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Borrowings and Credit Arrangements - Interest Expense under Convertible Notes (Detail) - USD ($)
$ in Millions
3 Months Ended
Dec. 30, 2017
Dec. 31, 2016
Debt Conversion [Line Items]    
Non-cash interest expense $ 8.7 $ 14.3
Cash accrued interest percentage 2.00% 2.00%
Convertible Notes Payable    
Debt Conversion [Line Items]    
Amortization of debt discount $ 2.9 $ 5.2
Amortization of deferred financing costs 0.1 0.2
Principal accretion 1.6 4.6
Non-cash interest expense 4.6 10.0
2.00% accrued interest (cash) 1.1 2.0
Interest expense, net $ 5.7 $ 12.0