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Retirement Benefits (Tables)
 3 Months Ended
Mar. 31, 2017
Compensation and Retirement Disclosure [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:

 
 
 
2017
 
2016
 
(Dollars in thousands)
Service cost
$
3,168

 
$
3,313

Interest cost
6,270

 
6,434

Expected return on plan assets
(15,713
)
 
(14,583
)
Amortization of prior service cost
80

 
151

Amortization of actuarial losses
1,853

 
2,083

Net periodic benefit credit
$
(4,342
)
 
$
(2,602
)
 
The components of the net periodic other postretirement benefit credit for the three months ended March 31 were as follows:
 
 
 
2017
 
2016
 
(Dollars in thousands)
Service cost
$
36

 
$
67

Interest cost
176

 
254

Amortization of prior service credit
(853
)
 
(850
)
Amortization of actuarial gains
(139
)
 
(118
)
Net periodic benefit credit
$
(780
)
 
$
(647
)