Employee Stock Plans - Net Share-Based Compensation Expense (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 28, 2018	Sep. 29, 2017	Sep. 30, 2016
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation expense	$ 46.4	$ 39.2	$ 42.2
Income tax benefit for share-based compensation	(10.5)	(11.5)	(12.8)
Cost of revenues - Product
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation expense	3.1	3.0	3.3
Cost of revenues - Service
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation expense	4.2	4.1	4.1
Research and development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation expense	4.8	5.1	5.3
Selling, general and administrative expenses
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation expense	$ 34.3	$ 27.0	$ 29.5