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Components of net periodic benefit cost
 3 Months Ended
Mar. 31, 2017
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,
 

2017
2016
Service cost
 
$
1,224

$
1,270

Interest cost
 
3,332

3,480

Expected return on plan assets
 
(4,749
)
(4,816
)
Amortization of prior service costs
 
27

28

Amortization of net loss
 
1,232

1,043

Net periodic benefit cost
 
$
1,066

$
1,005


 
Other Postretirement Benefits ("OPEB")
 
 
Three months ended March 31,
 
 
2017
2016
Service cost
 
$
247

$
330

Interest cost
 
1,366

1,456

Amortization of prior service cost
 
(338
)
(695
)
Amortization of net loss
 
911

876

Net periodic benefit cost
 
$
2,186

$
1,967



Employer contributions
During the three months ended March 31, 2017, we made contributions of $437 to the qualified defined benefit and unqualified supplemental executive retirement benefit ("SERB") plans that we sponsor.