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Borrowings - Weighted Average Cost and Amount of Advances (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Debt Disclosure [Abstract]
Weighted average interest rate, including cash flow hedges, at end of year	3.32%	3.83%	2.02%
Weighted daily average interest rate, including cash flow hedges, during the year	3.78%	3.42%	1.66%
Daily average of FHLB advances during the year	$ 2,952,872	$ 2,916,849	$ 1,731,110
Maximum amount of FHLB advances at any month end	4,338,731	3,425,000	2,125,000
Interest expense during the year (including swap interest income and expense)	$ 111,574	$ 99,631	$ 28,729

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- Definition

The calculated weighted average interest rate for Federal Home Loan Bank advances outstanding as of the Balance Sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13)(3)(a))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

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The calculated weighted average interest rate for Federal Home Loan Bank advances for the period ended as of the Balance Sheet date.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13)(3)(a))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

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The maximum balance outstanding at any month end during the period as of the Balance Sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13)(3)(a))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

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Federal Home Loan Bank, Advances, Activity for Year, Daily Average Balance of Agreements Outstanding

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