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Debt - Activity (Details) - USD ($)
$ in Millions
Dec. 31, 2022
Dec. 31, 2021
Apr. 14, 2021
Debt Instrument [Line Items]      
Hancock County industrial revenue bonds (IRBs) due April 2028, interest payable quarterly (variable interest rates (not to exceed 12%)) $ 7.8 $ 7.8  
Grundartangi casthouse debt facility 49.4 0.0  
Convertible senior notes payable 84.4 84.0  
Total 527.7 451.2  
Industrial revenue bonds due 2028      
Debt Instrument [Line Items]      
Hancock County industrial revenue bonds (IRBs) due April 2028, interest payable quarterly (variable interest rates (not to exceed 12%)) $ 7.8 7.8  
Stated interest rate 3.80%    
Industrial revenue bonds due 2028 | Maximum      
Debt Instrument [Line Items]      
Effective interest rate (percent) 12.00%    
Senior Notes | Senior Secured Notes, 7.5%      
Debt Instrument [Line Items]      
Grundartangi casthouse debt facility $ 246.6 245.8  
Financing fees $ 3.4    
Stated interest rate 7.50%   7.50%
Senior Notes | Senior Convertible Notes, 2.75%      
Debt Instrument [Line Items]      
Convertible senior notes payable $ 84.4 84.0  
Financing fees $ 1.9    
Stated interest rate 2.75%    
U.S revolving credit facility | U.S. Revolving Credit Facility      
Debt Instrument [Line Items]      
U.S. revolving credit facility $ 90.0 63.6  
Stated interest rate 8.00%    
Iceland revolving credit facility | Iceland Revolving Credit Facility      
Debt Instrument [Line Items]      
U.S. revolving credit facility $ 35.0 50.0  
Stated interest rate 7.41%    
Iceland revolving credit facility | Iceland Term Facility      
Debt Instrument [Line Items]      
U.S. revolving credit facility $ 13.3 0.0  
Revolving credit facility 1.2 0.0  
Financing fees $ 0.0    
Stated interest rate 474.00%    
Iceland revolving credit facility | Casthouse Facility      
Debt Instrument [Line Items]      
Revolving credit facility $ 49.4 $ 0.0  
Financing fees $ 0.6    
Stated interest rate 773.00%