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Borrowings and Credit Arrangements - Interest Expense under Convertible Notes (Detail) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2017
Sep. 24, 2016
Sep. 26, 2015
Debt Conversion [Line Items]      
Non-cash interest expense $ 49.4 $ 52.1 $ 63.8
Convertible Notes Payable [Member]      
Debt Conversion [Line Items]      
Amortization of debt discount 17.9 22.3 34.9
Amortization of deferred financing costs 0.8 1.1 1.7
Principal accretion 15.6 16.6 15.9
Non-cash interest expense 34.3 40.0 52.5
2.00% accrued interest 6.7 10.0 18.2
Interest expense, net $ 41.0 $ 50.0 $ 70.7
Percentage of accrued interest on Convertible Notes 2.00% 2.00%