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Commitments and contingencies - Legal Contingencies (Details) - USD ($)		1 Months Ended		3 Months Ended	12 Months Ended				45 Months Ended
	Feb. 09, 2017	Mar. 31, 2015	Apr. 30, 2013	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2016	Sep. 30, 2016
Loss Contingencies [Line Items]
Pension contributions					$ 0	$ 34,595,000	$ 0
PBGC Agreement [Member]
Loss Contingencies [Line Items]
Required pension contributions above minimum			$ 17,400,000						$ 17,400,000
Pension contributions		$ 1,100,000			0	$ 1,100,000	$ 0	$ 6,700,000
Pension contributions above minimum remaining				$ 9,600,000	$ 9,600,000
Hawesville [Member]
Loss Contingencies [Line Items]
Power supply agreement, termination notice period				1 year	1 year
Ravenswood Retiree Medical Benefits Changes [Member]
Loss Contingencies [Line Items]
Litigation settlement amount	$ 23,000,000
Estimated litigation liability					$ 23,000,000				$ 23,000,000	$ 23,000,000
Litigation settlement period	10 years
Litigation settlement payable upon final approval of settlement	$ 5,000,000
Litigation settlement, annual installment	$ 2,000,000
Litigation settlement installment period	9 years

X

- Definition

Litigation Settlement, Annual Installment

+ References

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- Definition

Litigation Settlement Installment Period

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- Definition

Litigation Settlement, Payable upon Final Approval of Settlement

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- Definition

Litigation Settlement Period

+ References

No definition available.

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- Definition

Pursuant to the terms of the settlement agreement with the Pension Benefit Guaranty Corporation, this amount represents the remaining additional contributions (above any minimum required contributions) to our defined benefit pension plans.

+ References

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- Definition

Pursuant to the terms of the settlement agreement with the Pension Benefit Guaranty Corporation, we will make additional contributions (above any minimum required contributions) to our defined benefit pension plans.

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- Definition

Power Supply Agreement, Termination Notice Period

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No definition available.

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- Definition

Aggregate carrying amount of the estimated litigation liability for known or estimated probable loss from litigation, which may include attorneys' fees and other litigation costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.25)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Amount of judgment or settlement awarded to (against) the entity in respect of litigation.

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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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- Definition

The amount of cash or cash equivalents contributed by the entity to fund its pension plans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3536-108585

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