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Retirement Benefits | 9 Months Ended
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Sep. 30, 2021
Retirement Benefits [Abstract] |
Retirement Benefits | Retirement Benefits The components of the net periodic pension benefit credit were as follows:
| Three Months Ended |  | Nine Months Ended
| Sept. 30, 2021 |  | Sept. 30, 2020 |  | Sept. 30, 2021 |  | Sept. 30, 2020
| (Dollars in thousands)
Service cost | $ | 3,177 |  |  | $ | 2,983 |  |  | $ | 10,754 |  |  | $ | 9,958 |
Interest cost | 4,579 |  |  | 5,816 |  |  | 13,288 |  |  | 17,236 |
Expected return on plan assets | (19,892) |  |  | (18,104) |  |  | (59,590) |  |  | (54,091) |
Amortization of prior service cost | 67 |  |  | 43 |  |  | 183 |  |  | 154 |
Amortization of actuarial losses | 3,464 |  |  | 2,993 |  |  | 9,518 |  |  | 8,862 |
Net periodic benefit credit | $ | (8,605) |  |  | $ | (6,269) |  |  | $ | (25,847) |  |  | $ | (17,881) |
The components of the net periodic other postretirement benefit credit were as follows:
| Three Months Ended |  | Nine Months Ended
| Sept. 30, 2021 |  | Sept. 30, 2020 |  | Sept. 30, 2021 |  | Sept. 30, 2020
| (Dollars in thousands)
Service cost | $ | 25 |  |  | $ | 18 |  |  | $ | 81 |  |  | $ | 66 |
Interest cost | 82 |  |  | 142 |  |  | 272 |  |  | 425 |
Amortization of prior service credit | (460) |  |  | (487) |  |  | (1,373) |  |  | (1,454) |
Amortization of actuarial gains | (137) |  |  | (115) |  |  | (232) |  |  | (254) |
Net periodic benefit credit | $ | (490) |  |  | $ | (442) |  |  | $ | (1,252) |  |  | $ | (1,217) |

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