Summary of Significant Accounting Policies - Accounts Receivable Reserve Activity (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 24, 2016	Sep. 26, 2015
Allowance for Doubtful Accounts Receivable [Roll Forward]
Balance at Beginning of Period	$ 12.7	$ 11.1	$ 12.0
Charged to Costs and Expenses	1.8	2.0	1.6
Write- offs and Payments	(4.7)	(0.4)	(2.5)
Balance at End of Period	$ 9.8	$ 12.7	$ 11.1