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Securities, at Fair Value (Impact of AFS Securities on AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
AOCI from AFS Securities:
Unrealized gain on AFS securities at beginning of period	$ 66,163	$ 52,889	$ 79,607	$ 392,722
Unrealized (loss)/gain on AFS securities, net	(8,029)	15,082	(21,473)	408,585
Reclassification adjustment for MBS sales included in net income	0	(60)	0	(389,127)
Reclassification adjustment for impairment included in net income	0	0	0	(344,269)
Change in AOCI from AFS securities	(8,029)	15,022	(21,473)	(324,811)
Balance at end of period	58,134	67,911	58,134	67,911
Agency MBS
AOCI from AFS Securities:
Unrealized (loss)/gain on AFS securities, net	$ (8,029)	$ 15,082	$ (21,473)	$ 408,585

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