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Derivative and hedging instruments Part 2 (Details) (Not Designated as Hedging Instrument [Member], USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Primary Aluminum Sales Contracts [Member] | Net Gain (Loss) on Forward and Derivative Contracts [Member]
    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (loss) recognized in income from derivatives $ (152) $ (215)
Primary Aluminum Sales Contracts [Member] | Related Party Sales [Member]
    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (loss) recognized in income from derivatives 292 355
E.ON Contingent Obligation - Net [Member] | Net Gain (Loss) on Forward and Derivative Contracts [Member]
    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (loss) recognized in income from derivatives 353 15,722
E.ON Contingent Obligation - Net [Member] | Interest Expense - Third Party [Member]
    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (loss) recognized in income from derivatives (353) (353)
Midwest Premium Contracts [Member] | Net Gain (Loss) on Forward and Derivative Contracts [Member]
    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (loss) recognized in income from derivatives $ (1,080) $ 0