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Commitments and contingencies (Details) |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 18, 2015 | Mar. 31, 2017 MW LaborUnions | Dec. 31, 2016 MW labor_union
Labor Commitments [Abstract] |  |  |
Percentage of Company's work force represented by a union |  | 64.00% | 63.00%
Percentage of Grundartangi work force represented by the labor unions |  | 84.00% | 83.00%
Number of labor unions under new agreement |  | 5 | 5
Percentage of Vlissingen work force represented by the labor union |  | 100.00% | 100.00%
Percentage of U.S. based work force represented by a union |  | 54.00% | 54.00%
Sebree [Member] |  |  |
Loss Contingencies [Line Items] |  |  |
Power supply agreement, termination notice period |  | 1 year | 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 | 525
Grundartangi - Landsvirkjun [Member] |  |  |
Loss Contingencies [Line Items] |  |  |
Power available |  | 161 | 161

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- Definition The percentage of a Company's work force, based in the country of domicile, that are subject to collective bargaining agreements. + References No definition available. + Details
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