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Benefit Plans and Other Postretirement Benefits (Tables)
 3 Months Ended
Mar. 31, 2014
Compensation and Retirement Disclosure [Abstract]  
Components of Net Periodic Benefit Cost
Components of Net Periodic Benefit Cost
 
 
Three Months Ended March 31
 
 
2014
 
2013
 
2014
 
2013
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health Care Benefits
Service cost
 
$
22,086

 
$
24,071

 
$
864

 
$
1,182

Interest cost
 
39,155

 
35,172

 
8,507

 
8,417

Expected return on plan assets
 
(51,801
)
 
(49,613
)
 
(8,489
)
 
(8,253
)
Amortization of transition obligation
 
—

 
—

 
—

 
206

Amortization of prior service (credit) cost
 
(437
)
 
1,468

 
(2,672
)
 
(2,438
)
Amortization of net loss
 
29,191

 
36,038

 
2,935

 
5,646

Net periodic benefit cost
 
38,194

 
47,136

 
1,145

 
4,760

Costs not recognized due to the effects of regulation
 
(7,052
)
 
(7,847
)
 
—

 
—

Net benefit cost recognized for financial reporting
 
$
31,142

 
$
39,289

 
$
1,145

 
$
4,760