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Related party transactions - Narrative (Details) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2015 USD ($) | Dec. 31, 2015 USD ($) t | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($)
Related Party Transaction [Line Items] |  |  |  |
BHH impairment |  | $ 11,584 | $ 0 | $ 0
Glencore [Member] |  |  |  |
Related Party Transaction [Line Items] |  |  |  |
Ownership percentage | 47.50% | 47.50% |  |
BHH [Member] |  |  |  |
Related Party Transaction [Line Items] |  |  |  |
Ownership percentage, equity method | 40.00% | 40.00% |  |
BHH impairment | $ 11,584 | $ 11,584 |  |
Glencore [Member] | Long-Term Tolling Contracts [Member] |  |  |  |
Related Party Transaction [Line Items] |  |  |  |
Annual primary aluminum physical delivery contract, maximum (in tonnes) | t |  | 90,000 |  |

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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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