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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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