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LOANS - Summary of Commercial Real Estate Investments Held for Investment (Details) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | --- | ---
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items] |  |  |
Outstanding principal | $ 113,123,301 | $ 96,602,237 |
Loans | 669,713 |  |
Carrying value, unamortized origination fees | 8,300 | 7,600 |
Commercial Mortgage |  |  |
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items] |  |  |
Outstanding principal | 1,627,109 | 1,307,911 |
Loans | $ 1,606,091 | $ 1,296,803 | $ 1,029,327
Percentage of loan portfolio | 100.00% | 100.00% |
Commercial Mortgage | Senior Mortgages |  |  |
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items] |  |  |
Outstanding principal | $ 503,499 | $ 988,248 |
Loans | $ 499,690 | $ 981,202 | 625,900
Percentage of loan portfolio | 30.90% | 75.60% |
Commercial Mortgage | Senior Securitized Mortgages |  |  |
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items] |  |  |
Outstanding principal | $ 940,546 | $ 0 |
Loans | $ 936,378 | $ 0 |
Percentage of loan portfolio | 57.80% | 0.00% |
Commercial Mortgage | Preferred Equity |  |  |
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items] |  |  |
Loans |  | $ 0 | 8,985
Mezzanine Loans | Commercial Mortgage |  |  |
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items] |  |  |
Outstanding principal | $ 183,064 | 319,663 |
Loans | $ 170,023 | $ 315,601 | $ 394,442
Percentage of loan portfolio | 11.30% | 24.40% |

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