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Retirement Plans and Other Benefits (Tables)
 3 Months Ended
Mar. 31, 2015
Compensation and Retirement Disclosure [Abstract]  
Schedule of net periodic benefit costs and the portion of these costs charged to expense (including administrative costs and excluding amounts capitalized as overhead construction, billed to electric plant participants or charged or amortized to the regulatory asset)
The following table provides details of the plans’ net periodic benefit costs and the portion of these costs charged to expense (including administrative costs and excluding amounts capitalized as overhead construction, billed to electric plant participants or charged or amortized to the regulatory asset) (dollars in millions):

 
Pension Benefits
 
 
Other Benefits
 
Three Months Ended 
 March 31,
 
 
Three Months Ended 
 March 31,
 
2015
 
2014
 
 
2015
 
2014
Service cost — benefits earned during the period
$
16

 
$
15

 
 
$
4

 
$
5

Interest cost on benefit obligation
31

 
32

 
 
7

 
11

Expected return on plan assets
(45
)
 
(40
)
 
 
(9
)
 
(12
)
Amortization of:
 

 
 

 
 
 

 
 

Prior service cost
—

 
—

 
 
(10
)
 
—

Net actuarial loss
8

 
2

 
 
2

 
—

Net periodic benefit cost
$
10

 
$
9

 
 
$
(6
)
 
$
4

Portion of cost charged to expense
$
6

 
$
5

 
 
$
(2
)
 
$
3