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Debt - Activity (Details) - USD ($)
$ in Millions
Jun. 30, 2024
Dec. 31, 2023
Apr. 30, 2021
Debt Instrument [Line Items]      
Total debt $ 480.4 $ 479.2  
Industrial revenue bonds, variable      
Debt Instrument [Line Items]      
Maximum variable interest rate 12.00%    
Hancock County industrial revenue bonds ("IRBs") due April 1, 2028, interest payable quarterly $ 7.8 7.8  
Effective interest rate 4.14%    
Revolving credit facility | U.S. revolving credit facility      
Debt Instrument [Line Items]      
Revolving credit facility $ 0.0 23.7  
Effective interest rate 9.25%    
Revolving credit facility | Iceland revolving line of credit      
Debt Instrument [Line Items]      
Revolving credit facility $ 0.0 0.0  
Effective interest rate 8.84%    
Casthouse Facility | Iceland revolving line of credit      
Debt Instrument [Line Items]      
Revolving credit facility $ 11.3 5.5  
Financing fees 0.0    
Secured debt $ 118.7 98.8  
Stated interest rate, percentage 8.88%    
Iceland Term Facility | Iceland revolving line of credit      
Debt Instrument [Line Items]      
Revolving credit facility $ 0.0 1.3  
Stated interest rate, percentage 3.20%    
Vlissingen facility agreement | Iceland revolving line of credit      
Debt Instrument [Line Items]      
Revolving credit facility $ 10.0 10.0  
Secured debt $ 10.0    
Stated interest rate, percentage 8.75%    
Senior secured notes, 7.5% | Senior notes      
Debt Instrument [Line Items]      
Financing fees $ 2.3    
Stated interest rate, percentage 7.50%   7.50%
7.5% senior secured notes due April 1, 2028, net of financing fees of $2.3 million at June 30, 2024, interest payable semiannually $ 247.7 247.4  
Senior convertible notes, 2.75% | Senior notes      
Debt Instrument [Line Items]      
Financing fees $ 1.3    
Stated interest rate, percentage 2.75%   2.75%
2.75% convertible senior notes due May 1, 2028, net of financing fees of $1.3 million at June 30, 2024, interest payable semiannually $ 84.9 $ 84.7