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EMPLOYEE BENEFIT PLANS - COMPONENTS OF NET PERIODIC COST (Details) - USD ($) $ in Thousands	12 Months Ended
	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

X

- Definition

Defined Benefit Plan, Net Periodic Benefit Cost Including Special Termination Benefits

+ References

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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

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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

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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)
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Amount of cost recognized for passage of time related to defined benefit plan.

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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)(2)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
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