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Stock-based Compensation Plans - Additional Information (Detail) - USD ($)
12 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2021
Sep. 30, 2020
Stock-Based Compensation Activity [Line Items]        
Unrecognized compensation costs, nonvested awards, amount $ 10,700,000      
Unrecognized compensation costs, nonvested awards, weighted average recognition period 2 years 2 months 12 days      
Vesting period 3 years      
Stock-based compensation expense $ 7,605,000 $ 7,057,000 $ 5,863,000  
Dividend yield 0.00%      
Nonqualified Stock Options [Member]        
Stock-Based Compensation Activity [Line Items]        
Expiration period 7 years      
Stock-based compensation expense $ 3,700,000 3,400,000 2,800,000  
Aggregate intrinsic value, outstanding options 800,000      
Aggregate intrinsic value, exercisable options $ 600,000      
Weighted average remaining contractual life, options outstanding 4 years      
Weighted average remaining contractual remaining, options exercisable 2 years 10 months 24 days      
Intrinsic value, options exercised $ 300,000 1,000,000 7,100,000  
Nonqualified Stock Options [Member] | Employee [Member]        
Stock-Based Compensation Activity [Line Items]        
Vesting period 4 years      
Nonqualified Stock Options [Member] | Maximum [Member] | Director [Member]        
Stock-Based Compensation Activity [Line Items]        
Vesting period 1 year      
Restricted Stock Awards [Member]        
Stock-Based Compensation Activity [Line Items]        
Stock-based compensation expense $ 3,100,000 $ 2,700,000 $ 2,200,000  
Units granted 103,000 99,000 71,000  
Units outstanding 173,000 158,000 119,000 100,000
Weighted average grant date fair value $ 40.96 $ 42.98 $ 44.07  
Restricted Stock Units (RSUs) [Member]        
Stock-Based Compensation Activity [Line Items]        
Vesting period 3 years      
Stock-based compensation expense $ 500,000 $ 500,000 $ 500,000  
Units granted 16,000 14,000 17,000  
Units outstanding 74,000 65,000    
Weighted average grant date fair value $ 32.18 $ 33.14    
Deferred Stock Units [Member] | Director [Member]        
Stock-Based Compensation Activity [Line Items]        
Stock-based compensation expense $ 100,000 $ 100,000 $ 100,000  
Units outstanding 38,000 36,000    
Weighted average grant date fair value $ 30.86 $ 30.97    
1999 Employee Stock Purchase Plan [Member]        
Stock-Based Compensation Activity [Line Items]        
Stock-based compensation expense $ 300,000 $ 300,000 $ 200,000  
Annual compensation withheld, maximum limit 25,000      
Employee contributions in accrued compensation at period end $ 100,000 $ 100,000    
1999 Employee Stock Purchase Plan [Member] | Maximum [Member]        
Stock-Based Compensation Activity [Line Items]        
Common stock authorized, original authorization 600,000      
Annual compensation withheld 10.00%      
Vesting Anniversary [Member] | Nonqualified Stock Options [Member] | Employee [Member]        
Stock-Based Compensation Activity [Line Items]        
Vesting percentage 25.00%      
Vesting Anniversary [Member] | Restricted Stock Awards [Member]        
Stock-Based Compensation Activity [Line Items]        
Vesting percentage 33.00%      
Vesting Anniversary [Member] | Restricted Stock Units (RSUs) [Member] | Employee [Member]        
Stock-Based Compensation Activity [Line Items]        
Vesting percentage 33.00%      
2019 Equity Incentive Plan [Member]        
Stock-Based Compensation Activity [Line Items]        
Common stock authorized, original authorization 2,340,000      
Common stock authorized, shares remaining 912,000