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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
	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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- References

No definition available.

+ Details

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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

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