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Securities, at Fair Value - Impact of AFS Securities on AOCI (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
AOCI from AFS Securities: |  |  |
Unrealized gain on AFS securities at beginning of period | $ 79,607 | $ 392,722 | $ 417,167
Unrealized (losses)/gains on securities available-for-sale | (32,774) | 420,281 | 20,335
Reclassification adjustment for securities sales included in net income | 0 | (389,127) | (44,600)
Reclassification adjustment for impairments included in net income | 0 | (344,269) | (180)
Change in AOCI from AFS securities | (32,774) | (313,115) | (24,445)
Balance at end of period | 46,833 | 79,607 | 392,722
Agency MBS |  |  |
AOCI from AFS Securities: |  |  |
Unrealized (losses)/gains on securities available-for-sale | $ (32,774) | $ 420,281 | $ 20,335

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