Counterparty Risk (Exposure to Counterparty Risk) (Details) - Counterparty Risk [Member] $ in Thousands	3 Months Ended
Mar. 31, 2021 USD ($) counterparty	Mar. 31, 2020 USD ($) counterparty
Cash and Cash Equivalents [Member]
Concentration Risk [Line Items]
Amount of Exposure \| $	$ 149,350	$ 111,647
Number of Counterparties with Exposure \| counterparty	7	8
Percentage of Total Outstanding Unpaid Principal Balance	26.10%	40.10%
Collateral On Repurchase Agreements Held By Dealers [Member]
Concentration Risk [Line Items]
Amount of Exposure \| $	$ 2,226,054	$ 1,860,059
Number of Counterparties with Exposure \| counterparty	24	24
Percentage of Total Outstanding Unpaid Principal Balance	16.60%	15.30%
Due From Broker [Member]
Concentration Risk [Line Items]
Amount of Exposure \| $	$ 91,814	$ 63,147
Number of Counterparties with Exposure \| counterparty	23	22
Percentage of Total Outstanding Unpaid Principal Balance	25.40%	28.90%
Receivable For Securities Sold [Member]
Concentration Risk [Line Items]
Amount of Exposure \| $	$ 5,147	$ 1,416
Number of Counterparties with Exposure \| counterparty	3	2
Percentage of Total Outstanding Unpaid Principal Balance	95.80%	94.50%

Concentration Risk, Number Of Counterparties With Concentration Risk Exposure

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- Definition

Maximum amount of loss due to credit risk that, based on the gross fair value of the financial instrument, the entity would incur if parties to the financial instruments that make up the concentration failed completely to perform according to the terms of the contracts and the collateral or other security, if any, for the amount due proved to be of no value to the entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13537-108611

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- Definition

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.

+ References

No definition available.

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- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13531-108611

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-Name Accounting Standards Codification
-Topic 825
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-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13537-108611

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

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