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EMPLOYEE BENEFIT PLANS EMPLOYEE BENEFIT PLANS (Tables)
 6 Months Ended
Jun. 30, 2018
Defined Benefit Plan [Abstract]  
Components of net periodic benefit cost for pension and postretirement benefit plans
The following tables set forth the components of net periodic benefit cost for our pension and postretirement benefit plans for the periods indicated:
 
Pension Benefits
 
Three Months Ended
 
Six Months Ended
 
June 30,
 
June 30,
 
2018
 
2017
 
2018
 
2017
 
(Thousands of dollars)
Components of net periodic benefit cost
 
 
 
 
 
 
 
Service cost
$
1,832

 
$
1,722

 
$
3,664

 
$
3,444

Interest cost
4,408

 
4,655

 
8,816

 
9,310

Expected return on plan assets
(5,969
)
 
(5,336
)
 
(11,938
)
 
(10,672
)
Amortization of net loss
4,258

 
3,392

 
8,516

 
6,784

Net periodic benefit cost
$
4,529

 
$
4,433

 
$
9,058

 
$
8,866


 
Postretirement Benefits
 
Three Months Ended
 
Six Months Ended
 
June 30,
 
June 30,
 
2018
 
2017
 
2018
 
2017
 
(Thousands of dollars)
Components of net periodic benefit cost
 
 
 
 
 
 
 
Service cost
$
211

 
$
165

 
$
422

 
$
330

Interest cost
527

 
565

 
1,054

 
1,130

Expected return on plan assets
(672
)
 
(564
)
 
(1,344
)
 
(1,128
)
Amortization of prior service credit
(415
)
 
(415
)
 
(830
)
 
(830
)
Amortization of net loss
334

 
420

 
668

 
840

Net periodic benefit cost (income)
$
(15
)
 
$
171

 
$
(30
)
 
$
342