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Components of net periodic benefit cost
 3 Months Ended
Mar. 31, 2016
Components of net periodic benefit cost [Abstract]  
Components of net periodic benefit cost
Components of net periodic benefit cost
 
Pension Benefits
 

Three months ended March 31,
 

2016
2015
Service cost
 
$
1,270

$
1,704

Interest cost
 
3,480

3,388

Expected return on plan assets
 
(4,816
)
(5,494
)
Amortization of prior service costs
 
28

25

Amortization of net loss
 
1,043

620

Net periodic benefit cost
 
$
1,005

$
243


 
Other Postretirement Benefits ("OPEB")
 
 
Three months ended March 31,
 
 
2016
2015
Service cost
 
$
330

$
562

Interest cost
 
1,456

1,591

Amortization of prior service cost
 
(695
)
(961
)
Amortization of net loss
 
876

865

Net periodic benefit cost
 
$
1,967

$
2,057



Employer contributions
During the three months ended March 31, 2016, we made contributions of $464 to the qualified defined benefit and unqualified supplemental executive retirement benefits ("SERB") plans that we sponsor.