Securities (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Marketable Securities [Line Items]
Net loss from securities transactions	$ 0	$ 13,345	$ 192,622
Proceeds from sale of AFS securities	0	1,272,512	0
Gross loss from securities transaction		14,900
Gross gain from securities transaction		1,600
Net loss from securities transactions	0	(13,345)	(192,622)
Debt Securities, Available-for-Sale, Excluding Accrued Interest	(867,216)	(856,266)
Residential MBS [Member]
Marketable Securities [Line Items]
Debt Securities, Available-for-Sale, Excluding Accrued Interest	(793,800)	(713,300)
Commercial MBS [Member]
Marketable Securities [Line Items]
Debt Securities, Available-for-Sale, Excluding Accrued Interest	$ (69,700)	(70,200)
Debt Securities, Available-for-Sale, FV of Securities Sold
Marketable Securities [Line Items]
Debt Securities, Available-for-Sale, Excluding Accrued Interest		$ (1,300,000)	$ (1,300,000)

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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
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-Paragraph 1
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