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Employee Stock Purchase Plan - Weighted Average Assumptions of Employee Stock Purchase Plan (Detail) - Stock Purchase Plan [Member]
3 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Dividend yield 0.00% 0.00%
Risk-free interest rate 0.10% 1.80%
Volatility 70.20% 52.40%
Expected life 6 months 6 months