Summary of Significant Accounting Policies - Components of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 26, 2015	Sep. 27, 2014	Sep. 28, 2013
Accounting Policies [Abstract]
Other Comprehensive Income (Loss), before Reclassifications, before Tax	$ (34.5)	$ (17.8)
Foreign currency translation adjustment	(15.7)	(4.7)	$ 8.6
Unrealized gains on available-for-sale securities	6.9	8.9	12.1
Minimum pension liability, net of tax of $0.2 and $0.2, respectively	(1.8)	(1.6)	(0.3)
Accumulated other comprehensive Income Loss Hedge	(3.9)	0.0
Accumulated other comprehensive income	(14.5)	2.6	20.4
Accumulated Other Comprehensive Income Loss Defined Benefit Pension And Other Postretirement Plans Tax	0.3	0.2
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Tax	$ 2.5	$ 0.0	$ 0.0