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Retirement Plan and Other Post-Retirement Benefits - Schedule of Fair Value of Plan Assets (Details) - USD ($) $ in Thousands | Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2022
--- | --- | --- | --- | ---
Retirement Plan |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Miscellaneous Accruals, Interest Receivables, and Non-Interest Cash |  | $ (7,390) | $ (9,985) |  |
Total plan assets |  | 778,117 | 823,985 | $ 784,712 | $ 845,205
Retirement Plan | Total Retirement Plan Investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 871,600 | 917,652 |  |
Retirement Plan | Domestic Equities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [1] | 116 | 37,984 |  |
Retirement Plan | Global Equities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [2] | 79,470 | 38,234 |  |
Retirement Plan | Domestic Fixed Income |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [3],[4] | 645,955 | 681,755 |  |
Retirement Plan | Domestic Fixed Income | Derivative |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 6,200 | 8,500 |  |
Retirement Plan | International Fixed Income |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [5] | 9,609 | 8,587 |  |
Retirement Plan | Real Estate |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [6] | 102,373 | 109,383 |  |
Retirement Plan | Cash Held in Collective Trust Funds |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 34,077 | 41,709 |  |
Retirement Plan | Total Retirement Plan Investments (excluding 401(h) Investments) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 785,507 | 833,970 |  |
Retirement Plan | 401(h) Investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 86,093 | 83,682 |  |
Other Post-Retirement Benefits |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Miscellaneous Accruals, Interest Receivables, and Non-Interest Cash |  | (285) | 1,243 |  |
Total plan assets |  | 496,106 | 496,065 | $ 455,702 | $ 461,438
Other Post-Retirement Benefits | Cash Held in Collective Trust Funds |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 12,302 | 14,274 |  |
Other Post-Retirement Benefits | Total Investments (including 401(h) Investments) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 496,391 | 494,822 |  |
Other Post-Retirement Benefits | 401(h) Investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 86,093 | 83,682 |  |
Other Post-Retirement Benefits | Total VEBA Trust Investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 410,298 | 411,140 |  |
Other Post-Retirement Benefits | Collective Trust Funds — Global Equities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 232,395 | 82,814 |  |
Other Post-Retirement Benefits | Exchange Traded Funds — Fixed Income |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 165,601 | 314,052 |  |
Level 1 | Retirement Plan | Total Retirement Plan Investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 116 | 37,984 |  |
Level 1 | Retirement Plan | Domestic Equities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [1] | 116 | 37,984 |  |
Level 1 | Retirement Plan | Global Equities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [2] | 0 | 0 |  |
Level 1 | Retirement Plan | Domestic Fixed Income |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [3],[4] | 0 | 0 |  |
Level 1 | Retirement Plan | International Fixed Income |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [5] | 0 | 0 |  |
Level 1 | Retirement Plan | Real Estate |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [6] | 0 | 0 |  |
Level 1 | Retirement Plan | Cash Held in Collective Trust Funds |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 0 | 0 |  |
Level 1 | Retirement Plan | Total Retirement Plan Investments (excluding 401(h) Investments) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 104 | 34,484 |  |
Level 1 | Retirement Plan | 401(h) Investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 12 | 3,500 |  |
Level 1 | Other Post-Retirement Benefits | Cash Held in Collective Trust Funds |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 0 | 0 |  |
Level 1 | Other Post-Retirement Benefits | Total Investments (including 401(h) Investments) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 165,613 | 317,552 |  |
Level 1 | Other Post-Retirement Benefits | 401(h) Investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 12 | 3,500 |  |
Level 1 | Other Post-Retirement Benefits | Total VEBA Trust Investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 165,601 | 314,052 |  |
Level 1 | Other Post-Retirement Benefits | Collective Trust Funds — Global Equities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 0 | 0 |  |
Level 1 | Other Post-Retirement Benefits | Exchange Traded Funds — Fixed Income |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 165,601 | 314,052 |  |
Level 2 | Retirement Plan | Total Retirement Plan Investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 606,514 | 642,187 |  |
Level 2 | Retirement Plan | Domestic Equities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [1] | 0 | 0 |  |
Level 2 | Retirement Plan | Global Equities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [2] | 0 | 0 |  |
Level 2 | Retirement Plan | Domestic Fixed Income |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [3],[4] | 596,905 | 633,600 |  |
Level 2 | Retirement Plan | International Fixed Income |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [5] | 9,609 | 8,587 |  |
Level 2 | Retirement Plan | Real Estate |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [6] | 0 | 0 |  |
Level 2 | Retirement Plan | Cash Held in Collective Trust Funds |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 0 | 0 |  |
Level 2 | Retirement Plan | Total Retirement Plan Investments (excluding 401(h) Investments) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 546,135 | 583,021 |  |
Level 2 | Retirement Plan | 401(h) Investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 60,379 | 59,166 |  |
Level 2 | Other Post-Retirement Benefits | Cash Held in Collective Trust Funds |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 0 | 0 |  |
