Marketable Securities (Details) - USD ($) $ in Millions	Mar. 29, 2019	Sep. 28, 2018
Other Current Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	$ 141.1	$ 294.1
Other Noncurrent Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	33.9	22.8
US Treasury and Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	32.4
US Treasury and Government [Member] | Other Current Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	27.3	65.0
US Treasury and Government [Member] | Other Noncurrent Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	5.1	0.0
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	65.6
Corporate Debt Securities [Member] | Other Current Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	53.7	204.1
Corporate Debt Securities [Member] | Other Noncurrent Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	11.9	12.0
US States and Political Subdivisions Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	73.8
US States and Political Subdivisions Debt Securities [Member] | Other Current Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	56.9	2.0
US States and Political Subdivisions Debt Securities [Member] | Other Noncurrent Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	16.9	0.8
Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	3.2
Foreign Government Debt Securities [Member] | Other Current Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	3.2	23.0
Foreign Government Debt Securities [Member] | Other Noncurrent Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	$ 0.0	$ 10.0