v3.25.3

Debt - Activity (Details) - USD ($) $ in Millions	Sep. 30, 2025	Jul. 31, 2025	Dec. 31, 2024	Oct. 01, 2024	Apr. 30, 2021
Debt Instrument [Line Items]
Total debt	$ 618.5		$ 528.2
Industrial revenue bonds, variable
Debt Instrument [Line Items]
Maximum variable interest rate	12.00%
Hancock County industrial revenue bonds ("IRBs") due April 1, 2028, interest payable quarterly (variable interest rates (not to exceed 12%))	$ 7.8		7.8
Revolving credit facility \| U.S. revolving credit facility
Debt Instrument [Line Items]
Revolving credit facility	$ 0.0		20.0
Stated interest rate, percentage	7.75%
Revolving credit facility \| Foreign line of credit
Debt Instrument [Line Items]
Revolving credit facility	$ 16.5		34.0
Stated interest rate, percentage	7.66%
Casthouse Facility \| Foreign line of credit
Debt Instrument [Line Items]
Financing fees	$ 1.5
Revolving credit facility	115.0		9.0
Secured debt	$ 0.0		114.2
Stated interest rate, percentage	7.89%
Vlissingen Credit Facility \| Foreign line of credit
Debt Instrument [Line Items]
Secured debt	$ 0.0		10.0
Stated interest rate, percentage	7.83%			8.75%
Senior secured notes, 7.5% \| Senior notes
Debt Instrument [Line Items]
Stated interest rate, percentage	7.50%
Secured long-term debt	$ 0.0		248.1
Senior convertible notes, 2.75% \| Senior notes
Debt Instrument [Line Items]
Financing fees	$ 0.9
Stated interest rate, percentage	2.75%				2.75%
2.75% convertible senior notes due May 1, 2028, net of financing fees of $0.9 million at September 30, 2025, interest payable semiannually	$ 85.4		85.1
Senior Secured Notes, 6.875% \| Senior notes
Debt Instrument [Line Items]
Financing fees	$ 6.1
Stated interest rate, percentage	6.875%	6.875%
Secured long-term debt	$ 393.8		$ 0.0
Industrial Revenue Bonds Due 2028
Debt Instrument [Line Items]
Stated interest rate, percentage	3.02%

Debt Instrument, Variable Interest Rate, Maximum Percentage

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CENTURY ALUMINUM CO 10-Q