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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, 2021
Mar. 31, 2020
Mar. 31, 2021
Mar. 31, 2020
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 0.10% 1.80% 0.10% 1.80%
Volatility 70.20% 52.40% 70.20% 52.40%
Expected life 6 months 6 months 6 months 6 months