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EMPLOYEE BENEFIT PLANS - ADDITIONAL INFORMATION (Details) - USD ($) | 6 Months Ended
--- | ---
Mar. 31, 2021 | Mar. 31, 2020
Pension |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Employer contributions | $ 0 | $ 0
OPEB | Minimum |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated future employer contributions | 5,000,000 |
OPEB | Maximum |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated future employer contributions | $ 10,000,000 |

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