Stockholders' Equity and Stock-Based Compensation - Stock-Based Compensation Expense in Consolidated Statement of Operations (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 26, 2015	Sep. 27, 2014	Sep. 28, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 59.3	$ 50.0	$ 52.3
Cost of Revenues [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	8.7	7.3	7.0
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	8.6	8.4	7.2
Selling and Marketing [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	8.8	8.2	8.9
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	29.1	19.5	20.2
Restructuring Charges [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 4.1	$ 6.6	$ 9.0