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Ravenswood impairment (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Jun. 30, 2015
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Dec. 31, 2014
Restructuring Cost and Reserve [Line Items]            
Ravenswood impairment   $ 0 $ 0 $ 30,850 $ 0  
Write-down of inventory       31,013 $ (1,247)  
Assets held for disposal   32,067   32,067   $ 0
Employee Severance and Postemployment Benefits [Member]            
Restructuring Cost and Reserve [Line Items]            
Expected restructuring cost $ 1,550          
Ravenswood Aluminum Smelter [Member]            
Restructuring Cost and Reserve [Line Items]            
Ravenswood impairment 30,850          
Impairment of assets 21,500          
Write-down of inventory $ 7,800          
Assets held for sale [Member]            
Restructuring Cost and Reserve [Line Items]            
Assets held for disposal   32,067   32,067    
Inventory [Member] | Assets held for sale [Member]            
Restructuring Cost and Reserve [Line Items]            
Assets held for disposal   9,968   9,968    
Property, plant and equipment [Member] | Assets held for sale [Member]            
Restructuring Cost and Reserve [Line Items]            
Assets held for disposal   $ 22,099   $ 22,099