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Commitments and Contingencies - Other Commitments (Details) | 12 Months Ended |
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Dec. 31, 2020 labor_union MW | Apr. 01, 2021
Labor Commitments [Abstract] |  |
Percentage of Company's work force represented by a union | 67.00% |
Percentage of Grundartangi work force represented by the labor unions | 86.00% |
Number of labor unions under new agreement | labor_union | 5 |
Percentage of U.S. based work force represented by a union | 58.00% |
Netherlands [Member] |  |
Labor Commitments [Abstract] |  |
Percentage of Company's work force represented by a union | 100.00% |
Hawesville [Member] |  |
Loss Contingencies [Line Items] |  |
Power supply agreement, termination notice period | 1 year |
Sebree [Member] |  |
Loss Contingencies [Line Items] |  |
Power supply agreement, termination notice period | 1 year |
Mt. Holly [Member] | Century Aluminum of South Carolina, Inc [Member] |  |
Loss Contingencies [Line Items] |  |
Short-term power agreement, supply from third party (percent) | 75.00% |
Mt. Holly [Member] | Santee Cooper [Member] |  |
Loss Contingencies [Line Items] |  |
Short-term power agreement, supply from third party (percent) | 25.00% |
Power supply agreement, term | 3 years |
Total production (percent) | 75.00% |
Mt. Holly [Member] | Santee Cooper [Member] | Forecast [Member] |  |
Loss Contingencies [Line Items] |  |
Power supply agreement (percent) |  | 100.00%
Grundartangi - HS, Landsvirkjun and OR [Member] |  |
Loss Contingencies [Line Items] |  |
Power available | 525 |
Grundartangi - Landsvirkjun [Member] |  |
Loss Contingencies [Line Items] |  |
Power available | 161 |
Helguvik [Member] |  |
Loss Contingencies [Line Items] |  |
Power available | 47.5 |

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