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Retirement Benefits (Tables)
 3 Months Ended
Mar. 31, 2019
Retirement Benefits [Abstract]  
Components of Net Periodic Benefit Cost

The components of the net periodic pension benefit credit for the three months ended March 31 were as follows:

 
2019
 
2018
 
(Dollars in thousands)
Service cost
$
3,258

 
$
3,721

Interest cost
7,049

 
6,309

Expected return on plan assets
(15,113
)
 
(17,123
)
Amortization of prior service cost
19

 
35

Amortization of actuarial losses
4,119

 
1,786

Net periodic benefit credit
$
(668
)
 
$
(5,272
)
 

The components of the net periodic other postretirement benefit credit for the three months ended March 31 were as follows:
 
2019
 
2018
 
(Dollars in thousands)
Service cost
$
22

 
$
31

Interest cost
185

 
163

Amortization of prior service credit
(582
)
 
(649
)
Amortization of actuarial gains
(167
)
 
(119
)
Net periodic benefit credit
$
(542
)
 
$
(574
)