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Benefit Plans and Other Postretirement Benefits (Tables)
 9 Months Ended
Sep. 30, 2016
Compensation and Retirement Disclosure [Abstract]  
Components of Net Periodic Benefit Cost (Credit)
Components of Net Periodic Benefit Cost (Credit)
 
 
Three Months Ended Sept. 30
 
 
2016
 
2015
 
2016
 
2015
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
6,487

 
$
7,065

 
$
192

 
$
232

Interest cost
 
13,852

 
12,714

 
4,518

 
4,375

Expected return on plan assets
 
(17,692
)
 
(18,147
)
 
(5,575
)
 
(5,951
)
Amortization of prior service credit
 
(801
)
 
(784
)
 
(1,562
)
 
(1,562
)
Amortization of net loss
 
6,692

 
9,094

 
483

 
618

Net periodic benefit cost (credit)
 
8,538

 
9,942

 
(1,944
)
 
(2,288
)
Credits (costs) not recognized due to the effects of regulation
 
682

 
(366
)
 
—

 
—

Net benefit cost (credit) recognized for financial reporting
 
$
9,220

 
$
9,576

 
$
(1,944
)
 
$
(2,288
)

 
 
 
 
 
 
 
 
 
 
 
Nine Months Ended Sept. 30
 
 
2016
 
2015
 
2016
 
2015
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
19,445

 
$
21,195

 
$
576

 
$
696

Interest cost
 
41,554

 
38,142

 
13,554

 
13,124

Expected return on plan assets
 
(53,076
)
 
(54,442
)
 
(16,725
)
 
(17,852
)
Amortization of prior service credit
 
(2,408
)
 
(2,352
)
 
(4,686
)
 
(4,686
)
Amortization of net loss
 
20,078

 
27,282

 
1,449

 
1,856

Net periodic benefit cost (credit)
 
25,593

 
29,825

 
(5,832
)
 
(6,862
)
Credits (cost) not recognized due to the effects of regulation
 
1,947

 
(1,098
)
 
—

 
—

Net benefit cost (credit) recognized for financial reporting
 
$
27,540

 
$
28,727

 
$
(5,832
)
 
$
(6,862
)