Borrowings and Credit Arrangements - Interest Expense under Convertible Notes (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 28, 2019	Sep. 29, 2018	Sep. 30, 2017
Debt Conversion [Line Items]
Non-cash interest expense	$ 8.6	$ 15.0	$ 49.4
Convertible Notes Payable
Debt Conversion [Line Items]
Amortization of debt discount		3.5	17.9
Amortization of deferred financing costs		0.2	0.8
Principal accretion		1.6	15.6
Non-cash interest expense		5.3	34.3
2.00% accrued interest		1.8	6.7
Interest expense, net		$ 7.1	$ 41.0