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Retirement Plans (Tables)
 6 Months Ended
Jun. 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 June 30,
 
Six months ended June 30,
 
2017
 
2016
 
2017
 
2016
 
 
 
 
 
 
 
 
Service cost-benefits earned during the period
$
311

 
$
158

 
$
436

 
$
408

Interest cost on benefit obligation
6,056

 
6,837

 
11,981

 
13,187

Expected return on plan assets
(6,511
)
 
(6,632
)
 
(13,161
)
 
(13,382
)
Amortization of prior service cost
167

 
190

 
317

 
340

Amortization of actuarial loss
2,187

 
2,194

 
4,162

 
3,894

Expense for company-sponsored retirement plans
$
2,210

 
$
2,747

 
$
3,735

 
$
4,447