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Postretirement Benefit Plans (Tables) | 9 Months Ended
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Sep. 30, 2025
Retirement Benefits [Abstract] |
Schedule of Debt Securities, Trading, and Equity Securities, FV-NI | The following table summarizes, by major security type, our marketable securities that are measured at fair value on a recurring basis and are categorized using the fair value hierarchy:
| Fair Value of Deferred Compensation Plan as of September 30, 2025
(Dollars in millions) | Adjusted Cost |  | Unrealized Gains |  | Unrealized Losses |  | Fair Value
Level 1 Securities: |  |  |  |  |  |  |
Mutual funds | $ | 4.0 |  |  | $ | 0.9 |  |  | $ | — |  |  | $ | 4.9 |
Level 2 Securities: |  |  |  |  |  |  |
Fixed income funds | $ | 0.1 |  |  | $ | — |  |  | $ | — |  |  | $ | 0.1 |
Total | $ | 4.1 |  |  | $ | 0.9 |  |  | $ | — |  |  | $ | 5.0 |
| Fair Value of Deferred Compensation Plan as of December 31, 2024
(Dollars in millions) | Adjusted Cost |  | Unrealized Gains |  | Unrealized Losses |  | Fair Value
Level 1 Securities: |  |  |  |  |  |  |
Mutual funds | $ | 4.6 |  |  | $ | 0.5 |  |  | $ | — |  |  | $ | 5.1 |
Level 2 Securities: |  |  |  |  |  |  |
Fixed income funds | $ | 0.2 |  |  | $ | — |  |  | $ | — |  |  | $ | 0.2 |
Total | $ | 4.8 |  |  | $ | 0.5 |  |  | $ | — |  |  | $ | 5.3 |

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- Definition Tabular disclosure of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6B -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-6B + Details
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