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Benefit Plans and Other Postretirement Benefits (Tables)
 3 Months Ended
Mar. 31, 2017
Compensation and Retirement Disclosure [Abstract]  
Components of Net Periodic Benefit Cost
Components of Net Periodic Benefit Cost
 
 
 
 
 
 
 
 
 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Three Months Ended March 31
 
 
2017
 
2016
 
2017
 
2016
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
1,154

 
$
1,104

 
$
7

 
$
6

Interest cost
 
1,554

 
1,704

 
148

 
163

Expected return on plan assets
 
(2,295
)
 
(2,289
)
 
(8
)
 
(6
)
Amortization of prior service cost (credit)
 
35

 
28

 
(88
)
 
(88
)
Amortization of net loss
 
1,462

 
1,348

 
109

 
83

Net benefit cost recognized for financial reporting
 
$
1,910

 
$
1,895

 
$
168

 
$
158