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Employee Stock Purchase Plan - Weighted Average Assumptions of Employee Stock Purchase Plan (Detail) - Stock Purchase Plan [Member]
3 Months Ended 6 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Mar. 31, 2020
Mar. 31, 2019
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 1.80% 2.40% 1.80% 2.40%
Volatility 52.40% 52.20% 52.40% 52.20%
Expected life 6 months 6 months 6 months 6 months