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Stock-based Compensation Plans - Additional Information (Detail) - USD ($)
12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2018
Stock-Based Compensation Activity [Line Items]            
Unrecognized compensation costs, nonvested awards, amount $ 8,400,000          
Unrecognized compensation costs, nonvested awards, weighted average recognition period 2 years 2 months 12 days          
Stock-based compensation expense $ 5,863,000 $ 5,453,000 $ 5,077,000      
Dividend yield   0.00%        
Nonqualified Stock Options [Member]            
Stock-Based Compensation Activity [Line Items]            
Expiration period 7 years          
Stock-based compensation expense $ 2,800,000 $ 2,500,000 2,200,000      
Aggregate intrinsic value, outstanding options 15,100,000          
Aggregate intrinsic value, exercisable options $ 7,900,000          
Weighted average remaining contractual life, options outstanding 4 years 7 months 6 days          
Weighted average remaining contractual remaining, options exercisable 3 years 4 months 24 days          
Intrinsic value, options exercised $ 7,100,000 2,000,000.0 300,000      
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]            
Vesting period 3 years          
Stock-based compensation expense $ 2,200,000 $ 2,000,000.0 $ 1,700,000      
Units granted 71,000 67,000 45,000      
Units outstanding 119,000 100,000 90,000     85,000
Weighted average grant date fair value $ 44.07 $ 38.74 $ 28.61      
Restricted Stock Units (RSUs) [Member]            
Stock-Based Compensation Activity [Line Items]            
Vesting period 3 years          
Stock-based compensation expense $ 500,000 $ 600,000 $ 600,000      
Units granted 17,000 18,000 12,000      
Units outstanding 61,000 65,000        
Weighted average grant date fair value $ 33.45 $ 31.12        
Deferred Stock Units [Member] | Director [Member]            
Stock-Based Compensation Activity [Line Items]            
Stock-based compensation expense $ 100,000 $ 100,000 $ 100,000      
Units outstanding 34,000 33,000        
Weighted average grant date fair value $ 30.32 $ 28.41        
Performance Shares [Member]            
Stock-Based Compensation Activity [Line Items]            
Vesting period       3 years 3 years  
Performance Shares [Member] | Fiscal 2017 (2017 - 2019) [Member]            
Stock-Based Compensation Activity [Line Items]            
Fair value of stock units granted       $ 1,200,000    
Performance Shares [Member] | Maximum [Member]            
Stock-Based Compensation Activity [Line Items]            
Stock-based compensation expense $ 0 $ 100,000 400,000      
1999 Employee Stock Purchase Plan [Member]            
Stock-Based Compensation Activity [Line Items]            
Stock-based compensation expense 200,000 100,000 $ 100,000      
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 1,100,000          
Common stock authorized, shares remaining 485,000