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Debt - Activity (Details) - USD ($) $ in Millions | Mar. 31, 2024 | Dec. 31, 2023 | Apr. 30, 2021
--- | --- | --- | ---
Debt Instrument [Line Items] |  |  |
Total debt | $ 528.0 | $ 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 | 3.77% |  |
Revolving credit facility | U.S. revolving credit facility |  |  |
Debt Instrument [Line Items] |  |  |
Revolving credit facility | $ 17.0 | 23.7 |
Effective interest rate | 9.25% |  |
Revolving credit facility | Iceland revolving line of credit |  |  |
Debt Instrument [Line Items] |  |  |
Revolving credit facility | $ 32.0 | 0.0 |
Effective interest rate | 8.83% |  |
Casthouse Facility | Iceland revolving line of credit |  |  |
Debt Instrument [Line Items] |  |  |
Revolving credit facility | $ 9.1 | 5.5 |
Financing fees | 1.2 |  |
Secured debt | $ 119.8 | 98.8 |
Stated interest rate, percentage | 8.84% |  |
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 | $ 2.5 |  |
Stated interest rate, percentage | 7.50% |  | 7.50%
7.5% senior secured notes due April 1, 2028, net of financing fees of $2.5 million at March 31, 2024, interest payable semiannually | $ 247.5 | 247.4 |
Senior convertible notes, 2.75% | Senior notes |  |  |
Debt Instrument [Line Items] |  |  |
Financing fees | $ 1.4 |  |
Stated interest rate, percentage | 2.75% |  | 2.75%
2.75% convertible senior notes due May 1, 2028, net of financing fees of $1.4 million at March 31, 2024, interest payable semiannually | $ 84.8 | $ 84.7 |

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