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Related Party Transactions - Narrative (Details) € in Millions, $ in Millions		1 Months Ended				3 Months Ended
	Oct. 01, 2024 USD ($)	Dec. 31, 2024 EUR (€) carbon_credit € / carbonCredit	Aug. 31, 2024 EUR (€) carbon_credit € / carbonCredit	Dec. 31, 2023 EUR (€) carbon_credit € / carbonCredit	Sep. 30, 2023 EUR (€) carbon_credit € / carbonCredit	Mar. 31, 2025 USD ($)	Mar. 31, 2024 USD ($)	Aug. 30, 2024 carbon_credit
Related Party Transaction [Line Items]
Net sales | $						$ 633.9	$ 489.5
Vlissingen Credit Facility | Affiliated Entity | Vlissingen
Related Party Transaction [Line Items]
Term extension	2 years
Credit facility maximum amount | $	$ 90.0
Stated interest rate, percentage	8.75%
Debt instrument, basis spread on variable rate	3.687%
Vlissingen Credit Facility | Affiliated Entity | Vlissingen | Maximum
Related Party Transaction [Line Items]
Variable interest rate	9.00%
Vlissingen Credit Facility | Affiliated Entity | Vlissingen | Minimum
Related Party Transaction [Line Items]
Variable interest rate	7.00%
Carbon Credit Sale | Affiliated Entity | Grundartangi
Related Party Transaction [Line Items]
Carbon credit sold in transaction		370,700			390,000
Sale price (Euro per carbon credit) | € / carbonCredit					82.18
Aggregate amount | €					€ 32.1
Carbon Credit Repurchase | Affiliated Entity | Grundartangi
Related Party Transaction [Line Items]
Sale price (Euro per carbon credit) | € / carbonCredit		69.16			83.72
Aggregate amount | €		€ 25.6			€ 32.7
Repurchase Agreement, Carbon Credits Settled | Affiliated Entity | Grundartangi
Related Party Transaction [Line Items]
Carbon credit sold in transaction		19,300
Second Repurchase Agreement | Affiliated Entity | Grundartangi
Related Party Transaction [Line Items]
Carbon credit sold in transaction			59,300
Sale price (Euro per carbon credit) | € / carbonCredit			186.74
Aggregate amount | €			€ 11.1
Carbon Credit Sale | Affiliated Entity | Grundartangi
Related Party Transaction [Line Items]
Carbon credit sold in transaction				40,000
Sale price (Euro per carbon credit) | € / carbonCredit				69.30
Carbon Credit Repurchase | Affiliated Entity | Grundartangi
Related Party Transaction [Line Items]
Sale price (Euro per carbon credit) | € / carbonCredit				70.71
Aggregate amount | €				€ 2.8
Carbon Credit Increase | Affiliated Entity | Grundartangi
Related Party Transaction [Line Items]
Carbon credit sold in transaction			19,300
Carbon Credit Settled | Affiliated Entity | Grundartangi
Related Party Transaction [Line Items]
Carbon credit sold in transaction								59,300
Glencore | Supply Commitment
Related Party Transaction [Line Items]
Net sales | $						$ 78.4	$ 35.5
Glencore | Consolidated sales | Customer concentration risk
Related Party Transaction [Line Items]
Major customer, percentage of revenue, net (percent)						59.70%	64.30%
Century Aluminum | Glencore
Related Party Transaction [Line Items]
Ownership percentage by noncontrolling owners						42.90%
Economic ownership percentage by related party						45.80%

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