COMMERCIAL REAL ESTATE INVESTMENTS - CRE Debt and Preferred Equity Investments -Based on Outstanding Principal (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Mortgage Loans on Real Estate [Roll Forward]
Net carrying value, beginning balance	$ 1,029,327	$ 970,505	$ 1,348,817
Originations & advances (principal)	407,363	273,708
Principal payments	(349,220)	(654,117)
Amortization & accretion of (premium) discounts	(16)	(314)
Net (increase) decrease in origination fees	(3,922)	(2,558)
Amortization of net origination fees	4,617	4,967	4,263
Net carrying value, ending balance	970,505	1,348,817
Preferred equity
Movement in Mortgage Loans on Real Estate [Roll Forward]
Net carrying value, beginning balance	8,985	8,967	121,773
Originations & advances (principal)	0	0
Principal payments	0	(113,444)
Amortization & accretion of (premium) discounts	0	0
Net (increase) decrease in origination fees	0	0
Amortization of net origination fees	18	638
Net carrying value, ending balance	8,967	121,773
Commercial Mortgage
Movement in Mortgage Loans on Real Estate [Roll Forward]
Net carrying value, beginning balance	1,029,327	970,505
Net carrying value, ending balance	970,505
Commercial Mortgage \| Mezzanine loans
Movement in Mortgage Loans on Real Estate [Roll Forward]
Net carrying value, beginning balance	394,442	451,467	578,503
Originations & advances (principal)	69,121	62,390
Principal payments	(127,799)	(191,291)
Amortization & accretion of (premium) discounts	28	(178)
Net (increase) decrease in origination fees	(605)	(472)
Amortization of net origination fees	2,230	2,515
Net carrying value, ending balance	451,467	578,503
Commercial Mortgage \| Senior mortgages
Movement in Mortgage Loans on Real Estate [Roll Forward]
Net carrying value, beginning balance	625,900	510,071	385,838
Originations & advances (principal)	338,242	211,318
Principal payments	(221,421)	(86,310)
Amortization & accretion of (premium) discounts	(44)	(136)
Net (increase) decrease in origination fees	(3,317)	(2,086)
Amortization of net origination fees	2,369	1,447
Net carrying value, ending balance	510,071	385,838
Commercial Mortgage \| Senior Securitized Mortgages
Movement in Mortgage Loans on Real Estate [Roll Forward]
Net carrying value, beginning balance		0	$ 262,703
Originations & advances (principal)		0
Principal payments		(263,072)
Amortization & accretion of (premium) discounts		0
Net (increase) decrease in origination fees		0
Amortization of net origination fees		369
Net carrying value, ending balance	0	262,703
Commercial Mortgage \| Preferred equity
Movement in Mortgage Loans on Real Estate [Roll Forward]
Net carrying value, beginning balance	8,985	$ 8,967
Net carrying value, ending balance	$ 8,967

Accretion (amortization) of discounts and premiums on loan purchases.

No definition available.

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- Definition

(Increase) Decrease in Lease Origination Costs

+ References

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
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.1)
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 20
-Section 30
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6378273&loc=d3e8446-111530

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 20
-Section 25
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6962437&loc=d3e7402-111529

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 20
-Section 35
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82894254&loc=d3e8622-111531

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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)
-URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 310
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-29)
-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
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-29.6)
-URI http://asc.fasb.org/extlink&oid=6589523&loc=d3e617274-123014

Reference 2: 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)
-URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674

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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
-Section S99
-Paragraph 6
-Subparagraph (SX 210.5-04.(c) Schedule IV)
-URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 310
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-29.6)
-URI http://asc.fasb.org/extlink&oid=6589523&loc=d3e617274-123014

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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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- Details

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