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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - PENSION AND POSTEMPLOYMENT PLANS (Details) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2015 USD ($) | Sep. 30, 2018 USD ($) plan | Sep. 30, 2017 USD ($) | Sep. 30, 2016 USD ($)
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Number of noncontributory defined benefit retirement plans (in plans) | plan |  |  | 2 |  |
Number of noncontributory medical and life insurance plans (in plans) | plan |  |  | 2 |  |
Pension |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Employer discretionary contributions |  | $ 30,000,000 | $ 0.0 | $ 0 |
Employer contributions | [1] |  | 137,000 | 74,000 |
OPEB |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Employer contributions |  |  | $ 6,222,000 | $ 6,049,000 | $ 3,200,000
[1] | Includes the Company’s PEP.

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