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EMPLOYEE BENEFIT PLANS - COMPONENTS OF NET PERIODIC COST (Details) - USD ($) $ in Thousands | 12 Months Ended
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Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015
Pension |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Service cost | $ 8,347 | $ 7,591 | $ 7,485
Interest cost | 9,771 | 11,342 | 10,199
Expected return on plan assets | (19,313) | (20,118) | (17,090)
Recognized actuarial loss | 8,827 | 7,281 | 6,985
Prior service cost (credit) amortization | 111 | 111 | 111
Net periodic benefit cost recognized as expense | 7,743 | 6,207 | 7,690
OPEB |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Service cost | 4,380 | 4,521 | 4,253
Interest cost | 5,545 | 6,256 | 5,739
Expected return on plan assets | (4,767) | (4,845) | (4,977)
Recognized actuarial loss | 4,370 | 3,274 | 2,943
Prior service cost (credit) amortization | (365) | (365) | (364)
Net periodic benefit cost recognized as expense | $ 9,163 | $ 8,841 | $ 7,594

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- Definition Defined Benefit Plan, Net Periodic Benefit Cost Including Special Termination Benefits + References No definition available. + Details
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- Definition Amount of gain (loss) recognized in net periodic benefit (cost) credit of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(4) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(5) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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