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Components of net periodic benefit cost
 6 Months Ended
Jun. 30, 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 June 30,

Six months ended June 30,
 
2017
 
2016

2017
 
2016
Service cost
$
968

 
$
1,270

 
$
2,192

 
$
2,540

Interest cost
3,323

 
3,482

 
6,655

 
6,962

Expected return on plan assets
(5,116
)
 
(4,816
)
 
(9,865
)
 
(9,632
)
Amortization of prior service costs
26

 
28

 
53

 
56

Amortization of net loss
1,506

 
1,041

 
2,738

 
2,084

Net periodic benefit cost
$
707

 
$
1,005

 
$
1,773

 
$
2,010


 
Other Postretirement Benefits ("OPEB")
 
Three months ended June 30,
 
Six months ended June 30,
 
2017
 
2016
 
2017
 
2016
Service cost
$
191

 
$
330

 
$
438

 
$
660

Interest cost
2,978

 
1,439

 
4,344

 
2,895

Amortization of prior service cost
(698
)
 
(695
)
 
(1,036
)
 
(1,390
)
Amortization of net loss
(929
)
 
879

 
(18
)
 
1,755

Net periodic benefit cost
$
1,542

 
$
1,953

 
$
3,728

 
$
3,920



Employer contributions
During the six months ended June 30, 2017, we made contributions of $874 to the qualified defined benefit and unqualified supplemental executive retirement benefit ("SERB") plans that we sponsor.