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Summary of Significant Accounting Policies - Fair Value of Shares Issued under Employee Stock Purchase Plan Estimated using Black-Scholes Option Pricing Model (Detail)	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	0.10%	0.10%	0.20%
Volatility	32.00%	45.00%	50.00%
Expected life	6 months	6 months	6 months
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	3.30%	2.75%	0.00%
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	3.40%	3.30%	3.00%

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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