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Securities (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
Securities (Narrative) (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Sep. 30, 2024
Marketable Securities [Line Items]
Debt Securities, Available-for-Sale, Excluding Accrued Interest	$ (956,229)		$ (956,229)		$ (856,266)
Proceeds from sale of AFS securities			0	$ 1,272,512
Gross loss from securities transaction				14,900
Gross gain from securities transaction				1,600
Net loss from securities transactions	0	$ 0	0	$ 13,345
Residential MBS [Member]
Marketable Securities [Line Items]
Debt Securities, Available-for-Sale, Excluding Accrued Interest	(827,800)		(827,800)		(713,300)
Commercial MBS [Member]
Marketable Securities [Line Items]
Debt Securities, Available-for-Sale, Excluding Accrued Interest	$ (69,700)		$ (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)