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Benefit Plans and Other Postretirement Benefits (Tables)
 3 Months Ended
Mar. 31, 2016
Compensation and Retirement Disclosure [Abstract]  
Components of Net Periodic Benefit Cost (Credit)
Components of Net Periodic Benefit Cost (Credit)
 
 
Three Months Ended March 31
 
 
2016
 
2015
 
2016
 
2015
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
2,440

 
$
2,752

 
$
194

 
$
239

Interest cost
 
5,315

 
5,046

 
455

 
436

Expected return on plan assets
 
(6,901
)
 
(7,153
)
 
(594
)
 
(635
)
Amortization of prior service cost (credit)
 
—

 
10

 
(100
)
 
(100
)
Amortization of net loss (gain)
 
2,997

 
3,772

 
(146
)
 
(160
)
Net periodic benefit cost (credit)
 
3,851

 
4,427

 
(191
)
 
(220
)
Credits recognized due to the effects of regulation
 
78

 
713

 
—

 
—

Net benefit cost (credit) recognized for financial reporting
 
$
3,929

 
$
5,140

 
$
(191
)
 
$
(220
)