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Pension Plan and Other Postretirement Benefits (Tables)
 3 Months Ended
Mar. 31, 2017
Compensation and Retirement Disclosure [Abstract]  
Schedule of components of net periodic benefit costs for the pension plans and other postretirement benefits
The following table lists components of net periodic benefit costs for the pension plans and other postretirement benefits. The data listed under “pension plan” includes the qualified pension plan and the non-qualified supplemental executive retirement plan. The data listed under “other benefits” is for all other postretirement benefits.
 
 
Three Months Ended March 31
 
Pension Plan
 
Other Benefits
 
2017
 
2016
 
2017
 
2016
Service cost
$
5,865

 
$
5,067

 
$
2,019

 
$
2,304

Interest cost
5,791

 
5,453

 
1,491

 
1,800

Expected return on plan assets
(6,029
)
 
(5,454
)
 
(1,218
)
 
(1,046
)
Amortization of prior service cost
1,445

 
1,555

 
11

 
11

Recognized net actuarial loss
1,752

 
1,293

 
649

 
1,261

Net periodic benefit cost
$
8,824

 
$
7,914

 
$
2,952

 
$
4,330