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Benefit Plans and Other Postretirement Benefits (Tables)
 3 Months Ended
Mar. 31, 2018
Retirement Benefits [Abstract]  
Components of Net Periodic Benefit Cost
Components of Net Periodic Benefit Cost
 
 
 
 
 
 
 
 
 
 
 
Three Months Ended March 31
 
 
2018
 
2017
 
2018
 
2017
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
1,195

 
$
1,154

 
$
9

 
$
7

Interest cost (a)
 
1,360

 
1,554

 
142

 
148

Expected return on plan assets (a)
 
(2,256
)
 
(2,295
)
 
(16
)
 
(8
)
Amortization of prior service cost (credit) (a)
 
(8
)
 
35

 
(88
)
 
(88
)
Amortization of net loss (a)
 
1,418

 
1,462

 
139

 
109

Net periodic benefit cost
 
$
1,709

 
$
1,910

 
$
186

 
$
168

Credits not recognized due to the effects of regulation

 
$
221

 
$
—

 
$
—

 
$
—

Net benefit cost recognized for financial reporting

 
$
1,930

 
$
1,910

 
$
186

 
$
168