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Debt - Activity (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Jul. 31, 2025 | Dec. 31, 2024 | Oct. 01, 2024 | Apr. 30, 2021
--- | --- | --- | --- | --- | ---
Debt Instrument [Line Items] |  |  |  |  |
Total debt | $ 618.5 |  | $ 528.2 |  |
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 (variable interest rates (not to exceed 12%)) | $ 7.8 |  | 7.8 |  |
Revolving credit facility | U.S. revolving credit facility |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Revolving credit facility | $ 0.0 |  | 20.0 |  |
Stated interest rate, percentage | 7.75% |  |  |  |
Revolving credit facility | Foreign line of credit |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Revolving credit facility | $ 16.5 |  | 34.0 |  |
Stated interest rate, percentage | 7.66% |  |  |  |
Casthouse Facility | Foreign line of credit |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Financing fees | $ 1.5 |  |  |  |
Revolving credit facility | 115.0 |  | 9.0 |  |
Secured debt | $ 0.0 |  | 114.2 |  |
Stated interest rate, percentage | 7.89% |  |  |  |
Vlissingen Credit Facility | Foreign line of credit |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Secured debt | $ 0.0 |  | 10.0 |  |
Stated interest rate, percentage | 7.83% |  |  | 8.75% |
Senior secured notes, 7.5% | Senior notes |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Stated interest rate, percentage | 7.50% |  |  |  |
Secured long-term debt | $ 0.0 |  | 248.1 |  |
Senior convertible notes, 2.75% | Senior notes |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Financing fees | $ 0.9 |  |  |  |
Stated interest rate, percentage | 2.75% |  |  |  | 2.75%
2.75% convertible senior notes due May 1, 2028, net of financing fees of $0.9 million at September 30, 2025, interest payable semiannually | $ 85.4 |  | 85.1 |  |
Senior Secured Notes, 6.875% | Senior notes |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Financing fees | $ 6.1 |  |  |  |
Stated interest rate, percentage | 6.875% | 6.875% |  |  |
Secured long-term debt | $ 393.8 |  | $ 0.0 |  |
Industrial Revenue Bonds Due 2028 |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Stated interest rate, percentage | 3.02% |  |  |  |

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