COMMERCIAL MORTGAGE LOANS HELD-FOR-INVESTMENT - Additional Portfolio Information (Details) - Commercial real estate portfolio segment - USD ($)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 544,586,537	$ 0
Multi-Family | Winston-Salem, NC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 4,675,000
Coupon	6.00%
Max Remaining Term (Years)	1 year 9 months 18 days
LTV(1)	83.50%
Multi-Family | Tyler, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 3,743,000
Coupon	5.50%
Max Remaining Term (Years)	1 year 4 months 24 days
LTV(1)	66.80%
Multi-Family | Pascagoula, MS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 6,035,000
Coupon	4.50%
Max Remaining Term (Years)	2 years 2 months 12 days
LTV(1)	72.90%
Multi-Family | New Orleans, LA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 6,370,000
Coupon	4.10%
Max Remaining Term (Years)	4 years 2 months 12 days
LTV(1)	75.50%
Multi-Family | Hattiesburg, MS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 14,715,000
Coupon	4.80%
Max Remaining Term (Years)	4 years 2 months 12 days
LTV(1)	81.50%
Multi-Family | North Highlands, CA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 9,295,085
Coupon	4.00%
Max Remaining Term (Years)	4 years 4 months 24 days
LTV(1)	79.00%
Multi-Family | Dallas, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 5,653,628
Coupon	4.80%
Max Remaining Term (Years)	3 years 9 months 18 days
LTV(1)	79.30%
Multi-Family | Phoenix, AZ
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 30,505,000
Coupon	3.80%
Max Remaining Term (Years)	4 years 3 months 18 days
LTV(1)	75.50%
Multi-Family | League City, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 24,000,000
Coupon	4.50%
Max Remaining Term (Years)	2 years
LTV(1)	76.40%
Multi-Family | Austell, GA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 11,720,232
Coupon	4.20%
Max Remaining Term (Years)	4 years 1 month 6 days
LTV(1)	80.40%
Multi-Family | Greensboro, NC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 10,250,000
Coupon	5.30%
Max Remaining Term (Years)	1 year 2 months 12 days
LTV(1)	88.70%
Multi-Family | Seattle, WA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 15,250,000
Coupon	4.50%
Max Remaining Term (Years)	1 year
LTV(1)	54.10%
Multi-Family | Various, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 23,145,934
Coupon	5.50%
Max Remaining Term (Years)	9 months 18 days
LTV(1)	76.00%
Multi-Family | Tuscon, AZ
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 19,110,000
Coupon	4.50%
Max Remaining Term (Years)	4 years 3 months 18 days
LTV(1)	81.00%
Multi-Family | Dallas, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 31,678,000
Coupon	3.70%
Max Remaining Term (Years)	4 years 10 months 24 days
LTV(1)	81.20%
Multi-Family | Tulsa, OK
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 6,360,000
Coupon	4.50%
Max Remaining Term (Years)	4 years 2 months 12 days
LTV(1)	76.60%
Multi-Family | Philadelphia, PA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 7,600,000
Coupon	4.10%
Max Remaining Term (Years)	4 years 3 months 18 days
LTV(1)	79.80%
Multi-Family | Baltimore, MD
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 28,476,000
Coupon	3.10%
Max Remaining Term (Years)	4 years 10 months 24 days
LTV(1)	77.60%
Multi-Family | Palatine, IL
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 31,690,000
Coupon	4.30%
Max Remaining Term (Years)	4 years 9 months 18 days
LTV(1)	70.90%
Multi-Family | Woodridge, IL
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 24,521,093
Coupon	3.80%
Max Remaining Term (Years)	4 years 9 months 18 days
LTV(1)	77.80%
Multi-Family | Richmond, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 22,500,000
Coupon	3.90%
Max Remaining Term (Years)	2 years 3 months 18 days
LTV(1)	73.50%
Multi-Family | Omaha, NE
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 19,430,000
Coupon	3.70%
Max Remaining Term (Years)	4 years 9 months 18 days
LTV(1)	77.30%
Multi-Family | Greenville, SC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 14,800,000
Coupon	3.90%
Max Remaining Term (Years)	4 years 9 months 18 days
LTV(1)	76.30%
Multi-Family | Fresno, CA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 12,920,000
Coupon	3.90%
Max Remaining Term (Years)	4 years 4 months 24 days
LTV(1)	82.30%
Multi-Family | Austin, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 11,070,000
Coupon	3.60%
Max Remaining Term (Years)	4 years 9 months 18 days
LTV(1)	80.20%
Multi-Family | Phoenix, AZ
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 9,440,000
Coupon	3.90%
Max Remaining Term (Years)	4 years 9 months 18 days
LTV(1)	69.40%
Multi-Family | Atlanta, GA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 8,262,000
Coupon	4.30%
Max Remaining Term (Years)	4 years 6 months
LTV(1)	80.20%
Multi-Family | Little Rock, AR
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 8,070,000
Coupon	4.30%
Max Remaining Term (Years)	4 years 6 months
LTV(1)	86.80%
Multi-Family | Chicago, IL
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 5,322,870
Coupon	4.10%
Max Remaining Term (Years)	4 years 9 months 18 days
LTV(1)	80.50%
Multi-Family | Wichita, KS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 3,580,000
Coupon	5.00%
Max Remaining Term (Years)	4 years 8 months 12 days
LTV(1)	69.00%
Multi-Family | Birmingham, AL
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 7,645,000
Coupon	3.50%
Max Remaining Term (Years)	5 years
LTV(1)	78.00%
Mixed-Use | Akron, OH
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 12,226,811
Coupon	5.30%
Max Remaining Term (Years)	2 years 4 months 24 days
LTV(1)	56.70%
Mixed-Use | Washington, D.C.
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 4,325,000
Coupon	4.70%
Max Remaining Term (Years)	4 years 9 months 18 days
LTV(1)	73.30%
Office | Stafford, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 4,618,553
Coupon	5.50%
Max Remaining Term (Years)	3 years 3 months 18 days
LTV(1)	56.40%
Office | Rochelle Park, NJ
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 13,600,000
Coupon	4.00%
Max Remaining Term (Years)	4 years 7 months 6 days
LTV(1)	76.80%
Office | Chicago, IL
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 12,075,000
Coupon	3.80%
Max Remaining Term (Years)	4 years 10 months 24 days
LTV(1)	72.70%
Office | Little Rock, AR
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 6,874,000
Coupon	4.90%
Max Remaining Term (Years)	4 years 7 months 6 days
LTV(1)	72.40%
Retail | Austin, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 15,050,568
Coupon	7.30%
Max Remaining Term (Years)	3 years 9 months 18 days
LTV(1)	70.80%
Retail | Austin, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 8,824,763
Coupon	5.20%
Max Remaining Term (Years)	3 years 8 months 12 days
LTV(1)	32.20%
Retail | Goldsboro, NC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 6,500,000
Coupon	4.00%
Max Remaining Term (Years)	4 years 10 months 24 days
LTV(1)	56.50%
Self-Storage | Las Vegas, NV
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 5,547,000
Coupon	4.30%
Max Remaining Term (Years)	4 years 3 months 18 days
LTV(1)	86.00%
Student Housing | Waco, TX
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 9,112,000
Coupon	4.80%
Max Remaining Term (Years)	4 years 7 months 6 days
LTV(1)	72.90%
Student Housing | Blacksburg, VA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 8,000,000
Coupon	3.90%
Max Remaining Term (Years)	5 years
LTV(1)	72.70%