Marketable Securities (Details) - USD ($) $ in Millions	Jun. 29, 2018	Sep. 29, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	$ 490.7
Other Current Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	384.4	$ 0.0
Other Noncurrent Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	106.3	0.0
US Treasury and Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	84.8
US Treasury and Government [Member] | Other Current Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	80.5	0.0
US Treasury and Government [Member] | Other Noncurrent Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	4.3	0.0
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	151.8
Corporate Debt Securities [Member] | Other Current Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	121.4	0.0
Corporate Debt Securities [Member] | Other Noncurrent Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	30.4	0.0
US States and Political Subdivisions Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	227.1
US States and Political Subdivisions Debt Securities [Member] | Other Current Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	163.0	0.0
US States and Political Subdivisions Debt Securities [Member] | Other Noncurrent Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	64.1	0.0
Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	27.0
Foreign Government Debt Securities [Member] | Other Current Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	19.5	0.0
Foreign Government Debt Securities [Member] | Other Noncurrent Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	$ 7.5	$ 0.0