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Borrowings - Weighted Average Cost and Amount of Advances (Details) - USD ($) $ in Thousands | 12 Months Ended
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Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2021
Debt Disclosure [Abstract] |  |  |
Weighted average interest rate, including cash flow hedges, at end of year | 3.83% | 2.02% | 1.51%
Weighted daily average interest rate, including cash flow hedges, during the year | 3.42% | 1.66% | 1.98%
Daily average of FHLB advances during the year | $ 2,916,849 | $ 1,731,110 | $ 2,234,027
Maximum amount of FHLB advances at any month end | 3,425,000 | 2,125,000 | 2,700,000
Interest expense during the year (including swap interest income and expense) | $ 99,631 | $ 28,729 | $ 44,188

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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//1943274/2147479853/942-210-S99-1 + Details
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- Definition The calculated weighted average interest rate for Federal Home Loan Bank advances for the period ended 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//1943274/2147479853/942-210-S99-1 + Details
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