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Retirement Benefits
 3 Months Ended
Mar. 31, 2016
Compensation and Retirement Disclosure [Abstract]  
Retirement Benefits
Retirement Benefits

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

 
 
 
2016
 
2015
 
(Dollars in thousands)
Service cost
$
3,313

 
$
4,050

Interest cost
6,434

 
7,149

Expected return on plan assets
(14,583
)
 
(15,655
)
Amortization of prior service cost
151

 
246

Amortization of actuarial losses
2,083

 
1,833

Net periodic benefit credit
$
(2,602
)
 
$
(2,377
)
 
The components of the net periodic other postretirement benefits credit for the three months ended March 31 were as follows:
 
 
 
2016
 
2015
 
(Dollars in thousands)
Service cost
$
67

 
$
143

Interest cost
254

 
360

Amortization of prior service credit
(850
)
 
(736
)
Amortization of actuarial gains
(118
)
 
(64
)
Net periodic benefit credit
$
(647
)
 
$
(297
)