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Changes in shareholders' equity - additional information (Details) - USD ($)
$ / shares in Units, shares in Millions
3 Months Ended 9 Months Ended
Sep. 27, 2015
Sep. 28, 2014
Sep. 27, 2015
Sep. 28, 2014
Dec. 31, 2007
Shareholders Equity [Line Items]          
Repurchase of outstanding common stock, authorized amount         $ 300,000,000
Debt instrument, stated interest rate 3.875% 3.875% 3.875% 3.875%  
Equity Option          
Shareholders Equity [Line Items]          
Weighted average antidilutive which were not included in the calculation of earnings per share 5.4 6.3 5.6 6.4  
Convertible Note          
Shareholders Equity [Line Items]          
Weighted average antidilutive which were not included in the calculation of earnings per share 3.5 2.8 3.3 2.7  
Per share market price of common stock $ 61.32   $ 61.32    
Strike price of warrents     $ 74.65    
Conversion of convertible notes, shares issued upon conversion 2.9 2.0 2.6 1.8