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Derivative and hedging instruments (Details) (USD $) In Thousands, unless otherwise specified | 6 Months Ended |  |  |  |  |  |  |  |
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Jun. 30, 2013 | Jun. 30, 2013 Designated as Hedging Instrument [Member] | Dec. 31, 2012 Designated as Hedging Instrument [Member] | Jun. 30, 2013 E.ON Contingent Obligation [Member] Other Liabilities [Member] Designated as Hedging Instrument [Member] | Dec. 31, 2012 E.ON Contingent Obligation [Member] Other Liabilities [Member] Designated as Hedging Instrument [Member] | Jun. 30, 2013 Aluminum Sales Premium Contract [Member] Accrued and Other Current Liabilities [Member] Designated as Hedging Instrument [Member] | Dec. 31, 2012 Aluminum Sales Premium Contract [Member] Accrued and Other Current Liabilities [Member] Designated as Hedging Instrument [Member] | Jun. 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 LIABILITIES |  | $ 773 | $ 16,539 | $ 0 | [1] | $ 15,369 | [1] | $ 773 | $ 1,170 |  |  |  |
Outstanding forward contracts not designated as hedging instruments [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Aluminum forward contract outstanding (in metric tons) |  |  |  |  |  |  |  |  |  | 10,430 | [2] | 20,400 | [2]
Accumulated other comprehensive loss to be reclassified to earnings over the next 12-month period | $ 153 |  |  |  |  |  |  |  |  |  |  |  |
[1] | Based on the LME forward market prices for primary aluminum at June 30, 2013 and management's estimate of the LME forward market for periods beyond the quoted periods, we believe that we will not have any payment obligations for the E.ON contingent obligation through the term of the agreement which expires in 2028. See Note 10 Debt for additional information about the E.ON contingent obligation fair value.
[2] | Represents the remaining physical deliveries under the Glencore Metal Agreement.

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