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Shareholders' equity - Summary of Performance Shares (Details) - Performance Shares - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Shares      
Unvested at beginning of year (in shares) 1,018,950 1,385,940 2,100,115
Granted (in shares) 307,950 392,589 285,458
Settled (in shares) (774,267) (687,125) (925,640)
Canceled (in shares) (68,573) (72,454) (123,621)
Adjustment due to spin-off of Publishing (in shares) 0 0 49,628
Unvested at end of year (in shares) 662,835 1,018,950 1,385,940
Weighted average fair value      
Unvested at beginning of year (in dollars per share) $ 35.60 $ 29.21 $ 20.95
Granted (in dollars per share) 23.92 30.69 39.47
Settled (in dollars per share) 36.94 20.12 14.23
Canceled (in dollars per share) 31.80 34.96 29.84
Unvested at end of year (in dollars per share) $ 25.87 $ 35.60 $ 29.21
Cars.com      
Shares      
Adjustment due to spin-off of Publishing (in shares) 178,775 0 0