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STOCK-BASED COMPENSATION - Weighted-Average Assumptions Used in Calculation of Fair Value of Performance Share Units (Details) - Performance Share Units
12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk-free interest rate 0.20% 1.60% 2.70%
Expected stock volatility 62.30% 34.80% 35.90%
Expected term (in years) 3 years 1 month 6 days 3 years 2 months 12 days 3 years