LOANS - Rollforward of the Activity of Commercial Real Estate Investments (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2021	Dec. 31, 2020
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Principal payments	$ (275,487)
Gains / (losses)	(11,177)
Allowance for loan losses
Beginning allowance	(169,500)
Ending allowance	(33,400)	$ (169,500)
Cumulative Effect, Period of Adoption, Adjustment
Allowance for loan losses
Beginning allowance	(37,400)
Ending 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	115,939	218,464
Principal payments	(112,368)	(221,730)
Principal write off		(7,000)
Transfers	(121,325)	(109,599)
Net (increase) decrease in origination fees	(1,403)	(1,788)
Gains / (losses)		204
Amortization of net origination fees	916	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	1,384,122	1,372,430
Commercial Mortgage \| Cumulative Effect, Period of Adoption, Adjusted Balance
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance	1,372,430
Allowance for loan losses
Ending balance		1,372,430
Commercial Mortgage \| Cumulative Effect, Period of Adoption, Adjustment
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	301	12,374
Principal payments	(9,922)	(78)
Principal write off		(7,000)
Transfers	(58,491)	(7,100)
Net (increase) decrease in origination fees	0	(80)
Gains / (losses)		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	112,960	124,156
Commercial Mortgage \| Mezzanine Loans \| Cumulative Effect, Period of Adoption, Adjusted Balance
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance	124,156
Allowance for loan losses
Ending balance		124,156
Commercial Mortgage \| Mezzanine Loans \| Cumulative Effect, Period of Adoption, Adjustment
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	115,569	206,090
Principal payments	(54,037)	(77,344)
Principal write off		0
Transfers	(36,195)	(245,120)
Net (increase) decrease in origination fees	(1,403)	(1,055)
Gains / (losses)		204
Amortization of net origination fees	396	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	409,166	373,925
Commercial Mortgage \| Senior Mortgages \| Cumulative Effect, Period of Adoption, Adjusted Balance
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance	373,925
Allowance for loan losses
Ending balance		373,925
Commercial Mortgage \| Senior Mortgages \| Cumulative Effect, Period of Adoption, Adjustment
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	69	0
Principal payments	(48,409)	(144,308)
Principal write off		0
Transfers	(26,639)	142,621
Net (increase) decrease in origination fees	0	(653)
Gains / (losses)		0
Amortization of net origination fees	477	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	861,996	874,349
Commercial Mortgage \| Senior Securitized Mortgages \| Cumulative Effect, Period of Adoption, Adjusted Balance
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance	$ 874,349
Allowance for loan losses
Ending balance		874,349
Commercial Mortgage \| Senior Securitized Mortgages \| Cumulative Effect, Period of Adoption, Adjustment
Allowance for loan losses
Beginning allowance		$ (4,166)

(Increase) Decrease in Lease Origination Costs

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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.

+ 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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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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Amount of writeoff of financing receivable, charged against allowance for credit loss.

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

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