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Commitments and contingencies (Details) $ in Thousands	1 Months Ended	12 Months Ended	45 Months Ended	168 Months Ended
	Mar. 31, 2015 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2016 USD ($)	Jun. 30, 2015 USD ($) LaborUnions	Dec. 31, 2014 USD ($)	Dec. 31, 2011 MW
Environmental Contingencies [Abstract]
Environmental-related accrued liabilities				$ 1,296	$ 1,101
Environmental remediation expense for Hydrocarbon Recovery Plan				1,033
OR Power Agreement [Abstract]
Power prepayments				$ 1,910
Power currently available under the power purchase agreement (in megawatts) | MW						47.5
Labor Commitments [Abstract]
Percentage of total work force in union				59.00%
Percentage of Grundartangi work force represented by the labor unions (in hundredths)				81.00%
Number of labor unions Grundartangi subsidiary entered into a new labor agreement with | LaborUnions				5
Percentage of Vlissingen work force represented by the labor union (in hundredths)				80.00%
Percentage of U.S. based work force represented by a union (in hundredths)				54.00%
PBGC Agreement [Member]
PBCG Settlement [Abstract]
Pension contributions	$ 1,076	$ 6,700
PBGC Forecast [Member] | PBGC Agreement [Member]
PBCG Settlement [Abstract]
Pension contributions above required minimum, total			$ 17,400

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

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The power currently available under the power purchase agreement.

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Ravenswood Pension Plan [Abstract]

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Total costs accrued as of the balance sheet date for environmental loss contingencies.

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