LOANS - Commercial Real Estate Held for Investment (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2020	Dec. 31, 2019
Allowance for loan losses
Principal payments	$ (621,804)
Realized gain	(106,481)
Beginning allowance	(20,100)
Ending allowance	(168,000)	$ (20,100)
Commercial Mortgage
Allowance for loan losses
Beginning balance	1,606,091	1,296,803
Originations & advances	195,818	593,913
Principal payments	(115,692)	(316,663)
Principal write off	(7,000)
Transfers (3)	(110,252)	40,058
Net (increase) decrease in origination fees	(892)	(4,384)
Realized gain	204
Amortization of net origination fees	3,743	5,571
Net (increase) decrease in allowance		(9,207)
Beginning allowance	(12,703)
Current period allowance	(131,482)
Write offs	23,687
Ending allowance	(128,264)	(12,703)
Ending balance	1,456,459	1,606,091
Mezzanine loans \| Commercial Mortgage
Allowance for loan losses
Beginning balance	170,023	315,601
Originations & advances	12,206	21,709
Principal payments	0	(149,633)
Principal write off	(7,000)
Transfers (3)	(7,100)	(8,675)
Net (increase) decrease in origination fees	(80)	(184)
Realized gain	0
Amortization of net origination fees	140	412
Net (increase) decrease in allowance		(9,207)
Beginning allowance	(12,703)
Current period allowance	(67,213)
Write offs	23,687
Ending allowance	(57,565)	(12,703)
Ending balance	123,327	170,023
Impact of adopting CECL \| Commercial Mortgage
Allowance for loan losses
Beginning allowance	(7,766)
Ending allowance		(7,766)
Impact of adopting CECL \| Mezzanine loans \| Commercial Mortgage
Allowance for loan losses
Beginning allowance	(1,336)
Ending allowance		(1,336)
CECL provision, adjusted balance \| Commercial Mortgage
Allowance for loan losses
Beginning balance	1,618,794
Ending balance		1,618,794
CECL provision, adjusted balance \| Mezzanine loans \| Commercial Mortgage
Allowance for loan losses
Beginning balance	182,726
Ending balance		182,726
Senior Mortgages \| Commercial Mortgage
Allowance for loan losses
Beginning balance	499,690	981,202
Originations & advances	183,612	572,204
Principal payments	(59,675)	(16,785)
Principal write off	0
Transfers (3)	(157,623)	(1,034,754)
Net (increase) decrease in origination fees	(812)	(4,200)
Realized gain	204
Amortization of net origination fees	1,695	2,023
Net (increase) decrease in allowance		0
Beginning allowance	0
Current period allowance	(14,650)
Write offs	0
Ending allowance	(16,914)	0
Ending balance	450,177	499,690
Senior Mortgages \| Impact of adopting CECL \| Commercial Mortgage
Allowance for loan losses
Beginning allowance	(2,264)
Ending allowance		(2,264)
Senior Mortgages \| CECL provision, adjusted balance \| Commercial Mortgage
Allowance for loan losses
Beginning balance	499,690
Ending balance		499,690
Senior Securitized Mortgages \| Commercial Mortgage
Allowance for loan losses
Beginning balance	936,378	0
Originations & advances	0	0
Principal payments	(56,017)	(150,245)
Principal write off	0
Transfers (3)	54,471	1,083,487
Net (increase) decrease in origination fees	0	0
Realized gain	0
Amortization of net origination fees	1,908	3,136
Net (increase) decrease in allowance		0
Beginning allowance	0
Current period allowance	(49,619)
Write offs	0
Ending allowance	(53,785)	0
Ending balance	882,955	936,378
Senior Securitized Mortgages \| Impact of adopting CECL \| Commercial Mortgage
Allowance for loan losses
Beginning allowance	(4,166)
Ending allowance		(4,166)
Senior Securitized Mortgages \| CECL provision, adjusted balance \| Commercial Mortgage
Allowance for loan losses
Beginning balance	$ 936,378
Ending balance		$ 936,378

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Mortgage Loans on Real Estate, Gains (Losses)

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Mortgage Loans on Real Estate, Transfers

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SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Principal Writeoff

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Amount of decrease in investment in mortgage loan on real estate by entity with substantial portion of business acquiring and holding investment real estate or interest in real estate, from collection of principal.

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The net increase(decrease) in interest income during the period representing the allocation of deferred loan origination fees less deferred loan origination costs using the effective interest method over the term of the debt arrangement to which they pertain taking into account the effect of prepayments.

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

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of investment in mortgage loan on real estate by entity with substantial portion of business acquiring and holding investment real estate or interest in real estate.

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Amount of addition to investment in mortgage loan on real estate by entity with substantial portion of business acquiring and holding investment real estate or interest in real estate, classified as other.

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