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Related party transactions - Summary of related party transactions (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Related Party Transaction [Line Items]    
Net sales to Glencore $ 280,377 $ 575,729
Glencore [Member]    
Related Party Transaction [Line Items]    
Net sales to Glencore 280,377 575,729
Purchases from Glencore 44,013 107,814
BHH [Member]    
Related Party Transaction [Line Items]    
Purchases from BHH $ 2,383 $ 14,820