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Components of net periodic benefit cost (Tables)
 9 Months Ended
Sep. 30, 2017
Components of net periodic benefit cost [Abstract]  
Schedule of net periodic benefit cost
 
Pension Benefits
 
Three months ended September 30,

Nine months ended September 30,
 
2017
 
2016

2017
 
2016
Service cost
$
1,096

 
$
1,270

 
$
3,288

 
$
3,810

Interest cost
3,327

 
3,478

 
9,982

 
10,440

Expected return on plan assets
(4,750
)
 
(4,813
)
 
(14,615
)
 
(14,445
)
Amortization of prior service costs
27

 
28

 
80

 
84

Amortization of net loss
1,187

 
1,041

 
3,925

 
3,125

Net periodic benefit cost
$
887

 
$
1,004

 
$
2,660

 
$
3,014


 
Other Postretirement Benefits ("OPEB")
 
Three months ended September 30,
 
Nine months ended September 30,
 
2017
 
2016
 
2017
 
2016
Service cost
$
219

 
$
330

 
$
657

 
$
990

Interest cost
1,382

 
1,469

 
5,726

 
4,364

Amortization of prior service cost
(695
)
 
(695
)
 
(1,731
)
 
(2,085
)
Amortization of net loss
958

 
878

 
940

 
2,633

Net periodic benefit cost
$
1,864

 
$
1,982

 
$
5,592

 
$
5,902