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Stock-Based Compensation - Assumptions Used to Value Stock Options (Detail) - $ / shares
6 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions And Methodology [Abstract]    
Risk-free interest rate, minimum 0.50% 2.30%
Risk-free interest rate, maximum 1.80% 3.10%
Volatility   115.00%
Volatility, minimum 90.50%  
Volatility, maximum 91.80%  
Expected life (in years) 6 years 3 months 6 years 3 months
Weighted average grant date fair value per share of options granted $ 40.70 $ 11.42