LOANS - Summary of Commercial Real Estate Investments Held for Investment (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Mortgage Loans on Real Estate [Line Items]
Outstanding Principal	$ 106,622,443	$ 96,602,237
Carrying value, unamortized origination fees	8,300	7,600
Commercial Mortgage
Mortgage Loans on Real Estate [Line Items]
Outstanding Principal	1,474,288	1,307,911
Carrying Value	$ 1,456,826	$ 1,296,803	$ 1,029,327
Percentage of Loan Portfolio	100.00%	100.00%
Commercial Mortgage \| Senior mortgages
Mortgage Loans on Real Estate [Line Items]
Outstanding Principal	$ 405,968	$ 988,248
Carrying Value	$ 403,497	$ 981,202	625,900
Percentage of Loan Portfolio	27.60%	75.60%
Commercial Mortgage \| Senior Securitized Mortgages
Mortgage Loans on Real Estate [Line Items]
Outstanding Principal	$ 739,058	$ 0
Carrying Value	$ 733,864	$ 0
Percentage of Loan Portfolio	50.10%	0.00%
Mezzanine loans \| Commercial Mortgage
Mortgage Loans on Real Estate [Line Items]
Outstanding Principal	$ 329,262	$ 319,663
Carrying Value	$ 319,465	$ 315,601	$ 394,442
Percentage of Loan Portfolio	22.30%	24.40%

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

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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.

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

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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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