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Stock-based Compensation - Additional Information (Detail) - USD ($)
3 Months Ended 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Stock-Based Compensation Activity [Line Items]          
Unrecognized compensation costs related to non-vested awards $ 6,200,000        
Weighted average period for recognition of compensation costs related to non-vested awards 2 years 6 months        
Performance share awards fully expensed $ 2,600,000        
Weighted average per share fair value of stock options $ 7.59 $ 6.87      
Dividend yield 0.00% 0.00%      
Vesting period       3 years 3 years
Stock compensation expenses recognized $ 789,000 $ 684,000      
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        
Scenario Forecast [Member]          
Stock-Based Compensation Activity [Line Items]          
Vesting period     3 years    
Maximum [Member]          
Stock-Based Compensation Activity [Line Items]          
Total pre-tax intrinsic value of options exercised $ 100,000 100,000      
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%        
Nonqualified Stock Options [Member]          
Stock-Based Compensation Activity [Line Items]          
Stock compensation expenses recognized $ 300,000 300,000      
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        
Nonqualified Stock Options [Member] | Vesting Anniversary [Member] | Employee [Member]          
Stock-Based Compensation Activity [Line Items]          
Vesting percentage 25.00%        
Restricted Stock Awards          
Stock-Based Compensation Activity [Line Items]          
Restricted stock expense $ 100,000 100,000      
Performance Shares [Member]          
Stock-Based Compensation Activity [Line Items]          
Stock compensation expenses recognized $ 200,000 200,000      
Issuance of shares 38,505        
Performance Shares [Member] | Fiscal 2015 (2015 - 2017) [Member]          
Stock-Based Compensation Activity [Line Items]          
Fair value of performance shares for grants awarded         $ 900,000
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] | Scenario Forecast [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 78,606        
1999 Employee Stock Purchase Plan [Member]          
Stock-Based Compensation Activity [Line Items]          
Annual compensation withheld, maximum limit $ 25,000        
Employee contributions 100,000     $ 100,000  
1999 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%        
Restricted Stock Units (RSUs) [Member] | 2009 Equity Incentive Plan [Member]          
Stock-Based Compensation Activity [Line Items]          
Number of stock units awarded 6,570 8,916      
Number of stock units forfeited 74        
Number of stock units outstanding 35,378     32,101  
Number of stock units outstanding estimated fair value $ 900,000     $ 900,000  
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 1,971 1,842      
Number of stock units outstanding 23,048     21,077  
Number of stock units outstanding estimated fair value $ 600,000     $ 600,000  
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