Stock-Based Compensation - Stock-Based Compensation Expense in Consolidated Statements of Operations (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 26, 2020	Dec. 28, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 18.6	$ 18.1
Cost of revenues
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	2.2	2.0
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	2.5	2.4
Selling and marketing
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	2.7	2.8
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 11.2	$ 10.9