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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
 
$
7,065

 
$
5,985

 
$
232

 
$
479

Interest cost
 
12,714

 
13,319

 
4,375

 
5,926

Expected return on plan assets
 
(18,148
)
 
(17,677
)
 
(5,951
)
 
(7,554
)
Amortization of prior service credit
 
(784
)
 
(773
)
 
(1,562
)
 
(1,562
)
Amortization of net loss
 
9,094

 
8,473

 
619

 
1,609

Net periodic benefit cost (credit)
 
9,941

 
9,327

 
(2,287
)
 
(1,102
)
Cost not recognized due to the effects of regulation
 
(366
)
 
—

 
—

 
—

Net benefit cost (credit) recognized for financial reporting
 
$
9,575

 
$
9,327

 
$
(2,287
)
 
$
(1,102
)