Marketable Securities Current Noncurrent (Details) - USD ($) $ in Millions	Sep. 28, 2018	Sep. 29, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	$ 316.9
US Treasury and Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	65.0
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	216.0
US States and Political Subdivisions Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	2.8
Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	33.1
Other Current Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	294.1	$ 0.0
Other Current Assets [Member] | US Treasury and Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	65.0	0.0
Other Current Assets [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	204.1	0.0
Other Current Assets [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	2.0	0.0
Other Current Assets [Member] | Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	23.0	0.0
Other Noncurrent Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	22.8	0.0
Other Noncurrent Assets [Member] | US Treasury and Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	0.0	0.0
Other Noncurrent Assets [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	12.0	0.0
Other Noncurrent Assets [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	0.8	0.0
Other Noncurrent Assets [Member] | Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	$ 10.0	$ 0.0