Fair Value of Financial Instruments - Summary of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Assets at Fair Value:
Agency Securities	$ 1,387,845	$ 1,970,902
Debt securities, trading	3,217,509	3,207,420
Derivatives, at fair value	199,073	54,686
Liabilities at Fair Value:
Derivatives, at fair value	10,900	1,217
US Treasury Securities
Assets at Fair Value:
Debt securities, trading	198,833
Fair Value, Measurements, Recurring
Assets at Fair Value:
Agency Securities	4,406,521	5,178,322
Derivatives, at fair value	199,073	54,686
Liabilities at Fair Value:
Derivatives, at fair value	10,900	1,217
Fair Value, Measurements, Recurring \| US Treasury Securities
Assets at Fair Value:
Debt securities, trading	198,833
Fair Value, Measurements, Recurring \| (Level 1)
Assets at Fair Value:
Agency Securities	0	0
Derivatives, at fair value	0	0
Liabilities at Fair Value:
Derivatives, at fair value	0	0
Fair Value, Measurements, Recurring \| (Level 1) \| US Treasury Securities
Assets at Fair Value:
Debt securities, trading	198,833
Fair Value, Measurements, Recurring \| (Level 2)
Assets at Fair Value:
Agency Securities	4,406,521	5,178,322
Derivatives, at fair value	199,073	54,686
Liabilities at Fair Value:
Derivatives, at fair value	10,900	1,217
Fair Value, Measurements, Recurring \| (Level 2) \| US Treasury Securities
Assets at Fair Value:
Debt securities, trading	0
Fair Value, Measurements, Recurring \| (Level 3)
Assets at Fair Value:
Agency Securities	0	0
Derivatives, at fair value	0	0
Liabilities at Fair Value:
Derivatives, at fair value	0	$ 0
Fair Value, Measurements, Recurring \| (Level 3) \| US Treasury Securities
Assets at Fair Value:
Debt securities, trading	$ 0

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