Forward delivery contracts and financial instruments (Details)	9 Months Ended
	Sep. 30, 2013 Primary Aluminum Sales Contracts [Member] Glencore Metal Agreement [Member] t		Sep. 30, 2013 Primary Aluminum Sales Contracts [Member] Glencore Sweep Agreement [Member]		Sep. 30, 2013 Primary Aluminum Sales Contracts [Member] Glencore Nordural Metal Agreement [Member] t	Sep. 30, 2013 Primary Aluminum Sales Contracts [Member] Southwire Metal Agreement [Member] lb		Sep. 30, 2013 Primary Aluminum Sales Contracts [Member] RTA Metal Agreement [Member] t	Sep. 30, 2013 Long-Term Tolling Conctracts [Member] Billiton Tolling Agreement [Member] t		Sep. 30, 2013 Long-Term Tolling Conctracts [Member] Glencore Toll Agreement 1 [Member] t		Sep. 30, 2013 Long-Term Tolling Conctracts [Member] Glencore Toll Agreement 2 [Member] t		Sep. 30, 2013 Other Forward Delivery Contracts [Member] t	Dec. 31, 2012 Other Forward Delivery Contracts [Member] t	Sep. 30, 2013 Glencore [Member] Other Forward Delivery Contracts [Member] t	Dec. 31, 2012 Glencore [Member] Other Forward Delivery Contracts [Member] t
Supply Commitment [Line Items]
Customer	Glencore	[1]	Glencore	[2]	Glencore	Southwire	[3]	Rio Tinto Alcan	BHP Billiton	[4]	Glencore	[4]	Glencore	[4]
Sales Volume Description	20,400 metric tons per year ("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	130,000 mtpy	[4]	90,000 mtpy	[4]	40,000 mtpy	[4]
Volume maximum (in mtpy, metric tons, and pounds per year)	20,400	[1]			19,000	240,000	[3]	111,000	130,000	[4]	90,000	[4]	40,000	[4]
Volume minimum (in mtpy, metric tons, and pounds per year)						220,000	[3]
Term	Through December 31, 2013	[1]	Through December 31, 2013	[2]	Through December 31, 2013	Through December 31, 2013	[3]	Through December 31, 2013	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	LME-based	[4]	LME-based	[4]	LME-based	[4]
Other forward delivery contracts to sell primary aluminum (in metric tons)															32,994	88,827	1,775	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. Despite reaching power agreements, certain regulatory approvals have not yet been received and CAKY could curtail all smelter operations if these approvals are not ultimately received.

[4] Grundartangi’s tolling revenues include a premium based on the European Union (“EU”) import duty for primary aluminum.