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Stock-based Compensation - Additional Information (Detail) - USD ($)
3 Months Ended 6 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Mar. 31, 2019
Mar. 31, 2018
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Stock-Based Compensation Activity [Line Items]              
Unrecognized compensation costs related to non-vested awards $ 9,200,000   $ 9,200,000        
Weighted average period for recognition of compensation costs related to non-vested awards     2 years 4 months 24 days        
Performance share awards fully expensed $ 200,000   $ 200,000        
Weighted average per share fair value of stock options $ 18.12 $ 9.00 $ 18.28 $ 10.32      
Dividend yield 0.00% 0.00% 0.00% 0.00%      
Vesting period     3 years 3 years     3 years
Stock compensation expenses (benefit) recognized $ 959,000 $ 1,101,000 $ 2,190,000 $ 2,003,000      
Total pre-tax intrinsic value of options exercised     $ 100,000 3,300,000      
Fiscal 2016 (2016 - 2018) [Member]              
Stock-Based Compensation Activity [Line Items]              
Performance period start year     2016        
Performance period end year     2018        
Fiscal 2017 (2017 - 2019) [Member]              
Stock-Based Compensation Activity [Line Items]              
Performance period start year     2017        
Performance period end year     2019        
Maximum [Member]              
Stock-Based Compensation Activity [Line Items]              
Total pre-tax intrinsic value of options exercised 100,000 3,100,000          
Nonqualified Stock Options [Member]              
Stock-Based Compensation Activity [Line Items]              
Stock compensation expenses (benefit) recognized $ 500,000 400,000 $ 1,000,000 $ 800,000      
Nonqualified Stock Options [Member] | Employee [Member]              
Stock-Based Compensation Activity [Line Items]              
Vesting period     4 years        
Nonqualified Stock Options [Member] | Minimum [Member] | Board of Director [Member]              
Stock-Based Compensation Activity [Line Items]              
Stock option expiration term upon expiration of employment or service     7 years        
Nonqualified Stock Options [Member] | Maximum [Member] | Board of Director [Member]              
Stock-Based Compensation Activity [Line Items]              
Stock option exercisable on a pro-rata basis     1 year        
Restricted Stock Awards [Member]              
Stock-Based Compensation Activity [Line Items]              
Number of stock units awarded     43,713 53,455      
Number of stock units forfeited     800 3,482      
Number of stock units outstanding 94,971   94,971   85,424    
Restricted stock expense $ 400,000 300,000 $ 800,000 $ 500,000      
Restricted Stock Awards [Member] | Minimum [Member]              
Stock-Based Compensation Activity [Line Items]              
Vesting period     1 year        
Restricted Stock Awards [Member] | Maximum [Member]              
Stock-Based Compensation Activity [Line Items]              
Vesting period     3 years        
Performance Shares [Member]              
Stock-Based Compensation Activity [Line Items]              
Stock compensation expenses (benefit) recognized (200,000) 200,000 $ (100,000) 400,000      
Issuance of common stock, shares     76,396        
Performance Shares [Member] | Fiscal 2017 (2017 - 2019) [Member]              
Stock-Based Compensation Activity [Line Items]              
Fair value of performance shares for grants awarded           $ 1,200,000  
Performance Shares [Member] | Fiscal 2018 (2018 - 2020) [Member]              
Stock-Based Compensation Activity [Line Items]              
Fair value of performance shares for grants awarded         $ 0    
Performance Shares [Member] | Fiscal 2019 (2019 - 2021) [Member]              
Stock-Based Compensation Activity [Line Items]              
Fair value of performance shares for grants awarded     $ 0        
Performance Shares [Member] | Maximum [Member]              
Stock-Based Compensation Activity [Line Items]              
Issuance of common stock, shares     132,676        
Employee Stock Purchase Plan [Member]              
Stock-Based Compensation Activity [Line Items]              
Annual compensation withheld, maximum limit     $ 25,000        
Employee Stock Purchase Plan [Member] | Maximum [Member]              
Stock-Based Compensation Activity [Line Items]              
Stock compensation expenses (benefit) recognized $ 100,000 100,000 $ 100,000 100,000      
Common stock authorized, shares 600,000   600,000        
Annual compensation withheld     10.00%        
Employee contributions $ 100,000   $ 100,000   $ 100,000    
Restricted Stock Units (RSUs) [Member] | 2009 Equity Incentive Plan [Member]              
Stock-Based Compensation Activity [Line Items]              
Stock compensation expenses (benefit) recognized $ 100,000 100,000 $ 300,000 $ 200,000      
Number of stock units awarded     11,871 21,265      
Number of stock units outstanding 62,420   62,420   60,440    
Deferred Stock Units [Member] | 2009 Equity Incentive Plan [Member] | Director [Member]              
Stock-Based Compensation Activity [Line Items]              
Stock compensation expenses (benefit) recognized $ 100,000 $ 100,000 $ 100,000 $ 100,000      
Number of stock units awarded     1,422 1,432      
Number of stock units outstanding 28,413   28,413   26,991    
Deferred Stock Units [Member] | Maximum [Member] | 2009 Equity Incentive Plan [Member] | Director [Member]              
Stock-Based Compensation Activity [Line Items]              
Number of stock units outstanding estimated fair value     $ 100,000 $ 100,000      
Vesting Anniversary [Member] | Nonqualified Stock Options [Member] | Employee [Member]              
Stock-Based Compensation Activity [Line Items]              
Vesting percentage     25.00%        
Performance Level [Member] | Minimum [Member]              
Stock-Based Compensation Activity [Line Items]              
Vesting percentage     20.00%        
Performance Level [Member] | Maximum [Member]              
Stock-Based Compensation Activity [Line Items]              
Performance share awards, estimated vesting percentage     200.00%