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Commitments and contingencies (Details)				1 Months Ended	9 Months Ended		12 Months Ended				345 Months Ended
	Sep. 30, 2017 USD ($) LaborUnions MW	Aug. 18, 2017 USD ($)	Dec. 18, 2015	Sep. 30, 2017 USD ($) LaborUnions MW	Sep. 30, 2017 USD ($) LaborUnions MW	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2017 USD ($) LaborUnions MW
Environmental Contingencies [Abstract]
Environmental-related accrued liabilities	$ 1,154,000			$ 1,154,000	$ 1,154,000		$ 998,000				$ 1,154,000
Environmental remediation expense for Hydrocarbon Recovery Plan											$ 1,085,000
Ravenswood litigation [Abstract]
Gain recognized					$ 5,500,000	$ (3,830,000)
Labor Commitments [Abstract]
Percentage of total work force in union	65.00%			65.00%	65.00%						65.00%
Percentage of Grundartangi work force represented by the labor unions (in hundredths)	85.00%			85.00%	85.00%						85.00%
Number of labor unions Grundartangi subsidiary entered into a new labor agreement with | LaborUnions	5			5	5						5
Percentage of Vlissingen work force represented by the labor union (in hundredths)	100.00%			100.00%	100.00%						100.00%
Percentage of U.S. based work force represented by a union (in hundredths)	57.00%			57.00%	57.00%						57.00%
PBGC Agreement [Member]
PBGC Settlement [Abstract]
Required pension contributions above minimum										$ 17,400,000
Payment for Pension Benefits					$ 0		$ 0	$ 1,100,000	$ 0	$ 6,700,000
Pension contributions above minimum remaining					$ 9,600,000
Hawesville [Member]
Power Contingencies [Abstract]
Power supply agreement, termination notice period					1 year
Sebree [Member]
Power Contingencies [Abstract]
Power supply agreement, termination notice period					1 year
South Carolina Public Service Authority [Member]
Power Contingencies [Abstract]
Power supply agreement, termination notice period			60 days
Power agreement, power supply from source one, percentage			25.00%
Power agreement, power supply from source two, percentage			75.00%
Grundartangi - HS, Landsvirkjun and OR [Member]
Power Contingencies [Abstract]
Power currently available under the power purchase agreement (in megawatts) | MW	525			525	525						525
Grundartangi - Landsvirkjun [Member]
Power Contingencies [Abstract]
Power currently available under the power purchase agreement (in megawatts) | MW	161			161	161						161
Helguvik [Member]
Power Contingencies [Abstract]
Power currently available under the power purchase agreement (in megawatts) | MW	47.5			47.5	47.5						47.5
Ravenswood Retiree Medical Benefits Changes [Member]
Ravenswood litigation [Abstract]
Ravenswood litigation settlement amount		$ 23,000,000
Ravenswood litigation settlement installment period	9 years	10 years
Litigation payment to trust				$ 5,000,000
Gain recognized				5,500,000
Loss contingency accrual	$ 12,500,000			$ 12,500,000	$ 12,500,000						$ 12,500,000
Litigation Settlement, Amount Awarded to Other Party, Annual Payments					$ 2,000,000

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