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Retirement Benefits
 3 Months Ended
Mar. 31, 2018
Retirement Benefits [Abstract]  
Retirement Benefits
Retirement Benefits

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

 
2018
 
2017
 
(Dollars in thousands)
Service cost
$
3,721

 
$
3,168

Interest cost
6,309

 
6,270

Expected return on plan assets
(17,123
)
 
(15,713
)
Amortization of prior service cost
35

 
80

Amortization of actuarial losses
1,786

 
1,853

Net periodic benefit credit
$
(5,272
)
 
$
(4,342
)
 
The components of the net periodic other postretirement benefit credit for the three months ended March 31 were as follows:
 
 
 
2018
 
2017
 
(Dollars in thousands)
Service cost
$
31

 
$
36

Interest cost
163

 
176

Amortization of prior service credit
(649
)
 
(853
)
Amortization of actuarial gains
(119
)
 
(139
)
Net periodic benefit credit
$
(574
)
 
$
(780
)