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Commitments and contingencies - Other Commitments (Details) | 12 Months Ended
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Dec. 31, 2017 labor_union MW
Labor Commitments [Abstract] |
Percentage of Company's work force represented by a union | 65.00%
Percentage of Grundartangi work force represented by the labor unions | 87.00%
Number of labor unions under new agreement | labor_union | 5
Percentage of U.S. based work force represented by a union | 52.00%
Hawesville [Member] |
Loss Contingencies [Line Items] |
Power supply agreement, termination notice period | 1 year
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
Helguvik [Member] |
Loss Contingencies [Line Items] |
Power available | 47.5
NETHERLANDS |
Labor Commitments [Abstract] |
Percentage of Vlissingen work force represented by the labor union | 100.00%

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