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Benefit Plans and Other Postretirement Benefits (Tables)
 3 Months Ended
Mar. 31, 2015
Compensation and Retirement Disclosure [Abstract]  
Components of Net Periodic Benefit Cost (Credit)
Components of Net Periodic Benefit Cost (Credit)
 
 
Three Months Ended March 31
 
 
2015
 
2014
 
2015
 
2014
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
2,752

 
$
2,296

 
$
239

 
$
312

Interest cost
 
5,046

 
5,111

 
436

 
643

Expected return on plan assets
 
(7,153
)
 
(6,545
)
 
(635
)
 
(812
)
Amortization of prior service cost (credit)
 
10

 
14

 
(100
)
 
(100
)
Amortization of net loss (gain)
 
3,772

 
3,332

 
(160
)
 
(80
)
Net periodic benefit cost (credit)
 
4,427

 
4,208

 
(220
)
 
(37
)
Credits recognized due to the effects of regulation
 
713

 
707

 
—

 
—

Net benefit cost (credit) recognized for financial reporting
 
$
5,140

 
$
4,915

 
$
(220
)
 
$
(37
)