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Retirement Plans (Tables)
 9 Months Ended
Sep. 30, 2017
Retirement Plans  
Defined Benefit Plan Disclosure [Line Items]  
Benefit costs
Our pension costs, which include costs for the qualified TRP plan and the nonqualified SERP plan, are presented in the following table (in thousands):
 
Quarter ended Sept. 30,
 
Nine months ended Sept. 30,
 
2017
 
2016
 
2017
 
2016
 
 
 
 
 
 
 
 
Service cost-benefits earned during the period
$
218

 
$
204

 
$
654

 
$
612

Interest cost on benefit obligation
5,990

 
6,449

 
17,971

 
19,636

Expected return on plan assets
(6,580
)
 
(6,691
)
 
(19,741
)
 
(20,073
)
Amortization of prior service cost
159

 
165

 
476

 
505

Amortization of actuarial loss
2,081

 
1,846

 
6,242

 
5,740

Expense for company-sponsored retirement plans
$
1,868

 
$
1,973

 
$
5,602

 
$
6,420