LOANS - Summary of Commercial Real Estate Investments Held for Investment (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Real Estate [Line Items]
Outstanding Principal	$ 96,782,059	$ 95,582,210
Carrying value, unamortized origination fees	5,700	3,800
Commercial Mortgage
Real Estate [Line Items]
Outstanding Principal	1,443,203	1,033,158
Carrying Value	$ 1,435,865	$ 1,029,327	$ 970,505
Percentage of Loan Portfolio	100.00%	100.00%
Commercial Mortgage \| Mezzanine Loans
Real Estate [Line Items]
Outstanding Principal	$ 343,354	$ 395,015
Carrying Value	$ 342,700	$ 394,442	451,467
Percentage of Loan Portfolio	23.80%	38.20%
Commercial Mortgage \| Senior Mortgages
Real Estate [Line Items]
Outstanding Principal	$ 1,090,849	$ 629,143
Carrying Value	$ 1,084,167	$ 625,900	510,071
Percentage of Loan Portfolio	75.60%	60.90%
Commercial Mortgage \| Preferred Equity
Real Estate [Line Items]
Outstanding Principal	$ 9,000	$ 9,000
Carrying Value	$ 8,998	$ 8,985	$ 8,967
Percentage of Loan Portfolio	0.60%	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
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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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