COMMERCIAL MORTGAGE LOANS HELD-FOR-INVESTMENT - Summary of Commercial Loan Risk Ratings (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) mortgage_loan	Dec. 31, 2018 USD ($) mortgage_loan
Financing Receivable, Credit Quality Indicator [Line Items]
Commercial mortgage loans held-for-investment, at amortized cost	$ 599,770,362	$ 555,172,891
Commercial real estate portfolio segment
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans | mortgage_loan	47	44
Unpaid Principal Balance	$ 599,770,362	$ 555,172,891
Commercial mortgage loans held-for-investment, at amortized cost	$ 599,770,362	$ 555,172,891
Commercial real estate portfolio segment | Risk rating, 1
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans | mortgage_loan	0	0
Unpaid Principal Balance	$ 0	$ 0
Commercial mortgage loans held-for-investment, at amortized cost	$ 0	$ 0
Commercial real estate portfolio segment | Risk rating, 2
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans | mortgage_loan	10	5
Unpaid Principal Balance	$ 100,585,761	$ 51,589,000
Commercial mortgage loans held-for-investment, at amortized cost	$ 100,585,761	$ 51,589,000
Commercial real estate portfolio segment | Risk rating, 3
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans | mortgage_loan	33	34
Unpaid Principal Balance	$ 435,463,192	$ 455,323,082
Commercial mortgage loans held-for-investment, at amortized cost	$ 435,463,192	$ 455,323,082
Average risk rating, percentage	89.40%	91.30%
Commercial real estate portfolio segment | Risk rating, 4
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans | mortgage_loan	4	5
Unpaid Principal Balance	$ 63,721,409	$ 48,260,809
Commercial mortgage loans held-for-investment, at amortized cost	$ 63,721,409	$ 48,260,809
Commercial real estate portfolio segment | Risk rating, 5
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans | mortgage_loan	0	0
Unpaid Principal Balance	$ 0	$ 0
Commercial mortgage loans held-for-investment, at amortized cost	$ 0	$ 0