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COMMITMENTS AND CONTINGENCIES - Narrative (Details) | 12 Months Ended
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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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