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Debt - Activity (Details) - USD ($) | Sep. 30, 2020 | Jun. 30, 2020 | Dec. 31, 2019 | Sep. 30, 2019
--- | --- | --- | --- | ---
Debt Instrument [Line Items] |  |  |  |
Hawesville term loan - current portion | $ 20,000,000.0 |  | $ 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 |
Hawesville term loan - less current portion, interest payable monthly | 5,000,000.0 |  | 20,000,000.0 |
Total debt | 325,600,000 |  | 301,000,000.0 |
Hawesville [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Hawesville term loan - current portion | 20,000,000.0 |  | 20,000,000.0 |
Hawesville term loan - less current portion, interest payable monthly | 5,000,000.0 |  | 20,000,000.0 |
Long-term Debt [Member] | Senior secured notes due June 01, 2021 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Senior securities notes | 0 |  | $ 249,200,000 |
Stated interest rate, percentage |  | 7.50% | 7.50% |
Debt discount |  |  | $ 800,000 |
Long-term Debt [Member] | Senior Secured Notes Due June 01, 2025 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Senior securities notes | $ 242,800,000 |  | 0 |
Stated interest rate, percentage | 12.00% |  |  |
Debt discount |  |  |  | $ 2,400,000
Financing fees |  |  |  | $ 4,800,000
Short-term Debt [Member] | Industrial revenue bonds due April 1, 2028 [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 |
Stated interest rate, percentage | 0.34% |  |  |
Short-term Debt [Member] | Industrial revenue bonds due April 1, 2028 [Member] | Maximum [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Effective interest rate, maximum | 12.00% |  |  |
Short-term Debt [Member] | Revolving credit facility [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Stated interest rate, percentage | 5.375% |  |  |
Interest payable outstanding | $ 0 |  |  |
Principal payment | 15,000,000.0 |  |  |
Interest payable | $ 1,600,000 |  |  |
Short-term Debt [Member] | Revolving credit facility [Member] | Revolving Credit Facility, Portion Subject To LIBOR Rate [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Stated interest rate, percentage | 0.30% |  |  |
U.S. revolving credit facility [Member] | Short-term Debt [Member] | Revolving credit facility [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Revolving credit facility | $ 5,000,000.0 |  | 4,000,000.0 |
Stated interest rate, percentage | 4.00% |  |  |
Iceland revolving line of credit [Member] | Long-term Debt [Member] | Revolving credit facility [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Revolving credit facility | $ 45,000,000.0 |  | $ 0 |
Stated interest rate, percentage | 3.20% |  |  |

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