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Pension Plan and Other Postretirement Benefits (Tables)
 6 Months Ended
Jun. 30, 2018
Retirement Benefits [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 June 30
 
Pension Plan
 
Other Benefits
 
2018
 
2017
 
2018
 
2017
Service cost
$
7,402

 
$
5,865

 
$
2,550

 
$
2,019

Interest cost
5,995

 
5,791

 
1,484

 
1,491

Expected return on plan assets
(6,862
)
 
(6,029
)
 
(1,416
)
 
(1,218
)
Amortization of prior service cost
1,263

 
1,445

 
11

 
11

Recognized net actuarial loss
2,797

 
1,752

 
773

 
649

Net periodic benefit cost
$
10,595

 
$
8,824

 
$
3,402

 
$
2,952


 
Six Months Ended June 30
 
Pension Plan
 
Other Benefits
 
2018
 
2017
 
2018
 
2017
Service cost
$
14,804

 
$
11,730

 
$
5,100

 
$
4,038

Interest cost
11,989

 
11,581

 
2,969

 
2,982

Expected return on plan assets
(13,725
)
 
(12,058
)
 
(2,832
)
 
(2,436
)
Amortization of prior service cost
2,526

 
2,890

 
21

 
22

Recognized net actuarial loss
5,595

 
3,504

 
1,547

 
1,298

Net periodic benefit cost
$
21,189

 
$
17,647

 
$
6,805

 
$
5,904