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Stock-based Compensation - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Stock-Based Compensation Activity [Line Items]              
Unrecognized compensation costs related to non-vested awards $ 6,300,000   $ 6,300,000        
Weighted average period for recognition of compensation costs related to non-vested awards     2 years 2 months 12 days        
Performance share awards fully expensed $ 800,000   $ 800,000        
Weighted average per share fair value of stock options $ 13.94 $ 7.22 $ 10.42 $ 7.57      
Dividend yield 0.00% 0.00% 0.00% 0.00%      
Vesting period         3 years 3 years 3 years
Total pre-tax intrinsic value of options exercised $ 6,100,000   $ 9,400,000        
Fiscal 2015 (2015 - 2017) [Member]              
Stock-Based Compensation Activity [Line Items]              
Performance period start year     2015        
Performance period end year     2017        
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   $ 100,000      
Nonqualified Stock Options [Member]              
Stock-Based Compensation Activity [Line Items]              
Stock compensation expenses recognized $ 400,000 300,000 $ 1,200,000 $ 900,000      
Nonqualified Stock Options [Member] | Board of Director [Member]              
Stock-Based Compensation Activity [Line Items]              
Stock option exercisable on a pro-rata basis     1 year        
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 expiration term upon expiration of employment or service     10 years        
Restricted Stock Awards [Member]              
Stock-Based Compensation Activity [Line Items]              
Number of stock units awarded     54,068 42,522      
Number of stock units forfeited     10,020 1,520      
Number of stock units outstanding 87,745   87,745   67,917    
Restricted stock expense $ 200,000 100,000 $ 700,000 $ 400,000      
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 recognized $ 500,000 400,000 $ 900,000 1,000,000      
Issuance of shares     51,478        
Performance shares forfeited 12,217   12,217        
Performance Shares [Member] | Fiscal 2016 (2016 - 2018) [Member]              
Stock-Based Compensation Activity [Line Items]              
Fair value of performance shares for grants awarded           $ 1,300,000  
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] | Maximum [Member]              
Stock-Based Compensation Activity [Line Items]              
Issuance of shares     84,398        
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 recognized $ 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 recognized $ 100,000 100,000 $ 300,000 $ 200,000      
Number of stock units awarded     21,265 16,004      
Number of stock units forfeited     257 446      
Number of stock units outstanding 60,183   60,183   44,440    
Deferred Stock Units [Member] | 2009 Equity Incentive Plan [Member] | Director [Member]              
Stock-Based Compensation Activity [Line Items]              
Stock compensation expenses recognized     $ 100,000 $ 100,000      
Number of stock units awarded     2,075 2,953      
Number of stock units outstanding 26,516   26,516   24,441    
Deferred Stock Units [Member] | Maximum [Member] | 2009 Equity Incentive Plan [Member] | Director [Member]              
Stock-Based Compensation Activity [Line Items]              
Stock compensation expenses recognized $ 100,000 $ 100,000          
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%