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Commitments and contingencies (Details) |  | 12 Months Ended
--- | --- | ---
Dec. 18, 2015 | Dec. 31, 2016 MW labor_union
Labor Commitments [Abstract] |  |
Percentage of Company's work force represented by a union |  | 63.00%
Percentage of Grundartangi work force represented by the labor unions |  | 83.00%
Number of labor unions under new agreement | labor_union |  | 5
Percentage of Vlissingen work force represented by the labor union |  | 100.00%
Percentage of U.S. based work force represented by a union |  | 54.00%
Sebree [Member] |  |
Loss Contingencies [Line Items] |  |
Power supply agreement, termination notice period |  | 1 year
South Carolina Public Service Authority [Member] |  |
Loss Contingencies [Line Items] |  |
Power supply agreement, termination notice period | 60 days |
Short-term power agreement, supply from South Carolina Public Service Authority (percent) | 25.00% |
Short-term power agreement, supply from third party supplier (percent) | 75.00% |
Grundartangi - HS, Landsvirkjun and OR [Member] |  |
Loss Contingencies [Line Items] |  |
Power available |  | 525
Grundartangi - Landsvirkjun [Member] |  |
Loss Contingencies [Line Items] |  |
Power available |  | 161

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