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, 2024	Sep. 30, 2023	Sep. 30, 2022
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	4 years 2 months 23 days	4 years 2 months 1 day	4 years 1 month 9 days
Risk-free rate of return (in percent)	4.40%	4.00%	1.10%
Expected volatility (in percent)	24.10%	28.60%	27.10%
Expected dividend yield (in percent)	0.80%	0.80%	0.70%
Fair value per option granted (in dollars per share)	$ 62.55	$ 57.31	$ 43.16