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Commitments and Contingencies (Details) |  | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Aug. 18, 2017 USD ($) | Sep. 30, 2017 USD ($) | Mar. 31, 2019 USD ($) Labor_Union MW | Mar. 31, 2018 USD ($) | Dec. 31, 2013 USD ($)
Labor Commitments [Abstract] |  |  |  |  |
Percentage of total work force in union |  |  | 65.00% |  |
Percentage of Grundartangi work force represented by the labor unions (in hundredths) |  |  | 85.00% |  |
Number of labor unions Grundartangi subsidiary entered into a new labor agreement with | Labor_Union |  |  | 5 |  |
Percentage of U.S. based work force represented by a union (in hundredths) |  |  | 57.00% |  |
Ravenswood Retiree Medical Benefits Changes [Member] |  |  |  |  |
Ravenswood litigation [Abstract] |  |  |  |  |
Gain (loss) related to litigation settlement |  | $ 5,500,000 |  |  |
Loss contingency accrual |  | 12,500,000 |  |  |
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 |  |  | 120 days |  |
Power agreement, power supply from source one, percentage |  |  | 25.00% |  |
Power agreement, power supply from source two, percentage |  |  | 75.00% |  |
Other Power Suppliers [Member] |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |
Power supply agreement, termination notice period |  |  | 60 days |  |
Grundartangi - HS, Landsvirkjun and OR [Member] |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |
Power currently available under the power purchase agreement (in megawatts) | MW |  |  | 525 |  |
Grundartangi - Landsvirkjun [Member] |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |
Power currently available under the power purchase agreement (in megawatts) | MW |  |  | 161 |  |
Helguvik [Member] |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |
Power currently available under the power purchase agreement (in megawatts) | MW |  |  | 47.5 |  |
NETHERLANDS |  |  |  |  |
Labor Commitments [Abstract] |  |  |  |  |
Percentage of Vlissingen work force represented by the labor union (in hundredths) |  |  | 100.00% |  |
Ravenswood Retiree Medical Benefits Changes [Member] |  |  |  |  |
Ravenswood litigation [Abstract] |  |  |  |  |
Ravenswood litigation settlement amount | $ 23,000,000 |  |  |  |
Ravenswood litigation settlement installment period | 10 years |  | 9 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 |  |  | $ 10,000,000 |  |

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