Borrowings and Credit Arrangements - Interest Expense under Convertible Notes (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 26, 2015	Sep. 27, 2014	Sep. 28, 2013
Debt Conversion [Line Items]
Non-cash interest expense	$ 63.8	$ 68.7	$ 81.2
Convertible Notes Payable [Member]
Debt Conversion [Line Items]
Amortization of debt discount	34.9	37.1	52.7
Amortization of deferred financing costs	1.7	1.9	3.0
Principal accretion	15.9	15.3	9.2
Non-cash interest expense	52.5	54.3	64.9
2.00% accrued interest	18.2	22.3	34.4
Interest expense, net	$ 70.7	$ 76.6	$ 99.3
Percentage of accrued interest on Convertible Notes	2.00%	2.00%