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Share-based Compensation - Assumptions Used to Estimate the Fair Value of Each Stock Option on the Date of Grant Using a Black-Scholes Option Pricing Model (Detail) - Stock Option - $ / shares
 12 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected term (in years) 4 years 2 months 1 day 4 years 1 month 9 days 4 years 25 days
Risk-free rate of return (in percent) 4.00% 1.10% 0.30%
Expected volatility (in percent) 28.60% 27.10% 25.10%
Expected dividend yield (in percent) 0.80% 0.70% 0.60%
Fair value per performance-based share granted (in dollars per share) $ 57.31 $ 43.16 $ 39.51