Level 2 | Other Post-Retirement Benefits | Total Investments (including 401(h) Investments) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 60,379 | 59,166 |  |
Level 2 | Other Post-Retirement Benefits | 401(h) Investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 60,379 | 59,166 |  |
Level 2 | Other Post-Retirement Benefits | Total VEBA Trust Investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 0 | 0 |  |
Level 2 | Other Post-Retirement Benefits | Collective Trust Funds — Global Equities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 0 | 0 |  |
Level 2 | Other Post-Retirement Benefits | Exchange Traded Funds — Fixed Income |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 0 | 0 |  |
Level 3 | Retirement Plan | Total Retirement Plan Investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 0 | 0 |  |
Level 3 | Retirement Plan | Domestic Equities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [1] | 0 | 0 |  |
Level 3 | Retirement Plan | Global Equities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [2] | 0 | 0 |  |
Level 3 | Retirement Plan | Domestic Fixed Income |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [3],[4] | 0 | 0 |  |
Level 3 | Retirement Plan | International Fixed Income |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [5] | 0 | 0 |  |
Level 3 | Retirement Plan | Real Estate |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [6] | 0 | 0 |  |
Level 3 | Retirement Plan | Cash Held in Collective Trust Funds |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 0 | 0 |  |
Level 3 | Retirement Plan | Total Retirement Plan Investments (excluding 401(h) Investments) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 0 | 0 |  |
Level 3 | Retirement Plan | 401(h) Investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 0 | 0 |  |
Level 3 | Other Post-Retirement Benefits | Cash Held in Collective Trust Funds |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 0 | 0 |  |
Level 3 | Other Post-Retirement Benefits | Total Investments (including 401(h) Investments) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 0 | 0 |  |
Level 3 | Other Post-Retirement Benefits | 401(h) Investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 0 | 0 |  |
Level 3 | Other Post-Retirement Benefits | Total VEBA Trust Investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 0 | 0 |  |
Level 3 | Other Post-Retirement Benefits | Collective Trust Funds — Global Equities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 0 | 0 |  |
Level 3 | Other Post-Retirement Benefits | Exchange Traded Funds — Fixed Income |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals |  | 0 | 0 |  |
Measured at NAV | Retirement Plan | Total Retirement Plan Investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [7] | 264,970 | 237,481 |  |
Measured at NAV | Retirement Plan | Domestic Equities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [1],[7] | 0 | 0 |  |
Measured at NAV | Retirement Plan | Global Equities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [2],[7] | 79,470 | 38,234 |  |
Measured at NAV | Retirement Plan | Domestic Fixed Income |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [3],[4],[7] | 49,050 | 48,155 |  |
Measured at NAV | Retirement Plan | International Fixed Income |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [5],[7] | 0 | 0 |  |
Measured at NAV | Retirement Plan | Real Estate |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [6],[7] | 102,373 | 109,383 |  |
Measured at NAV | Retirement Plan | Cash Held in Collective Trust Funds |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [7] | 34,077 | 41,709 |  |
Measured at NAV | Retirement Plan | Total Retirement Plan Investments (excluding 401(h) Investments) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [7] | 239,268 | 216,465 |  |
Measured at NAV | Retirement Plan | 401(h) Investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [7] | 25,702 | 21,016 |  |
Measured at NAV | Other Post-Retirement Benefits | Cash Held in Collective Trust Funds |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [7] | 12,302 | 14,274 |  |
Measured at NAV | Other Post-Retirement Benefits | Total Investments (including 401(h) Investments) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [7] | 270,399 | 118,104 |  |
Measured at NAV | Other Post-Retirement Benefits | 401(h) Investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [7] | 25,702 | 21,016 |  |
Measured at NAV | Other Post-Retirement Benefits | Total VEBA Trust Investments |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [7] | 244,697 | 97,088 |  |
Measured at NAV | Other Post-Retirement Benefits | Collective Trust Funds — Global Equities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [7] | 232,395 | 82,814 |  |
Measured at NAV | Other Post-Retirement Benefits | Exchange Traded Funds — Fixed Income |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Plan assets, subtotals | [7] | $ 0 | $ 0 |  |
[1] | Domestic Equities include mostly collective trust funds, common stock, and exchange traded funds.
[2] | Global Equities are comprised of collective trust funds.
[3] | Domestic Fixed Income securities include $6.2 million and $8.5 million of derivative instruments used as part of the Company’s overall liability-driven investment strategy as a way to assist in matching the duration of the assets of the Retirement Plan investments with its liability to plan participants as of September 30, 2025 and September 30, 2024, respectively.
[4] | Domestic Fixed Income securities include mostly collective trust funds, corporate/government bonds and mortgages, and exchange traded funds.
[5] | International Fixed Income securities are comprised mostly of corporate/government bonds.
[6] | Real Estate consists of investments held in a collective trust fund and a partnership.
[7] | Reflects the authoritative guidance related to investments measured at net asset value (NAV).

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