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Pension Benefits and Other Postretirement Benefit Plans (Tables) | 6 Months Ended
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Jun. 30, 2017
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract] |
Components of net periodic benefit cost | The components of net periodic (benefit) cost for the periods indicated were:
(Dollars in thousands) | Pension Benefits |  | Retirement Health and Life Insurance Benefits
Quarter Ended |  | Six Months Ended |  | Quarter Ended |  | Six Months Ended
Change in benefit obligation: | June 30, 2017 |  | June 30, 2016 |  | June 30, 2017 |  | June 30, 2016 |  | June 30, 2017 |  | June 30, 2016 |  | June 30, 2017 |  | June 30, 2016
Service cost | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | 17 |  |  | $ | 37 |  |  | $ | 56 |  |  | $ | 74 |
Interest cost | 1,841 |  |  | 1,893 |  |  | 3,682 |  |  | 3,786 |  |  | 13 |  |  | 19 |  |  | 31 |  |  | 38 |
Expected return on plan assets | (2,309 | ) |  | (2,706 | ) |  | (4,618 | ) |  | (5,412 | ) |  | — |  |  | — |  |  | — |  |  | — |
Amortization of prior service cost (credit) | — |  |  | — |  |  | — |  |  | — |  |  | (407 | ) |  | (373 | ) |  | (780 | ) |  | (746 | )
Amortization of net loss (gain) | 433 |  |  | 447 |  |  | 866 |  |  | 894 |  |  | (17 | ) |  | (19 | ) |  | (31 | ) |  | (38 | )
Net periodic (benefit) cost | $ | (35 | ) |  | $ | (366 | ) |  | $ | (70 | ) |  | $ | (732 | ) |  | $ | (394 | ) |  | $ | (336 | ) |  | $ | (724 | ) |  | $ | (672 | )

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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=65897934&loc=d3e1928-114920 + Details
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