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Related party transactions (Tables)
 9 Months Ended
Sep. 30, 2017
Related Party Transactions [Abstract]  
Schedule of related party transactions
A summary of the aforementioned significant related party transactions for the three and nine months ended September 30, 2017 and 2016 is as follows: 
 
Three months ended September 30,
 
Nine months ended September 30,
 
2017
 
2016
 
2017
 
2016
Net sales to Glencore
$
299,235

 
$
301,111

 
$
876,079

 
$
871,771

Purchases from Glencore
81,905

 
62,317

 
195,073

 
156,507

Purchases from BHH
3,248

 
2,568

 
9,137

 
7,666