Share-Based Compensation - Assumptions for Estimation of Fair Values of Stock Appreciation Rights Granted During Reporting Periods (Detail) - Stock Appreciation Rights (SARs) - $ / shares	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	3.78%	1.41%	0.68%
Expected volatility	21.00%	22.00%	23.00%
Expected dividend yield	1.53%	1.42%	1.46%
Expected life	7 years	7 years 3 months 18 days	7 years 4 months 24 days
Fair value derived (USD per share)	$ 57.80	$ 49.45	$ 44.38