Commitments and Contingencies - Other Commitments (Details)	12 Months Ended
Dec. 31, 2018 labor_union MW
Labor Commitments [Abstract]
Percentage of Company's work force represented by a union	65.00%
Percentage of Grundartangi work force represented by the labor unions	84.00%
Number of labor unions under new agreement \| labor_union	5
Percentage of U.S. based work force represented by a union	57.00%
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
South Carolina Public Service Authority [Member]
Loss Contingencies [Line Items]
Short-term power agreement, supply from South Carolina Public Service Authority (percent)	25.00%
Short-term power agreement, supply from third party supplier (percent)	75.00%
South Carolina Public Service Authority [Member] \| Santee Cooper [Member]
Loss Contingencies [Line Items]
Power supply agreement, termination notice period	120 days
South Carolina Public Service Authority [Member] \| Other Power Suppliers [Member]
Loss Contingencies [Line Items]
Power supply agreement, termination notice period	60 days
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
Netherlands [Member]
Labor Commitments [Abstract]
Percentage of Vlissingen work force represented by the labor union	100.00%

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- Definition

The number of labor unions representing a group of employees.

+ References

No definition available.

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- Definition

The percentage of a Company's work force, based in the country of domicile, that are subject to collective bargaining agreements.

+ References

No definition available.

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- Definition

Percentage Of Foreign Netherlands Work Force Represented By The Labor Union

+ References

No definition available.

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- Definition

The percentage of a Company's Icelandic work force that are subject to collective bargaining agreements.

+ References

No definition available.

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The percentage of a Company's work force that are subject to collective bargaining agreements.

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No definition available.

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- Definition

The power currently available under the power purchase agreement.

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Power Supply Agreement, Termination Notice Period

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Short-term Power Agreement, Supply from Source One, Percentage

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Short-term Power Agreement, Supply from Source Two, Percentage

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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