Counterparty Risk (Exposure to Counterparty Risk) (Details) - Counterparty Risk [Member] $ in Thousands	6 Months Ended	9 Months Ended
Jun. 30, 2023 USD ($) counterparty	Sep. 30, 2022 USD ($) counterparty
Cash and Cash Equivalents [Member]
Concentration Risk [Line Items]
Amount of Exposure \| $	$ 194,595	$ 217,053
Number of Counterparties with Exposure \| counterparty	13	13
Percentage of Total Outstanding Unpaid Principal Balance	44.90%	41.30%
Collateral On Repurchase Agreements Held By Dealers [Member]
Concentration Risk [Line Items]
Amount of Exposure \| $	$ 3,221,191	$ 3,247,276
Number of Counterparties with Exposure \| counterparty	26	26
Percentage of Total Outstanding Unpaid Principal Balance	23.10%	21.60%
Due From Broker [Member]
Concentration Risk [Line Items]
Amount of Exposure \| $	$ 33,118	$ 36,761
Number of Counterparties with Exposure \| counterparty	25	20
Percentage of Total Outstanding Unpaid Principal Balance	14.80%	22.80%
Receivable For Securities Sold [Member]
Concentration Risk [Line Items]
Amount of Exposure \| $	$ 30,334	$ 21,439
Number of Counterparties with Exposure \| counterparty	5	6
Percentage of Total Outstanding Unpaid Principal Balance	87.10%	36.40%

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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

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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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 954
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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://www.xbrl.org/2003/role/disclosureRef
-Topic 280
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-Section 50
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