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Employee 401(k) Profit Sharing Plans - Additional Information (Detail) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Compensation And Retirement Disclosure [Abstract]      
Duration of profit sharing plan 90 days    
Introductory period of profit sharing April 1 or October 1    
Employees eligibility age for participating in profit sharing plan 21 years    
Total participants contributions in Profit sharing plan 50.00%    
Maximum of each participants compensation 5.00%    
Contribution under the Profit sharing plan $ 2.7 $ 2.3 $ 1.9