COMMERCIAL MORTGAGE LOANS HELD-FOR-INVESTMENT - Summary of Commercial Loan Risk Ratings (Details)	9 Months Ended
	Sep. 30, 2018 USD ($) mortgage_loan	Dec. 31, 2017 USD ($)
Financing Receivable, Recorded Investment [Line Items]
Commercial mortgage loans held-for-investment, at amortized cost	$ 544,586,537	$ 0
Commercial real estate portfolio segment
Financing Receivable, Recorded Investment [Line Items]
Number of Loans | mortgage_loan	43
Unpaid Principal Balance	$ 544,586,537	0
Commercial mortgage loans held-for-investment, at amortized cost	$ 544,586,537	$ 0
Commercial real estate portfolio segment | Risk rating, 1
Financing Receivable, Recorded Investment [Line Items]
Number of Loans | mortgage_loan	0
Unpaid Principal Balance	$ 0
Commercial mortgage loans held-for-investment, at amortized cost	$ 0
Commercial real estate portfolio segment | Risk rating, 2
Financing Receivable, Recorded Investment [Line Items]
Number of Loans | mortgage_loan	9
Unpaid Principal Balance	$ 105,814,995
Commercial mortgage loans held-for-investment, at amortized cost	$ 105,814,995
Commercial real estate portfolio segment | Risk rating, 3
Financing Receivable, Recorded Investment [Line Items]
Number of Loans | mortgage_loan	30
Unpaid Principal Balance	$ 393,620,797
Commercial mortgage loans held-for-investment, at amortized cost	$ 393,620,797
Average risk rating, percentage	91.70%
Commercial real estate portfolio segment | Risk rating, 4
Financing Receivable, Recorded Investment [Line Items]
Number of Loans | mortgage_loan	4
Unpaid Principal Balance	$ 45,150,745
Commercial mortgage loans held-for-investment, at amortized cost	$ 45,150,745
Commercial real estate portfolio segment | Risk rating, 5
Financing Receivable, Recorded Investment [Line Items]
Number of Loans | mortgage_loan	0
Unpaid Principal Balance	$ 0
Commercial mortgage loans held-for-investment, at amortized cost	$ 0