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Shareholders' equity - Additional Information (Details) - USD ($)
$ / shares in Units, $ in Thousands, shares in Millions
3 Months Ended
Apr. 02, 2017
Mar. 27, 2016
Dec. 31, 2016
Shareholders Equity [Line Items]      
Investment warrants, exercise price (in dollars per share) $ 74.65    
Conversion of convertible notes, shares issued upon conversion 0.4 3.0  
Convertible notes - redeemable equity component $ 0   $ 1,824
Equity Option      
Shareholders Equity [Line Items]      
Weighted average antidilutive shares which were not included in the calculation of earnings per share 0.5 5.2  
Convertible Note      
Shareholders Equity [Line Items]      
Weighted average antidilutive shares which were not included in the calculation of earnings per share 0.5 3.6