COMMERCIAL MORTGAGE LOANS HELD-FOR-INVESTMENT - Summary of Commercial Loan Risk Ratings (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) mortgage_loan	Dec. 31, 2018 USD ($) mortgage_loan
Financing Receivable, Recorded Investment [Line Items]
Commercial mortgage loans held-for-investment, at amortized cost	$ 585,770,803	$ 555,172,891
Commercial real estate portfolio segment
Financing Receivable, Recorded Investment [Line Items]
Number of Loans | mortgage_loan	45	44
Unpaid Principal Balance	$ 585,770,803	$ 555,172,891
Commercial mortgage loans held-for-investment, at amortized cost	$ 585,770,803	$ 555,172,891
Commercial real estate portfolio segment | Risk rating, 1
Financing Receivable, Recorded Investment [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, Recorded Investment [Line Items]
Number of Loans | mortgage_loan	7	5
Unpaid Principal Balance	$ 63,522,869	$ 51,589,000
Commercial mortgage loans held-for-investment, at amortized cost	$ 63,522,869	$ 51,589,000
Commercial real estate portfolio segment | Risk rating, 3
Financing Receivable, Recorded Investment [Line Items]
Number of Loans | mortgage_loan	34	34
Unpaid Principal Balance	$ 483,865,447	$ 455,323,082
Commercial mortgage loans held-for-investment, at amortized cost	$ 483,865,447	$ 455,323,082
Average risk rating, percentage	90.90%	91.30%
Commercial real estate portfolio segment | Risk rating, 4
Financing Receivable, Recorded Investment [Line Items]
Number of Loans | mortgage_loan	4	5
Unpaid Principal Balance	$ 38,382,487	$ 48,260,809
Commercial mortgage loans held-for-investment, at amortized cost	$ 38,382,487	$ 48,260,809
Commercial real estate portfolio segment | Risk rating, 5
Financing Receivable, Recorded Investment [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