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Employee Stock Purchase Plan - Weighted Average Assumptions of Employee Stock Purchase Plan (Detail) - Stock Purchase Plan [Member]
3 Months Ended 9 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Dividend yield 0.00% 0.00% 0.00% 0.00%
Risk-free interest rate 0.90% 0.40% 0.70% 0.20%
Volatility 46.10% 56.00% 40.70% 51.40%
Expected life 6 months 6 months 6 months 6 months