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Components of net periodic benefit cost (Tables)
 6 Months Ended
Jun. 30, 2015
Components of net periodic benefit cost [Abstract]  
Schedule of net periodic benefit cost
 
Pension Benefits
 
Three months ended June 30,

Six months ended June 30,
 
2015
2014

2015
2014
Service cost
$
1,509

$
1,079

 
$
3,213

$
2,666

Interest cost
3,267

3,101

 
6,655

5,741

Expected return on plan assets
(5,269
)
(3,701
)
 
(10,763
)
(7,098
)
Amortization of prior service costs
30

9

 
55

18

Amortization of net loss
1,405

167

 
2,024

953

Curtailment
(12
)
—

 
(12
)
—

Net periodic benefit cost
$
930

$
655

 
$
1,172

$
2,280

 
Other Postretirement Benefits ("OPEB")
 
Three months ended June 30,
 
Six months ended June 30,
 
2015
2014
 
2015
2014
Service cost
$
487

$
117

 
$
1,049

$
794

Interest cost
1,390

1,695

 
2,981

3,206

Amortization of prior service cost
(961
)
(961
)
 
(1,922
)
(1,922
)
Amortization of net loss
1,234

828

 
2,099

1,852

Net periodic benefit cost
$
2,150

$
1,679

 
$
4,207

$
3,930