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Commitments and contingencies (Details) $ in Thousands	Dec. 18, 2015 source	Dec. 31, 2015 USD ($) MW labor_union
OR Power Purchase Agreement Abstract]
Power prepayments | $		$ 1,477
Power currently available under the power purchase agreement (in megawatts) | MW		47.5
Labor Commitments [Abstract]
Percentage of Company's work force represented by a union		63.00%
Percentage of Grundartangi work force represented by the labor unions		84.00%
Number of labor unions under new agreement | labor_union		5
Percentage of Vlissingen work force represented by the labor union		100.00%
Percentage of U.S. based work force represented by a union		53.00%
South Carolina Public Service Authority [Member]
Short-term Agreement for Power Supply [Line Items]
Operating capacity under agreement, percentage	50.00%
Number of power sources | source	2
Power agreement, power supply from source one, percentage	25.00%
Power agreement, power supply from source two, percentage	75.00%
Agreement termination notice period	60 days

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

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

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The percentage of a Company's Netherlands 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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Short-term Agreement, Operating Capacity, Percentage

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[Line Items] for Short-term Agreement for Power Supply [Table]

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Short-term Power Agreement, Number of Sources

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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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Short-term Power Agreement, Termination Notice Period

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Sum of the carrying amounts as of the balance sheet date of amounts paid in advance for expenses which will be charged against earnings in periods after one year or beyond the operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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