Marketable Securities (Details) - USD ($) $ in Millions	Jun. 26, 2020	Sep. 27, 2019
Other Current Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	$ 347.0	$ 203.3
Other Noncurrent Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	24.1	27.6
US Treasury and Government [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	69.3
US Treasury and Government [Member] | Other Current Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	53.9	34.2
US Treasury and Government [Member] | Other Noncurrent Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	15.4	20.0
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	261.1
Corporate Debt Securities [Member] | Other Current Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	257.9	66.2
Corporate Debt Securities [Member] | Other Noncurrent Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	3.2	5.9
US States and Political Subdivisions Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	40.7
US States and Political Subdivisions Debt Securities [Member] | Other Current Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	35.2	102.9
US States and Political Subdivisions Debt Securities [Member] | Other Noncurrent Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	$ 5.5	$ 1.7