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Debt (Details) - USD ($) | Jun. 30, 2020 | Dec. 31, 2019 | Jun. 30, 2019 | Jun. 30, 2013
--- | --- | --- | --- | ---
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 | 10,000,000.0 | 20,000,000.0 |  |
Total debt | 377,300,000 | 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 | 10,000,000.0 | 20,000,000.0 |  |
Long-term Debt [Member] | Senior secured notes due June 01, 2021 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
7.5% senior secured notes due June 2021, net of debt discount of $0.5 million and $0.8 million, respectively, interest payable semiannually (5) | 249,500,000 | 249,200,000 |  |
Stated interest rate, percentage |  |  |  | 7.50%
Debt discount | 500,000 | 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 | 0.39% |  |  |
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 | 10,000,000.0 |  |  |
Interest payable | $ 1,200,000 |  |  |
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 | 0.40% |  |  |
U.S. revolving credit facility [Member] | Short-term Debt [Member] | Revolving credit facility [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Revolving credit facility | $ 45,000,000.0 |  | $ 4,000,000.0 |
Stated interest rate, percentage | 2.50% |  |  |
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 | 3.80% |  |  |

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