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EMPLOYEE BENEFIT PLANS - FAIR VALUE (Details) - USD ($) $ in Thousands | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
--- | --- | --- | ---
Pension |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | $ 307,968 | $ 288,634 | $ 279,410
OPEB |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 96,406 | 83,926 | $ 77,980
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 | 292,886 | 276,014 |
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 | 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 | 95,542 | 89,374 |
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 | 21,085 | 16,548 |
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 | 56,912 | 49,929 |
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 | 16,008 | 15,794 |
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 | Opportunistic 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 | Ultra Short Duration |  |  |
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 | 26,303 | 24,328 |
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 | 77,036 | 80,041 |
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 | 92,295 | 80,484 |
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 | 15 | 21 |
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 | 29,908 | 25,474 |
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 | 6,470 | 5,036 |
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 | 17,390 | 14,564 |
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 | 4,958 | 4,764 |
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 | 11,146 | 10,570 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | OPEB | Opportunistic Income |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 7,128 | 6,365 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | OPEB | Ultra Short Duration |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 7,057 | 6,340 |
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 | 8,223 | 7,350 |
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 |
Fair Value, Inputs, Level 1, 2 and 3 | Pension |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 292,886 | 276,014 |
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 | 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 | 95,542 | 89,374 |
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 | 21,085 | 16,548 |
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 | 56,912 | 49,929 |
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 | 16,008 | 15,794 |
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 | Opportunistic 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 | Ultra Short Duration |  |  |
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 | 26,303 | 24,328 |
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 | 77,036 | 80,041 |
Fair Value, Inputs, Level 1, 2 and 3 | OPEB |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 92,295 | 80,484 |
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 | 15 | 21 |
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 | 29,908 | 25,474 |
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 | 6,470 | 5,036 |
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 | 17,390 | 14,564 |
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 | 4,958 | 4,764 |
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 | 11,146 | 10,570 |
Fair Value, Inputs, Level 1, 2 and 3 | OPEB | Opportunistic Income |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 7,128 | 6,365 |
Fair Value, Inputs, Level 1, 2 and 3 | OPEB | Ultra Short Duration |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 7,057 | 6,340 |
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 | 8,223 | 7,350 |
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 |
Investments measured at net asset value | Pension | Common collective trusts |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 15,082 | 12,620 |
Investments measured at net asset value | OPEB | Common collective trusts |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | $ 4,111 | $ 3,442 |

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