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Retirement Benefits | 6 Months Ended
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Jun. 30, 2021
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, 2021 |  | June 30, 2020 |  | June 30, 2021 |  | June 30, 2020
| (Dollars in thousands)
Service cost | $ | 3,789 |  |  | $ | 3,489 |  |  | $ | 7,577 |  |  | $ | 6,975 |
Interest cost | 4,355 |  |  | 5,710 |  |  | 8,709 |  |  | 11,420 |
Expected return on plan assets | (19,850) |  |  | (17,993) |  |  | (39,698) |  |  | (35,987) |
Amortization of prior service cost | 58 |  |  | 56 |  |  | 116 |  |  | 111 |
Amortization of actuarial losses | 3,028 |  |  | 2,934 |  |  | 6,054 |  |  | 5,869 |
Net periodic benefit credit | $ | (8,620) |  |  | $ | (5,804) |  |  | $ | (17,242) |  |  | $ | (11,612) |
The components of the net periodic other postretirement benefit credit were as follows:
| Three Months Ended |  | Six Months Ended
| June 30, 2021 |  | June 30, 2020 |  | June 30, 2021 |  | June 30, 2020
| (Dollars in thousands)
Service cost | $ | 28 |  |  | $ | 25 |  |  | $ | 56 |  |  | $ | 48 |
Interest cost | 95 |  |  | 141 |  |  | 190 |  |  | 283 |
Amortization of prior service credit | (457) |  |  | (484) |  |  | (913) |  |  | (967) |
Amortization of actuarial gains | (47) |  |  | (69) |  |  | (95) |  |  | (139) |
Net periodic benefit credit | $ | (381) |  |  | $ | (387) |  |  | $ | (762) |  |  | $ | (775) |

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