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Commitments and contingencies (Details) |  |  | 1 Months Ended | 3 Months Ended | 12 Months Ended | 351 Months Ended
--- | --- | --- | --- | --- | --- | ---
Mar. 31, 2018 LaborUnions MW | Aug. 18, 2017 USD ($) | Sep. 30, 2017 USD ($) | Mar. 31, 2018 USD ($) LaborUnions MW | Mar. 31, 2017 USD ($) | Dec. 31, 2013 USD ($) | Mar. 31, 2018 USD ($) LaborUnions MW
Environmental Contingencies [Abstract] |  |  |  |  |  |  |
Environmental remediation expense for Hydrocarbon Recovery Plan |  |  |  |  |  |  | $ 1,100,000
Labor Commitments [Abstract] |  |  |  |  |  |  |
Percentage of total work force in union | 63.00% |  |  | 63.00% |  |  | 63.00%
Percentage of Grundartangi work force represented by the labor unions (in hundredths) | 85.00% |  |  | 85.00% |  |  | 85.00%
Number of labor unions Grundartangi subsidiary entered into a new labor agreement with | LaborUnions | 5 |  |  | 5 |  |  | 5
Percentage of U.S. based work force represented by a union (in hundredths) | 51.00% |  |  | 51.00% |  |  | 51.00%
PBGC Agreement [Member] |  |  |  |  |  |  |
PBGC Settlement [Abstract] |  |  |  |  |  |  |
Required pension contributions above minimum |  |  |  |  |  | $ 17,400,000 |
Payment for pension benefits |  |  |  | $ 0 | $ 0 |  |
Pension contributions above minimum remaining |  |  |  | $ 9,600,000 |  |  |
Hawesville [Member] |  |  |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |  |  |
Power supply agreement, termination notice period |  |  |  | 1 year |  |  |
Sebree [Member] |  |  |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |  |  |
Power supply agreement, termination notice period |  |  |  | 1 year |  |  |
South Carolina Public Service Authority [Member] |  |  |  |  |  |  |
Power 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] |  |  |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |  |  |
Power currently available under the power purchase agreement (in megawatts) | MW | 525 |  |  | 525 |  |  | 525
Grundartangi - Landsvirkjun [Member] |  |  |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |  |  |
Power currently available under the power purchase agreement (in megawatts) | MW | 161 |  |  | 161 |  |  | 161
Helguvik [Member] |  |  |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |  |  |
Power currently available under the power purchase agreement (in megawatts) | MW | 47.5 |  |  | 47.5 |  |  | 47.5
NETHERLANDS |  |  |  |  |  |  |
Labor Commitments [Abstract] |  |  |  |  |  |  |
Percentage of Vlissingen work force represented by the labor union (in hundredths) | 100.00% |  |  | 100.00% |  |  | 100.00%
Ravenswood Retiree Medical Benefits Changes [Member] |  |  |  |  |  |  |
Ravenswood litigation [Abstract] |  |  |  |  |  |  |
Ravenswood litigation settlement amount |  | $ 23,000,000 |  |  |  |  |
Ravenswood litigation settlement installment period | 9 years | 10 years |  |  |  |  |
Litigation payment to trust |  |  | $ 5,000,000 |  |  |  |
Litigation settlement, amount awarded to other party, annual payments |  |  |  | $ 2,000,000 |  |  |
Other Current Liabilities [Member] | Ravenswood Retiree Medical Benefits Changes [Member] |  |  |  |  |  |  |
Ravenswood litigation [Abstract] |  |  |  |  |  |  |
Litigation settlement, amount awarded to other party, annual payments |  |  |  | 2,000,000 |  |  |
Other Liabilities [Member] | Ravenswood Retiree Medical Benefits Changes [Member] |  |  |  |  |  |  |
Ravenswood litigation [Abstract] |  |  |  |  |  |  |
Litigation settlement, amount awarded to other party, annual payments |  |  |  | $ 11,100,000 |  |  |

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