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Retirement Benefits
 3 Months Ended
Mar. 31, 2024
Retirement Benefits [Abstract]  
Retirement Benefits Retirement Benefits
The components of the net periodic pension benefit cost for the three months ended March 31 were as follows:
20242023
 (Dollars in thousands)
Service cost$2,165 $2,229 
Interest cost8,413 8,709 
Expected return on plan assets(10,771)(10,189)
Amortization of prior service cost 23 28 
Amortization of actuarial losses1,852 2,945 
Net periodic benefit cost $1,682 $3,722 
 
The components of the net periodic other postretirement benefit cost (credit) for the three months ended March 31 were as follows:
20242023
(Dollars in thousands)
Service cost$7 $14 
Interest cost165 189 
Amortization of prior service credit(5)(235)
Amortization of actuarial gains(84)(161)
Net periodic benefit cost (credit)$83 $(193)