Commitments and Contingencies (Details)		1 Months Ended	9 Months Ended		12 Months Ended
	Aug. 18, 2017 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2020 USD ($) Labor_Union MW	Sep. 30, 2019 USD ($)	Dec. 31, 2013 USD ($)
Labor Commitments [Abstract]
Percentage of total work force in union			66.00%
Percentage of Grundartangi work force represented by the labor unions (in hundredths)			86.00%
Number of labor unions Grundartangi subsidiary entered into a new labor agreement with | Labor_Union			5
Percentage of U.S. based work force represented by a union (in hundredths)			57.00%
Ravenswood Retiree Medical Benefits Changes [Member]
Ravenswood litigation [Abstract]
Gain (loss) related to litigation settlement		$ 5,500,000
Loss contingency accrual		12,500,000
PBGC Agreement [Member]
PBGC Settlement [Abstract]
Required pension contributions above minimum					$ 17,400,000
Payment for pension benefits			$ 0	$ 0
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 agreement, power supply from source one, percentage			25.00%
Power agreement, power supply from source two, percentage			75.00%
Other Power Suppliers [Member]
Power Contingencies [Abstract]
Power supply agreement, termination notice period			60 days
Grundartangi - HS, Landsvirkjun and OR [Member]
Power Contingencies [Abstract]
Power currently available under the power purchase agreement (in megawatts) | MW			525
Grundartangi - Landsvirkjun [Member]
Power Contingencies [Abstract]
Power currently available under the power purchase agreement (in megawatts) | MW			161
Helguvik [Member]
Power Contingencies [Abstract]
Power currently available under the power purchase agreement (in megawatts) | MW			47.5
Netherlands [Member]
Labor Commitments [Abstract]
Percentage of Vlissingen work force represented by the labor union (in hundredths)			100.00%
Ravenswood Retiree Medical Benefits Changes [Member]
Ravenswood litigation [Abstract]
Ravenswood litigation settlement amount	$ 23,000,000.0
Ravenswood litigation settlement installment period	10 years		9 years
Litigation payment to trust		$ 5,000,000.0
Litigation settlement, amount awarded to other party, annual payments			$ 2,000,000.0
Other Current Liabilities [Member] | Ravenswood Retiree Medical Benefits Changes [Member]
Ravenswood litigation [Abstract]
Litigation settlement, amount awarded to other party, annual payments			2,000,000.0
Other Liabilities [Member] | Ravenswood Retiree Medical Benefits Changes [Member]
Ravenswood litigation [Abstract]
Litigation settlement, amount awarded to other party, annual payments			$ 7,300,000