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Retirement Benefits | 6 Months Ended
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Jun. 30, 2019
Retirement Benefits [Abstract] |
Retirement Benefits | Retirement Benefits The components of the net periodic pension benefit credit were as follows:
| Three Months Ended |  | Six Months Ended
| June 30, 2019 |  | June 30, 2018 |  | June 30, 2019 |  | June 30, 2018
| (Dollars in thousands)
Service cost | $ | 3,254 |  |  | $ | 3,710 |  |  | $ | 6,512 |  |  | $ | 7,431 |
Interest cost | 7,043 |  |  | 6,296 |  |  | 14,092 |  |  | 12,605 |
Expected return on plan assets | (15,112 | ) |  | (17,122 | ) |  | (30,225 | ) |  | (34,245 | )
Amortization of prior service cost | 21 |  |  | 34 |  |  | 40 |  |  | 69 |
Amortization of actuarial losses | 4,121 |  |  | 1,787 |  |  | 8,240 |  |  | 3,573 |
Net periodic benefit credit | $ | (673 | ) |  | $ | (5,295 | ) |  | $ | (1,341 | ) |  | $ | (10,567 | )
The components of the net periodic other postretirement benefit credit were as follows:
| Three Months Ended |  | Six Months Ended
| June 30, 2019 |  | June 30, 2018 |  | June 30, 2019 |  | June 30, 2018
| (Dollars in thousands)
Service cost | $ | 22 |  |  | $ | 32 |  |  | $ | 44 |  |  | $ | 63 |
Interest cost | 184 |  |  | 162 |  |  | 369 |  |  | 325 |
Amortization of prior service credit | (581 | ) |  | (650 | ) |  | (1,163 | ) |  | (1,299 | )
Amortization of actuarial gains | (166 | ) |  | (119 | ) |  | (333 | ) |  | (238 | )
Net periodic benefit credit | $ | (541 | ) |  | $ | (575 | ) |  | $ | (1,083 | ) |  | $ | (1,149 | )

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- Definition The entire disclosure for pension and other postretirement benefits. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -URI http://asc.fasb.org/topic&trid=2235017 + Details
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