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STOCK-BASED COMPENSATION - Summary of Compensation Cost for Stock-Based Payment Arrangements (Details) - USD ($) $ in Thousands | 12 Months Ended
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Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Stock-based compensation expense | $ 36,329 | $ 34,292 | $ 31,687
Stock options |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Stock-based compensation expense | 1,753 | 3,721 | 7,913
Restricted stock |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Stock-based compensation expense | 30,605 | 26,149 | 23,774
Performance share units |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Stock-based compensation expense | 7,454 | 4,422 | 0
Stock-based compensation benefit included in restructuring charges |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Stock-based compensation expense | $ (3,483) | $ 0 | $ 0

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