Borrowings and Credit Arrangements - Convertible Notes and Related Equity Components (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 24, 2016	Sep. 26, 2015
Two Thousand And Ten Notes [Member]
Schedule Of Borrowings [Line Items]
Debt Instrument, Face Amount	$ 12.3	$ 150.0
Unamortized discount and issuance costs (1)	(0.1)	(8.2)
Convertible Notes Payable	12.2	141.8
Debt Instrument, Convertible, Carrying Amount of Equity Component	1.6	20.0
Two Thousand And Twelve Notes [Member]
Schedule Of Borrowings [Line Items]
Debt Instrument, Face Amount	363.4	500.0
Unamortized discount and issuance costs (1)	(9.5)	(21.8)
Convertible Notes Payable	353.9	478.2
Debt Instrument, Convertible, Carrying Amount of Equity Component	35.8	49.2
Two Thousand And Thirteen Notes [Member]
Schedule Of Borrowings [Line Items]
Debt Instrument, Face Amount	370.0	370.0
Unamortized discount and issuance costs (1)	(17.5)	(29.9)
Convertible Notes Payable	409.6	380.6
Debt Instrument, Convertible, Carrying Amount of Equity Component	131.5	131.5
Base Principal Amount For Accretion Value	$ 57.1	$ 40.5