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Stock-based Compensation - Additional Information (Detail) - USD ($)
3 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Stock-Based Compensation Activity [Line Items]          
Unrecognized compensation costs related to non-vested awards $ 8,000,000        
Weighted average period for recognition of compensation costs related to non-vested awards 2 years 8 months 13 days        
Performance share awards fully expensed $ 1,300,000        
Weighted average per share fair value of stock options $ 10.57 $ 7.59      
Dividend yield 0.00% 0.00%      
Vesting period     3 years 3 years 3 years
Total pre-tax intrinsic value of options exercised $ 300,000        
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      
Nonqualified Stock Options [Member]          
Stock-Based Compensation Activity [Line Items]          
Stock compensation expenses recognized $ 400,000 $ 300,000      
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 57,635 39,958      
Number of stock units forfeited 2,220        
Number of stock units outstanding 100,557   67,917    
Restricted stock expense $ 200,000 $ 100,000      
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 shares 51,478        
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        
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]          
Number of stock units awarded 5,626 6,570      
Number of stock units outstanding 28,444   44,391    
Restricted Stock Units (RSUs) [Member] | Maximum [Member] | 2009 Equity Incentive Plan [Member]          
Stock-Based Compensation Activity [Line Items]          
Stock compensation expenses recognized $ 100,000 $ 100,000      
Deferred Stock Units [Member] | 2009 Equity Incentive Plan [Member] | Director [Member]          
Stock-Based Compensation Activity [Line Items]          
Number of stock units awarded 500 1,971      
Number of stock units outstanding 24,941   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%