Balance Sheet Components - Available-for-Sale Securities (Detail) - USD ($) $ in Millions	Jul. 03, 2015	Sep. 26, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 91.2	$ 75.6
Gross Unrealized Losses	(0.3)	0.0
Fair value	90.9	75.6
Corporate debt securities: CPTC loans
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	81.9	75.6
Gross Unrealized Losses	0.0	0.0
Fair value	81.9	$ 75.6
Corporate debt securities: Other
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	8.6
Gross Unrealized Losses	(0.2)
Fair value	8.4
Non-U.S. government security
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	0.7
Gross Unrealized Losses	(0.1)
Fair value	$ 0.6