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Employee Stock Purchase Plan - Additional Information (Detail) - USD ($)
1 Months Ended 12 Months Ended
Feb. 29, 2012
Sep. 30, 2018
Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Common stock, shares issued   27,141,267 26,314,066
Stock Purchase Plan [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Additional common shares authorized 500,000    
Common stock available for issuance   1,000,000  
Stock Purchase Plan, requisite continuous service   1 year  
Annual offerings description   Implementation of up to 20 annual offerings beginning on the first day of October starting in 2008, with each offering terminating on September 30 of the following year.  
Closing price of common stock on the first and last day of the offering   85.00%  
Percentage not exceeding to periodic payment of purchase price   10.00%  
Maximum common stock value purchased by participant annually   $ 25,000  
Common stock, shares issued   860,535  
1998 Employee Stock Purchase Plan [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Additional Common Shares Authorized   52,837