Stockholders' Equity and Stock-Based Compensation - Stock-Based Compensation Expense in Consolidated Statement of Operations (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 28, 2019	Sep. 29, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 62.0	$ 65.0	$ 68.2
Cost of revenues
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	7.1	8.3	10.7
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	9.2	9.5	11.2
Selling and marketing
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	10.2	10.3	11.9
General and Administrative Expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	35.5	35.6	34.4
Restructuring Charges
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 0.0	$ 1.3	$ 0.0