COMMERCIAL MORTGAGE LOANS HELD-FOR-INVESTMENT - Summary of Commercial Loan Risk Ratings (Details)	9 Months Ended			12 Months Ended
	Sep. 30, 2021 USD ($) mortgage_loan		Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) mortgage_loan
Financing Receivable, Credit Quality Indicator [Line Items]
Commercial mortgage loans held-for-investment, at amortized cost	$ 803,074,256	[1]		$ 547,345,334
Average risk rating, moderate	2.3			3.1
Purchase of commercial loans held for investment	$ 647,459,391		$ 52,580,443
Commercial real estate portfolio segment
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans | mortgage_loan	53			40
Unpaid Principal Balance	$ 803,043,750			$ 547,345,334
Commercial mortgage loans held-for-investment, at amortized cost	803,074,256			$ 547,345,334
Purchase of commercial loans held for investment	$ 647,459,391
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	30			14
Unpaid Principal Balance	$ 463,032,213			$ 168,401,366
Commercial mortgage loans held-for-investment, at amortized cost	462,940,447			$ 168,401,366
Commercial loans that paid off	77,000,000
Purchase of commercial loans held for investment	$ 357,800,000
Commercial real estate portfolio segment | Risk rating, 3
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans | mortgage_loan	21			20
Unpaid Principal Balance	$ 322,567,733			$ 309,726,343
Commercial mortgage loans held-for-investment, at amortized cost	$ 322,690,005			$ 309,726,343
Average risk rating, percentage (as a percent)	97.80%			84.40%
Commercial loans that paid off	$ 231,900,000
Purchase of commercial loans held for investment	$ 224,200,000
Commercial real estate portfolio segment | Risk rating, 4
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans | mortgage_loan	2			6
Unpaid Principal Balance	$ 17,443,804			$ 69,217,625
Commercial mortgage loans held-for-investment, at amortized cost	17,443,804			$ 69,217,625
Commercial loans that paid off	$ 17,400,000
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

[1] Our consolidated balance sheets include assets and liabilities of consolidated variable interest entities ("VIEs") as the Company was the primary beneficiary of these VIEs. As of September 30, 2021 and December 31, 2020, assets of consolidated VIEs totaled $1,003,113,182 and $591,318,506, respectively and the liabilities of consolidated VIEs totaled $826,707,857 and $463,411,967 respectively. See Note 4 for further discussion.