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Marketable Securities Schedule of Available-for-sale Securities (Details) - USD ($)
$ in Millions
6 Months Ended
Mar. 27, 2020
Sep. 27, 2019
Debt Securities, Available-for-sale [Line Items]    
Document Period End Date Mar. 27, 2020  
US Treasury and Government [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax $ 0.7  
Available-for-sale Securities 84.2  
Corporate Debt Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 0.1  
Available-for-sale Securities 89.8  
US States and Political Subdivisions Debt Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities $ 21.6  
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax   $ (0.1)