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Summary of Significant Accounting Policies - Accounts Receivable Reserve Activity (Detail) - USD ($)
$ in Millions
12 Months Ended
Sep. 29, 2018
Sep. 30, 2017
Sep. 24, 2016
Allowance for Doubtful Accounts Receivable [Roll Forward]      
Balance at Beginning of Period $ 9.8 $ 12.7 $ 11.1
Charged to Costs and Expenses 7.0 1.8 2.0
Write- offs and Payments (0.6) (4.7) (0.4)
Balance at End of Period $ 16.2 $ 9.8 $ 12.7