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Retirement Benefits | 6 Months Ended
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Jun. 30, 2020
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, 2020 |  | June 30, 2019 |  | June 30, 2020 |  | June 30, 2019
| (Dollars in thousands) |  |  |  |  |  |
Service cost | $ | 3,489 |  |  | $ | 3,254 |  |  | $ | 6,975 |  |  | $ | 6,512 |
Interest cost | 5,710 |  |  | 7,043 |  |  | 11,420 |  |  | 14,092 |
Expected return on plan assets | (17,993) |  |  | (15,112) |  |  | (35,987) |  |  | (30,225) |
Amortization of prior service cost | 56 |  |  | 21 |  |  | 111 |  |  | 40 |
Amortization of actuarial losses | 2,934 |  |  | 4,121 |  |  | 5,869 |  |  | 8,240 |
Net periodic benefit credit | $ | (5,804) |  |  | $ | (673) |  |  | $ | (11,612) |  |  | $ | (1,341) |
The components of the net periodic other postretirement benefit credit were as follows:
| Three Months Ended |  |  |  | Six Months Ended |  |
| June 30, 2020 |  | June 30, 2019 |  | June 30, 2020 |  | June 30, 2019
| (Dollars in thousands) |  |  |  |  |  |
Service cost | $ | 25 |  |  | $ | 22 |  |  | $ | 48 |  |  | $ | 44 |
Interest cost | 141 |  |  | 184 |  |  | 283 |  |  | 369 |
Amortization of prior service credit | (484) |  |  | (581) |  |  | (967) |  |  | (1,163) |
Amortization of actuarial gains | (69) |  |  | (166) |  |  | (139) |  |  | (333) |
Net periodic benefit credit | $ | (387) |  |  | $ | (541) |  |  | $ | (775) |  |  | $ | (1,083) |

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