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Commitments and contingencies (Details) (USD $)
In Thousands, unless otherwise specified
 1 Months Ended 6 Months Ended 6 Months Ended 45 Months Ended
Apr. 30, 2013
Jan. 31, 2010
Jun. 30, 2013
Dec. 31, 2012
Dec. 31, 2011
MW
Mar. 31, 2011
Plaintiffs
Dec. 31, 2010
Plaintiffs
Apr. 30, 2010
LaborUnions
Nov. 30, 2009
Retirees
Dec. 31, 2006
SucceedingOwners
Jun. 30, 2013
Logan Kruger [Member]
May 01, 2013
Ingram Barge Case [Member]
Apr. 03, 2013
Grundartangi Power Agreement [Member]
Dec. 31, 2016
CAWV Total Pension Contribution over Agreement [Member]
Environmental Contingencies [Abstract]                            
Aggregate environmental-related accrued liabilities     $ 1,396 $ 906                    
Environmental Remediation Expense Minimum Amount Entitling Reimbursement by Third Party     400                      
Amount expended on hydrocarbon recovery plan under EPA Administrative Order     985                      
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          
Damages sought for breach of contract and wrongful termination                     20,000      
Loss Contingency, Settlement Agreement, Consideration                       1,575 1,470  
OR Power 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%            
Pension Contributions                           17,400
Pension Expense $ 5,900