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Commitments and contingencies (Details) $ in Thousands |  |  | 1 Months Ended | 6 Months Ended | 12 Months Ended | 342 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Feb. 09, 2017 USD ($) | Dec. 18, 2015 | Mar. 31, 2015 USD ($) | Apr. 30, 2013 USD ($) | Jun. 30, 2017 USD ($) MW LaborUnions | Dec. 31, 2013 USD ($) | Jun. 30, 2017 USD ($) MW LaborUnions | Dec. 31, 2016 USD ($) | Sep. 30, 2016 USD ($)
Environmental Contingencies [Abstract] |  |  |  |  |  |  |  |  |
Environmental-related accrued liabilities |  |  |  |  | $ 1,222 |  | $ 1,222 | $ 998 |
Environmental remediation expense for Hydrocarbon Recovery Plan |  |  |  |  |  |  | $ 1,085 |  |
Labor Commitments [Abstract] |  |  |  |  |  |  |  |  |
Percentage of total work force in union |  |  |  |  | 66.00% |  | 66.00% |  |
Percentage of Grundartangi work force represented by the labor unions (in hundredths) |  |  |  |  | 86.00% |  | 86.00% |  |
Number of labor unions Grundartangi subsidiary entered into a new labor agreement with | LaborUnions |  |  |  |  | 5 |  | 5 |  |
Percentage of Vlissingen work force represented by the labor union (in hundredths) |  |  |  |  | 100.00% |  | 100.00% |  |
Percentage of U.S. based work force represented by a union (in hundredths) |  |  |  |  | 53.00% |  | 53.00% |  |
PBGC Agreement [Member] |  |  |  |  |  |  |  |  |
Environmental Contingencies [Abstract] |  |  |  |  |  |  |  |  |
Required pension contributions above minimum |  |  |  | $ 17,400 |  |  |  |  |
Pension Contributions |  |  | $ 1,100 |  |  | $ 6,700 |  |  |
Pension contributions above minimum remaining |  |  |  |  | $ 9,600 |  |  |  |
Hawesville [Member] |  |  |  |  |  |  |  |  |
Legal Contingencies [Abstract] |  |  |  |  |  |  |  |  |
Power supply agreement, termination notice period |  |  |  |  | 1 year |  |  |  |
Sebree [Member] |  |  |  |  |  |  |  |  |
Legal Contingencies [Abstract] |  |  |  |  |  |  |  |  |
Power supply agreement, termination notice period |  |  |  |  | 1 year |  |  |  |
South Carolina Public Service Authority [Member] |  |  |  |  |  |  |  |  |
Legal Contingencies [Abstract] |  |  |  |  |  |  |  |  |
Power supply agreement, termination notice period |  | 60 days |  |  |  |  |  |  |
Power agreement, power supply from source one, percentage |  | 25.00% |  |  |  |  |  |  |
Power agreement, power supply from source two, percentage |  | 75.00% |  |  |  |  |  |  |
Grundartangi - HS, Landsvirkjun and OR [Member] |  |  |  |  |  |  |  |  |
OR Power Agreement [Abstract] |  |  |  |  |  |  |  |  |
Power currently available under the power purchase agreement (in megawatts) | MW |  |  |  |  | 525 |  | 525 |  |
Grundartangi - Landsvirkjun [Member] |  |  |  |  |  |  |  |  |
OR Power Agreement [Abstract] |  |  |  |  |  |  |  |  |
Power currently available under the power purchase agreement (in megawatts) | MW |  |  |  |  | 161 |  | 161 |  |
Helguvik [Member] |  |  |  |  |  |  |  |  |
OR Power Agreement [Abstract] |  |  |  |  |  |  |  |  |
Power currently available under the power purchase agreement (in megawatts) | MW |  |  |  |  | 47.5 |  | 47.5 |  |
Ravenswood Retiree Medical Benefits Changes [Member] |  |  |  |  |  |  |  |  |
Environmental Contingencies [Abstract] |  |  |  |  |  |  |  |  |
Ravenswood litigation settlement amount | $ 23,000 |  |  |  |  |  |  |  |
Ravenswood litigation settlement period | 10 years |  |  |  |  |  |  |  |
Ravenswood settlement payment to be made upon final approval from court | $ 5,000 |  |  |  |  |  |  |  |
Ravenswood settlement, annual payment amount | $ 2,000 |  |  |  |  |  |  |  |
Ravenswood litigation settlement installment period | 9 years |  |  |  |  |  |  |  |
Ravenswood retiree medical benefits settlement liability |  |  |  |  |  |  |  |  | $ 23,000

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