Forward delivery contracts and financial instruments (Details)	9 Months Ended
	Sep. 30, 2012 Primary Aluminum Sales Contracts [Member] Glencore Metal Agreement [Member] T		Sep. 30, 2012 Primary Aluminum Sales Contracts [Member] Glencore Sweep Agreement [Member]		Sep. 30, 2012 Primary Aluminum Sales Contracts [Member] Glencore Nordural Metal Agreement [Member] T	Sep. 30, 2012 Primary Aluminum Sales Contracts [Member] Southwire Metal Agreement [Member] lb		Sep. 30, 2012 Long-Term Tolling Conctracts [Member] Billiton Tolling Agreement [Member] T		Sep. 30, 2012 Long-Term Tolling Conctracts [Member] Glencore Toll Agreement 1 [Member] T		Sep. 30, 2012 Long-Term Tolling Conctracts [Member] Glencore Toll Agreement 2 [Member] T		Sep. 30, 2012 Other Forward Delivery Contracts [Member]	Dec. 31, 2011 Other Forward Delivery Contracts [Member]	Sep. 30, 2012 Glencore [Member] Other Forward Delivery Contracts [Member]	Dec. 31, 2011 Glencore [Member] Other Forward Delivery Contracts [Member]
Supply Commitment [Line Items]
Customer	Glencore	[1]	Glencore	[2]	Glencore	Southwire	[3]	BHP Billiton	[4]	Glencore	[4]	Glencore	[4]
Volume	20,400 mtpy	[1]	Surplus primary aluminum produced in the United States	[2]	Approximately 16,000 metric tons	240 million pounds per year (high conductivity molten aluminum)	[3]	130,000 mtpy	[4]	90,000 mtpy	[4]	40,000 mtpy	[4]
Volume (in mtpy, metric tons, and pounds per year)	20,400				16,000	240,000,000		130,000		90,000		40,000
Term	Through December 31, 2013	[1]	Through December 31, 2012	[2]	Through December 31, 2012	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]	LME-based	[4]	LME-based	[4]	LME-based	[4]
Other forward delivery contracts to sell primary aluminum (in metric tons)														28,511	41,504	1,420	3,423
Fixed price commitments on these other forward delivery contracts (in metric tons)														20	41

[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 2012, 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.