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Commitments and contingencies (Details)			1 Months Ended	3 Months Ended		12 Months Ended	351 Months Ended
	Mar. 31, 2018 LaborUnions MW	Aug. 18, 2017 USD ($)	Sep. 30, 2017 USD ($)	Mar. 31, 2018 USD ($) LaborUnions MW	Mar. 31, 2017 USD ($)	Dec. 31, 2013 USD ($)	Mar. 31, 2018 USD ($) 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	63.00%			63.00%			63.00%
Percentage of Grundartangi work force represented by the labor unions (in hundredths)	85.00%			85.00%			85.00%
Number of labor unions Grundartangi subsidiary entered into a new labor agreement with | LaborUnions	5			5			5
Percentage of U.S. based work force represented by a union (in hundredths)	51.00%			51.00%			51.00%
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				60 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			525
Grundartangi - Landsvirkjun [Member]
Power Contingencies [Abstract]
Power currently available under the power purchase agreement (in megawatts) | MW	161			161			161
Helguvik [Member]
Power Contingencies [Abstract]
Power currently available under the power purchase agreement (in megawatts) | MW	47.5			47.5			47.5
NETHERLANDS
Labor Commitments [Abstract]
Percentage of Vlissingen work force represented by the labor union (in hundredths)	100.00%			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	9 years	10 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				$ 11,100,000

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The number of labor unions representing a group of employees.

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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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Power Supply Agreement, Termination Notice Period

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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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-Name Accounting Standards Codification
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