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Debt - Activity (Details) - USD ($) $ in Millions	Dec. 31, 2021	Apr. 30, 2021	Apr. 14, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Hawesville term loan - current portion	$ 0.0			$ 20.0
Hancock County industrial revenue bonds (IRBs) due April 2028, interest payable quarterly (variable interest rates (not to exceed 12%))	7.8			7.8
Convertible senior notes payable	84.0			0.0
Total	451.2			315.9
Industrial revenue bonds due 2028
Debt Instrument [Line Items]
Hancock County industrial revenue bonds (IRBs) due April 2028, interest payable quarterly (variable interest rates (not to exceed 12%))	$ 7.8			7.8
Stated interest rate	0.30%
Revolving Credit Facility
Debt Instrument [Line Items]
Principal payment	$ 40.0
Interest payable	4.6
Senior Notes
Debt Instrument [Line Items]
Debt discount		$ 2.2
Senior Notes | Senior Secured Notes, 12.0%
Debt Instrument [Line Items]
Senior secured notes	0.0			$ 243.1
Stated interest rate		12.00%		12.00%
Debt discount				$ 2.3
Financing fees				4.6
Senior Notes | Senior Secured Notes, 7.5%
Debt Instrument [Line Items]
Senior secured notes	$ 245.8			0.0
Stated interest rate	7.50%		7.50%
Financing fees	$ 4.2
Senior Notes | Senior Convertible Notes, 2.75%
Debt Instrument [Line Items]
Convertible senior notes payable	$ 84.0			0.0
Stated interest rate	2.75%
Financing fees	$ 2.3
U.S revolving credit facility | Revolving Credit Facility
Debt Instrument [Line Items]
U.S. revolving credit facility	$ 63.6			0.0
Stated interest rate	4.00%
Iceland revolving credit facility | Revolving Credit Facility
Debt Instrument [Line Items]
U.S. revolving credit facility	$ 50.0			0.0
Revolving credit facility	$ 0.0			45.0
Stated interest rate	3.054%
Hawesville
Debt Instrument [Line Items]
Hawesville term loan - current portion	$ 0.0			$ 20.0

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