Securities (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Marketable Securities [Line Items]
Net loss from securities transactions	$ 13,345	$ 192,622	$ 0
Proceeds from sale of AFS securities	1,272,512	0	0
Gross loss from securities transaction	14,900
Gross gain from securities transaction	1,600
Net loss from securities transactions	(13,345)	(192,622)	$ 0
Debt Securities, Available-for-Sale, Excluding Accrued Interest	(856,266)	(1,384,482)
Residential MBS [Member]
Marketable Securities [Line Items]
Debt Securities, Available-for-Sale, Excluding Accrued Interest	(713,300)	(900,734)
Commercial MBS [Member]
Marketable Securities [Line Items]
Debt Securities, Available-for-Sale, Excluding Accrued Interest	(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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