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Benefit Plans and Other Postretirement Benefits (Tables)
 9 Months Ended
Sep. 30, 2017
Retirement Benefits [Abstract]  
Components of Net Periodic Benefit Cost (Credit)
Components of Net Periodic Benefit Cost (Credit)
 
 
 
 
 
 
 
 
 
 
 
Three Months Ended Sept. 30
 
 
2017
 
2016
 
2017
 
2016
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
6,820

 
$
6,487

 
$
192

 
$
192

Interest cost
 
12,639

 
13,852

 
4,191

 
4,518

Expected return on plan assets
 
(17,134
)
 
(17,692
)
 
(5,476
)
 
(5,575
)
Amortization of prior service credit
 
(803
)
 
(801
)
 
(1,562
)
 
(1,562
)
Amortization of net loss
 
7,089

 
6,692

 
961

 
483

Net periodic benefit cost (credit)
 
8,611

 
8,538

 
(1,694
)
 
(1,944
)
Credits not recognized due to the effects of regulation
 
736

 
682

 
—

 
—

Net benefit cost (credit) recognized for financial reporting
 
$
9,347

 
$
9,220

 
$
(1,694
)
 
$
(1,944
)

 
 
 
 
 
 
 
 
 
 
 
Nine Months Ended Sept. 30
 
 
2017
 
2016
 
2017
 
2016
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
20,460

 
$
19,445

 
$
576

 
$
576

Interest cost
 
37,919

 
41,554

 
12,573

 
13,554

Expected return on plan assets
 
(51,402
)
 
(53,076
)
 
(16,428
)
 
(16,725
)
Amortization of prior service credit
 
(2,409
)
 
(2,408
)
 
(4,686
)
 
(4,686
)
Amortization of net loss
 
21,267

 
20,078

 
2,883

 
1,449

Net periodic benefit cost (credit)
 
25,835

 
25,593

 
(5,082
)
 
(5,832
)
Credits not recognized due to the effects of regulation
 
1,898

 
1,947

 
—

 
—

Net benefit cost (credit) recognized for financial reporting
 
$
27,733

 
$
27,540

 
$
(5,082
)
 
$
(5,832
)