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Fair value measurements (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Derivative Financial Instruments, Assets [Member]
    
Level 3 fair value measurements [Roll Forward]    
Beginning balance $ (13,802) $ (1,632)
Total gain (loss) (realized/unrealized) included in earnings (1,382) 23
Purchases, issuances and settlements 424 (12,193)
Ending balance (14,760) (13,802)
Amount of total (gain) loss included in earnings attributable to the change in unrealized gains/losses relating to assets and liabilities held at December 31, (1,382) 23
Fair Value, Measurements, Recurring [Member]
    
ASSETS:    
Cash equivalents 176,284 294,269
Trust assets 15,889 0
Surety bonds - workers comp insurance 2,391 0
Primary aluminum put option contracts 9,331 4,691
Natural gas forward financial purchase contracts 0 79
Power contract 106 72
TOTAL 204,001 299,111
LIABILITIES:    
Natural gas forward financial purchase contracts 281 0
E.ON contingent obligation - net 13,958 13,091
Primary aluminum sales contract - premium collar 908 783
TOTAL 15,147 13,874
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
    
ASSETS:    
Cash equivalents 176,284 294,269
Trust assets 15,889 0
Surety bonds - workers comp insurance 2,391 0
Primary aluminum put option contracts 0 0
Natural gas forward financial purchase contracts 0 0
Power contract 0 0
TOTAL 194,564 294,269
LIABILITIES:    
Natural gas forward financial purchase contracts 0 0
E.ON contingent obligation - net 0 0
Primary aluminum sales contract - premium collar 0 0
TOTAL 0 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
    
ASSETS:    
Cash equivalents 0 0
Trust assets 0 0
Surety bonds - workers comp insurance 0 0
Primary aluminum put option contracts 9,331 4,691
Natural gas forward financial purchase contracts 0 79
Power contract 0 0
TOTAL 9,331 4,770
LIABILITIES:    
Natural gas forward financial purchase contracts 281 0
E.ON contingent obligation - net 0 0
Primary aluminum sales contract - premium collar 0 0
TOTAL 281 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
    
ASSETS:    
Cash equivalents 0 0
Trust assets 0 0
Surety bonds - workers comp insurance 0 0
Primary aluminum put option contracts 0 0
Natural gas forward financial purchase contracts 0 0
Power contract 106 72
TOTAL 106 72
LIABILITIES:    
Natural gas forward financial purchase contracts 0 0
E.ON contingent obligation - net 13,958 13,091
Primary aluminum sales contract - premium collar 908 783
TOTAL $ 14,866 $ 13,874