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Deferred income taxes - Additional Information (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Deferred tax credit from US tax reform $ 6.0    
Unrecognized deferred income tax assets for which no deferred tax asset is recognized 13.3 $ 12.3  
Unused tax losses for which no deferred tax asset recognised 81.9 72.1 $ 82.9
Temporary differences of unremitted earnings of overseas subsidiaries and joint ventures and associates for which deferred tax liabilities have not been recognised 42.4 54.9  
Indefinite      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Unused tax losses for which no deferred tax asset recognised 10.3 8.8  
Between 2028 and 2034      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Unused tax losses for which no deferred tax asset recognised $ 3.0 $ 3.5