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Forward delivery contracts and financial instruments (Details) - t
 9 Months Ended
Sep. 30, 2015
Dec. 31, 2014
Primary Aluminum Sales Contracts [Member] | Glencore Nordural Metal Agreement [Member]    
Supply Commitment [Line Items]    
Customer Glencore  
Volume All primary aluminum produced at Grundartangi, net of tolling and other sales commitments  
Term January 1, 2014 through December 31, 2017  
Pricing Variable, based on LME and European Duty Paid premium  
Primary Aluminum Sales Contracts [Member] | Glencore U.S. Aluminum Sales Agreement [Member]    
Supply Commitment [Line Items]    
Customer Glencore  
Volume Substantially all of the primary aluminum produced in North America  
Term January 1, 2015 through December 31, 2016  
Pricing Variable, based on LME and Midwest premium and product premiums, as applicable  
Long-Term Tolling Conctracts [Member] | Glencore Toll Agreement [Member]    
Supply Commitment [Line Items]    
Customer Glencore  
Volume 90,000 tonnes per year ("tpy")  
Volume (in tonnes) 90,000  
Term Through July 2016  
Pricing Variable, based on LME and European Duty Paid premium  
Other Forward Delivery Contracts [Member]    
Supply Commitment [Line Items]    
Other forward delivery contracts to sell primary aluminum (in tonnes) 6,486 6,108
Glencore [Member] | Other Forward Delivery Contracts [Member]    
Supply Commitment [Line Items]    
Other forward delivery contracts to sell primary aluminum (in tonnes) 0 4,058