Employee Benefits - Black-Scholes Option Pricing Model to Determine Grant-date Fair Value for Stock Options (Detail)	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Risk-free interest rates	2.10%	1.34%	1.63%
Dividend yield	3.30%	4.10%	4.40%
Life of option	6 years 5 months 19 days	6 years 5 months 8 days	6 years 4 months 20 days
Share price volatility	30.00%	27.00%	31.00%
Forfeitures (by employee group) Minimum	0.00%	0.00%	0.00%
Forfeitures (by employee group) Maximum	16.00%	19.00%	16.00%

No definition available.

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- Definition

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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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.

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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 share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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-Paragraph 2
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Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Forfeitures Rate Maximum

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Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Forfeitures Rate Minimum

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