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Quarterly information (Unaudited) - Financial Results Overview (Details) - USD ($) $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effect of Fourth Quarter Events [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax		$ (16,137)			$ (16,549)	$ 3,487	$ 1,600
Helguvik (gains) losses						(152,220)
Ravenswood retiree legal settlement					$ 5,000	$ (23,000)	$ 0
Helguvik [Member]
Effect of Fourth Quarter Events [Line Items]
Gain on extinguishment of liability	$ (7,310)
Helguvik (gains) losses			$ (152,200)
Ravenswood Retiree Medical Benefits Changes [Member]
Effect of Fourth Quarter Events [Line Items]
Ravenswood retiree legal settlement				$ (23,000)

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- Definition

Gain (Loss) on Extinguishment of Liability

+ References

No definition available.

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- Definition

Increase (Decrease) in Estimated Litigation Liability

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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- Definition

Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5618551-113959

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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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-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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