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Residential Mortgage Securities and MSR Related Assets (Impact of AFS Securities on AOCI) (Details) - USD ($) $ in Thousands	12 Months Ended
	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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Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment Excluding Cumulative Effect, Net of Tax, Portion Attributable to Parent

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