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Debt - Activity (Details) - USD ($) $ in Millions | Dec. 31, 2021 | Apr. 30, 2021 | Apr. 14, 2021 | Dec. 31, 2020
--- | --- | --- | --- | ---
Debt Instrument [Line Items] |  |  |  |
Hawesville term loan - current portion | $ 0.0 |  |  | $ 20.0
Hancock County industrial revenue bonds (IRBs) due April 2028, interest payable quarterly (variable interest rates (not to exceed 12%)) | 7.8 |  |  | 7.8
Convertible senior notes payable | 84.0 |  |  | 0.0
Total | 451.2 |  |  | 315.9
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 | 0.30% |  |  |
Revolving Credit Facility |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Principal payment | $ 40.0 |  |  |
Interest payable | 4.6 |  |  |
Senior Notes |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt discount |  | $ 2.2 |  |
Senior Notes | Senior Secured Notes, 12.0% |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Senior secured notes | 0.0 |  |  | $ 243.1
Stated interest rate |  | 12.00% |  | 12.00%
Debt discount |  |  |  | $ 2.3
Financing fees |  |  |  | 4.6
Senior Notes | Senior Secured Notes, 7.5% |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Senior secured notes | $ 245.8 |  |  | 0.0
Stated interest rate | 7.50% |  | 7.50% |
Financing fees | $ 4.2 |  |  |
Senior Notes | Senior Convertible Notes, 2.75% |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Convertible senior notes payable | $ 84.0 |  |  | 0.0
Stated interest rate | 2.75% |  |  |
Financing fees | $ 2.3 |  |  |
U.S revolving credit facility | Revolving Credit Facility |  |  |  |
Debt Instrument [Line Items] |  |  |  |
U.S. revolving credit facility | $ 63.6 |  |  | 0.0
Stated interest rate | 4.00% |  |  |
Iceland revolving credit facility | Revolving Credit Facility |  |  |  |
Debt Instrument [Line Items] |  |  |  |
U.S. revolving credit facility | $ 50.0 |  |  | 0.0
Revolving credit facility | $ 0.0 |  |  | 45.0
Stated interest rate | 3.054% |  |  |
Hawesville |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Hawesville term loan - current portion | $ 0.0 |  |  | $ 20.0

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