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Summary of Significant Accounting Policies - Summary of Marketable Securities (Detail) - Equity securities - USD ($) $ in Millions | Sep. 30, 2017 | Sep. 24, 2016 | Sep. 26, 2015
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Schedule Of Marketable Securities [Line Items] |  |  |
Cost | $ 0.7 | $ 2.4 | $ 16.1
Gross Unrealized Gains | 0.0 | 0.0 | 7.2
Gross Unrealized Losses | (0.6) | (0.3) | (0.3)
Other than temporary impairment | 0.0 | (1.1) | (7.8)
Fair Value | $ 0.1 | $ 1.0 | $ 15.2

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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in equity securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563 + Details
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