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The Incentive Stock Plans - Additional Information (Detail) - USD ($)
1 Months Ended 12 Months Ended
Feb. 28, 2017
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Jan. 31, 2011
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Weighted average grant fair value of options granted   $ 8.42   $ 6.88  
Options granted     0    
Total intrinsic value of options exercised   $ 6,300,000 $ 1,600,000 $ 3,600,000  
Unrecognized compensation costs related to non-vested options   100,000 200,000    
Fair value of options vested   $ 1,300,000 $ 2,500,000 $ 200,000  
Incentive Stock Plan 2011 [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Common stock, shares authorized         2,200,456
Additional common shares authorized 1,000,000        
Expiration of Plan 2011   2021-01      
Contractual term of plan 2011   10 years      
Incentive Stock Plan 2011 [Member] | Maximum [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Common stock, shares authorized 3,200,456        
Incentive Stock Plan 2011 [Member] | Subject To Award [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Common stock, shares authorized   3,000,000      
Incentive Stock Plan 2007 [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Number of Common stock shares available   200,456      
Incentive Stock Plans [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Weighted average period unrecognized compensation costs related to non-vested options are expected to be recognized   1 year 9 months 18 days 1 year 9 months 18 days