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LOANS - Additional Information (Details) - USD ($) | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2018
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items] |  |  |  |  |  |  |
Assets |  | $ 128,956,120,000 |  | $ 128,956,120,000 |  | $ 105,787,527,000 | [1]
Loans receivable with variable rates of interest |  |  |  | 91.00% |  | 88.00% |
Loan loss provision recorded during period |  | 3,504,000 | $ 0 | $ 9,207,000 | $ 0 |  |
Balance of the loan loss provision |  | 12,700,000 |  | 12,700,000 |  | $ 3,500,000 |
Loans | [2] | 3,946,614,000 |  | 3,946,614,000 |  | 4,585,975,000 | [1]
Commercial Mortgage Loan |  |  |  |  |  |  |
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items] |  |  |  |  |  |  |
Loans |  | 0 | $ 0 | 0 | $ 0 | $ 42,200,000 |
Residential mortgage loans |  |  |  |  |  |  |
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items] |  |  |  |  |  |  |
Assets |  | $ 1,200,000,000 |  | $ 1,200,000,000 |  |  |
Percent of adjustable-rate loans |  | 38.00% |  | 38.00% |  | 47.00% |
Loans |  | $ 76,300,000 |  | $ 76,300,000 |  | $ 97,500,000 |
Minimum |  |  |  |  |  |  |
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items] |  |  |  |  |  |  |
Senior secured loans, stated maturity |  |  |  | 5 years |  |  |
Maximum |  |  |  |  |  |  |
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items] |  |  |  |  |  |  |
Senior secured loans, stated maturity |  |  |  | 7 years |  |  |
Commercial Mortgage Loan |  |  |  |  |  |  |
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items] |  |  |  |  |  |  |
Mortgage Principal |  | $ 491,435,000 |  | $ 491,435,000 |  | $ 516,834,000 |
[1] | (1) Derived from the audited consolidated financial statements at December 31, 2018.
[2] | (4) Includes $76.3 million and $97.5 million of residential mortgage loans held for sale and $0 and $42.2 million of commercial mortgage loans held for sale at September 30, 2019 and December 31, 2018, respectively.

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