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Debt - Activity (Details) - USD ($)
$ in Millions
Mar. 31, 2022
Dec. 31, 2021
Apr. 14, 2021
Debt Instrument [Line Items]      
Hancock County industrial revenue bonds ("IRBs") due April 1, 2028, interest payable quarterly $ 7.8 $ 7.8  
Senior secured notes 39.4 0.0  
Convertible senior notes 84.1 84.0  
Total debt 447.6 451.2  
Industrial revenue bonds, variable [Member]      
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 4.00%    
Stated interest rate, percentage 0.64%    
Revolving credit facility [Member] | U.S. revolving credit facility [Member]      
Debt Instrument [Line Items]      
Revolving credit facility $ 35.3 63.6  
Stated interest rate, percentage 4.25%    
Revolving credit facility [Member] | Iceland revolving line of credit [Member]      
Debt Instrument [Line Items]      
Revolving credit facility $ 35.0 50.0  
Stated interest rate, percentage 3.51%    
Casthouse Facility [Member] | Iceland revolving line of credit [Member]      
Debt Instrument [Line Items]      
Grundartangi casthouse debt facility $ 39.4 0.0  
Stated interest rate, percentage 3.64%    
Financing fees $ 0.6    
Senior notes [Member] | Senior secured notes, 7.5% [Member]      
Debt Instrument [Line Items]      
Senior secured notes $ 246.0 245.8  
Stated interest rate, percentage 7.50%   7.50%
Financing fees $ 4.0    
Senior notes [Member] | Senior convertible notes, 2.75% [Member]      
Debt Instrument [Line Items]      
Convertible senior notes $ 84.1 $ 84.0  
Stated interest rate, percentage 2.75%    
Financing fees $ 2.2