LOANS - CRE Debt and Preferred Equity Investments - Based on Outstanding Principal (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Real Estate [Line Items]
Principal payments	$ (66,755)
Commercial Mortgage
Real Estate [Line Items]
Net carrying value, beginning balance	1,296,803	$ 1,029,327
Originations & advances (principal)	905,673	628,177
Principal payments	(385)	(353,424)
Transfers	(738,910)
Net (increase) decrease in origination fees	(1,857)	(6,994)
Amortization of net origination fees	1,205	3,213
Allowance for loan losses	(5,703)	(3,496)
Net carrying value, ending balance	1,456,826	1,296,803
Commercial Mortgage \| Senior mortgages
Real Estate [Line Items]
Net carrying value, beginning balance	981,202	625,900
Originations & advances (principal)	148,341	575,953
Principal payments	(385)	(216,849)
Transfers	(730,235)
Net (increase) decrease in origination fees	3,815	(6,624)
Amortization of net origination fees	759	2,822
Allowance for loan losses	0	0
Net carrying value, ending balance	403,497	981,202
Commercial Mortgage \| Senior Securitized Mortgages
Real Estate [Line Items]
Net carrying value, beginning balance	0
Originations & advances (principal)	739,058
Principal payments	0
Transfers	0
Net (increase) decrease in origination fees	(5,488)
Amortization of net origination fees	294
Allowance for loan losses	0
Net carrying value, ending balance	733,864	0
Commercial Mortgage \| Preferred equity
Real Estate [Line Items]
Net carrying value, beginning balance	0	8,985
Originations & advances (principal)	0	0
Principal payments	0	(9,000)
Transfers	0
Net (increase) decrease in origination fees	0	0
Amortization of net origination fees	0	15
Allowance for loan losses	0	0
Net carrying value, ending balance	0	0
Mezzanine loans \| Commercial Mortgage
Real Estate [Line Items]
Net carrying value, beginning balance	315,601	394,442
Originations & advances (principal)	18,274	52,224
Principal payments	0	(127,575)
Transfers	(8,675)
Net (increase) decrease in origination fees	(184)	(370)
Amortization of net origination fees	152	376
Allowance for loan losses	(5,703)	(3,496)
Net carrying value, ending balance	$ 319,465	$ 315,601

(Increase) Decrease in Lease Origination Costs

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- Definition

Mortgage Loans on Real Estate, Change In Loan Loss Allowance

+ References

No definition available.

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- Definition

Mortgage Loans on Real Estate, Transfers

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No definition available.

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- Definition

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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
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-Name Accounting Standards Codification
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- Definition

The carrying amount of the outstanding balance due under the mortgage loan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 10
-Section S99
-Paragraph 6
-Subparagraph (SX 210.5-04.(c) Schedule IV)
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-URI http://asc.fasb.org/extlink&oid=6589523&loc=d3e617274-123014

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- Definition

The amount of principal collected on mortgage loans on real estate during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 310
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-Paragraph 1
-Subparagraph (SX 210.12-29.6)
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- Definition

Total amount of additions to mortgage loans other than new loans during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 10
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-Paragraph 6
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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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