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Stock-Based Compensation - Summary of Option Activity under Stock-Based Compensation Plan (Detail) - $ / shares
12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of Shares, Options granted 365,000      
2009 Plan [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of Shares, Options granted 0 75,000    
Number of Shares, Options exercised (43,750) (37,875) (28,125)  
Number of Shares, Options outstanding 440,000 483,750 446,625 474,750
Average Exercise Price Per Share, Options granted   $ 8.760    
Average Exercise Price Per Share, Options exercised $ 5.126 5.126 $ 4.839  
Average Exercise Price Per Share, Options outstanding $ 5.739 $ 5.684 $ 5.120 $ 5.103
1997 Plan [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of Shares, Options exercised   (30,000)    
Number of Shares, Options expired   (11,250)    
Number of Shares, Options outstanding 0   41,250 41,250
Average Exercise Price Per Share, Options exercised   $ 6.213    
Average Exercise Price Per Share, Options expired   $ 6.213    
Average Exercise Price Per Share, Options outstanding     $ 6.213 $ 6.213