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Derivative and hedging instruments Part 2 (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended
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Mar. 31, 2013 | Mar. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items] |  |
Unrealized gain (loss) on E.ON obligation | $ 15,722 | $ 0
Not Designated as Hedging Instrument [Member] | Primary Aluminum Put Option Contracts [Member] | Net Gain (Loss) on Forward Contracts [Member] |  |
Derivatives not designated as hedging instruments [Abstract] |  |
Gain (loss) recognized in income from derivatives | 0 | (4,695)
Not Designated as Hedging Instrument [Member] | Aluminum Sales Premium Contracts [Member] | Net Gain (Loss) on Forward Contracts [Member] |  |
Derivatives not designated as hedging instruments [Abstract] |  |
Gain (loss) recognized in income from derivatives | (215) | (465)
Not Designated as Hedging Instrument [Member] | Aluminum Sales Premium Contracts [Member] | Related Party Sales [Member] |  |
Derivatives not designated as hedging instruments [Abstract] |  |
Gain (loss) recognized in income from derivatives | 355 | 204
Not Designated as Hedging Instrument [Member] | E.ON Contingent Obligation [Member] | Net Gain (Loss) on Forward Contracts [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Unrealized gain (loss) on E.ON obligation | 15,722 | 0
Not Designated as Hedging Instrument [Member] | E.ON Contingent Obligation [Member] | Interest Expense [Member] |  |
Derivatives not designated as hedging instruments [Abstract] |  |
Gain (loss) recognized in income from derivatives | $ (353) | $ (353)

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- Definition The net change in the difference between the fair value and the carrying value on the contingent obligation held at each balance sheet date, that was included in earnings for the period. + References No definition available. + Details
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- Definition The amount of net gains and losses recognized in income during the period on derivative instruments designated and qualifying as hedging instruments in fair value hedges and related hedged items designated and qualifying in fair value hedges, on derivative instruments designated and qualifying as hedging instruments in cash flow hedges, and on derivative instruments not designated as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (a),(c),(d),(e) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624171-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 205G -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 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 44C -Subparagraph b -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 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5618551-113959 + Details
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