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Fair Value Measurements - Estimated Fair Values of Convertible Notes (Detail) - USD ($)
$ in Millions
Sep. 24, 2016
Sep. 26, 2015
Estimated Fair Value Of Financial Instruments [Line Items]    
Estimated fair values of debt instruments $ 960.9 $ 1,424.1
2010 Notes [Member]    
Estimated Fair Value Of Financial Instruments [Line Items]    
Estimated fair values of debt instruments 20.2 264.1
2012 Notes [Member]    
Estimated Fair Value Of Financial Instruments [Line Items]    
Estimated fair values of debt instruments 481.9 688.2
2013 Notes [Member]    
Estimated Fair Value Of Financial Instruments [Line Items]    
Estimated fair values of debt instruments $ 458.8 $ 471.8