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Commitments and contingencies - Matters relating to the St. Croix Alumina Refining Facility (Details) - USD ($) $ in Thousands | 336 Months Ended | 339 Months Ended
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Dec. 31, 2016 | Mar. 31, 2017
Accrual for Environmental Loss Contingencies [Abstract] |  |
Environmental remediation expense for Hydrocarbon Recovery Plan | $ 1,085 | $ 1,085

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- Definition The charge against earnings in the period for known or estimated future costs arising from requirements to perform environmental remediation activities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13296-110859 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585 + Details
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