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Derivative and hedging instruments (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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Mar. 31, 2012 | Mar. 31, 2012 Aluminum Sales Premium Contracts [Member] T | Mar. 31, 2012 Designated as Hedging Instrument [Member] | Dec. 31, 2011 Designated as Hedging Instrument [Member] | Mar. 31, 2012 Primary Aluminum Put Option Contracts [Member] Due From Affiliates [Member] Designated as Hedging Instrument [Member] | Dec. 31, 2011 Primary Aluminum Put Option Contracts [Member] Due From Affiliates [Member] Designated as Hedging Instrument [Member] | Mar. 31, 2012 Primary Aluminum Put Option Contracts [Member] Prepaid and Other Current Assets [Member] Designated as Hedging Instrument [Member] | Dec. 31, 2011 Primary Aluminum Put Option Contracts [Member] Prepaid and Other Current Assets [Member] Designated as Hedging Instrument [Member] | Mar. 31, 2012 Power Contracts [Member] Not Designated as Hedging Instrument [Member] MWh | Dec. 31, 2011 Power Contracts [Member] Not Designated as Hedging Instrument [Member] MWh | Mar. 31, 2012 Power Contracts [Member] Prepaid and Other Current Assets [Member] Designated as Hedging Instrument [Member] | Dec. 31, 2011 Power Contracts [Member] Prepaid and Other Current Assets [Member] Designated as Hedging Instrument [Member] | Mar. 31, 2012 E.ON Contingent Obligation [Member] Other Liabilities [Member] Designated as Hedging Instrument [Member] | Dec. 31, 2011 E.ON Contingent Obligation [Member] Other Liabilities [Member] Designated as Hedging Instrument [Member] | Mar. 31, 2012 Aluminum Sales Premium Contract [Member] Other Liabilities [Member] Designated as Hedging Instrument [Member] | Dec. 31, 2011 Aluminum Sales Premium Contract [Member] Other Liabilities [Member] Designated as Hedging Instrument [Member] | Mar. 31, 2012 Aluminum Sales Premium Contract [Member] Accrued and Other Current Liabilities [Member] Designated as Hedging Instrument [Member] | Dec. 31, 2011 Aluminum Sales Premium Contract [Member] Accrued and Other Current Liabilities [Member] Designated as Hedging Instrument [Member] | Mar. 31, 2012 Natural Gas Forward Contracts [Member] Accrued and Other Current Liabilities [Member] Designated as Hedging Instrument [Member] | Dec. 31, 2011 Natural Gas Forward Contracts [Member] Accrued and Other Current Liabilities [Member] Designated as Hedging Instrument [Member] | Mar. 31, 2012 Primary Aluminum Sales Contract Premium [Member] Not Designated as Hedging Instrument [Member] t | Dec. 31, 2011 Primary Aluminum Sales Contract Premium [Member] Not Designated as Hedging Instrument [Member] t | Mar. 31, 2012 Primary Aluminum Put Option Contracts [Member] Not Designated as Hedging Instrument [Member] t | Dec. 31, 2011 Primary Aluminum Put Option Contracts [Member] Not Designated as Hedging Instrument [Member] t | Mar. 31, 2012 Primary Aluminum Put Option Contracts [Member] Not Designated as Hedging Instrument [Member] Related Party [Member] t | Dec. 31, 2011 Primary Aluminum Put Option Contracts [Member] Not Designated as Hedging Instrument [Member] Related Party [Member] t
Fair value of derivative assets and liabilities by balance sheet location [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
DERIVATIVE ASSETS |  |  | $ 3,034 | $ 9,437 | $ 1,801 | $ 5,439 | $ 1,180 | $ 3,892 |  |  |  |  | $ 53 | $ 106 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
DERIVATIVE LIABILITIES |  |  | 15,715 | 15,147 |  |  |  |  |  |  |  |  |  |  | 14,310 | 13,958 | 350 | 301 | 819 | 607 | 236 | 281 |  |  |  |  |  |  |  |
Outstanding forward contracts not designated as hedging instruments [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power contracts (in megawatt hours (MWH)) |  |  |  |  |  |  |  |  | 1,886 | [1] | 3,772 | [1] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aluminum forward contract outstanding (in metric tons) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 35,700 | [2] | 40,870 | [2] | 7,500 | 15,000 | 9,000 | 18,000
Primary aluminim physical delivery contract through Decmeber 31, 2013 (in metric tons per year) |  | 20,400 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accumulated other comprehensive loss to be reclassified to earnings over the next 12-month period | $ 170 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | Represents our expected usage during the remaining term of the Ravenswood power contract. In June 2011, the West Virginia PSC extended the term of this contract for an additional year.
[2] | Represents the remaining physical deliveries under our 2013 Glencore Metal Agreement.

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- Definition Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements. + References Reference 1: 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=6947722&loc=d3e13495-108611 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 FASB Staff Position (FSP) -Number FIN39-1 -Paragraph 10A, 10B -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. 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 Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 4, 17 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 6: 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=6947722&loc=d3e13433-108611 + Details
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