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Related party transactions - Narrative (Details) | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
BHH [Member] |  |  |  |
Related Party Transaction [Line Items] |  |  |  |
Ownership percentage | 40.00% |  | 40.00% |
Consolidated Sales [Member] | Customer Concentration Risk [Member] | Affiliated Entity [Member] |  |  |  |
Related Party Transaction [Line Items] |  |  |  |
Major customer, percentage of revenue, net (percent) | 63.00% | 75.00% | 63.00% | 76.00%
Glencore [Member] |  |  |  |
Related Party Transaction [Line Items] |  |  |  |
Ownership percentage by majority shareholder | 47.40% |  | 47.40% |
Common Stock [Member] | Glencore [Member] |  |  |  |
Related Party Transaction [Line Items] |  |  |  |
Ownership percentage by majority shareholder | 42.90% |  | 42.90% |

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- Definition For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13531-108611 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 21 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13537-108611 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592 + Details
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