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EMPLOYEE BENEFIT PLANS - ADDITIONAL INFORMATION (Details) | 6 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2020 USD ($) plan | Mar. 31, 2019 USD ($) | Sep. 30, 2019 USD ($)
Defined Benefit Plan Disclosure [Line Items] |  |  |
Number of plans | plan | 2 |  |
Required length of service | 1 year |  |
Basis period | 60 months |  |
Employer contributions | $ 0 | $ 0 |
OPEB |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Employer contributions |  |  | $ 7,926,000
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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