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Fair value measurements (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Derivative Liabilities - Net [Member]
    
Level 3 fair value measurements [Roll Forward]    
Beginning balance, January 1, $ (16,539) $ (14,760)
Total gain (loss) included in earnings 16,539 (1,684)
Settlements 0 (95)
Ending balance, December 31, 0 (16,539)
Amount of gain (loss) included in earnings attributable to the change in unrealized losses relating to assets and liabilities held at December 31, 16,539 (1,684)
Fair Value, Measurements, Recurring [Member]
    
ASSETS:    
Cash equivalents 49,658 168,309
Trust assets 11,151 14,254
Surety bonds 2,002 2,123
Midwest premium contracts 140  
TOTAL 62,951 184,686
LIABILITIES:    
E.ON contingent obligation – net 0 15,369
Primary aluminum sales contract 140 1,170
TOTAL 140 16,539
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
    
ASSETS:    
Cash equivalents 49,658 168,309
Trust assets 11,151 14,254
Surety bonds 2,002 2,123
Midwest premium contracts 0  
TOTAL 62,811 184,686
LIABILITIES:    
E.ON contingent obligation – net 0 0
Primary aluminum sales contract 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 0 0
Midwest premium contracts 0  
TOTAL 0 0
LIABILITIES:    
E.ON contingent obligation – net 0 0
Primary aluminum sales contract 0 0
TOTAL 0 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
    
ASSETS:    
Cash equivalents 0 0
Trust assets 0 0
Surety bonds 0 0
Midwest premium contracts 140  
TOTAL 140 0
LIABILITIES:    
E.ON contingent obligation – net 0 15,369
Primary aluminum sales contract 140 1,170
TOTAL $ 140 $ 16,539