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Components of net periodic benefit cost
 9 Months Ended
Sep. 30, 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 September 30,

Nine months ended September 30,
 
2016
2015

2016
2015
Service cost
$
1,270

$
2,074

 
$
3,810

$
5,287

Interest cost
3,478

3,348

 
10,440

10,003

Expected return on plan assets
(4,813
)
(5,262
)
 
(14,445
)
(16,025
)
Amortization of prior service costs
28

28

 
84

83

Amortization of net loss
1,041

1,025

 
3,125

3,049

Curtailment
—

377

 
—

365

Net periodic benefit cost
$
1,004

$
1,590

 
$
3,014

$
2,762


 
Other Postretirement Benefits ("OPEB")
 
Three months ended September 30,
 
Nine months ended September 30,
 
2016
2015
 
2016
2015
Service cost
$
330

$
468

 
$
990

$
1,517

Interest cost
1,469

1,542

 
4,364

4,523

Amortization of prior service cost
(695
)
(1,007
)
 
(2,085
)
(2,929
)
Amortization of net loss
878

874

 
2,633

2,973

Net periodic benefit cost
$
1,982

$
1,877

 
$
5,902

$
6,084



Employer contributions
During the nine months ended September 30, 2016, we made contributions of $1,338 to the qualified defined benefit and unqualified supplemental executive retirement benefit ("SERB") plans that we sponsor.