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FAIR VALUE MEASUREMENT OF FINANCIAL INSTRUMENTS - Narrative (Details) - USD ($) $ in Thousands, shares in Millions | 1 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Gain (loss) resulting from the change in the fair value of investments |  | $ (8,700) |  |
Investment shares sold (in shares) | 1.6 |  |  |
Proceeds from sale of marketable securities | $ 12,000 | 0 | $ 11,999 | $ 0
Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Assets held in a non-qualified supplemental savings plan | $ 15,700 | $ 19,800 | $ 15,700 |

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