COMMERCIAL REAL ESTATE INVESTMENTS - CRE Debt and Preferred Equity Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Real Estate [Line Items]
Carrying Value	$ 1,251,138	$ 1,029,327	$ 970,505
Carrying value, unamortized origination fees	5,700	3,800
Preferred Equity Interests
Real Estate [Line Items]
Carrying Value	8,994	8,985	8,967
Commercial Mortgage
Real Estate [Line Items]
Outstanding Principal	1,256,823	1,033,158
Carrying Value	$ 1,251,138	$ 1,029,327
Percentage of Loan Portfolio	100.00%	100.00%
Commercial Mortgage \| Mezzanine Loans
Real Estate [Line Items]
Outstanding Principal	$ 360,095	$ 395,015
Carrying Value	$ 359,574	$ 394,442	451,467
Percentage of Loan Portfolio	28.70%	38.20%
Commercial Mortgage \| Senior Mortgages
Real Estate [Line Items]
Outstanding Principal	$ 887,728	$ 629,143
Carrying Value	$ 882,570	$ 625,900	$ 510,071
Percentage of Loan Portfolio	70.60%	60.90%
Commercial Mortgage \| Preferred Equity Interests
Real Estate [Line Items]
Outstanding Principal	$ 9,000	$ 9,000
Carrying Value	$ 8,994	$ 8,985
Percentage of Loan Portfolio	0.70%	0.90%

For investments which are quantified by principal amount, the principle balance held at close of period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 6
-Subparagraph (SX 210.12-14.Column B)
-URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611322-123010

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=63488139&loc=d3e12121-115841

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-Publisher FASB
-Name Accounting Standards Codification
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Percent of net assets at close of period. For schedules of investments that are categorized, each category has a percent of net assets for the aggregated value of the Investments in the category.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 210
-Section 50
-Paragraph 6
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611133-123010

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Amount of deferred fees paid by borrowers and unamortized costs incurred to originate loans and leases, unamortized loan commitments and loan syndication fees, and premiums over or discounts from face amounts of loans that are being amortized into income as an adjustment to yield. Excludes amounts for loans and leases covered under loss sharing agreements.

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