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Related Party Transactions - Narrative (Details) € in Millions, $ in Millions |  | 1 Months Ended | 3 Months Ended | 9 Months Ended |
--- | --- | --- | --- | --- | ---
Oct. 01, 2024 USD ($) | Dec. 31, 2024 EUR (€) carbon_credit € / carbonCredit | Aug. 31, 2024 EUR (€) carbon_credit € / carbonCredit | Dec. 31, 2023 EUR (€) carbon_credit € / carbonCredit | Sep. 30, 2023 EUR (€) carbon_credit € / carbonCredit | Sep. 30, 2025 USD ($) | Sep. 30, 2024 USD ($) | Sep. 30, 2025 USD ($) | Sep. 30, 2024 USD ($) | Aug. 31, 2025 carbon_credit
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |
Net sales | $ |  |  |  |  |  | $ 632.2 | $ 539.1 | $ 1,894.2 | $ 1,589.4 |
Vlissingen | Vlissingen Credit Facility | Affiliated Entity |  |  |  |  |  |  |  |  |  |
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 | Vlissingen Credit Facility | Affiliated Entity | Maximum |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |
Variable interest rate | 9.00% |  |  |  |  |  |  |  |  |
Vlissingen | Vlissingen Credit Facility | Affiliated Entity | Minimum |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |
Variable interest rate | 7.00% |  |  |  |  |  |  |  |  |
Grundartangi | Carbon Credit Sale | Affiliated Entity |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |
Grundartangi | Carbon Credit Repurchase | Affiliated Entity |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |
Sale price (Euro per carbon credit) | € / carbonCredit |  | 69.16 |  |  | 83.72 |  |  |  |  |
Aggregate amount | € |  | € 25.6 |  |  | € 32.7 |  |  |  |  |
Grundartangi | Repurchase Agreement, Carbon Credits Settled | Affiliated Entity |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |
Carbon credit sold in transaction |  | 19,300 |  |  |  |  |  |  |  | 370,700
Grundartangi | Second Repurchase Agreement | Affiliated Entity |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |
Grundartangi | Carbon Credit Sale | Affiliated Entity |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |
Carbon credit sold in transaction |  |  |  | 40,000 |  |  |  |  |  |
Sale price (Euro per carbon credit) | € / carbonCredit |  |  |  | 69.30 |  |  |  |  |  |
Grundartangi | Carbon Credit Repurchase | Affiliated Entity |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |
Sale price (Euro per carbon credit) | € / carbonCredit |  |  |  | 70.71 |  |  |  |  |  |
Aggregate amount | € |  |  |  | € 2.8 |  |  |  |  |  |
Grundartangi | Carbon Credit Increase | Affiliated Entity |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |
Carbon credit sold in transaction |  |  | 19,300 |  |  |  |  |  |  |
Grundartangi | Carbon Credit Settled | Affiliated Entity |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |
Carbon credit sold in transaction |  |  | 59,300 |  |  |  |  |  |  |
Glencore | Supply Commitment |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |
Net sales | $ |  |  |  |  |  | $ 34.8 | $ 31.5 | $ 161.2 | $ 117.3 |
Glencore | Consolidated sales | Customer concentration risk |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |
Major customer, percentage of revenue, net (percent) |  |  |  |  |  | 50.60% | 61.60% | 56.30% | 61.00% |
Century Aluminum | Glencore |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |
Ownership percentage by noncontrolling owners |  |  |  |  |  | 42.90% |  | 42.90% |  |
Economic ownership percentage by related party |  |  |  |  |  | 45.80% |  | 45.80% |  |

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