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Related party transactions (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Related Party Transaction [Line Items]
Net sales to Glencore	$ 513,681	$ 288,573	$ 1,089,410	$ 574,156
Glencore [Member]
Related Party Transaction [Line Items]
Net sales to Glencore	513,681	288,573	1,089,410	574,156
Purchases from Glencore	133,642	60,998	241,456	124,855
BHH [Member]
Related Party Transaction [Line Items]
Purchases from BHH	$ 13,434	$ 13,117	$ 28,253	$ 27,723
BHH [Member]
Related Party Transaction [Line Items]
Ownership percentage	40.00%		40.00%

X

- Definition

Direct costs arising from transactions with related parties who are joint ventures. These costs are categorized as cost of goods sold

+ References

No definition available.

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- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef



-Publisher FASB



-Name Accounting Standards Codification



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- Definition

Direct costs arising from transactions with related parties who are not affiliates or joint Ventures. These costs are categorized as cost of goods sold.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef



-Publisher FASB



-Name Accounting Standards Codification



-Topic 235



-SubTopic 10



-Section S99



-Paragraph 1



-Subparagraph (SX 210.4-08.(k)(1))



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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef



-Publisher FASB



-Name Accounting Standards Codification



-Topic 225



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-Paragraph 2



-Subparagraph (SX 210.5-03.1(e))



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-Publisher FASB



-Name Accounting Standards Codification



-Glossary Related Parties



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Reference 3: http://www.xbrl.org/2003/role/presentationRef



-Publisher FASB



-Name Accounting Standards Codification



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