LOANS - CRE Debt and Preferred Equity Investments - Based on Outstanding Principal (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Real Estate [Roll Forward]
Principal payments	$ (809,045)
Realized gain	(101,248)
Allowance for loan losses
Beginning allowance	(20,100)
Ending allowance	(169,500)	$ (20,100)
Ending balance, carrying value	498,081
Cumulative Effect, Period of Adoption, Adjustment
Allowance for loan losses
Beginning allowance	(37,400)
Ending allowance		(37,400)
Commercial Mortgage
Real Estate [Roll Forward]
Beginning balance, carrying value	1,606,091	1,296,803
Originations & advances (principal)	218,464	593,913
Principal payments	(221,730)	(316,663)
Principal write off	(7,000)
Transfers	(109,599)	40,058
Net (increase) decrease in origination fees	(1,788)	(4,384)
Realized gain	204
Amortization of net origination fees	5,018	5,571
Allowance for loan losses
Beginning allowance	(12,703)
Current period allowance	(133,152)	(9,207)
Write offs	23,687
Ending allowance	(129,933)	(12,703)
Ending balance, carrying value	1,372,430	1,606,091
Commercial Mortgage \| Cumulative Effect, Period of Adoption, Adjusted Balance
Real Estate [Roll Forward]
Beginning balance, carrying value	1,618,794
Allowance for loan losses
Ending balance, carrying value		1,618,794
Commercial Mortgage \| Cumulative Effect, Period of Adoption, Adjustment
Allowance for loan losses
Beginning allowance	(7,765)
Ending allowance		(7,765)
Commercial Mortgage \| Senior Mortgages
Real Estate [Roll Forward]
Beginning balance, carrying value	499,690	981,202
Originations & advances (principal)	206,090	572,204
Principal payments	(77,344)	(16,785)
Principal write off	0
Transfers	(245,120)	(1,034,754)
Net (increase) decrease in origination fees	(1,055)	(4,200)
Realized gain	204
Amortization of net origination fees	2,371	2,023
Allowance for loan losses
Beginning allowance	0
Current period allowance	(8,648)	0
Write offs	0
Ending allowance	(10,911)	0
Ending balance, carrying value	373,925	499,690
Commercial Mortgage \| Senior Mortgages \| Cumulative Effect, Period of Adoption, Adjusted Balance
Real Estate [Roll Forward]
Beginning balance, carrying value	499,690
Allowance for loan losses
Ending balance, carrying value		499,690
Commercial Mortgage \| Senior Mortgages \| Cumulative Effect, Period of Adoption, Adjustment
Allowance for loan losses
Beginning allowance	(2,263)
Ending allowance		(2,263)
Commercial Mortgage \| Senior Securitized Mortgages
Real Estate [Roll Forward]
Beginning balance, carrying value	936,378	0
Originations & advances (principal)	0	0
Principal payments	(144,308)	(150,245)
Principal write off	0
Transfers	142,621	1,083,487
Net (increase) decrease in origination fees	(653)	0
Realized gain	0
Amortization of net origination fees	2,460	3,136
Allowance for loan losses
Beginning allowance	0
Current period allowance	(57,983)	0
Write offs	0
Ending allowance	(62,149)	0
Ending balance, carrying value	874,349	936,378
Commercial Mortgage \| Senior Securitized Mortgages \| Cumulative Effect, Period of Adoption, Adjusted Balance
Real Estate [Roll Forward]
Beginning balance, carrying value	936,378
Allowance for loan losses
Ending balance, carrying value		936,378
Commercial Mortgage \| Senior Securitized Mortgages \| Cumulative Effect, Period of Adoption, Adjustment
Allowance for loan losses
Beginning allowance	(4,166)
Ending allowance		(4,166)
Mezzanine Loans \| Commercial Mortgage
Real Estate [Roll Forward]
Beginning balance, carrying value	170,023	315,601
Originations & advances (principal)	12,374	21,709
Principal payments	(78)	(149,633)
Principal write off	(7,000)
Transfers	(7,100)	(8,675)
Net (increase) decrease in origination fees	(80)	(184)
Realized gain	0
Amortization of net origination fees	187	412
Allowance for loan losses
Beginning allowance	(12,703)
Current period allowance	(66,521)	(9,207)
Write offs	23,687
Ending allowance	(56,873)	(12,703)
Ending balance, carrying value	124,156	170,023
Mezzanine Loans \| Commercial Mortgage \| Cumulative Effect, Period of Adoption, Adjusted Balance
Real Estate [Roll Forward]
Beginning balance, carrying value	182,726
Allowance for loan losses
Ending balance, carrying value		182,726
Mezzanine Loans \| Commercial Mortgage \| Cumulative Effect, Period of Adoption, Adjustment
Allowance for loan losses
Beginning allowance	$ (1,336)
Ending allowance		$ (1,336)

(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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Real Estate [Roll Forward]

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

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