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Debt - Activity (Details) - USD ($) $ in Millions | Dec. 31, 2022 | Dec. 31, 2021 | Apr. 14, 2021
--- | --- | --- | ---
Debt Instrument [Line Items] |  |  |
Hancock County industrial revenue bonds (IRBs) due April 2028, interest payable quarterly (variable interest rates (not to exceed 12%)) | $ 7.8 | $ 7.8 |
Grundartangi casthouse debt facility | 49.4 | 0.0 |
Convertible senior notes payable | 84.4 | 84.0 |
Total | 527.7 | 451.2 |
Industrial revenue bonds due 2028 |  |  |
Debt Instrument [Line Items] |  |  |
Hancock County industrial revenue bonds (IRBs) due April 2028, interest payable quarterly (variable interest rates (not to exceed 12%)) | $ 7.8 | 7.8 |
Stated interest rate | 3.80% |  |
Industrial revenue bonds due 2028 | Maximum |  |  |
Debt Instrument [Line Items] |  |  |
Effective interest rate (percent) | 12.00% |  |
Senior Notes | Senior Secured Notes, 7.5% |  |  |
Debt Instrument [Line Items] |  |  |
Grundartangi casthouse debt facility | $ 246.6 | 245.8 |
Financing fees | $ 3.4 |  |
Stated interest rate | 7.50% |  | 7.50%
Senior Notes | Senior Convertible Notes, 2.75% |  |  |
Debt Instrument [Line Items] |  |  |
Convertible senior notes payable | $ 84.4 | 84.0 |
Financing fees | $ 1.9 |  |
Stated interest rate | 2.75% |  |
U.S revolving credit facility | U.S. Revolving Credit Facility |  |  |
Debt Instrument [Line Items] |  |  |
U.S. revolving credit facility | $ 90.0 | 63.6 |
Stated interest rate | 8.00% |  |
Iceland revolving credit facility | Iceland Revolving Credit Facility |  |  |
Debt Instrument [Line Items] |  |  |
U.S. revolving credit facility | $ 35.0 | 50.0 |
Stated interest rate | 7.41% |  |
Iceland revolving credit facility | Iceland Term Facility |  |  |
Debt Instrument [Line Items] |  |  |
U.S. revolving credit facility | $ 13.3 | 0.0 |
Revolving credit facility | 1.2 | 0.0 |
Financing fees | $ 0.0 |  |
Stated interest rate | 474.00% |  |
Iceland revolving credit facility | Casthouse Facility |  |  |
Debt Instrument [Line Items] |  |  |
Revolving credit facility | $ 49.4 | $ 0.0 |
Financing fees | $ 0.6 |  |
Stated interest rate | 773.00% |  |

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