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Residential Mortgage Securities and MSR Related Assets (Impact of AFS Securities on AOCI) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
AOCI from AFS Securities: |  |  |
Unrealized gain on AFS securities at beginning of period | $ 392,722 | $ 417,167 | $ 620,648
Unrealized (loss)/gain on AFS securities, net | 420,281 | 20,335 | (150,642)
Reclassification adjustment for MBS sales included in net income | (389,127) | (44,600) | (51,580)
Reclassification adjustment for impairments included in net income | (344,269) | (180) | (1,259)
Change in AOCI from AFS securities | (313,115) | (24,445) | (203,481)
Balance at end of period | 79,607 | 392,722 | 417,167
Agency MBS |  |  |
AOCI from AFS Securities: |  |  |
Unrealized (loss)/gain on AFS securities, net | (161) | 21,844 | (17,891)
Mortgage-backed Securities, Issued by Private Enterprises [Member] |  |  |
AOCI from AFS Securities: |  |  |
Unrealized (loss)/gain on AFS securities, net | 367,469 | (6,682) | (131,939)
Unrealized gain/(loss) on MSR term notes, net |  |  |
AOCI from AFS Securities: |  |  |
Unrealized (loss)/gain on AFS securities, net | $ 52,973 | $ 5,173 | $ (812)

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