Employee Stock Plans - Net Share-Based Compensation Expense (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 29, 2017	Sep. 30, 2016	Oct. 02, 2015
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation expense	$ 39.2	$ 42.2	$ 39.9
Income tax benefit for share-based compensation	(11.5)	(12.8)	(12.0)
Cost of revenues - Product
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation expense	3.0	3.3	3.5
Cost of revenues - Service
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation expense	4.1	4.1	4.0
Research and development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation expense	5.1	5.3	5.4
Selling, general and administrative expenses
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation expense	$ 27.0	$ 29.5	$ 27.0