term (Years)	4 years 3 months 18 days
LTV(2)	73.30%
Multi-Family | Wichita, KS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 4,080,000
Unpaid Principal Balance	$ 3,793,542
Coupon	5.00%
Max Remaining Term (Years)	4 years 2 months 12 days
LTV(2)	69.00%
Mixed Use | Washington, DC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Coupon	4.70%
Office | Stafford, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 5,000,000
Unpaid Principal Balance	$ 4,675,039
Coupon	5.50%
Max Remaining Term (Years)	2 years 9 months 18 days
LTV(2)	56.40%
Office | Rochelle Park, NJ
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 19,235,000
Unpaid Principal Balance	$ 14,212,713
Coupon	4.00%
Max Remaining Term (Years)	4 years 1 month 6 days
LTV(2)	76.80%
Office | Chicago, IL
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 16,200,000
Unpaid Principal Balance	$ 12,432,514
Coupon	3.80%
Max Remaining Term (Years)	4 years 4 months 24 days
LTV(2)	72.70%
Office | Little Rock, AR
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 7,332,000
Unpaid Principal Balance	$ 6,874,000
Coupon	4.90%
Max Remaining Term (Years)	4 years 1 month 6 days
LTV(2)	72.40%
Office | Decatur, GA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 8,250,000
Unpaid Principal Balance	$ 4,714,340
Coupon	4.10%
Max Remaining Term (Years)	4 years 8 months 12 days
LTV(2)	56.80%
Retail | Austin, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 24,123,000
Unpaid Principal Balance	$ 17,000,000
Coupon	4.10%
Max Remaining Term (Years)	4 years 9 months 18 days
LTV(2)	60.50%
Retail | Goldsboro, NC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 10,000,000
Unpaid Principal Balance	$ 6,860,637
Coupon	4.00%
Max Remaining Term (Years)	4 years 4 months 24 days
LTV(2)	56.50%
Retail | Austin, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Commitment(1)	$ 20,850,000
Unpaid Principal Balance	$ 18,000,000
Coupon	3.90%
Max Remaining Term (Years)	4 years 9 months 18 days
LTV(2)	71.40%
Self-Storage | Las Vegas, NV
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 5,547,000
Coupon	4.30%