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Marketable Securities - Reconciles Cost Basis to Fair Market Value of Company's Equity Security (Detail) - Equity securities - USD ($)
$ in Millions
Mar. 26, 2016
Sep. 26, 2015
Schedule Of Marketable Securities [Line Items]    
Available-for-sale Equity Securities, Amortized Cost Basis $ 2.4 $ 16.1
Gross Unrealized Gains 0.0 7.2
Gross Unrealized Losses (1.5) (0.3)
Other than temporary impairment 0.0 7.8
Fair Value $ 0.9 $ 15.2