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Employee Stock Purchase and Stock Option Plans (Stock Issuance Activity Under the 1989 ESPP and the GESPP) (Details) - USD ($)
 12 Months Ended
Jan. 19, 2017
Sep. 28, 2018
Sep. 29, 2017
Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Discount (percent) available to employees on the purchase of common stock   5.00%    
Aggregate Purchase Price Paid for Shares Sold:   $ 23,831,467 $ 23,190,491 $ 26,291,308
Aggregate Number of Shares Sold:   394,304 443,300 627,657
Number of remaining shares reserved for issuance at September 28, 2018   7,398,130    
Under the 1989 ESPP        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Aggregate Purchase Price Paid for Shares Sold:   $ 21,590,858 $ 21,084,657 $ 23,631,241
Aggregate Number of Shares Sold:   357,899 403,652 564,461
Increase in number of shares authorized 4,350,000      
Number of remaining shares reserved for issuance at September 28, 2018   4,187,955    
Under the GESPP        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Aggregate Purchase Price Paid for Shares Sold:   $ 2,240,609 $ 2,105,834 $ 2,660,067
Aggregate Number of Shares Sold:   36,405 39,648 63,196
Increase in number of shares authorized 150,000      
Number of remaining shares reserved for issuance at September 28, 2018   138,575