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FINANCIAL INSTRUMENTS - Summary of Characteristics of Financial Instruments (Details) - USD ($) $ in Thousands | Dec. 31, 2021 | Dec. 31, 2020
--- | --- | ---
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Assets | $ 76,764,064 | $ 88,455,103
Liabilities | 63,568,739 | 74,433,307
Total securities |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Assets | 63,655,674 | 75,652,396
Agency mortgage-backed securities, recognized through other comprehensive income |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Assets | 59,939,383 | 73,562,972
Agency mortgage-backed securities, recognized through earnings |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Assets | 586,222 | 504,087
Residential credit risk transfer securities |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Assets | 936,228 | 532,403
Non-agency mortgage-backed securities |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Assets | 1,663,336 | 972,192
Commercial real estate debt investments, CMBS, recognized through other comprehensive income |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Assets | 0 | 31,603
Commercial real estate debt investments, CMBS, recognized through earnings |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Assets | 521,440 | 45,254
Commercial real estate debt investments, credit risk transfer securities |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Assets | 9,065 | 3,885
Total loans, net |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Assets | 4,242,043 | 3,083,821
Residential mortgage loans |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Assets | 2,272,072 | 345,810
Residential mortgage loan warehouse facility |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Assets | 980 | 0
Commercial real estate debt and preferred equity, held for investment (4) |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Assets | 0 | 498,081
Corporate debt, held for investment |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Assets | 1,968,991 | 2,239,930
Total assets transferred or pledged to securitization vehicles |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Assets | 6,086,308 | 6,910,020
Excess spread agreements, recognized through earnings |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Assets | 69,316 | 0
Mortgage-backed securities, recognized through other comprehensive income |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Assets | 589,873 | 620,347
Residential mortgage loans, recognized through earnings |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Assets | 5,496,435 | 3,249,251
Commercial mortgage loans, recognized through earnings |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Assets | 0 | 2,166,073
Commercial mortgage loans |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Assets | 0 | 874,349
Repurchase agreements |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Liabilities | 54,769,643 | 64,825,239
Other secured financing |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Liabilities | 903,255 | 917,876
Debt issued by securitization vehicles |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Liabilities | 5,155,633 | 5,652,982
Participations issued |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Liabilities | 1,049,066 | 39,198
Mortgages payable |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Liabilities | $ 0 | $ 426,256

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