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Borrowings and Credit Arrangements - Interest Expense under Convertible Notes (Detail) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Mar. 31, 2018
Apr. 01, 2017
Mar. 31, 2018
Apr. 01, 2017
Debt Conversion [Line Items]        
Non-cash interest expense     $ 11.2 $ 26.9
Cash accrued interest percentage 2.00% 2.00% 2.00% 2.00%
Convertible Notes Payable        
Debt Conversion [Line Items]        
Amortization of debt discount $ 0.6 $ 4.9 $ 3.5 $ 10.1
Amortization of deferred financing costs 0.1 0.2 0.2 0.5
Principal accretion 0.0 4.3 1.6 8.9
Non-cash interest expense 0.7 9.4 5.3 19.5
2.00% accrued interest (cash) 0.7 1.8 1.8 3.8
Interest expense, net $ 1.4 $ 11.2 $ 7.1 $ 23.3