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Debt - Activity (Details) - USD ($) $ in Millions | Mar. 31, 2022 | Dec. 31, 2021 | Apr. 14, 2021
--- | --- | --- | ---
Debt Instrument [Line Items] |  |  |
Hancock County industrial revenue bonds ("IRBs") due April 1, 2028, interest payable quarterly | $ 7.8 | $ 7.8 |
Senior secured notes | 39.4 | 0.0 |
Convertible senior notes | 84.1 | 84.0 |
Total debt | 447.6 | 451.2 |
Industrial revenue bonds, variable [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Hancock County industrial revenue bonds ("IRBs") due April 1, 2028, interest payable quarterly | $ 7.8 | 7.8 |
Effective interest rate, maximum | 4.00% |  |
Stated interest rate, percentage | 0.64% |  |
Revolving credit facility [Member] | U.S. revolving credit facility [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Revolving credit facility | $ 35.3 | 63.6 |
Stated interest rate, percentage | 4.25% |  |
Revolving credit facility [Member] | Iceland revolving line of credit [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Revolving credit facility | $ 35.0 | 50.0 |
Stated interest rate, percentage | 3.51% |  |
Casthouse Facility [Member] | Iceland revolving line of credit [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Grundartangi casthouse debt facility | $ 39.4 | 0.0 |
Stated interest rate, percentage | 3.64% |  |
Financing fees | $ 0.6 |  |
Senior notes [Member] | Senior secured notes, 7.5% [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Senior secured notes | $ 246.0 | 245.8 |
Stated interest rate, percentage | 7.50% |  | 7.50%
Financing fees | $ 4.0 |  |
Senior notes [Member] | Senior convertible notes, 2.75% [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Convertible senior notes | $ 84.1 | $ 84.0 |
Stated interest rate, percentage | 2.75% |  |
Financing fees | $ 2.2 |  |

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- Definition Carrying value as of the balance sheet date of the long-term debt associated with industrial revenue bonds. For classified balance sheets, these bonds are considered current assets because they are remarketed weekly and the bonds could be required to be repaid upon demand if there is a failed remarketing. + References No definition available. + Details
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