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Fair Value Measurements - Estimated Fair Values of Convertible Notes (Detail) - USD ($)
$ in Millions
Sep. 26, 2015
Sep. 27, 2014
Estimated Fair Value Of Financial Instruments [Line Items]    
Estimated fair values of debt instruments $ 1,424.1 $ 1,469.4
2010 Notes [Member]    
Estimated Fair Value Of Financial Instruments [Line Items]    
Estimated fair values of debt instruments 264.1 536.6
2012 Notes [Member]    
Estimated Fair Value Of Financial Instruments [Line Items]    
Estimated fair values of debt instruments 688.2 531.7
2013 Notes [Member]    
Estimated Fair Value Of Financial Instruments [Line Items]    
Estimated fair values of debt instruments $ 471.8 $ 401.1