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EMPLOYEE BENEFIT PLANS - ASSUMED HCCTR (Details) - OPEB - USD ($) $ in Thousands | 12 Months Ended
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Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
HCCTR | 7.90% | 8.30% | 8.50%
Ultimate HCCTR | 4.50% | 4.50% | 4.50%
Year ultimate HCCTR reached | 2024 | 2025 | 2025
Effect of a 1 percentage point increase in the HCCTR on: |  |  |
Year-end benefit obligation | $ 36,260 | $ 32,019 | $ 28,803
Total service and interest cost | 2,482 | 2,468 | 2,331
Effect of a 1 percentage point decrease in the HCCTR on: |  |  |
Year-end benefit obligation | (28,743) | (25,466) | (22,862)
Total service and interest costs | $ (1,937) | $ (1,909) | $ (1,801)

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- Definition Defined Benefit Plan, Effect of One-Percentage Point Increase in Assumed Health Care Cost Trend Rates [Abstract] + References No definition available. + Details
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- Definition Amount of decrease in accumulated postretirement benefit obligation from one-percentage-point decrease in assumed health care cost trend rate. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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- Definition Amount of decrease in service and interest cost components of net periodic postretirement benefit cost from one-percentage-point decrease in assumed health care cost trend rate. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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