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Assumptions Used to Estimate Fair Value of Stock Options (Detail)
12 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]      
Average expected term (years) 4 years 13 days 4 years 1 month 6 days 4 years 3 months 22 days
Expected volatility (range), minimum 35.00% 35.00% 38.00%
Expected volatility (range), maximum 36.00% 37.00% 41.00%
Weighted average volatility 35.00% 37.00% 40.00%
Risk free interest rate, minimum 0.80% 0.50% 0.50%
Risk free interest rate, maximum 1.20% 1.10% 1.50%
Average expected dividend yield 0.20% 0.20% 0.30%
Expected dividend yield (range), minimum 0.20% 0.20% 0.20%
Expected dividend yield (range), maximum     0.30%