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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 (Credit)
Components of Net Periodic Benefit Cost (Credit)
 
 
 
 
 
 
 
 
 
 
 
Three Months Ended March 31
 
 
2018
 
2017
 
2018
 
2017
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
7,271

 
$
6,820

 
$
152

 
$
192

Interest cost (a)
 
11,814

 
12,640

 
3,749

 
4,191

Expected return on plan assets (a)
 
(17,130
)
 
(17,134
)
 
(5,675
)
 
(5,476
)
Amortization of prior service credit (a)
 
(845
)
 
(803
)
 
(1,545
)
 
(1,562
)
Amortization of net loss (a)
 
7,815

 
7,089

 
1,021

 
961

Net periodic benefit cost (credit)
 
8,925

 
8,612

 
(2,298
)
 
(1,694
)
Credits not recognized due to the effects of regulation
 
1,475

 
736

 
—

 
—

Net benefit cost (credit) recognized for financial reporting
 
$
10,400

 
$
9,348

 
$
(2,298
)
 
$
(1,694
)