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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
 
$
6,466

 
$
7,065

 
$
192

 
$
232

Interest cost
 
13,849

 
12,714

 
4,518

 
4,375

Expected return on plan assets
 
(17,692
)
 
(18,148
)
 
(5,575
)
 
(5,951
)
Amortization of prior service credit
 
(807
)
 
(784
)
 
(1,562
)
 
(1,562
)
Amortization of net loss
 
6,693

 
9,094

 
483

 
619

Net periodic benefit cost (credit)
 
8,509

 
9,941

 
(1,944
)
 
(2,287
)
Credits (costs) not recognized due to the effects of regulation
 
766

 
(366
)
 
—

 
—

Net benefit cost (credit) recognized for financial reporting
 
$
9,275

 
$
9,575

 
$
(1,944
)
 
$
(2,287
)