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Debt - Activity (Details) - USD ($)
$ in Millions
Mar. 31, 2023
Dec. 31, 2022
Apr. 30, 2021
Debt Instrument [Line Items]      
Hancock County industrial revenue bonds ("IRBs") due April 1, 2028, interest payable quarterly $ 7.8 $ 7.8  
Grundartangi casthouse debt facility 59.3 49.4  
Convertible senior notes 84.5 84.4  
Total debt 518.8 527.7  
Industrial revenue bonds, variable      
Debt Instrument [Line Items]      
Hancock County industrial revenue bonds ("IRBs") due April 1, 2028, interest payable quarterly $ 7.8 7.8  
Effective interest rate, maximum 12.00%    
Stated interest rate, percentage 4.11%    
Revolving credit facility | U.S. revolving credit facility      
Debt Instrument [Line Items]      
Revolving credit facility $ 63.1 90.0  
Stated interest rate, percentage 8.75%    
Revolving credit facility | Iceland revolving line of credit      
Debt Instrument [Line Items]      
Revolving credit facility $ 45.0 35.0  
Stated interest rate, percentage 7.66%    
Casthouse facility | Iceland revolving line of credit      
Debt Instrument [Line Items]      
Secured Debt $ 59.3 49.4  
Stated interest rate, percentage 8.08%    
Financing fees $ 0.6    
Iceland term facility | Iceland revolving line of credit      
Debt Instrument [Line Items]      
Revolving credit facility 12.3 13.3  
Secured Debt $ 0.0 1.2  
Stated interest rate, percentage 5.77%    
Financing fees $ 0.0    
Senior notes | Senior secured notes, 7.5%      
Debt Instrument [Line Items]      
Grundartangi casthouse debt facility $ 246.8 246.6  
Stated interest rate, percentage 7.50%   7.50%
Financing fees $ 3.2    
Senior notes | Senior convertible notes, 2.75%      
Debt Instrument [Line Items]      
Convertible senior notes $ 84.5 $ 84.4  
Stated interest rate, percentage 2.75%    
Financing fees $ 1.8