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Borrowings and Credit Arrangements - Interest Expense under Convertible Notes (Detail) - USD ($)
$ in Millions
12 Months Ended
Sep. 29, 2018
Sep. 30, 2017
Sep. 24, 2016
Debt Conversion [Line Items]      
Non-cash interest expense $ 15.0 $ 49.4 $ 52.1
Convertible Notes Payable      
Debt Conversion [Line Items]      
Amortization of debt discount 3.5 17.9 22.3
Amortization of deferred financing costs 0.2 0.8 1.1
Principal accretion 1.6 15.6 16.6
Non-cash interest expense 5.3 34.3 40.0
2.00% accrued interest 1.8 6.7 10.0
Interest expense, net $ 7.1 $ 41.0 $ 50.0