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Retirement Plans and Other Postretirement Benefits (Tables)
 9 Months Ended
Sep. 30, 2017
Retirement Benefits [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 or billed to electric plant participants) (dollars in thousands):

 
Pension Benefits
 
Other Benefits
 
Three Months Ended 
 September 30,
 
Nine Months Ended 
 September 30,
 
Three Months Ended 
 September 30,
 
Nine Months Ended 
 September 30,
 
2017
 
2016
 
2017
 
2016
 
2017
 
2016
 
2017
 
2016
Service cost — benefits earned during the period
$
13,715

 
$
13,448

 
$
41,144

 
$
40,344

 
$
4,280

 
$
3,748

 
$
12,839

 
$
11,245

Interest cost on benefit obligation
32,439

 
32,912

 
97,316

 
98,735

 
7,490

 
7,430

 
22,470

 
22,291

Expected return on plan assets
(43,568
)
 
(43,477
)
 
(130,703
)
 
(130,429
)
 
(13,350
)
 
(9,123
)
 
(40,051
)
 
(27,371
)
Amortization of:
 

 
 
 
 

 
 

 
 

 
 

 
 

 
 

Prior service cost (credit)
20

 
132

 
61

 
395

 
(9,461
)
 
(9,471
)
 
(28,382
)
 
(28,413
)
Net actuarial loss
11,975

 
10,179

 
35,924

 
30,538

 
1,279

 
1,147

 
3,838

 
3,442

Net periodic benefit cost
$
14,581

 
$
13,194

 
$
43,742

 
$
39,583

 
$
(9,762
)
 
$
(6,269
)
 
$
(29,286
)
 
$
(18,806
)
Portion of cost charged to expense
$
7,231

 
$
6,476

 
$
21,692

 
$
19,427

 
$
(4,841
)
 
$
(3,077
)
 
$
(14,523
)
 
$
(9,230
)