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Commitments and Contingencies (Details)
$ in Millions
1 Months Ended 9 Months Ended 12 Months Ended
Aug. 18, 2017
USD ($)
Oct. 31, 2021
USD ($)
Sep. 30, 2017
USD ($)
Sep. 30, 2025
USD ($)
laborUnion
MW
Dec. 31, 2013
USD ($)
Labor Commitments [Abstract]          
Percentage of total work force in union       58.00%  
Percentage of Grundartangi work force represented by the labor unions       87.00%  
Number of labor unions Grundartangi subsidiary entered into a new labor agreement with | laborUnion       5  
Percentage of domestic based work force represented by a union       37.00%  
Percentage of foreign work force, represented by the union       62.00%  
Contingent consideration, accrued interest and principal       $ 33.4  
E ON Contingent Obligation | Long-Term Debt          
Labor Commitments [Abstract]          
Stated interest rate, percentage       10.94%  
Netherlands          
Labor Commitments [Abstract]          
Percentage of total work force in union       100.00%  
PBGC          
PBGC Settlement [Abstract]          
Required pension contributions above minimum         $ 17.4
Pension contributions, amended term, annual contribution   $ 2.4      
Pension contributions, amended term, total contribution   $ 9.6      
Pension contributions, term   4 years      
Kenergy | Century Marketer, LLC          
Power Contingencies [Abstract]          
Extension term       1 year  
Required termination period       1 year  
Hawesville | Kenergy          
Power Contingencies [Abstract]          
Extension term       1 year  
Required termination period       1 year  
Sebree | Kenergy          
Power Contingencies [Abstract]          
Extension term       1 year  
Required termination period       1 year  
Santee Cooper          
Power Contingencies [Abstract]          
Power agreement, power supply, percentage       100.00%  
Grundartangi - HS, Landsvirkjun and OR          
Power Contingencies [Abstract]          
Power currently available under the power purchase agreement, available (in megawatts) | MW       545  
Grundartangi - Landsvirkjun          
Power Contingencies [Abstract]          
Power currently available under the power purchase agreement, available (in megawatts) | MW       25  
Ravenswood Retiree Medical Benefits Changes          
Ravenswood litigation [Abstract]          
Litigation settlement amount $ 23.0        
Ravenswood litigation settlement installment period 10 years        
Litigation payment to trust     $ 5.0    
Gain (loss) related to litigation settlement     5.5    
Loss contingency accrual     $ 12.5    
Litigation settlement, annual installment       $ 2.0  
Litigation settlement installment period       9 years  
Other current liabilities       $ 1.9