Summary of Significant Accounting Policies - Accounts Receivable Reserve Activity (Detail) - USD ($)	12 Months Ended
	Sep. 26, 2020	Sep. 28, 2019	Sep. 29, 2018
Allowance for Doubtful Accounts Receivable [Roll Forward]
Balance at Beginning of Period	$ 17,800,000	$ 16,200,000	$ 9,800,000
Charged to Costs and Expenses	26,800,000	4,400,000	7,000,000.0
Accounts receivable reserve divested	(5,800,000)	0	0
Write- offs and Payments	(7,200,000)	(2,800,000)	(600,000)
Balance at End of Period	$ 31,600,000	$ 17,800,000	$ 16,200,000