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Forward delivery contracts and financial instruments (Details)
 6 Months Ended
Jun. 30, 2013
Primary Aluminum Sales Contracts [Member]
Glencore Metal Agreement [Member]
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Jun. 30, 2013
Primary Aluminum Sales Contracts [Member]
Glencore Sweep Agreement [Member]
Jun. 30, 2013
Primary Aluminum Sales Contracts [Member]
Glencore Nordural Metal Agreement [Member]
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Jun. 30, 2013
Primary Aluminum Sales Contracts [Member]
Southwire Metal Agreement [Member]
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Jun. 30, 2013
Primary Aluminum Sales Contracts [Member]
RTA Metal Agreement [Member]
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Jun. 30, 2013
Long-Term Tolling Conctracts [Member]
Billiton Tolling Agreement [Member]
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Jun. 30, 2013
Long-Term Tolling Conctracts [Member]
Glencore Toll Agreement 1 [Member]
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Jun. 30, 2013
Long-Term Tolling Conctracts [Member]
Glencore Toll Agreement 2 [Member]
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Jun. 30, 2013
Other Forward Delivery Contracts [Member]
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Dec. 31, 2012
Other Forward Delivery Contracts [Member]
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Jun. 30, 2013
Glencore [Member]
Other Forward Delivery Contracts [Member]
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Dec. 31, 2012
Glencore [Member]
Other Forward Delivery Contracts [Member]
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Supply Commitment [Line Items]                        
Customer Glencore [1] Glencore [2] Glencore Southwire [3] Rio Tinto Alcan [3] BHP Billiton [4] Glencore [4] Glencore [4]        
Sales Volume Description 20,400 mtpy [1] Surplus primary aluminum produced in the United States [2] Approximately 19,000 metric tons 220 to 240 million pounds per year (high conductivity molten aluminum) [3] Approximately 111,000 metric tons [3] 130,000 mtpy [4] 90,000 mtpy [4] 40,000 mtpy [4]        
Volume (in mtpy, metric tons, and pounds per year) 20,400,000 [1]   19,000,000 240,000,000 111,000,000 130,000 90,000 40,000        
Annual Primary Aluminim Physical Delivery Contract_Minimum       220,000,000                
Term Through December 31, 2013 [1] Through December 31, 2013 [2] Through December 31, 2013 Through December 31, 2013 [3] Through December 31, 2013 [3] Through December 31, 2013 [4] Through July 31, 2016 [4] Through December 31, 2014 [4]        
Pricing Variable, based on U.S. Midwest market [1] Variable, based on U.S. Midwest market [2] Variable, based on LME Variable, based on U.S. Midwest market [3] Variable, based on U.S. Midwest market [3] LME-based [4] LME-based [4] LME-based [4]        
Other forward delivery contracts to sell primary aluminum (in metric tons)                 50,181 88,827 2,010 1,811
[1] We account for the Glencore Metal Agreement as a derivative instrument under ASC 815. Under the Glencore Metal Agreement, pricing is based on then-current Midwest market prices, adjusted by a negotiated U.S. Midwest premium with a cap and a floor as applied to the current U.S. Midwest premium.
[2] The Glencore Sweep Agreement is for all metal produced in the U.S. in 2013, less existing sales agreements and high-purity metal sales. The term of the contract may be extended for one year upon mutual agreement.
[3] The Southwire Metal Agreement contains termination rights in the event of a partial or full curtailment of the Hawesville facility.
[4] Grundartangi’s tolling revenues include a premium based on the European Union (“EU”) import duty for primary aluminum.