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Commitments and Contingencies (Details)
 1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Apr. 01, 2021
Aug. 18, 2017
USD ($)
Oct. 31, 2021
USD ($)
Sep. 30, 2017
USD ($)
Sep. 30, 2023
USD ($)
laborUnion
MW
Sep. 30, 2022
USD ($)
MW
Sep. 30, 2023
USD ($)
laborUnion
MW
Dec. 31, 2013
USD ($)
Jul. 31, 2021
MW
Labor Commitments [Abstract]                  
Percentage of total work force in union         59.00%   59.00%    
Percentage of Grundartangi work force represented by the labor unions         87.00%   87.00%    
Number of labor unions Grundartangi subsidiary entered into a new labor agreement with | laborUnion         5   5    
Percentage of domestic based work force represented by a union         41.00%   41.00%    
Percentage of foreign work force, represented by the union         0.62   0.62    
Contingent consideration, accrued interest and principal         $ 30,500,000   $ 30,500,000    
Netherlands                  
Labor Commitments [Abstract]                  
Percentage of Vlissingen work force represented by the labor union         100.00%   100.00%    
PBGC                  
PBGC Settlement [Abstract]                  
Required pension contributions above minimum               $ 17,400,000  
Pension contributions, amended term, annual contribution     $ 2,400,000            
Pension contributions, amended term, total contribution     $ 9,600,000   $ 6,900,000   $ 6,900,000    
Pension contributions, term     4 years            
Payment for pension benefits         $ 4,500,000 $ 0      
Santee Cooper                  
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                  
Power Contingencies [Abstract]                  
Power currently available under the power purchase agreement, available (in megawatts) | MW         545   545    
Grundartangi - Landsvirkjun                  
Power Contingencies [Abstract]                  
Power currently available under the power purchase agreement, available (in megawatts) | MW           25      
Power currently available under the power purchase agreement, extension (in megawatts) | MW           42     161
Power currently available under the power purchase agreement, requested (in megawatts) | MW                 182
Power rate | MW           119      
Ravenswood Retiree Medical Benefits Changes                  
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         $ 2,000,000   2,000,000    
Other liabilities         $ 3,200,000   $ 3,200,000