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Share capital - Treasury shares (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jun. 30, 2015
Disclosure of classes of share capital [line items]        
Beginning balance $ 141,900,000 $ 169,700,000 $ 175,400,000  
Purchase of own shares   (6,300,000) (1,900,000)  
Ending balance $ 162,300,000 $ 141,900,000 169,700,000  
Authorized stock repurchase amount       $ 10,000,000
Authorized stock repurchase, number of shares       2,700,000
Number of shares repurchased 0 634,185    
Number of treasury shares held 527,616 665,424    
Treasury shares        
Disclosure of classes of share capital [line items]        
Beginning balance $ (7,100,000) $ (1,300,000) 0  
Purchase of own shares 800,000 (6,300,000) (1,900,000)  
Utilization of treasury shares 500,000 500,000 600,000  
Ending balance $ (5,800,000) $ (7,100,000) $ (1,300,000)