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Securities (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
Securities (Narrative) (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024	Sep. 30, 2024
Marketable Securities [Line Items]
Debt Securities, Available-for-Sale, Excluding Accrued Interest	$ (961,417)		$ (961,417)		$ (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	(818,300)		(818,300)		(713,300)
Commercial MBS [Member]
Marketable Securities [Line Items]
Debt Securities, Available-for-Sale, Excluding Accrued Interest	$ (69,400)		$ (69,400)		(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)