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Retirement Benefits
 6 Months Ended
Jun. 30, 2019
Retirement Benefits [Abstract]  
Retirement Benefits Retirement Benefits

The components of the net periodic pension benefit credit were as follows:

 
Three Months Ended
 
Six Months Ended
 
June 30,
2019
 
June 30,
2018
 
June 30,
2019
 
June 30,
2018
 
(Dollars in thousands)
Service cost
$
3,254

 
$
3,710

 
$
6,512

 
$
7,431

Interest cost
7,043

 
6,296

 
14,092

 
12,605

Expected return on plan assets
(15,112
)
 
(17,122
)
 
(30,225
)
 
(34,245
)
Amortization of prior service cost
21

 
34

 
40

 
69

Amortization of actuarial losses
4,121

 
1,787

 
8,240

 
3,573

Net periodic benefit credit
$
(673
)
 
$
(5,295
)
 
$
(1,341
)
 
$
(10,567
)
 

The components of the net periodic other postretirement benefit credit were as follows:
 
Three Months Ended
 
Six Months Ended
 
June 30,
2019
 
June 30,
2018
 
June 30,
2019
 
June 30,
2018
 
(Dollars in thousands)
Service cost
$
22

 
$
32

 
$
44

 
$
63

Interest cost
184

 
162

 
369

 
325

Amortization of prior service credit
(581
)
 
(650
)
 
(1,163
)
 
(1,299
)
Amortization of actuarial gains
(166
)
 
(119
)
 
(333
)
 
(238
)
Net periodic benefit credit
$
(541
)
 
$
(575
)
 
$
(1,083
)
 
$
(1,149
)