Investment In Securities - Summary of Investment in Securities (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2021	Dec. 31, 2020
Investment In Securities, Fair Value [Roll Forward]
Beginning balance	$ 12,825,367	$ 5,178,322	$ 12,825,367
Purchases		2,253,829	10,340,353
Proceeds from sales		(1,760,064)	(16,491,693)
Principal repayments		(870,985)	(1,262,930)
Gains (losses)		(147,642)	(181,206)
Credit loss expense			(1,012)
(Amortization) accretion		(48,106)	(50,557)
Ending balance		$ 4,605,354	$ 5,178,322
Percentage of Portfolio		100.00%	100.00%
Agency Securities
Investment In Securities, Fair Value [Roll Forward]
Beginning balance	11,941,766	$ 1,970,902	$ 11,941,766
Purchases		0	1,768,688
Proceeds from sales		(167,202)	(10,800,879)
Principal repayments		(339,393)	(873,650)
Gains (losses)		(61,106)	(32,565)
Credit loss expense	(1,012)		(1,012)
(Amortization) accretion		(15,356)	(31,446)
Ending balance		$ 1,387,845	$ 1,970,902
Percentage of Portfolio		30.14%	38.06%
Agency Trading Securities
Investment In Securities, Fair Value [Roll Forward]
Beginning balance	0	$ 3,207,420	$ 0
Purchases		1,265,942	3,711,961
Proceeds from sales		(813,178)	(158,708)
Principal repayments		(531,592)	(343,514)
Gains (losses)		(77,145)	19,557
Credit loss expense			0
(Amortization) accretion		(32,771)	(21,876)
Ending balance		$ 3,018,676	$ 3,207,420
Percentage of Portfolio		65.55%	61.94%
Credit Risk and Non-Agency Securities
Investment In Securities, Fair Value [Roll Forward]
Beginning balance	883,601	$ 0	$ 883,601
Purchases		0	237,928
Proceeds from sales		0	(889,057)
Principal repayments		0	(45,766)
Gains (losses)		0	(189,555)
Credit loss expense			0
(Amortization) accretion		0	2,849
Ending balance		$ 0	$ 0
Percentage of Portfolio		0.00%	0.00%
US Treasury Securities
Investment In Securities, Fair Value [Roll Forward]
Beginning balance	$ 0	$ 0	$ 0
Purchases		987,887	4,621,776
Proceeds from sales		(779,684)	(4,643,049)
Principal repayments		0	0
Gains (losses)		(9,391)	21,357
Credit loss expense			0
(Amortization) accretion		21	(84)
Ending balance		$ 198,833	$ 0
Percentage of Portfolio		4.31%	0.00%