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Stock-Based Compensation Plans - Additional Information (Detail) - USD ($)
1 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Stock-Based Compensation Activity [Line Items]          
Unrecognized compensation costs related to non-vested awards   $ 5,700,000      
Weighted average period for recognition of compensation costs related to non-vested awards   2 years 1 month 6 days      
Performance share awards fully expensed   $ 600,000      
Weighted average per share fair value of stock options   $ 10.91 $ 7.63 $ 6.95  
Dividend rates   0.00% 0.00% 0.00%  
Stock option exercisable on a pro-rata basis   3 years 9 months 18 days      
Vesting period   3 years 3 years 3 years  
Aggregate intrinsic value of the option shares outstanding   $ 34,400,000      
Aggregate intrinsic value of the option shares exercisable   $ 12,400,000      
Remaining contractual life of options outstanding   4 years 10 months 24 days      
Total pre-tax intrinsic value of options exercised   $ 12,100,000 $ 100,000 $ 5,100,000  
Fiscal 2014 (2014 - 2016) [Member]          
Stock-Based Compensation Activity [Line Items]          
Performance period start year   2014      
Performance period end year   2016      
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 2016 (2016 - 2018) [Member] | Scenario Forecast [Member]          
Stock-Based Compensation Activity [Line Items]          
Maximum estimated shares earned 76,400        
Fiscal 2017 (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]          
Maximum estimated shares earned   78,226      
Nonqualified Stock Options [Member]          
Stock-Based Compensation Activity [Line Items]          
Stock compensation expenses recognized   $ 1,600,000 1,300,000 1,200,000  
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]          
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]          
Vesting period   3 years      
Restricted stock expense   $ 1,000,000 $ 500,000 $ 300,000  
Number of stock units awarded   56,244 51,559 20,108  
Number of stock units forfeited   10,020 2,278 2,439  
Number of stock units outstanding   85,424 67,917 33,133 27,775
Performance Shares [Member]          
Stock-Based Compensation Activity [Line Items]          
Stock compensation expenses recognized   $ 1,500,000 $ 1,200,000 $ 1,900,000  
Performance shares forfeited   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 (2016 - 2018) [Member]          
Stock-Based Compensation Activity [Line Items]          
Fair value of performance shares for grants awarded     1,200,000    
1999 Employee Stock Purchase Plan [Member]          
Stock-Based Compensation Activity [Line Items]          
Annual compensation withheld, maximum limit   $ 25,000      
1999 Employee Stock Purchase Plan [Member] | Maximum [Member]          
Stock-Based Compensation Activity [Line Items]          
Common stock authorized, shares   600,000      
Stock compensation expenses recognized   $ 100,000 100,000 100,000  
Annual compensation withheld   10.00%      
Employee contributions   $ 100,000 100,000    
Vesting Anniversary [Member] | Nonqualified Stock Options [Member] | Employee [Member]          
Stock-Based Compensation Activity [Line Items]          
Vesting percentage   25.00%      
Vesting Anniversary [Member] | Restricted Stock Awards [Member]          
Stock-Based Compensation Activity [Line Items]          
Vesting percentage   33.00%      
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%      
2009 Equity Incentive Plan [Member]          
Stock-Based Compensation Activity [Line Items]          
Common stock authorized, shares   2,000,000      
Number of shares available for future awards   675,587      
2009 Equity Incentive Plan [Member] | Restricted Stock Units (RSUs) [Member]          
Stock-Based Compensation Activity [Line Items]          
Stock compensation expenses recognized   $ 500,000 $ 300,000 200,000  
Number of stock units awarded   21,535 16,004    
Number of stock units forfeited   528 495    
Number of stock units outstanding   60,182 44,391    
2009 Equity Incentive Plan [Member] | Deferred Stock Units [Member] | Director [Member]          
Stock-Based Compensation Activity [Line Items]          
Stock compensation expenses recognized   $ 100,000 $ 100,000 $ 200,000  
Number of stock units outstanding   26,991 24,441    
Number of stock units outstanding estimated fair value   $ 2,000,000 $ 800,000