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Securities, at Fair Value (Impact of AFS Securities on AOCI) (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2021 | Mar. 31, 2020
AOCI from AFS Securities: |  |
Unrealized gain on AFS securities at beginning of period | $ 79,607 | $ 392,722
Unrealized (loss)/gain on AFS securities, net | (3,855) | 124,410
Reclassification adjustment for MBS sales included in net income | 0 | (23,953)
Reclassification adjustment for impairment included in net income | 0 | (344,269)
Change in AOCI from AFS securities | (3,855) | (243,812)
Balance at end of period | 75,752 | 148,910
Agency MBS |  |
AOCI from AFS Securities: |  |
Unrealized (loss)/gain on AFS securities, net | $ (3,855) | $ 124,410

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