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Benefit Plans and Other Postretirement Benefits (Tables)
 9 Months Ended
Sep. 30, 2013
Compensation and Retirement Disclosure [Abstract]  
Components of Net Periodic Benefit Cost
Components of Net Periodic Benefit Cost
 
 
Three Months Ended Sept. 30
 
 
2013
 
2012
 
2013
 
2012
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
6,302

 
$
5,679

 
$
803

 
$
706

Interest cost
 
11,540

 
12,776

 
5,934

 
6,131

Expected return on plan assets
 
(15,955
)
 
(16,326
)
 
(7,307
)
 
(6,264
)
Amortization of transition obligation
 
—

 
—

 
196

 
2,751

Amortization of prior service (credit) cost
 
(266
)
 
57

 
(1,229
)
 
(1,288
)
Amortization of net loss
 
10,855

 
8,552

 
3,490

 
2,734

Net periodic benefit cost
 
12,476

 
10,738

 
1,887

 
4,770

Additional cost recognized due to the effects of regulation
 
—

 
—

 
—

 
972

Net benefit cost recognized for financial reporting
 
$
12,476

 
$
10,738

 
$
1,887

 
$
5,742

 
 
Nine Months Ended Sept. 30
 
 
2013
 
2012
 
2013
 
2012
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
18,905

 
$
17,039

 
$
2,409

 
$
2,119

Interest cost
 
34,620

 
38,329

 
17,802

 
18,395

Expected return on plan assets
 
(47,865
)
 
(48,977
)
 
(21,921
)
 
(18,792
)
Amortization of transition obligation
 
—

 
—

 
588

 
8,253

Amortization of prior service (credit) cost
 
(798
)
 
171

 
(3,687
)
 
(3,863
)
Amortization of net loss
 
32,563

 
25,652

 
10,469

 
8,198

Net periodic benefit cost
 
37,425

 
32,214

 
5,660

 
14,310

Additional cost recognized due to the effects of regulation
 
—

 
—

 
—

 
2,918

Net benefit cost recognized for financial reporting
 
$
37,425

 
$
32,214

 
$
5,660

 
$
17,228