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Commitments and contingencies (Details) (USD $)
 1 Months Ended 3 Months Ended
Jan. 31, 2010
Mar. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
MW
Mar. 31, 2011
Plaintiffs
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   $ 987,000 $ 906,000              
Environmental Remediation Expense Minimum Amount Entitling Reimbursement by Third Party   400,000                
Amount expended on hydrocarbon recovery plan under EPA Administrative Order   940,000                
Expected future potential payments under indemnification plan in order to comply with EPA Administrative Order   500,000                
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          
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%      
Logan Kruger [Member]
                    
Ravenswood Retiree Medical Benefits [Abstract]                    
Damages sought for breach of contract and wrongful termination   20,000,000                
Ingram Barge Case [Member]
                    
Ravenswood Retiree Medical Benefits [Abstract]                    
Damages sought for breach of contract and wrongful termination   $ 4,600