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Benefit Plans and Other Postretirement Benefits (Tables)
 6 Months Ended
Jun. 30, 2016
Compensation and Retirement Disclosure [Abstract]  
Components of Net Periodic Benefit Cost (Credit)
Components of Net Periodic Benefit Cost (Credit)
 
 
Three Months Ended  
 June 30
 
 
2016
 
2015
 
2016
 
2015
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
2,440

 
$
2,751

 
$
194

 
$
238

Interest cost
 
5,315

 
5,046

 
455

 
437

Expected return on plan assets
 
(6,901
)
 
(7,152
)
 
(594
)
 
(635
)
Amortization of prior service cost (credit)
 
—

 
10

 
(100
)
 
(100
)
Amortization of net loss (gain)
 
2,997

 
3,772

 
(146
)
 
(160
)
Net periodic benefit cost (credit)
 
3,851

 
4,427

 
(191
)
 
(220
)
Credits recognized due to the effects of regulation
 
638

 
686

 
—

 
—

Net benefit cost (credit) recognized for financial reporting
 
$
4,489

 
$
5,113

 
$
(191
)
 
$
(220
)

 
 
 
 
 
 
 
 
 
 
 
Six Months Ended 
 June 30
 
 
2016
 
2015
 
2016
 
2015
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
4,880

 
$
5,503

 
$
388

 
$
477

Interest cost
 
10,630

 
10,092

 
910

 
873

Expected return on plan assets
 
(13,802
)
 
(14,305
)
 
(1,188
)
 
(1,270
)
Amortization of prior service cost (credit)
 
—

 
20

 
(200
)
 
(200
)
Amortization of net loss (gain)
 
5,994

 
7,544

 
(292
)
 
(320
)
Net periodic benefit cost (credit)
 
7,702

 
8,854

 
(382
)
 
(440
)
Credits recognized due to the effects of regulation
 
716

 
1,399

 
—

 
—

Net benefit cost (credit) recognized for financial reporting
 
$
8,418

 
$
10,253

 
$
(382
)
 
$
(440
)