The cost basis of the derivative instruments when contract was entered into.

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Nominal or face amount used to calculate payments on the derivative asset.

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Average fixed interest rate related to the group of interest rate derivatives.

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Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as an asset.

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Amount of realized and unrealized gain of derivative instruments not designated or qualifying as hedging instruments.

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Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

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Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as a liability.

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Amount of realized and unrealized loss of derivative instruments not designated or qualifying as hedging instruments.

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Nominal or face amount used to calculate payments on the derivative liability.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Derivatives (Details) - USD ($) $ in Thousands		12 Months Ended
Derivatives (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net		$ (172,290)	$ (186,949)	$ (3,461)
Derivative liabilities, fair value	$ (974)	(1,634)	(974)
Derivative Asset, Fair Value, Gross Asset	4,290	11,342	4,290
Derivative Liability, Fair Value, Gross Liability	974	1,634	974
Derivative assets, fair value	4,290	11,342	4,290
Interest rate swaps
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain				10,363
Derivative Instruments Not Designated as Hedging Instruments, Loss		(182,942)	(202,450)
Interest rate swaption
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain		680
Derivative Instruments Not Designated as Hedging Instruments, Loss			(5,607)	0
Derivative instrument, cost basis		13,000
Derivative Asset, Fair Value, Gross Asset	573		573
Derivative Liability, Fair Value, Gross Liability		$ (107)
Derivative, Average Fixed Interest Rate		1.07%
Derivative Asset, Notional Amount		$ 1,250,000
Derivative assets, fair value		1,253
U.S. Treasury futures
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain			2,250
Derivative Instruments Not Designated as Hedging Instruments, Loss		(15,046)		(2,722)
Derivative Liability, Fair Value, Gross Liability		(1,527)
Options on U.S. Treasury futures
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Loss		(26,186)	(1,422)	(658)
Derivative Asset, Fair Value, Gross Asset	2,883		2,883
Long position \| TBA securities
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain		61,245	20,020
Derivative Instruments Not Designated as Hedging Instruments, Loss				(10,737)
Derivative Asset, Fair Value, Gross Asset	834	8,888	834
Short position \| TBA securities
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain			260	$ 293
Derivative Instruments Not Designated as Hedging Instruments, Loss		(10,041)
Derivative Liability, Fair Value, Gross Liability	974		974
Not Designated as Hedging Instrument, Economic Hedge \| Interest rate swaps
Derivative [Line Items]
Derivative Asset, Notional Amount	4,225,000	0	4,225,000
Not Designated as Hedging Instrument, Economic Hedge \| Interest rate swaption
Derivative [Line Items]
Derivative liabilities, fair value		(107)
Derivative Asset, Notional Amount	750,000	1,250,000	750,000
Derivative assets, fair value	573	1,360	573
Not Designated as Hedging Instrument, Economic Hedge \| Interest rate swaption \| 6 months or less
Derivative [Line Items]
Derivative liabilities, fair value	0		0
Derivative instrument, cost basis	$ 6,180	$ 6,312	$ 6,180
Derivative, Average Fixed Interest Rate	2.07%	1.02%	2.07%
Derivative Asset, Notional Amount	$ 750,000	$ 750,000	$ 750,000
Derivative, Average Remaining Maturity		3 months	1 month
Derivative assets, fair value	573	$ 1,161	$ 573
Not Designated as Hedging Instrument, Economic Hedge \| Interest rate swaption \| Greater than 6 months
Derivative [Line Items]
Derivative instrument, cost basis		$ 6,688
Derivative, Average Fixed Interest Rate		1.16%
Derivative Asset, Notional Amount		$ 500,000
Derivative, Average Remaining Maturity		8 months
Derivative assets, fair value		$ 92
Not Designated as Hedging Instrument, Economic Hedge \| U.S. Treasury futures
Derivative [Line Items]
Derivative liabilities, fair value	0		0
Not Designated as Hedging Instrument, Economic Hedge \| Options on U.S. Treasury futures
Derivative [Line Items]
Derivative instrument, cost basis	2,883		2,883
Derivative Asset, Notional Amount	$ 1,350,000	$ 500,000	1,350,000
Derivative, Average Remaining Maturity	2 months	1 month
Derivative assets, fair value	$ 2,883	$ 1,094	2,883
Not Designated as Hedging Instrument, Economic Hedge \| Short position \| U.S. Treasury futures
Derivative [Line Items]
Derivative liabilities, fair value		$ (1,527)
Derivative, Average Remaining Maturity		2 months
Derivative Liability, Notional Amount	0	$ 825,000	0
Not Designated as Hedging Instrument, Economic Hedge \| Short position \| TBA securities
Derivative [Line Items]
Derivative liabilities, fair value	(974)	0	(974)
Derivative Liability, Notional Amount	500,000	0	500,000
Not Designated as Hedging Instrument, Trading \| Long position \| TBA securities
Derivative [Line Items]
Derivative Asset, Notional Amount	435,000	1,515,000	435,000
Derivative assets, fair value	$ 834	$ 8,888	$ 834