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Debt - Activity (Details) - USD ($) $ in Millions | Jun. 30, 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 | 69.3 | 49.4 |
Convertible senior notes | 84.5 | 84.4 |
Total debt | 553.4 | 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 |
Maximum variable interest rate | 12.00% |  |
Effective interest rate | 4.14% |  |
Revolving credit facility | U.S. revolving credit facility |  |  |
Debt Instrument [Line Items] |  |  |
Revolving credit facility | $ 63.2 | 90.0 |
Effective interest rate | 9.00% |  |
Revolving credit facility | Iceland revolving line of credit |  |  |
Debt Instrument [Line Items] |  |  |
Revolving credit facility | $ 73.0 | 35.0 |
Effective interest rate | 8.16% |  |
Casthouse Facility | Iceland revolving line of credit |  |  |
Debt Instrument [Line Items] |  |  |
Secured debt | $ 69.3 | 49.4 |
Financing fees | $ 0.7 |  |
Effective interest rate | 8.63% |  |
Iceland term facility | Iceland revolving line of credit |  |  |
Debt Instrument [Line Items] |  |  |
Revolving credit facility | $ 8.6 | 13.3 |
Secured debt | 0.0 | 1.2 |
Financing fees | $ 0.0 |  |
Stated interest rate, percentage | 3.20% |  |
Effective interest rate | 6.46% |  |
Senior secured notes, 7.5% | Senior notes |  |  |
Debt Instrument [Line Items] |  |  |
Grundartangi casthouse debt facility | $ 247.0 | 246.6 |
Financing fees | $ 3.0 |  |
Stated interest rate, percentage | 7.50% |  | 7.50%
Senior convertible notes, 2.75% | Senior notes |  |  |
Debt Instrument [Line Items] |  |  |
Convertible senior notes | $ 84.5 | $ 84.4 |
Financing fees | $ 1.7 |  |
Stated interest rate, percentage | 2.75% |  |

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