Summary of Significant Accounting Policies - Summary of Marketable Securities (Detail) - USD ($) $ in Millions	Sep. 26, 2015	Sep. 27, 2014	Sep. 28, 2013
Schedule Of Marketable Securities [Line Items]
Other than temporary impairment	$ (7.8)
Equity Securities [Member]
Schedule Of Marketable Securities [Line Items]
Cost	16.1	$ 15.5	$ 5.9
Gross Unrealized Gains	7.2	10.2	12.2
Gross Unrealized Losses	(0.3)	(1.3)	0.0
Other than temporary impairment	(7.8)	0.0	0.0
Marketable securities	$ 15.2	$ 24.4	$ 18.1

Amount of equity securities categorized neither as held-to-maturity nor as trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 320

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803

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