Fair Value of Financial Instruments - Summary of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets at Fair Value:
Agency Securities	$ 12,007,021	$ 7,051,954
Derivatives	7,794	111,913
Liabilities at Fair Value:
Derivatives	115,807	24,505
Credit Risk and Non-Agency Securities
Assets at Fair Value:
Securities, trading	777,487	819,915
Interest-Only Securities
Assets at Fair Value:
Securities, trading	0	20,623
US Treasury Securities
Assets at Fair Value:
Securities, trading	0	98,646
Fair Value, Measurements, Recurring
Assets at Fair Value:
Agency Securities	12,007,021	7,051,954
Derivatives	7,794	111,913
Liabilities at Fair Value:
Derivatives	115,807	24,505
Fair Value, Measurements, Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets at Fair Value:
Agency Securities	0	0
Derivatives	0	0
Liabilities at Fair Value:
Derivatives	0	0
Fair Value, Measurements, Recurring \| Significant Observable Inputs (Level 2)
Assets at Fair Value:
Agency Securities	12,007,021	7,051,954
Derivatives	7,794	111,913
Liabilities at Fair Value:
Derivatives	115,807	24,505
Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3)
Assets at Fair Value:
Agency Securities	0	0
Derivatives	0	0
Liabilities at Fair Value:
Derivatives	0	0
Fair Value, Measurements, Recurring \| Credit Risk and Non-Agency Securities
Assets at Fair Value:
Securities, trading	777,487	819,915
Fair Value, Measurements, Recurring \| Credit Risk and Non-Agency Securities \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets at Fair Value:
Securities, trading	0	0
Fair Value, Measurements, Recurring \| Credit Risk and Non-Agency Securities \| Significant Observable Inputs (Level 2)
Assets at Fair Value:
Securities, trading	777,487	819,915
Fair Value, Measurements, Recurring \| Credit Risk and Non-Agency Securities \| Significant Unobservable Inputs (Level 3)
Assets at Fair Value:
Securities, trading	$ 0	0
Fair Value, Measurements, Recurring \| Interest-Only Securities
Assets at Fair Value:
Securities, trading		20,623
Fair Value, Measurements, Recurring \| Interest-Only Securities \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets at Fair Value:
Securities, trading		0
Fair Value, Measurements, Recurring \| Interest-Only Securities \| Significant Observable Inputs (Level 2)
Assets at Fair Value:
Securities, trading		20,623
Fair Value, Measurements, Recurring \| Interest-Only Securities \| Significant Unobservable Inputs (Level 3)
Assets at Fair Value:
Securities, trading		0
Fair Value, Measurements, Recurring \| US Treasury Securities
Assets at Fair Value:
Securities, trading		98,646
Fair Value, Measurements, Recurring \| US Treasury Securities \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets at Fair Value:
Securities, trading		98,646
Fair Value, Measurements, Recurring \| US Treasury Securities \| Significant Observable Inputs (Level 2)
Assets at Fair Value:
Securities, trading		0
Fair Value, Measurements, Recurring \| US Treasury Securities \| Significant Unobservable Inputs (Level 3)
Assets at Fair Value:
Securities, trading		$ 0

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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