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Stock-Based Employee Benefit Plans - Assumptions Used to Estimate Fair Value of Stock Options (Details)
12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted average volatility    
Equity Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted average expected term (years) 4 years 5 months 19 days 4 years 5 months 15 days 4 years 3 months 3 days
Expected volatility (range), minimum 33.60% 30.00% 31.10%
Expected volatility (range), maximum 34.40% 35.90% 32.40%
Weighted average volatility 33.90% 30.60% 32.20%
Risk free interest rate, minimum 0.29% 0.36% 2.50%
Risk free interest rate, maximum 0.73% 1.68% 2.68%
Weighted average expected dividend yield 0.00% 0.00% 0.00%