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Stock Option and Incentive Plan - Assumptions Used in Calculating Fair Value of Options (Detail) - $ / shares
 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Disclosure of Stock Option and Incentive Plan [Abstract]    
Risk-free interest rate [1],[2] 0.30% 1.41%
Expected life of stock options [2],[3] 4 years 6 months 4 years 6 months
Expected volatility [2],[4] 21.30% 17.30%
Expected dividend yield [2],[5] 2.22% 1.75%
Fair value per option [2] $ 7.82 $ 8.85
[1] Risk-free interest rate is based upon U.S. Treasury yield curve appropriate for the term of the Company’s employee stock options.
[2] There were no options grants during fiscal year 2022.
[3] Expected life of stock options is based upon historical experience.
[4] Expected volatility is based on blended volatility.
[5] Expected dividend yield is based on the Company’s history and future expectation of dividend payouts.