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LOANS - Schedule of Commercial Real Estate Investments (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
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SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items] |  |  |
Outstanding Principal | $ 69,291,915 | $ 72,253,465 |
Commercial Mortgage |  |  |
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items] |  |  |
Outstanding Principal | 1,510,815 | 1,507,244 |
Loans | $ 1,384,122 | $ 1,372,430 | $ 1,618,794
Percentage of Loan Portfolio | 100.00% | 100.00% |
Unamortized Loan Commitment and Origination Fees and Unamortized Discounts or Premiums | $ 5,400 | $ 4,900 |
Commercial Mortgage | Mezzanine loans |  |  |
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items] |  |  |
Outstanding Principal | 171,639 | 181,261 |
Loans | $ 112,960 | $ 124,156 | 182,726
Percentage of Loan Portfolio | 11.40% | 12.00% |
Commercial Mortgage | Senior mortgages |  |  |
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items] |  |  |
Outstanding Principal | $ 432,736 | $ 387,124 |
Loans | $ 409,166 | $ 373,925 | 499,690
Percentage of Loan Portfolio | 28.60% | 25.70% |
Commercial Mortgage | Senior Securitized Mortgages |  |  |
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items] |  |  |
Outstanding Principal | $ 906,440 | $ 938,859 |
Loans | $ 861,996 | $ 874,349 | $ 936,378
Percentage of Loan Portfolio | 60.00% | 62.30% |

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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-12B(Column D)) -URI http://asc.fasb.org/extlink&oid=120401592&loc=d3e611197-123010 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 210 -Section 50 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=99383244&loc=d3e12121-115841 + Details
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- Definition Amount of investment in mortgage loan on real estate by entity with substantial portion of business acquiring and holding investment real estate or interest in real estate. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 948 -SubTopic 310 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-29(Column G)) -URI http://asc.fasb.org/extlink&oid=120402547&loc=d3e617274-123014 + Details
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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://fasb.org/us-gaap/role/ref/legacyRef -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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