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Retirement Plan and Other Post-Retirement Benefits - Schedule of Fair Value of Plan Assets (Details) - USD ($) $ in Thousands | Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2021
--- | --- | --- | --- | ---
Retirement Plan |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Miscellaneous Accruals, Interest Receivables, and Non-Interest Cash |  | $ (9,985) |  | $ 3,075 |  |
Total plan assets |  | (823,985) |  | (784,712) | $ (845,205) | $ (1,095,729)
Retirement Plan | Total Retirement Plan Investments |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals |  | 917,652 |  | 854,956 |  |
Retirement Plan | Domestic Equities |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals | [1] | 37,984 |  | 37,611 |  |
Retirement Plan | Global Equities |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals | [2] | 38,234 |  | 36,088 |  |
Retirement Plan | Domestic Fixed Income |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals | [3] | 681,755 | [4] | 612,820 |  |
Retirement Plan | Domestic Fixed Income | Derivative |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals |  | 8,500 |  |  |  |
Retirement Plan | International Fixed Income |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals | [5] | 8,587 |  | 7,778 |  |
Retirement Plan | Real Estate |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals | [6] | 109,383 |  | 123,859 |  |
Retirement Plan | Cash Held in Collective Trust Funds |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals |  | 41,709 |  | 36,800 |  |
Retirement Plan | Total Retirement Plan Investments (excluding 401(h) Investments) |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals |  | 833,970 |  | 781,637 |  |
Retirement Plan | 401(h) Investments |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals |  | 83,682 |  | 73,319 |  |
Other Post-Retirement Benefits |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Miscellaneous Accruals, Interest Receivables, and Non-Interest Cash |  | 1,243 |  | 10,795 |  |
Total plan assets |  | (496,065) |  | (455,702) | $ (461,438) | $ (575,565)
Other Post-Retirement Benefits | Cash Held in Collective Trust Funds |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals |  | 14,274 |  | 9,637 |  |
Other Post-Retirement Benefits | Total Investments (including 401(h) Investments) |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals |  | 494,822 |  | 444,907 |  |
Other Post-Retirement Benefits | 401(h) Investments |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals |  | 83,682 |  | 73,319 |  |
Other Post-Retirement Benefits | Total VEBA Trust Investments |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals |  | 411,140 |  | 371,588 |  |
Other Post-Retirement Benefits | Collective Trust Funds — Global Equities |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals |  | 82,814 |  | 72,285 |  |
Other Post-Retirement Benefits | Exchange Traded Funds — Fixed Income |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals |  | 314,052 |  | 289,666 |  |
Level 1 | Retirement Plan | Total Retirement Plan Investments |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals |  | 37,984 |  | 37,611 |  |
Level 1 | Retirement Plan | Domestic Equities |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals | [1] | 37,984 |  | 37,611 |  |
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] | 0 | [4] | 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 |  | 34,484 |  | 34,399 |  |
Level 1 | Retirement Plan | 401(h) Investments |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals |  | 3,500 |  | 3,212 |  |
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 |  | 317,552 |  | 292,878 |  |
Level 1 | Other Post-Retirement Benefits | 401(h) Investments |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals |  | 3,500 |  | 3,212 |  |
Level 1 | Other Post-Retirement Benefits | Total VEBA Trust Investments |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals |  | 314,052 |  | 289,666 |  |
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 |  | 314,052 |  | 289,666 |  |
Level 2 | Retirement Plan | Total Retirement Plan Investments |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals |  | 642,187 |  | 564,282 |  |
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] | 633,600 | [4] | 556,504 |  |
Level 2 | Retirement Plan | International Fixed Income |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals | [5] | 8,587 |  | 7,778 |  |
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 |  | 583,021 |  | 516,098 |  |
Level 2 | Retirement Plan | 401(h) Investments |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals |  | 59,166 |  | 48,184 |  |
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 |  | 59,166 |  | 48,184 |  |
Level 2 | Other Post-Retirement Benefits | 401(h) Investments |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals |  | 59,166 |  | 48,184 |  |
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] | 0 | [4] | 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] | 237,481 |  | 253,063 |  |
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] | 38,234 |  | 36,088 |  |
Measured at NAV | Retirement Plan | Domestic Fixed Income |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals | [3],[7] | 48,155 | [4] | 56,316 |  |
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] | 109,383 |  | 123,859 |  |
Measured at NAV | Retirement Plan | Cash Held in Collective Trust Funds |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals | [7] | 41,709 |  | 36,800 |  |
Measured at NAV | Retirement Plan | Total Retirement Plan Investments (excluding 401(h) Investments) |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals | [7] | 216,465 |  | 231,140 |  |
Measured at NAV | Retirement Plan | 401(h) Investments |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals | [7] | 21,016 |  | 21,923 |  |
Measured at NAV | Other Post-Retirement Benefits | Cash Held in Collective Trust Funds |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals | [7] | 14,274 |  | 9,637 |  |
Measured at NAV | Other Post-Retirement Benefits | Total Investments (including 401(h) Investments) |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals | [7] | 118,104 |  | 103,845 |  |
Measured at NAV | Other Post-Retirement Benefits | 401(h) Investments |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals | [7] | 21,016 |  | 21,923 |  |
Measured at NAV | Other Post-Retirement Benefits | Total VEBA Trust Investments |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals | [7] | 97,088 |  | 81,922 |  |
Measured at NAV | Other Post-Retirement Benefits | Collective Trust Funds — Global Equities |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plan assets, subtotals | [7] | 82,814 |  | 72,285 |  |
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 mostly collective trust funds, corporate/government bonds and mortgages, and exchange traded funds.
[4] | Domestic Fixed Income securities include $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.
[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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