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Debt - Activity (Details) - USD ($) | Sep. 30, 2021 | Apr. 30, 2021 | Apr. 14, 2021 | Dec. 31, 2020
--- | --- | --- | --- | ---
Debt Instrument [Line Items] |  |  |  |
Hawesville term loan - current portion | $ 5,000,000.0 |  |  | $ 20,000,000.0
Hancock County industrial revenue bonds ("IRBs") due April 1, 2028, interest payable quarterly (variable interest rates (not to exceed 12%)) | 7,800,000 |  |  | 7,800,000
Convertible senior notes | 83,900,000 |  |  | 0
Total debt | 389,600,000 |  |  | 315,900,000
Industrial revenue bonds, variable, not to exceed to 12% [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Hancock County industrial revenue bonds ("IRBs") due April 1, 2028, interest payable quarterly (variable interest rates (not to exceed 12%)) | $ 7,800,000 |  |  | 7,800,000
Effective interest rate, maximum | 12.00% |  |  |
Stated interest rate, percentage | 0.21% |  |  |
Revolving credit facility [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Stated interest rate, percentage | 5.375% |  |  |
Interest payable outstanding | $ 0 |  |  |
Principal payment | 35,000,000 |  |  |
Interest payable | $ 4,600,000 |  |  |
Revolving credit facility [Member] | Revolving credit facility, portion subject To LIBOR rate [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Stated interest rate, percentage | 0.125% |  |  |
Revolving credit facility [Member] | U.S. revolving credit facility [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Revolving credit facility | $ 17,300,000 |  |  | 0
Stated interest rate, percentage | 4.00% |  |  |
Revolving credit facility [Member] | Iceland revolving line of credit [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Revolving credit facility | $ 30,000,000 |  |  | 0
Revolving credit facility | $ 0 |  |  | 45,000,000.0
Stated interest rate, percentage | 3.088% |  |  |
Senior notes [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt discount |  | $ 2,200,000 |  |
Senior notes [Member] | Senior secured notes, 12.0% [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Senior secured notes | $ 0 |  |  | $ 243,100,000
Stated interest rate, percentage |  | 12.00% |  | 12.00%
Debt discount |  |  |  | $ 2,300,000
Financing fees |  |  |  | 4,600,000
Senior notes [Member] | Senior secured notes, 7.5% [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Senior secured notes | $ 245,600,000 |  |  | 0
Stated interest rate, percentage | 7.50% |  | 7.50% |
Financing fees | $ 4,400,000 |  |  |
Senior notes [Member] | Senior convertible notes, 2.75% [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Convertible senior notes | $ 83,900,000 |  |  | 0
Stated interest rate, percentage | 2.75% |  |  |
Financing fees | $ 2,400,000 |  |  |
Hawesville [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Hawesville term loan - current portion | $ 5,000,000.0 |  |  | $ 20,000,000.0

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