Stockholders' Equity and Stock-Based Compensation - Stock-Based Compensation Expense in Consolidated Statement of Operations (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 24, 2016	Sep. 26, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 68.2	$ 65.4	$ 59.3
Cost of Sales [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	10.7	10.5	8.7
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	11.2	10.8	8.6
Selling and Marketing [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	11.9	10.9	8.8
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	34.4	32.8	29.1
Restructuring Charges [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 0.0	$ 0.4	$ 4.1