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Fair Value Measurements (Tables) | 12 Months Ended
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Dec. 31, 2024
Fair Value Disclosures [Abstract] |
Schedule of financial assets and liabilities at fair value on a recurring basis |
Recurring Fair Value Measurements |  | As of December 31, 2024
|  | Level 1 |  | Level 2 |  | Level 3 |  | Total
ASSETS: |  |  |  |  |  |  |  |
Cash equivalents |  | $ | 7.9 |  |  | $ | — |  |  | $ | — |  |  | $ | 7.9 |
Trust assets (1) |  | 0.3 |  | — |  |  | — |  |  | 0.3
Derivative instruments |  | — |  |  | 4.5 |  |  | — |  |  | 4.5 |
TOTAL |  | $ | 8.2 |  |  | $ | 4.5 |  |  | $ | — |  |  | $ | 12.7 |
LIABILITIES: |  |  |  |  |  |  |  |
Derivative instruments |  | — |  |  | 4.4 |  |  | — |  |  | 4.4 |
TOTAL |  | $ | — |  |  | $ | 4.4 |  |  | $ | — |  |  | $ | 4.4 |
Recurring Fair Value Measurements |  | As of December 31, 2023
|  | Level 1 |  | Level 2 |  | Level 3 |  | Total
ASSETS: |  |  |  |  |  |  |  |
Cash equivalents |  | $ | 16.8 |  |  | $ | — |  |  | $ | — |  |  | $ | 16.8 |
Trust assets (1) |  | 0.2 |  | — |  |  | — |  |  | 0.2 |
Derivative instruments |  | — |  |  | 2.9 |  |  | — |  |  | 2.9 |
TOTAL |  | $ | 17.0 |  |  | $ | 2.9 |  |  | $ | — |  |  | $ | 19.9 |
LIABILITIES: |  |  |  |  |  |  |  |
Derivative instruments |  | — |  |  | 7.9 |  |  | — |  |  | 7.9 |
TOTAL |  | $ | — |  |  | $ | 7.9 |  |  | $ | — |  |  | $ | 7.9 |
(1)Trust assets are currently invested in money market funds. These trust assets are held to fund the non-qualified supplemental executive pension benefit obligations for certain of our officers.
Schedule of valuation methodology for assets and liabilities at fair value | The following section describes the valuation techniques and inputs used for fair value measurements categorized within Level 2 of the fair value hierarchy:
Level 2 Fair Value Measurements:
Asset / Liability |  | Valuation Techniques |  | Inputs
LME forward financial sales contracts |  | Discounted cash flows |  | Quoted LME forward market, Secured Overnight Financing Rate ("SOFR") discount rate
Midwest Premium ("MWP") forward financial sales contracts |  | Discounted cash flows |  | Quoted MWP forward market, SOFR discount rate
Fixed for floating swaps |  | Discounted cash flows |  | Quoted LME forward market, quoted MWP forward market
Indiana Hub power price swaps |  | Discounted cash flows |  | Quoted Indiana Hub forward market, SOFR discount rate
FX swaps |  | Discounted cash flows |  | Euro/USD forward exchange rate
Casthouse currency hedges |  | Discounted cash flows |  | Euro/USD forward exchange rate; ISK/USD forward exchange rate
Heavy Fuel Oil ("HFO") price swaps |  | Discounted cash flows |  | Quoted HFO forward market
Schedule of fair value reconciliation of Level 3 assets and liabilities measured at fair value | The following table presents the fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis.
For the twelve months ended December 31, 2023 and 2024 |  | Level 3 Assets |  | Level 3 Liabilities
| LME forward financial sales contracts |  | LME forward financial sales contracts |  |
Balance as of January 1, 2023 |  | $ | 1.8 |  |  | $ | (4.6) |  |  |
Transfers out of Level 3 (1) |  | (1.8) |  |  | 4.6 |  |  |
Balance as of December 31, 2023 |  | $ | — |  |  | $ | — |  |  |
Transactions during 2024 |  | $ | — |  |  | $ | — |  |  |
Balance as of December 31, 2024 |  | $ | — |  |  | $ | — |  |  |
Change in unrealized gains (losses) 2023 (2) |  | $ | — |  |  | $ | — |  |  |
Change in unrealized gains (losses) 2024 (2) |  | $ | — |  |  | $ | — |  |  |
(1)Transfer out of Level 3 due to period of time remaining in derivative contract. (2)Gains and losses are presented in the Consolidated Statements of Operations within the line item "Net gain (loss) on forward and derivative contracts."

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- Definition Tabular disclosure of input and valuation technique used to measure fair value and change in valuation approach and technique for each separate class of asset and liability measured on recurring and nonrecurring basis. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 103 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-103 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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- Definition Tabular disclosure of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset) using recurring unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-3 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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- Definition Tabular disclosure of assets and liabilities, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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