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Derivative and hedging instruments (Details) (USD $) In Thousands, unless otherwise specified | 9 Months Ended |  |  |  |  |  |  |  |  |  |  |  |
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Sep. 30, 2013 | Sep. 30, 2013 Not Designated as Hedging Instrument [Member] | Dec. 31, 2012 Not Designated as Hedging Instrument [Member] | Sep. 30, 2013 Midwest Premium Contracts [Member] Not Designated as Hedging Instrument [Member] t | Dec. 31, 2012 Midwest Premium Contracts [Member] Not Designated as Hedging Instrument [Member] t | Sep. 30, 2013 Midwest Premium Contracts [Member] Prepaid and Other Current Assets [Member] Not Designated as Hedging Instrument [Member] | Dec. 31, 2012 Midwest Premium Contracts [Member] Prepaid and Other Current Assets [Member] Not Designated as Hedging Instrument [Member] | Sep. 30, 2013 Aluminum Sales Premium Contract Current [Member] Accrued and Other Current Liabilities [Member] Not Designated as Hedging Instrument [Member] | Dec. 31, 2012 Aluminum Sales Premium Contract Current [Member] Accrued and Other Current Liabilities [Member] Not Designated as Hedging Instrument [Member] | Sep. 30, 2013 E.ON Contingent Obligation [Member] Other Liabilities [Member] Not Designated as Hedging Instrument [Member] | Dec. 31, 2012 E.ON Contingent Obligation [Member] Other Liabilities [Member] Not Designated as Hedging Instrument [Member] | Sep. 30, 2013 Primary Aluminum Sales Contract Premium [Member] Not Designated as Hedging Instrument [Member] t | Dec. 31, 2012 Primary Aluminum Sales Contract Premium [Member] Not Designated as Hedging Instrument [Member] t
Fair value of derivative assets and liabilities by balance sheet location [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
DERIVATIVE ASSETS |  | $ 363 | $ 0 |  |  | $ 363 | [1] | $ 0 | [1] |  |  |  |  |  |  |  |  |  |
DERIVATIVE LIABILITIES |  | 771 | 16,539 |  |  |  |  |  |  | 771 | 1,170 | 0 | [2] | 15,369 | [2] |  |  |  |
Outstanding forward contracts not designated as hedging instruments [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aluminum forward contract outstanding (in metric tons) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5,993 | [2] | 20,400 | [2]
Midwest premium contracts (in metric tons) |  |  |  | 6,000 | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accumulated other comprehensive loss to be reclassified to earnings over the next 12-month period | $ 153 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | We entered into a fixed-price forward contract that settles monthly from January 2014 to March 2014 based on the Midwest Premium price published in the Platts Metals Week for the applicable period.
[2] | Represents the remaining physical deliveries under the Glencore Metal Agreement.

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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 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13433-108611 Reference 2: 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 3: 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=28364263&loc=d3e13495-108611 Reference 4: 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=20225523&loc=SL20225862-175312 Reference 5: 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 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=28370219&loc=SL20226008-175313 + Details
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