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Commitments and contingencies (Details) $ in Thousands | 1 Months Ended | 12 Months Ended | 45 Months Ended | 168 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Mar. 31, 2015 USD ($) | Dec. 31, 2013 USD ($) | Dec. 31, 2016 USD ($) | Jun. 30, 2015 USD ($) LaborUnions | Dec. 31, 2014 USD ($) | Dec. 31, 2011 MW
Environmental Contingencies [Abstract] |  |  |  |  |  |
Environmental-related accrued liabilities |  |  |  | $ 1,296 | $ 1,101 |
Environmental remediation expense for Hydrocarbon Recovery Plan |  |  |  | 1,033 |  |
OR Power Agreement [Abstract] |  |  |  |  |  |
Power prepayments |  |  |  | $ 1,910 |  |
Power currently available under the power purchase agreement (in megawatts) | MW |  |  |  |  |  | 47.5
Labor Commitments [Abstract] |  |  |  |  |  |
Percentage of total work force in union |  |  |  | 59.00% |  |
Percentage of Grundartangi work force represented by the labor unions (in hundredths) |  |  |  | 81.00% |  |
Number of labor unions Grundartangi subsidiary entered into a new labor agreement with | LaborUnions |  |  |  | 5 |  |
Percentage of Vlissingen work force represented by the labor union (in hundredths) |  |  |  | 80.00% |  |
Percentage of U.S. based work force represented by a union (in hundredths) |  |  |  | 54.00% |  |
PBGC Agreement [Member] |  |  |  |  |  |
PBCG Settlement [Abstract] |  |  |  |  |  |
Pension contributions | $ 1,076 | $ 6,700 |  |  |  |
PBGC Forecast [Member] | PBGC Agreement [Member] |  |  |  |  |  |
PBCG Settlement [Abstract] |  |  |  |  |  |
Pension contributions above required minimum, total |  |  | $ 17,400 |  |  |

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