Investments in Securities - Carrying Value of Trading Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Debt and Equity Securities, FV-NI [Line Items]
Principal Amount	$ 6,551,154	$ 3,110,168
Amortized Cost	6,720,691	3,271,186
Gross Unrealized Loss	(342,136)	(54,779)
Gross Unrealized Gain	1,456	1,102
Fair Value	6,380,011	3,217,509
Agency Securities
Debt and Equity Securities, FV-NI [Line Items]
Principal Amount	5,251,154	2,910,168
Amortized Cost	5,429,903	3,072,199
Gross Unrealized Loss	(309,368)	(54,625)
Gross Unrealized Gain	1,456	1,102
Fair Value	5,121,991	3,018,676
U.S. Treasuries
Debt and Equity Securities, FV-NI [Line Items]
Principal Amount	1,300,000	200,000
Amortized Cost	1,290,788	198,987
Gross Unrealized Loss	(32,768)	(154)
Gross Unrealized Gain	0	0
Fair Value	1,258,020	198,833
Fannie Mae | Agency Securities
Debt and Equity Securities, FV-NI [Line Items]
Principal Amount	4,210,224	2,253,393
Amortized Cost	4,354,922	2,382,146
Gross Unrealized Loss	(254,365)	(47,079)
Gross Unrealized Gain	1,256	1,056
Fair Value	4,101,813	2,336,123
Freddie Mac | Agency Securities
Debt and Equity Securities, FV-NI [Line Items]
Principal Amount	1,040,930	656,775
Amortized Cost	1,074,981	690,053
Gross Unrealized Loss	(55,003)	(7,546)
Gross Unrealized Gain	200	46
Fair Value	$ 1,020,178	$ 682,553