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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
 
$
23,547

 
$
22,920

 
$
465

 
$
432

Interest cost
 
36,702

 
40,023

 
5,984

 
6,527

Expected return on plan assets
 
(52,317
)
 
(52,575
)
 
(6,156
)
 
(6,249
)
Amortization of prior service credit
 
(442
)
 
(484
)
 
(2,671
)
 
(2,672
)
Amortization of net loss
 
26,670

 
24,385

 
1,672

 
1,011

Net periodic benefit cost (credit)
 
34,160

 
34,269

 
(706
)
 
(951
)
Costs not recognized due to the effects of regulation
 
(4,015
)
 
(4,452
)
 
—

 
—

Net benefit cost (credit) recognized for financial reporting
 
$
30,145

 
$
29,817

 
$
(706
)
 
$
(951
)