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Retirement Benefits (Tables) | 9 Months Ended
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Sep. 30, 2017
Retirement Benefits [Abstract] |
Components of Net Periodic Benefit Cost | The components of the net periodic pension benefit credit were as follows:
| Three Months Ended |  | Nine Months Ended
| Sept. 30, 2017 |  | Sept. 30, 2016 |  | Sept. 30, 2017 |  | Sept. 30, 2016
| (Dollars in thousands)
Service cost | $ | 3,532 |  |  | $ | 2,950 |  |  | $ | 9,967 |  |  | $ | 9,584 |
Interest cost | 6,482 |  |  | 6,473 |  |  | 19,114 |  |  | 19,351 |
Expected return on plan assets | (15,832 | ) |  | (14,653 | ) |  | (47,258 | ) |  | (43,819 | )
Amortization of prior service cost | 75 |  |  | 113 |  |  | 235 |  |  | 415 |
Amortization of actuarial losses | 1,501 |  |  | 2,326 |  |  | 5,208 |  |  | 6,493 |
Special termination benefits | — |  |  | — |  |  | — |  |  | 2,900 |
Curtailment loss | — |  |  | — |  |  | — |  |  | 180 |
Net periodic benefit credit | $ | (4,242 | ) |  | $ | (2,791 | ) |  | $ | (12,734 | ) |  | $ | (4,896 | )
The components of the net periodic other postretirement benefit credit were as follows:
| Three Months Ended |  | Nine Months Ended
| Sept. 30, 2017 |  | Sept. 30, 2016 |  | Sept. 30, 2017 |  | Sept. 30, 2016
| (Dollars in thousands)
Service cost | $ | 9 |  |  | $ | 52 |  |  | $ | 79 |  |  | $ | 188 |
Interest cost | 168 |  |  | 230 |  |  | 520 |  |  | 737 |
Amortization of prior service credit | (857 | ) |  | (816 | ) |  | (2,564 | ) |  | (2,516 | )
Amortization of actuarial gains | (172 | ) |  | (187 | ) |  | (447 | ) |  | (475 | )
Net periodic benefit credit | $ | (852 | ) |  | $ | (721 | ) |  | $ | (2,412 | ) |  | $ | (2,066 | )

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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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