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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, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
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 2.50% 1.90% 2.40% 1.60%
Volatility 45.30% 48.80% 48.20% 49.90%
Expected life 6 months 6 months 6 months 6 months