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Commitments and contingencies (Details) (USD $)
In Thousands, unless otherwise specified
 1 Months Ended 3 Months Ended
Nov. 30, 2011
Jan. 31, 2010
Mar. 31, 2012
Sites
MW
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     $ 933 $ 852          
Number of sites in which interim remediation measures have been completed (in sites)     2            
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            
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            
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                
Ravenswood Retiree Medical Benefits [Abstract]                  
Age at which retiree medical benefits will cease for retirees and dependents (age in 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  
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%            
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%