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Commitments and contingencies (Details)		1 Months Ended	9 Months Ended		12 Months Ended	357 Months Ended
	Aug. 18, 2017 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) LaborUnions MW	Sep. 30, 2017 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2018 USD ($) 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			65.00%			65.00%
Percentage of Grundartangi work force represented by the labor unions (in hundredths)			84.00%			84.00%
Number of labor unions Grundartangi subsidiary entered into a new labor agreement with | LaborUnions			5			5
Percentage of U.S. based work force represented by a union (in hundredths)			56.00%			56.00%
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			120 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
Grundartangi - Landsvirkjun [Member]
Power Contingencies [Abstract]
Power currently available under the power purchase agreement (in megawatts) | MW			161			161
Helguvik [Member]
Power Contingencies [Abstract]
Power currently available under the power purchase agreement (in megawatts) | MW			47.5			47.5
NETHERLANDS
Labor Commitments [Abstract]
Percentage of Vlissingen work force represented by the labor union (in hundredths)			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	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			$ 9,600,000

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The number of labor unions representing a group of employees.

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

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The charge against earnings in the period for known or estimated future costs arising from requirements to perform environmental remediation activities.

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