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EMPLOYEE BENEFIT PLANS - ASSUMED HCCTR (Details) - OPEB - USD ($) $ in Thousands | 12 Months Ended
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Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
HCCTR | 7.60% | 7.60% | 7.90%
Ultimate HCCTR | 4.50% | 4.50% | 4.50%
Effect of a 1 percentage point increase in the HCCTR on: |  |  |
Year-end benefit obligation | $ 49,106 | $ 49,061 | $ 36,260
Total service and interest cost | 2,799 | 2,923 | 2,482
Effect of a 1 percentage point decrease in the HCCTR on: |  |  |
Year-end benefit obligation | (38,844) | (38,747) | (28,743)
Total service and interest costs | $ (2,151) | $ (2,250) | $ (1,937)

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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/otherTransitionRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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