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Residential Mortgage Securities and MSR Related Assets (Narrative) (Details) - USD ($) | 12 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2020 | Jun. 30, 2020 | Mar. 31, 2020 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |  |  |  |
Minimum term of fixed rate mortgages underlying MBS | 15 years |  |  |  |  |  |  |  |
Maximum term of fixed rate mortgages underlying MBS | 30 years |  |  |  |  |  |  |  |
Available-for-sale, unrealized loss position, accumulated loss | $ 1,183,000 |  |  |  |  |  |  |  |
Allowance for credit loss | 86,833,000 | $ 3,025,000 | $ 968,000 | $ 106,246,000 | $ 136,589,000 | $ 218,011,000 | $ 2,271,000 | $ 1,986,000 | $ 1,651,000
Impairment and other losses on securities available-for-sale and other assets | (425,082,000) | (180,000) | $ (1,259,000) |  |  |  |  |  |
Non-Agency MBS |  |  |  |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |  |  |  |
Available-for-sale, unrealized loss position, accumulated loss | 861,000 |  |  |  |  |  |  |  |
Allowance for credit loss | 0 | $ 0 |  |  |  |  |  |  |
MBS and MSR-Related Assets |  |  |  |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |  |  |  |
Impairment and other losses on securities available-for-sale and other assets | $ (63,500,000) |  |  |  |  |  |  |  |
Minimum | RPL/NPL MBS |  |  |  |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, basis spread on variable rate | 3.00% |  |  |  |  |  |  |  |
Debt instrument, coupon step-up period | 36 months |  |  |  |  |  |  |  |
Maximum | RPL/NPL MBS |  |  |  |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, basis spread on variable rate | 4.00% |  |  |  |  |  |  |  |
Debt instrument, coupon step-up period | 48 months |  |  |  |  |  |  |  |

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