Marketable Securities (Details) - USD ($) $ in Millions	Jun. 28, 2019	Sep. 28, 2018
Other Current Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	$ 192.2	$ 294.1
Other Noncurrent Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	35.1	22.8
US Treasury and Government [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	54.4
US Treasury and Government [Member] | Other Current Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	44.4	65.0
US Treasury and Government [Member] | Other Noncurrent Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	10.0	0.0
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	94.4
Corporate Debt Securities [Member] | Other Current Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	83.9	204.1
Corporate Debt Securities [Member] | Other Noncurrent Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	10.5	12.0
US States and Political Subdivisions Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	77.2
US States and Political Subdivisions Debt Securities [Member] | Other Current Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	62.6	2.0
US States and Political Subdivisions Debt Securities [Member] | Other Noncurrent Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	14.6	0.8
Debt Security, Government, Non-US [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	1.3
Debt Security, Government, Non-US [Member] | Other Current Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	1.3	23.0
Debt Security, Government, Non-US [Member] | Other Noncurrent Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	$ 0.0	$ 10.0