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Pension and Postretirement Benefit Costs (Tables) | 6 Months Ended
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Jun. 30, 2019
Defined benefit pension plans |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |
Schedule of Net Benefit Costs | Net periodic pension cost (benefit) included the following components:
| Three Months Ended June 30, |  | Six Months Ended June 30,
| 2019 |  | 2018 |  | 2019 |  | 2018
| (Dollars in millions)
Service cost for benefits earned | $ | 0.5 |  |  | $ | 0.5 |  |  | $ | 1.0 |  |  | $ | 1.1 |
Interest cost on projected benefit obligation | 8.4 |  |  | 7.9 |  |  | 16.7 |  |  | 15.7 |
Expected return on plan assets | (7.8 | ) |  | (10.7 | ) |  | (15.6 | ) |  | (21.4 | )
Net periodic pension cost (benefit) | $ | 1.1 |  |  | $ | (2.3 | ) |  | $ | 2.1 |  |  | $ | (4.6 | )
Postretirement benefit plans |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |
Schedule of Net Benefit Costs | Net periodic postretirement benefit cost included the following components:
| Three Months Ended June 30, |  | Six Months Ended June 30,
| 2019 |  | 2018 |  | 2019 |  | 2018
| (Dollars in millions)
Service cost for benefits earned | $ | 1.2 |  |  | $ | 2.1 |  |  | $ | 2.4 |  |  | $ | 4.1 |
Interest cost on accumulated postretirement benefit obligation | 6.3 |  |  | 7.1 |  |  | 12.6 |  |  | 14.2 |
Expected return on plan assets | (0.1 | ) |  | — |  |  | (0.2 | ) |  | — |
Amortization of prior service credit | (2.2 | ) |  | — |  |  | (4.4 | ) |  | — |
Net periodic postretirement benefit cost | $ | 5.2 |  |  | $ | 9.2 |  |  | $ | 10.4 |  |  | $ | 18.3 |

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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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 -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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