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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
 
 
Three Months Ended March 31
 
 
2016
 
2015
 
2016
 
2015
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
7,077

 
$
7,889

 
$
31

 
$
40

Interest cost
 
11,358

 
10,804

 
981

 
954

Expected return on plan assets
 
(15,236
)
 
(15,708
)
 
(43
)
 
(30
)
Amortization of prior service cost (credit)
 
234

 
234

 
(759
)
 
(759
)
Amortization of net loss
 
9,194

 
11,548

 
401

 
523

Net periodic benefit cost
 
12,627

 
14,767

 
611

 
728

Costs not recognized due to the effects of regulation
 
(5,296
)
 
(7,843
)
 
—

 
—

Net benefit cost recognized for financial reporting
 
$
7,331

 
$
6,924

 
$
611

 
$
728