LOANS - Commercial Real Estate Held for Investment (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Principal payments	$ (134,622)
Realized gain	(136,583)
Beginning allowance	(20,100)
Ending allowance	(137,900)	$ (20,100)
Commercial Mortgage
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	1,606,091	1,296,803
Originations & advances	174,409	593,913
Principal payments	(91,958)	(316,663)
Principal write off	(7,000)
Transfers	(43,100)	40,058
Net (increase) decrease in origination fees	(892)	(4,384)
Realized gain	204
Amortization & accretion of (premium) discounts	1,445	5,571
Net (increase) decrease in allowance		9,207
Beginning allowance	(12,703)
Current period allowance	(75,299)
Write offs	(7,000)
Ending allowance	(88,768)	(12,703)
Ending balance	1,563,134	1,606,091	$ 1,296,803
Mezzanine loans \| Commercial Mortgage
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	170,023	315,601
Originations & advances	11,628	21,709
Principal payments	0	(149,633)
Principal write off	(7,000)
Transfers	0	(8,675)
Net (increase) decrease in origination fees	(80)	(184)
Realized gain	0
Amortization & accretion of (premium) discounts	46	412
Net (increase) decrease in allowance		9,207
Beginning allowance	(12,703)
Current period allowance	(49,825)
Write offs	(7,000)
Ending allowance	(56,864)	(12,703)
Ending balance	130,456	170,023	315,601
Impact of adopting CECL \| Commercial Mortgage
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning allowance	(7,766)
Ending allowance		(7,766)
Impact of adopting CECL \| Mezzanine loans \| Commercial Mortgage
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning allowance	(1,336)
Ending allowance		(1,336)
CECL provision, adjusted balance \| Commercial Mortgage
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	1,618,794
Ending balance		1,618,794
CECL provision, adjusted balance \| Mezzanine loans \| Commercial Mortgage
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	182,726
Ending balance		182,726
Senior Mortgages \| Commercial Mortgage
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	499,690	981,202
Originations & advances	162,781	572,204
Principal payments	(38,425)	(16,785)
Principal write off	0
Transfers	(95,730)	(1,034,754)
Net (increase) decrease in origination fees	(812)	(4,200)
Realized gain	204
Amortization & accretion of (premium) discounts	797	2,023
Net (increase) decrease in allowance		0
Beginning allowance	0
Current period allowance	(6,854)
Write offs	0
Ending allowance	(9,118)	0
Ending balance	519,387	499,690	981,202
Senior Mortgages \| Impact of adopting CECL \| Commercial Mortgage
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning allowance	(2,264)
Ending allowance		(2,264)
Senior Mortgages \| CECL provision, adjusted balance \| Commercial Mortgage
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	499,690
Ending balance		499,690
Senior Securitized Mortgages \| Commercial Mortgage
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	936,378	0
Originations & advances	0		0
Principal payments	(53,533)		(150,245)
Principal write off	0
Transfers	52,630	1,083,487
Net (increase) decrease in origination fees	0		0
Realized gain	0
Amortization & accretion of (premium) discounts	602		3,136
Net (increase) decrease in allowance		0
Beginning allowance	0
Current period allowance	(18,620)
Write offs	0
Ending allowance	(22,786)	0
Ending balance	913,291	936,378	$ 0
Senior Securitized Mortgages \| Impact of adopting CECL \| Commercial Mortgage
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning allowance	(4,166)
Ending allowance		(4,166)
Senior Securitized Mortgages \| CECL provision, adjusted balance \| Commercial Mortgage
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	$ 936,378
Ending balance		$ 936,378

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- Definition

Mortgage Loans on Real Estate, Gains (Losses)

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Mortgage Loans on Real Estate, Transfers

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- Definition

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
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-Name Accounting Standards Codification
-Topic 948
-SubTopic 310
-Section S99
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Amount of increase (decrease) of allowance reserve for credit loss.

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-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
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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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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Topic 310
-SubTopic 20
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-Paragraph 2
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
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A valuation allowance for financing receivables that are expected to be uncollectible.

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-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
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-Paragraph 11B
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- Definition

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 direct write-downs of financing receivables charged against the allowance.

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-SubTopic 10
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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 310
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-29(Column G))
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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 expense related to loss from loan transactions.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.11)
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