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Debt - Activity (Details) - USD ($)	Jun. 30, 2021	Apr. 30, 2021	Apr. 14, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Hawesville term loan - current portion	$ 10,000,000.0			$ 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	83,800,000			0
Total debt	386,600,000			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	0.20%
Revolving credit facility [Member]
Debt Instrument [Line Items]
Stated interest rate, percentage	5.375%
Interest payable outstanding	$ 0
Principal payment	30,000,000.0
Interest payable	$ 4,400,000
Revolving credit facility [Member] | Revolving credit facility, portion subject To LIBOR rate [Member]
Debt Instrument [Line Items]
Stated interest rate, percentage	0.20%
Revolving credit facility [Member] | U.S. revolving credit facility [Member]
Debt Instrument [Line Items]
Revolving credit facility	$ 39,600,000			0
Stated interest rate, percentage	3.75%
Revolving credit facility [Member] | Iceland revolving line of credit [Member]
Debt Instrument [Line Items]
Revolving credit facility	$ 0			45,000,000.0
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%
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	$ 245,400,000			0
Stated interest rate, percentage	7.50%		7.50%
Financing fees	$ 4,600,000
Senior notes [Member] | Senior convertible notes, 2.75% [Member]
Debt Instrument [Line Items]
Convertible senior notes	$ 83,800,000			0
Stated interest rate, percentage	2.75%
Financing fees	$ 2,500,000
Hawesville [Member]
Debt Instrument [Line Items]
Hawesville term loan - current portion	$ 10,000,000.0			$ 20,000,000.0

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