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Retirement Benefits
 6 Months Ended
Jun. 30, 2025
Retirement Benefits [Abstract]  
Retirement Benefits Retirement Benefits
The components of the net periodic pension benefit cost were as follows:
Three Months EndedSix Months Ended
June 30, 2025June 30, 2024June 30, 2025June 30, 2024
 (Dollars in thousands)
Service cost$1,968 $2,164 $3,912 $4,329 
Interest cost8,177 8,408 16,296 16,821 
Expected return on plan assets(10,256)(10,771)(20,513)(21,542)
Amortization of prior service cost 5 23 13 46 
Amortization of actuarial losses1,883 1,853 3,776 3,705 
Net periodic benefit cost $1,777 $1,677 $3,484 $3,359 
 
The components of the net periodic other postretirement benefit cost were as follows:
Three Months EndedSix Months Ended
June 30, 2025June 30, 2024June 30, 2025June 30, 2024
(Dollars in thousands)
Service cost$6 $7 $10 $14 
Interest cost151 167 302 332 
Amortization of prior service credit(14)(5)(29)(10)
Amortization of actuarial gains(86)(84)(172)(168)
Net periodic benefit cost $57 $85 $111 $168