Stockholders' Equity (AOCI Reclassifications) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized (loss)/gain on sales of residential mortgage securities and residential whole loans		$ (188,847)	$ 62,002	$ 61,307
Other, net		(18,885)	(1,375)	(2,877)
Net Income/(Loss) Available to Common Stock and Participating Securities		(709,186)	363,117	286,801
Amounts Reclassified from AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income/(Loss) Available to Common Stock and Participating Securities		(660,594)	(47,234)	(52,839)
Amounts Reclassified from AOCI | AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Including Noncontrolling Interest
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized (loss)/gain on sales of residential mortgage securities and residential whole loans		(389,127)	(44,600)	(51,580)
Other, net		(344,269)	(180)	(1,259)
Net Income/(Loss) Available to Common Stock and Participating Securities		(733,396)	(44,780)	(52,839)
Amounts Reclassified from AOCI | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other, net	$ (2,454)	72,802		0
Net Income/(Loss) Available to Common Stock and Participating Securities		$ 72,802	$ (2,454)	$ 0