Stock-Based Compensation - Weighted-Average Assumptions (Detail) - $ / shares	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Expected stock price volatility	28.00%	27.00%	28.00%
Risk free interest rate	1.90%	3.10%	2.20%
Expected life of options (years)		6 years	6 years
Expected annual dividends per year	$ 1.40	$ 1.32	$ 1.26

Share based compensation arrangement by share based payment award fair value assumptions expected annual dividend rate.

No definition available.

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- References

No definition available.

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- Definition

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

+ References

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

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- Definition

Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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