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Helguvik and Ravenswood gains and losses (Details) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Jan. 31, 2017	Jun. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Impaired Assets to be Disposed of by Method Other than Sale [Line Items]
Helguvik (gains) losses				$ 152,220
Gain project termination			$ 7,310	(152,220)	$ 0
Ravenswood Aluminum Smelter [Member]
Impaired Assets to be Disposed of by Method Other than Sale [Line Items]
Ravenswood charges		$ 30,850		$ 3,830
Proceeds from sale of Ravenswood facility and assets	$ 13,585

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- Definition

Gain (Loss) Related Project Termination, Including Asset Impairment

+ References

No definition available.

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- Definition

Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=68055307&loc=d3e2420-110228

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

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- Definition

The amount of impairment loss recognized in the period resulting from the write-down of the carrying amount of an intangible asset (excluding goodwill) to fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16373-109275

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- Definition

The cash inflow associated with the amount received from the sale of a portion of the company's business, for example a segment, division, branch or other business, during the period.

+ References

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-Name Accounting Standards Codification
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