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Related party transactions - Narrative (Details) $ in Thousands | 3 Months Ended
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Mar. 31, 2016 t | Dec. 31, 2015 USD ($)
Glencore [Member] |  |
Related Party Transaction [Line Items] |  |
Ownership percentage by majority shareholder | 47.50% |
BHH [Member] |  |
Related Party Transaction [Line Items] |  |
Ownership percentage | 40.00% |
Impairment of equity method investment | $ |  | $ 11,584
Agreement to Sell Molten Aluminum [Member] | Affiliated Entity [Member] |  |
Related Party Transaction [Line Items] |  |
Sales agreement, amount of agreement (tonnes) | t | 24,500 |
Common Stock [Member] | Glencore [Member] |  |
Related Party Transaction [Line Items] |  |
Ownership percentage by majority shareholder | 42.90% |

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- Definition This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 35 -Paragraph 32 -URI http://asc.fasb.org/extlink&oid=49116238&loc=d3e32787-111569 + Details
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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 -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571 + Details
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- Definition The parent entity's interest in net assets of the subsidiary, expressed as a percentage. + References No definition available. + Details
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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. + References No definition available. + Details
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- Details
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- Details
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- Details
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