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EMPLOYEE BENEFIT PLANS EMPLOYEE BENEFIT PLANS (Notes)
 6 Months Ended
Jun. 30, 2018
Defined Benefit Plan [Abstract]  
EMPLOYEE BENEFIT PLANS
EMPLOYEE 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