LOANS - Corporate Debt Held for Investment (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Principal payments	$ (134,622)
Sales	(1,294,348)
Corporate Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	2,144,850	$ 1,887,182
Originations & advances	387,082	888,036
Principal payments	(54,465)	(368,927)
Amortization & accretion of (premium) discounts	2,320	8,896
Loan restructuring	(16,732)
Sales	(271,007)	(262,974)
Net (increase) decrease in allowance		(7,363)
Beginning allowance	(7,363)
Current period allowance	(24,026)
Write offs	11,894
Ending allowance	(49,148)	(7,363)
Ending balance	2,150,263	2,144,850
Corporate Loans \| First lien loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	1,396,140	1,346,356
Originations & advances	387,082	542,463
Principal payments	(28,567)	(228,302)
Amortization & accretion of (premium) discounts	1,430	5,960
Loan restructuring	(19,550)
Sales	(223,625)	(262,974)
Net (increase) decrease in allowance		(7,363)
Beginning allowance	(7,363)
Current period allowance	(17,130)
Write offs	11,894
Ending allowance	(23,386)	(7,363)
Ending balance	1,496,887	1,396,140
Corporate Loans \| Second lien loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	748,710	540,826
Originations & advances	0	345,573
Principal payments	(25,898)	(140,625)
Amortization & accretion of (premium) discounts	890	2,936
Loan restructuring	2,818
Sales	(47,382)	0
Net (increase) decrease in allowance		0
Beginning allowance	0
Current period allowance	(6,896)
Write offs	0
Ending allowance	(25,762)	0
Ending balance	653,376	748,710
Impact of adopting CECL \| Corporate Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning allowance	(29,653)
Ending allowance		(29,653)
Impact of adopting CECL \| Corporate Loans \| First lien loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning allowance	(10,787)
Ending allowance		(10,787)
Impact of adopting CECL \| Corporate Loans \| Second lien loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning allowance	(18,866)
Ending allowance		(18,866)
CECL provision, adjusted balance \| Corporate Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	2,152,213
Ending balance		2,152,213
CECL provision, adjusted balance \| Corporate Loans \| First lien loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	1,403,503
Ending balance		1,403,503
CECL provision, adjusted balance \| Corporate Loans \| Second lien loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	$ 748,710
Ending balance		$ 748,710

Debt Securities, Held-to-maturity, Allowance for Credit Loss

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- Definition

Debt Securities, Held-to-maturity, Allowance for Credit Loss, Writeoff

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No definition available.

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- Definition

Debt Securities, Held-to-maturity, Credit Loss Expense (Reversal)

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No definition available.

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- Definition

Mortgage Loans on Real Estate, Restructuring

+ References

No definition available.

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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

Reference 2: 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(Footnote 6))
-URI http://asc.fasb.org/extlink&oid=6589523&loc=d3e617274-123014

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- Definition

Amount of increase (decrease) of allowance reserve for credit loss.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 20
-Section 35
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109123272&loc=d3e8622-111531

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

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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.

+ References

No definition available.

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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))
-URI http://asc.fasb.org/extlink&oid=6589523&loc=d3e617274-123014

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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 sale.

+ 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(Footnote 6))
-URI http://asc.fasb.org/extlink&oid=6589523&loc=d3e617274-123014

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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.

+ 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(Footnote 6))
-URI http://asc.fasb.org/extlink&oid=6589523&loc=d3e617274-123014

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