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Commitments and Contingencies (Details)			1 Months Ended	3 Months Ended		12 Months Ended
	Apr. 01, 2021	Aug. 18, 2017 USD ($)	Sep. 30, 2017 USD ($)	Mar. 31, 2021 USD ($) Labor_Union MW	Mar. 31, 2020 USD ($)	Dec. 31, 2013 USD ($)
Labor Commitments [Abstract]
Percentage of total work force in union				66.00%
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)				58.00%
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
Santee Cooper [Member] | Subsequent Event [Member]
Power Contingencies [Abstract]
Power agreement, power supply, percentage	100.00%
Power supply agreement, increase production, at full capacity, percentage	75.00%
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 [Member]
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		$ 23,000,000.0
Ravenswood litigation settlement installment period		10 years		9 years
Litigation payment to trust			$ 5,000,000.0
Litigation settlement, amount awarded to other party, annual payments				$ 2,000,000.0
Other Current Liabilities [Member] | Ravenswood Retiree Medical Benefits Changes [Member]
Ravenswood litigation [Abstract]
Litigation settlement, amount awarded to other party, annual payments				2,000,000.0
Other Liabilities [Member] | Ravenswood Retiree Medical Benefits Changes [Member]
Ravenswood litigation [Abstract]
Litigation settlement, amount awarded to other party, annual payments				$ 7,700,000

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Legal Contingencies [Abstract]

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Litigation Settlement, Amount Awarded to Other Party, Annual Payments

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The number of labor unions representing a group of employees.

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PBGC Settlement [Abstract]

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Pursuant to the terms of the settlement agreement with the Pension Benefit Guaranty Corporation, this amount represents the remaining additional contributions (above any minimum required contributions) to our defined benefit pension plans.

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Pursuant to the terms of the settlement agreement with the Pension Benefit Guaranty Corporation, we will make additional contributions (above any minimum required contributions) to our defined benefit pension plans.

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

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Percentage Of Foreign Netherlands Work Force Represented By The Labor Union

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

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Power Supply Agreement, Increase Production At Full Capacity, Percentage

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Power Supply Agreement, Termination Notice Period

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Ravenswood litigation [Abstract]

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Short-term Power Agreement, Supply Percentage

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The power currently available under the power purchase agreement.

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Amount of gain (loss) recognized in settlement of litigation and insurance claims. Excludes claims within an insurance entity's normal claims settlement process.

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Amount of loss contingency liability.

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Amount of cash outflow for pension benefit. Includes, but is not limited to, employer contribution to fund plan asset and payment to retiree. Excludes other postretirement benefit.

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