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Benefit Plans and Other Postretirement Benefits (Tables)
 6 Months Ended
Jun. 30, 2015
Compensation and Retirement Disclosure [Abstract]  
Components of Net Periodic Benefit Cost (Credit)
Components of Net Periodic Benefit Cost (Credit)
 
 
Three Months Ended June 30
 
 
2015
 
2014
 
2015
 
2014
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
2,751

 
$
2,296

 
$
238

 
$
311

Interest cost
 
5,046

 
5,111

 
437

 
643

Expected return on plan assets
 
(7,152
)
 
(6,545
)
 
(635
)
 
(811
)
Amortization of prior service cost (credit)
 
10

 
13

 
(100
)
 
(101
)
Amortization of net loss (gain)
 
3,772

 
3,331

 
(160
)
 
(81
)
Net periodic benefit cost (credit)
 
4,427

 
4,206

 
(220
)
 
(39
)
Credits recognized due to the effects of regulation
 
686

 
708

 
—

 
—

Net benefit cost (credit) recognized for financial reporting
 
$
5,113

 
$
4,914

 
$
(220
)
 
$
(39
)

 
 
 
 
 
 
 
 
 
 
 
Six Months Ended June 30
 
 
2015
 
2014
 
2015
 
2014
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
5,503

 
$
4,592

 
$
477

 
$
623

Interest cost
 
10,092

 
10,222

 
873

 
1,286

Expected return on plan assets
 
(14,305
)
 
(13,090
)
 
(1,270
)
 
(1,623
)
Amortization of prior service cost (credit)
 
20

 
27

 
(200
)
 
(201
)
Amortization of net loss (gain)
 
7,544

 
6,663

 
(320
)
 
(161
)
Net periodic benefit cost (credit)
 
8,854

 
8,414

 
(440
)
 
(76
)
Credits recognized due to the effects of regulation
 
1,399

 
1,415

 
—

 
—

Net benefit cost (credit) recognized for financial reporting
 
$
10,253

 
$
9,829

 
$
(440
)
 
$
(76
)