LOANS - Rollforward of the Activity of Commercial Real Estate Investments (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2021	Dec. 31, 2020
Allowance for loan losses
Beginning allowance	$ (169,500)
Ending allowance	(27,700)	$ (169,500)
Cumulative effect of change in accounting principle for credit losses
Allowance for loan losses
Beginning allowance		(37,400)
Commercial Mortgage
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance	1,372,430	1,618,794
Originations & advances (principal)	128,194	218,464
Principal payments	(172,182)	(221,730)
Principal write off		(7,000)
Transfers and sales	(1,405,318)	(109,599)
Net (increase) decrease in origination fees	(1,403)	(1,788)
Realized gain		204
Amortization of net origination fees	1,030	5,018
Allowance for loan losses
Beginning allowance	(129,933)	(12,703)
Current period (allowance) reversal	129,933	(133,152)
Write offs		23,687
Ending allowance	0	(129,933)
Ending balance	52,684	1,372,430
Commercial Mortgage \| Cumulative effect of change in accounting principle for credit losses
Allowance for loan losses
Beginning allowance		(7,765)
Commercial Mortgage \| Mezzanine Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance	124,156	182,726
Originations & advances (principal)	644	12,374
Principal payments	(9,922)	(78)
Principal write off		(7,000)
Transfers and sales	(171,794)	(7,100)
Net (increase) decrease in origination fees	0	(80)
Realized gain		0
Amortization of net origination fees	43	187
Allowance for loan losses
Beginning allowance	(56,873)	(12,703)
Current period (allowance) reversal	56,873	(66,521)
Write offs		23,687
Ending allowance	0	(56,873)
Ending balance	0	124,156
Commercial Mortgage \| Mezzanine Loans \| Cumulative effect of change in accounting principle for credit losses
Allowance for loan losses
Beginning allowance		(1,336)
Commercial Mortgage \| Senior Mortgages
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance	373,925	499,690
Originations & advances (principal)	127,481	206,090
Principal payments	(75,007)	(77,344)
Principal write off		0
Transfers and sales	(436,408)	(245,120)
Net (increase) decrease in origination fees	(1,403)	(1,055)
Realized gain		204
Amortization of net origination fees	501	2,371
Allowance for loan losses
Beginning allowance	(10,911)	0
Current period (allowance) reversal	10,911	(8,648)
Write offs		0
Ending allowance	0	(10,911)
Ending balance	0	373,925
Commercial Mortgage \| Senior Mortgages \| Cumulative effect of change in accounting principle for credit losses
Allowance for loan losses
Beginning allowance		(2,263)
Commercial Mortgage \| Senior Securitized Mortgages
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance	874,349	936,378
Originations & advances (principal)	69	0
Principal payments	(87,253)	(144,308)
Principal write off		0
Transfers and sales	(797,116)	142,621
Net (increase) decrease in origination fees	0	(653)
Realized gain		0
Amortization of net origination fees	486	2,460
Allowance for loan losses
Beginning allowance	(62,149)	0
Current period (allowance) reversal	62,149	(57,983)
Write offs		0
Ending allowance	0	(62,149)
Ending balance	$ 52,684	874,349
Commercial Mortgage \| Senior Securitized Mortgages \| Cumulative effect of change in accounting principle for credit losses
Allowance for loan losses
Beginning allowance		$ (4,166)

(Increase) Decrease in Lease Origination Costs

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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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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 12
-Footnote 6
-Subsection 29

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 310
-Section S99
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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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Amount excluding accrued interest, of allowance for credit loss on financing receivable.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
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Amount of writeoff of financing receivable, charged against allowance for credit loss.

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-Publisher FASB
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-Topic 326
-SubTopic 20
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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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-Publisher FASB
-Name Accounting Standards Codification
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- Definition

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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-Publisher FASB
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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 credit loss expense (reversal of expense) for financing receivable.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
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