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Commitments and Contingencies (Details)				1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
Commitments and Contingencies (Details)	Oct. 01, 2021 USD ($)	Apr. 01, 2021	Aug. 18, 2017 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2021 USD ($) Labor_Union MW	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) Labor_Union MW	Sep. 30, 2020 USD ($)	Dec. 31, 2013 USD ($)	Jul. 31, 2021 MW
Labor Commitments [Abstract]
Percentage of total work force in union					64.00%		64.00%
Percentage of Grundartangi work force represented by the labor unions					86.00%		86.00%
Number of labor unions Grundartangi subsidiary entered into a new labor agreement with \| Labor_Union					5		5
Percentage of U.S. based work force represented by a union					55.00%		55.00%
PBGC [Member]
PBGC Settlement [Abstract]
Required pension contributions above minimum									$ 17,400,000
Payment for pension benefits					$ 0	$ 0	$ 0	$ 0
PBGC [Member] \| Subsequent Event [Member]
PBGC Settlement [Abstract]
Pension contributions, amended term, annual contribution	$ 2,400,000
Pension contributions, amended term, total contribution	$ 9,600,000
Pension contributions, term	4 years
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
Santee Cooper [Member]
Power Contingencies [Abstract]
Power agreement, power supply, percentage		100.00%
Power supply agreement, increase production, at full capacity, percentage		75.00%
Grundartangi - HS, Landsvirkjun and OR [Member]
Power Contingencies [Abstract]
Power currently available under the power purchase agreement, available (in megawatts) \| MW					545		545
Grundartangi - Landsvirkjun [Member]
Power Contingencies [Abstract]
Power currently available under the power purchase agreement, available (in megawatts) \| MW					25		25
Power currently available under the power purchase agreement, extension (in megawatts) \| MW										161
Power currently available under the power purchase agreement, requested (in megawatts) \| MW										182
Netherlands [Member]
Labor Commitments [Abstract]
Percentage of Vlissingen work force represented by the labor union					100.00%		100.00%
Vernon [Member] \| Subsequent Event [Member]
Ravenswood litigation [Abstract]
Litigation settlement amount	$ 3,900,000
Ravenswood Retiree Medical Benefits Changes [Member]
Ravenswood litigation [Abstract]
Litigation settlement amount			$ 23,000,000
Ravenswood litigation settlement installment period			10 years				9 years
Litigation payment to trust				$ 5,000,000
Gain (loss) related to litigation settlement				5,500,000
Loss contingency accrual				$ 12,500,000
Litigation settlement, amount awarded to other party, annual payments							$ 2,000,000
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
Other liabilities [Member] \| Ravenswood Retiree Medical Benefits Changes [Member]
Ravenswood litigation [Abstract]
Litigation settlement, amount awarded to other party, annual payments							$ 6,000,000