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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
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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 |  |  |  |  |  |  |  |  |  |  |  |  |

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