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Related Party Transactions - Narrative (Details) € in Millions, $ in Millions					1 Months Ended			12 Months Ended
	Oct. 01, 2024 USD ($)	Aug. 30, 2024 EUR (€) carbon_credit	Dec. 18, 2023 EUR (€) carbon_credit € / carbonCredit	Sep. 28, 2023 EUR (€) carbon_credit € / carbonCredit	Dec. 31, 2024 EUR (€) carbon_credit € / carbonCredit	Aug. 31, 2024 EUR (€) carbon_credit € / carbonCredit	Mar. 31, 2024 EUR (€) € / carbonCredit	Dec. 31, 2024 USD ($) carbon_credit € / carbonCredit	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Related Party Transaction [Line Items]
Total net sales | $								$ 2,220.3	$ 2,185.4	$ 2,777.3
Vlissingen Facility Agreement | Affiliated Entity | Vlissingen
Related Party Transaction [Line Items]
Term extension	2 years
Credit facility face amount | $	$ 90.0
Stated interest rate, percentage	8.75%
Debt instrument, basis spread on variable rate	3.687%
Vlissingen Facility Agreement | Affiliated Entity | Vlissingen | Maximum
Related Party Transaction [Line Items]
Variable interest rate	9.00%
Vlissingen Facility Agreement | Affiliated Entity | Vlissingen | Minimum
Related Party Transaction [Line Items]
Variable interest rate	7.00%
Carbon Credit Sale | Affiliated Entity | Nordural Grundartangi ehf
Related Party Transaction [Line Items]
Carbon credit sold in transaction | carbon_credit			40,000	390,000	370,700	370,700		370,700
Sale price (Euro per carbon credit) | € / carbonCredit			69.30	82.18
Aggregate amount				€ 32.1
Carbon Credit Repurchase | Affiliated Entity | Nordural Grundartangi ehf
Related Party Transaction [Line Items]
Sale price (Euro per carbon credit) | € / carbonCredit			85.13	83.72	69.16	71.20	87.01	69.16
Aggregate amount			€ 33.2	€ 32.7	€ 25.6	€ 26.4	€ 33.9
Additional Carbon Credit Repurchase | Affiliated Entity | Nordural Grundartangi ehf
Related Party Transaction [Line Items]
Sale price (Euro per carbon credit) | € / carbonCredit			70.71
Aggregate amount			€ 2.8
Second Repurchase Agreement, Carbon Credit Repurchase | Affiliated Entity | Nordural Grundartangi ehf
Related Party Transaction [Line Items]
Sale price (Euro per carbon credit) | € / carbonCredit							72.59
Aggregate amount							€ 2.9
Repurchase Agreement, Carbon Credits Settled | Affiliated Entity | Nordural Grundartangi ehf
Related Party Transaction [Line Items]
Carbon credit sold in transaction | carbon_credit		59,300
Aggregate amount		€ 11.1
Glencore | Supply Commitment
Related Party Transaction [Line Items]
Total net sales | $								$ 191.3	$ 191.7	$ 24.9
Glencore | Sales Revenue | Customer Concentration Risk
Related Party Transaction [Line Items]
Concentration risk, percentage								59.10%	73.80%	60.20%
Century Aluminum
Related Party Transaction [Line Items]
Glencore beneficial ownership					42.90%			42.90%
Glencore economic ownership					45.80%			45.80%

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