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COMMITMENTS AND CONTINGENCIES - Narrative (Details)	12 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
Purchase Commitment, Excluding Long-term Commitment [Line Items]
Purchase orders outstanding for equipment, parts and supplies	$ 13,700,000
Total rent expense	$ 15,500,000	$ 13,700,000	$ 14,000,000.0
Number of renewal options	2
Lease renewal term	3 years
Contingent gains recognized in consolidated financial statements	$ 0	$ 0	$ 0
HPIDC Employee Injury Lawsuit - January 2018
Purchase Commitment, Excluding Long-term Commitment [Line Items]
Settlement amount	21,000,000.0
HPIDC Employee Injury Lawsuit - October 2017
Purchase Commitment, Excluding Long-term Commitment [Line Items]
Loss contingency accrual	$ 9,500,000

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Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income.

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