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Stock-based Compensation - Additional Information (Detail) - USD ($)
3 Months Ended 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Stock-Based Compensation Activity [Line Items]          
Unrecognized compensation costs related to non-vested awards $ 9,800,000        
Weighted average period for recognition of compensation costs related to non-vested awards 2 years 8 months 12 days        
Performance share awards fully expensed $ 500,000        
Weighted average per share fair value of stock options $ 18.30 $ 10.57      
Dividend yield 0.00% 0.00%      
Vesting period     3 years 3 years 3 years
Stock compensation expenses recognized $ 1,231,000 $ 903,000      
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 $ 100,000 300,000      
Nonqualified Stock Options [Member]          
Stock-Based Compensation Activity [Line Items]          
Stock compensation expenses recognized $ 500,000 $ 400,000      
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 expiration term upon expiration of employment or service 10 years        
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 42,248 57,635      
Number of stock units forfeited 507 2,220      
Number of stock units outstanding 94,624 85,424      
Restricted stock expense $ 400,000 $ 200,000      
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 $ 200,000 200,000      
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] | 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 recognized $ 100,000 100,000      
Common stock authorized, shares 600,000        
Annual compensation withheld 10.00%        
Employee contributions $ 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      
Number of stock units awarded 6,206 5,626      
Number of stock units outstanding 57,387   60,182    
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 751 500      
Number of stock units outstanding 27,742   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%