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Shareholders' equity - Additional Information (Details) - shares
shares in Millions
3 Months Ended 9 Months Ended
Sep. 27, 2020
Sep. 29, 2019
Sep. 27, 2020
Sep. 29, 2019
Equity Option        
Shareholders Equity [Line Items]        
Weighted average antidilutive shares which were not included in the calculation of earnings per share 0.1 0.1 0.1 0.1