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Shareholders' equity - Summary of Performance Shares (Details) - $ / shares
12 Months Ended
Jun. 29, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 28, 2014
Shares        
Granted (in shares) 125,000      
Performance Shares        
Shares        
Unvested at beginning of year (in shares)   1,385,940 2,100,115 1,760,488
Granted (in shares) 50,000 392,589 285,458 436,340
Settled (in shares)   (687,125) (925,640)  
Canceled (in shares)   (72,454) (123,621) (96,713)
Adjustment due to spin-off of Publishing (in shares)     49,628  
Unvested at end of year (in shares)   1,018,950 1,385,940 2,100,115
Weighted average fair value        
Unvested at beginning of year (in dollars per share)   $ 29.21 $ 20.95 $ 16.92
Granted (in dollars per share)   30.69 39.47 37.31
Settled (in dollars per share)   20.12 14.23  
Canceled (in dollars per share)   34.96 29.84 21.41
Unvested at end of year (in dollars per share)   $ 35.60 $ 29.21 $ 20.95