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Fair value measurements (Details) - USD ($) $ in Thousands | Mar. 31, 2017 | Dec. 31, 2016
--- | --- | ---
ASSETS: |  |
Derivatives | $ 1,610 |
Fair Value, Measurements, Recurring [Member] |  |
ASSETS: |  |
Cash equivalents | 103,286 | $ 79,014
Trust assets | 3,138 | 3,147
Surety bonds | 1,618 | 1,874
Derivatives | 1,610 | 925
TOTAL | 109,652 | 84,960
LIABILITIES: |  |
E.ON contingent obligation - net | 0 | 0
Derivatives | 14,915 | 253
TOTAL | 14,915 | 253
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] |  |
ASSETS: |  |
Cash equivalents | 103,286 | 79,014
Trust assets | 3,138 | 3,147
Surety bonds | 1,618 | 1,874
Derivatives | 0 | 0
TOTAL | 108,042 | 84,035
LIABILITIES: |  |
E.ON contingent obligation - net | 0 | 0
Derivatives | 0 | 0
TOTAL | 0 | 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] |  |
ASSETS: |  |
Cash equivalents | 0 | 0
Trust assets | 0 | 0
Surety bonds | 0 | 0
Derivatives | 0 | 0
TOTAL | 0 | 0
LIABILITIES: |  |
E.ON contingent obligation - net | 0 | 0
Derivatives | 13,015 | 0
TOTAL | 13,015 | 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] |  |
ASSETS: |  |
Cash equivalents | 0 | 0
Trust assets | 0 | 0
Surety bonds | 0 | 0
Derivatives | 1,610 | 925
TOTAL | 1,610 | 925
LIABILITIES: |  |
E.ON contingent obligation - net | 0 | 0
Derivatives | 1,900 | 253
TOTAL | $ 1,900 | $ 253

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- Definition Fair values as of the balance sheet date of all primary aluminum sales contracts that meet the criteria of being accounted for as liability derivative instruments that do not qualify for normal purchases/normal sales exclusion. + References No definition available. + Details
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- Definition Fair values as of the balance sheet date of all surety bonds for workers compensation insurance as of the balance sheet date. + References No definition available. + Details
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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash -URI http://asc.fasb.org/extlink&oid=6506951 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash Equivalents -URI http://asc.fasb.org/extlink&oid=6507016 + Details
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- Definition Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13433-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13495-108611 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226008-175313 + Details
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