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Assumptions Used in Stock Option Plans (Detail)	3 Months Ended		6 Months Ended
Assumptions Used in Stock Option Plans (Detail)	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013	Mar. 31, 2012
Share Based Payment Award Stock Options Valuation Assumptions [Line Items]
Risk-free interest rates	0.70%	0.70%	0.60%	0.80%
Expected life (years)	4 years 9 months 18 days	4 years 9 months 18 days	4 years 9 months 18 days	4 years 9 months 18 days
Expected volatility	49.10%	49.60%	49.20%	49.60%
Dividend yield	0.00%	0.00%	0.00%	0.00%

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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