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EMPLOYEE BENEFIT PLANS (Notes)
 3 Months Ended
Mar. 31, 2017
Employee Benefit Plans [Line Items]  
EMPLOYEE BENEFIT PLANS
8.
EMPLOYEE BENEFIT PLANS

The following tables set forth the components of net periodic benefit cost for our pension and other postemployment benefit plans for the periods indicated:
 
Pension Benefits
 
Three Months Ended
 
March 31,
 
2017
2016
 
(Thousands of dollars)
Components of net periodic benefit cost
 
 
Service cost
$
3,044

$
3,014

Interest cost
10,113

11,387

Expected return on assets
(14,624
)
(15,296
)
Amortization of net loss
9,027

8,886

Net periodic benefit cost
$
7,560

$
7,991


 
Other Postemployment Benefits
 
Three Months Ended
 
March 31,
 
2017
2016
 
(Thousands of dollars)
Components of net periodic benefit cost
 
 
Service cost
$
627

$
638

Interest cost
2,472

2,627

Expected return on assets
(3,147
)
(3,071
)
Amortization of unrecognized prior service cost
(1,149
)
(908
)
Amortization of net loss
1,621

1,151

Net periodic benefit cost
$
424

$
437



We recover qualified pension benefit plan and other postemployment benefit plan costs through rates charged to our customers. Certain utility commissions require that the recovery of these costs be based on specific guidelines. The difference between these regulatory-based amounts and the periodic benefit cost calculated pursuant to GAAP is deferred as a regulatory asset or liability and amortized to expense over periods in which this difference will be recovered in rates, as authorized by the applicable utility commission. Regulatory deferrals related to net periodic benefit cost were not material for the three months ended March 31, 2017.