Summary of Significant Accounting Policies - Summary of Marketable Securities (Detail) - USD ($) $ in Millions	Sep. 24, 2016	Sep. 26, 2015	Sep. 27, 2014
Schedule Of Marketable Securities [Line Items]
Gross Unrealized Gains		$ 7.2
Other than temporary impairment		(7.8)
Equity Securities [Member]
Schedule Of Marketable Securities [Line Items]
Cost	$ 2.4	16.1	$ 15.5
Gross Unrealized Gains	0.0		10.2
Gross Unrealized Losses	(0.3)	(0.3)	(1.3)
Other than temporary impairment	(1.1)	(7.8)	0.0
Marketable securities	$ 1.0	$ 15.2	$ 24.4