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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% |

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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6378536&loc=d3e10092-111533 + Details
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