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Employee Stock Purchase Plan - Weighted Average Assumptions of Employee Stock Purchase Plan (Detail) - Stock Purchase Plan [Member]
12 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2013
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Dividend yield 0.00% 0.00% 0.00%
Risk-free interest rate 0.10% 0.10% 0.10%
Volatility 36.10% 39.60% 54.80%
Expected life 6 months 6 months 6 months