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Fair Value Measurements (Tables) | 6 Months Ended
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Jun. 30, 2025
Fair Value Disclosures [Abstract] |
Schedule of financial assets and liabilities at fair value on a recurring basis |
Recurring Fair Value Measurements |  | As of June 30, 2025
|  | Level 1 |  | Level 2 |  | Level 3 |  | Total
ASSETS: |  |  |  |  |  |  |  |
Cash equivalents |  | $ | 2.8 |  |  | $ | — |  |  | $ | — |  |  | $ | 2.8 |
Trust assets(1) |  | 1.1 |  |  | — |  |  | — |  |  | 1.1 |
Derivative instruments |  | — |  |  | 4.6 |  |  | — |  |  | 4.6 |
TOTAL |  | $ | 3.9 |  |  | $ | 4.6 |  |  | $ | — |  |  | $ | 8.5 |
LIABILITIES: |  |  |  |  |  |  |  |
Derivative instruments |  | $ | — |  |  | $ | 19.8 |  |  | $ | — |  |  | $ | 19.8 |
TOTAL |  | $ | — |  |  | $ | 19.8 |  |  | $ | — |  |  | $ | 19.8 |
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 |
(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 or inputs 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
Heavy Fuel Oil ("HFO") price swaps |  | Discounted cash flows |  | Quoted HFO forward market

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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/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 Reference 2: 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 3: 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 + 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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