Commitments and contingencies (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended		3 Months Ended	32 Months Ended
	Nov. 30, 2011	Jan. 31, 2010	Sep. 30, 2012 Sites	Sep. 30, 2012 Sites	Dec. 31, 2011 MW	Dec. 31, 2010 Plaintiffs	Apr. 30, 2010 LaborUnions	Jan. 02, 2010	Nov. 30, 2009 Retirees	Dec. 31, 2006 SucceedingOwners
Environmental Contingencies [Abstract]
Aggregate environmental-related accrued liabilities			$ 1,095	$ 1,095	$ 852
Number of sites in which interim remediation measures have been completed (in sites)			2	2
Amount expended on hydrocarbon recovery plan under EPA Administrative Order			940
Expected future potential payments under indemnification plan in order to comply with EPA Administrative Order			500
Minimum amount expended by Company entitling them to reimbursement by third party for operating and maintaining water treatment system on behalf of third party			400
Number of succeeding owners of alumina facility named as defendants in a lawsuit filed by the Commissioner of the Department of Planning and Natural Resources										2
Number of plaintiffs to lawsuit alleging violations of territorial water pollution control laws in first lawsuit						2,300
Number of plaintiffs to lawsuit alleging violations of territorial water pollution control laws in second lawsuit						200
Damages sought for breach of contract and wrongful termination	20,000
loss contingency, estimated by Ingram			4,600	4,600
Ravenswood Retiree Medical Benefits [Abstract]
Age at which retiree medical benefits will cease for retirees and dependents (age in years)		65 years		65 years
Age at which bargaining unit retirees and qualified dependents are covered by the salary retiree medical plan (age in years)		65 years
Number of retirees in part of class action lawsuit regarding Company's rights to modify or terminate retiree medical benefits								4	4
Potential CAWV Accelearated Pension Plan Contributions Estimation			17,400	17,400
Loss Contingency, Range of Possible Loss, Minimum			0	0
Loss Contingency, Range of Possible Loss, Maximum			$ 5,000	$ 5,000
Helguvik Agreement [Abstract]
Power currently available under the power purchase agreement (in megawatts)					47.5
Labor Commitments [Abstract]
Percentage of U.S. based work force represented by a union (in hundredths)			75.00%	75.00%
Number of labor unions foreign subsidiary entered into a new labor agreement with							5
Percentage of foreign work force represented by the labor unions (in hundredths)							84.00%