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Benefit Plans and Other Postretirement Benefits (Tables)
 3 Months Ended
Mar. 31, 2017
Compensation and Retirement Disclosure [Abstract]  
Components of Net Periodic Benefit Cost (Credit)
Components of Net Periodic Benefit Cost (Credit)
 
 
 
 
 
 
 
 
 

 
 
 
 
 
 
 
 
 
 
 
Three Months Ended March 31
 
 
2017
 
2016
 
2017
 
2016
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
6,820

 
$
6,466

 
$
192

 
$
192

Interest cost
 
12,640

 
13,849

 
4,191

 
4,518

Expected return on plan assets
 
(17,134
)
 
(17,692
)
 
(5,476
)
 
(5,575
)
Amortization of prior service credit
 
(803
)
 
(807
)
 
(1,562
)
 
(1,562
)
Amortization of net loss
 
7,089

 
6,693

 
961

 
483

Net periodic benefit cost (credit)
 
8,612

 
8,509

 
(1,694
)
 
(1,944
)
Credits not recognized due to the effects of regulation
 
736

 
766

 
—

 
—

Net benefit cost (credit) recognized for financial reporting
 
$
9,348

 
$
9,275

 
$
(1,694
)
 
$
(1,944
)