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EMPLOYEE BENEFIT PLANS - FAIR VALUE (Details) - USD ($) $ in Thousands | Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2022
--- | --- | --- | ---
Pension |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | $ 342,710 | $ 298,361 | $ 284,347
OPEB |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 129,721 | 106,783 | $ 99,736
Fair Value, Inputs, Level 1, 2 and 3 | Pension |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 280,825 | 237,624 |
Fair Value, Inputs, Level 1, 2 and 3 | Pension | Money market funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 0 |  |
Fair Value, Inputs, Level 1, 2 and 3 | Pension | Large Cap Index |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 76,897 | 81,171 |
Fair Value, Inputs, Level 1, 2 and 3 | Pension | Extended Market Index |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 16,665 | 17,256 |
Fair Value, Inputs, Level 1, 2 and 3 | Pension | International Stock |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 50,549 | 48,557 |
Fair Value, Inputs, Level 1, 2 and 3 | Pension | Emerging Markets |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 13,354 | 11,471 |
Fair Value, Inputs, Level 1, 2 and 3 | Pension | Core Fixed Income |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 0 | 0 |
Fair Value, Inputs, Level 1, 2 and 3 | Pension | High Yield Bond Fund |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 16,704 | 20,685 |
Fair Value, Inputs, Level 1, 2 and 3 | Pension | Long Duration Fund |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 106,656 | 58,484 |
Fair Value, Inputs, Level 1, 2 and 3 | OPEB |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 112,089 | 89,492 |
Fair Value, Inputs, Level 1, 2 and 3 | OPEB | Money market funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 24 |  |
Fair Value, Inputs, Level 1, 2 and 3 | OPEB | Large Cap Index |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 38,040 | 30,884 |
Fair Value, Inputs, Level 1, 2 and 3 | OPEB | Extended Market Index |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 7,977 | 6,444 |
Fair Value, Inputs, Level 1, 2 and 3 | OPEB | International Stock |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 22,730 | 17,966 |
Fair Value, Inputs, Level 1, 2 and 3 | OPEB | Emerging Markets |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 5,358 | 4,306 |
Fair Value, Inputs, Level 1, 2 and 3 | OPEB | Core Fixed Income |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 28,765 | 22,241 |
Fair Value, Inputs, Level 1, 2 and 3 | OPEB | High Yield Bond Fund |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 9,195 | 7,651 |
Fair Value, Inputs, Level 1, 2 and 3 | OPEB | Long Duration Fund |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 0 | 0 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 280,825 | 237,624 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension | Money market funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 0 |  |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension | Large Cap Index |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 76,897 | 81,171 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension | Extended Market Index |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 16,665 | 17,256 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension | International Stock |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 50,549 | 48,557 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension | Emerging Markets |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 13,354 | 11,471 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension | Core Fixed Income |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 0 | 0 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension | High Yield Bond Fund |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 16,704 | 20,685 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension | Long Duration Fund |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 106,656 | 58,484 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | OPEB |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 112,089 | 89,492 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | OPEB | Money market funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 24 |  |
Quoted Prices in Active Markets for Identical Assets (Level 1) | OPEB | Large Cap Index |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 38,040 | 30,884 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | OPEB | Extended Market Index |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 7,977 | 6,444 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | OPEB | International Stock |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 22,730 | 17,966 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | OPEB | Emerging Markets |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 5,358 | 4,306 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | OPEB | Core Fixed Income |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 28,765 | 22,241 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | OPEB | High Yield Bond Fund |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 9,195 | 7,651 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | OPEB | Long Duration Fund |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 0 | 0 |
Investments measured at net asset value | Pension | Collective investment trusts |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 61,885 | 60,737 |
Investments measured at net asset value | OPEB | Collective investment trusts |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | $ 17,632 | $ 17,291 |

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