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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - PENSION AND POSTEMPLOYMENT PLANS (Details) $ in Thousands | 12 Months Ended
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Sep. 30, 2017 USD ($) plan | Sep. 30, 2016 USD ($) | Sep. 30, 2015 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 | $ 0 | $ 30,000 | $ 0
Employer contributions | 74 | 30,071 |
OPEB |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Employer contributions | $ 6,049 | $ 3,235 | $ 5,700

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