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Retirement Benefits (Tables)
 6 Months Ended
Jun. 30, 2024
Retirement Benefits [Abstract]  
Components of Net Periodic Benefit Cost (Credit)
The components of the net periodic pension benefit cost were as follows:
Three Months EndedSix Months Ended
June 30, 2024June 30, 2023June 30, 2024June 30, 2023
 (Dollars in thousands)
Service cost$2,164 $2,234 $4,329 $4,463 
Interest cost8,408 8,724 16,821 17,433 
Expected return on plan assets(10,771)(10,187)(21,542)(20,376)
Amortization of prior service cost 23 25 46 53 
Amortization of actuarial losses1,853 2,939 3,705 5,884 
Net periodic benefit cost $1,677 $3,735 $3,359 $7,457 
 
The components of the net periodic other postretirement benefit cost (credit) were as follows:
Three Months EndedSix Months Ended
June 30, 2024June 30, 2023June 30, 2024June 30, 2023
(Dollars in thousands)
Service cost$7 $12 $14 $26 
Interest cost167 189 332 378 
Amortization of prior service credit(5)(233)(10)(468)
Amortization of actuarial gains(84)(162)(168)(323)
Net periodic benefit cost (credit)$85 $(194)$168 $(387)