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Benefit Plans and Other Postretirement Benefits (Tables)
 6 Months Ended
Jun. 30, 2013
Compensation and Retirement Disclosure [Abstract]  
Components of Net Periodic Benefit Cost
Components of Net Periodic Benefit Cost
 
 
Three Months Ended June 30
 
 
2013
 
2012
 
2013
 
2012
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
2,403

 
$
2,127

 
$
342

 
$
302

Interest cost
 
4,477

 
4,920

 
588

 
745

Expected return on plan assets
 
(5,992
)
 
(6,217
)
 
(796
)
 
(674
)
Amortization of transition obligation
 
—

 
—

 
—

 
387

Amortization of prior service cost (credit)
 
217

 
359

 
(121
)
 
(37
)
Amortization of net loss (gain)
 
4,287

 
3,265

 
(1
)
 
345

Net periodic benefit cost
 
5,392

 
4,454

 
12

 
1,068

Costs not recognized due to the effects of regulation
 
(317
)
 
(1,075
)
 
—

 
—

Net benefit cost recognized for financial reporting
 
$
5,075

 
$
3,379

 
$
12

 
$
1,068


 
 
Six Months Ended June 30
 
 
2013
 
2012
 
2013
 
2012
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
4,807

 
$
4,260

 
$
684

 
$
630

Interest cost
 
8,954

 
9,800

 
1,176

 
1,416

Expected return on plan assets
 
(11,985
)
 
(12,464
)
 
(1,592
)
 
(1,351
)
Amortization of transition obligation
 
—

 
—

 
—

 
773

Amortization of prior service cost (credit)
 
435

 
719

 
(242
)
 
(74
)
Amortization of net loss (gain)
 
8,574

 
6,407

 
(3
)
 
628

Net periodic benefit cost
 
10,785

 
8,722

 
23

 
2,022

Costs not recognized due to the effects of regulation
 
(1,392
)
 
(2,150
)
 
—

 
—

Net benefit cost recognized for financial reporting
 
$
9,393

 
$
6,572

 
$
23

 
$
2,022