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EMPLOYEE BENEFIT PLANS - PENSION AND OTHER POSTEMPLOYMENT BENEFIT PLANS, NARRATIVE (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 Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Number of noncontributory defined benefit retirement plans (in plans) | plan | 2 |  |
Required number of years of service (more than) | 1 year |  |
Years of service and average compensation, basis period for plan benefits | 60 months |  |
Long-term real rate of return on assets (as a percent) | 5.00% |  |
OPEB |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Employer contributions | $ 6,049 | $ 3,235 | $ 5,700
OPEB | Minimum |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Estimated future employer contributions over the next five years | 4,000 |  |
OPEB | Maximum |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Estimated future employer contributions over the next five years | 7,000 |  |
Pension |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Employer discretionary contributions | 0 | 30,000 | $ 0
Employer contributions | $ 74 | $ 30,071 |

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- Definition Defined Benefit Plan, Benefits, Basis Period + References No definition available. + Details
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