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Benefit Plans and Other Postretirement Benefits (Tables) | 9 Months Ended
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Sep. 30, 2024
Benefit Plans and Other Postretirement Benefits |
Schedule of components of net pension expense | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended September 30, | ​ | Nine Months Ended September 30,
​ |  | 2024 |  | 2023 |  | 2024 |  | 2023
Service cost |  | $ | 0.9 | ​ | $ | 1.8 | ​ | $ | 2.7 | ​ | $ | 5.5
Interest cost | ​ |  | 6.1 | ​ |  | 6.5 | ​ |  | 18.1 | ​ |  | 19.5
Expected return on plan assets | ​ |  | (7.2) | ​ |  | (7.3) | ​ |  | (21.5) | ​ |  | (21.8)
Amortization of prior service cost | ​ |  | 0.3 | ​ |  | 0.4 | ​ |  | 0.9 | ​ |  | 1.3
Amortization of net actuarial losses | ​ |  | 0.8 | ​ |  | 0.6 | ​ |  | 2.5 | ​ |  | 1.6
Net pension expense |  | $ | 0.9 | ​ | $ | 2.0 | ​ | $ | 2.7 | ​ | $ | 6.1

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- Definition Tabular disclosure of the components of net benefit costs for pension plans and/or other employee benefit plans including service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) recognized due to settlements or curtailments. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 715 -SubTopic 20 -Subparagraph (h) -Name Accounting Standards Codification -Paragraph 1 -Section 50 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
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