COMMERCIAL MORTGAGE LOANS HELD-FOR-INVESTMENT - Loan Portfolio Activity (Details) - USD ($)	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018
Loans Receivable Held-For-Investment, Net, Reconciliation To Cash Flow [Roll Forward]
Balance at December 31, 2018	$ 555,172,891
Purchases	179,075,497	$ 323,416,036
Proceeds from principal repayments	(174,726,836)	$ (124,493,511)
Balance at September 30, 2019	559,521,552
Commercial real estate portfolio segment
Loans Receivable Held-For-Investment, Net, Reconciliation To Cash Flow [Roll Forward]
Balance at December 31, 2018	555,172,891
Purchases	179,075,497
Proceeds from principal repayments	(174,726,836)
Balance at September 30, 2019	$ 559,521,552

Loans Receivable Held-For-Investment, Net, Reconciliation To Cash Flow [Roll Forward]

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- Definition

Amount, after allowance for credit loss, of financing receivable. Excludes financing receivable covered under loss sharing agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919230-210447

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(3))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118952595&loc=d3e4428-111522

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- Definition

The cash outflow associated with purchasing loans held for investment purposes during the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3213-108585

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- Definition

The cash inflow from repayments of the balance excluding interest (principal) on loans receivable and leases held for investment purposes.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3179-108585

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